GEOLOGY is the science which investigates the history of the earth. Its object is to trace the progress of our planet from the earliest beginnings of its separate existence, through its various stages of growth, down to the present condition of things. It seeks to determine the manner in which the evolution of the earth’s great surface features has been effected. It unravels the complicated processes by which each continent has been built up. It follows, even into detail, the varied sculpture of mountain and valley, crag and ravine. Nor does it confine itself merely to changes in the inorganic world. Geology shows that the present races of plants and animals are the descendants of other and very different races which once peopled the earth. It teaches that there has been a progress of the inhabitants, as well as one of the globe on which they dwelt; that each successive period in the earth’s history, since the introduction of living things, has been marked by characteristic types of the animal and vegetable kingdoms: and that, however imperfectly they have been preserved or may be deciphered, materials exist for a history of life upon the planet. The geographical distribution of existing faunas and floras is often made clear and intelligible by geological evidence; and in the same way light is thrown upon some of the remoter phases in the history of man himself. A subject so comprehensive as this must require a wide and varied basis of evidence. It is one of the characteristics of geology to gather evidence from sources which at first sight seem far removed from its scope, and to seek aid from almost every other leading branch of science. Thus, in dealing with the earliest conditions of the planet, the geologist must fully avail himself of the labours of the astronomer. Whatever is ascertainable by telescope, spectroscope, or chemical analysis, regarding the constitution of other heavenly bodies, has a geological bearing. The experiments of the physicist, undertaken to determine conditions of matter and of energy, may sometimes be taken as the starting-points of geological investigation. The work of the chemical laboratory forms the foundation of a vast and increasing mass of geological inquiry. To the botanist, the zoologist, even to the unscientific, if observant, traveller by land or sea, the geologist turns for information and assistance.
But while thus culling freely from the dominions of other sciences, geology claims as its peculiar territory the rocky framework of the globe. In the materials composing that framework, their composition and arrangement, the processes of their formation, the changes which they have undergone, and the terrestrial revolutions to which they bear witness, lie the main data of geological history. It is the task of the geologist to group these elements in such a way that they may be made to yield up their evidence as to the march of events in the evolution of the planet. He finds that they have in large measure arranged themselves in chronological sequence,—the oldest lying at the bottom and the newest at the top. Relics of an ancient sea-floor are overlaid by traces of a vanished land-surface; these are in turn covered by the deposits of a former lake, above which once more appear proofs of the return of the sea. Among these rocky records lie the lavas and ashes of long-extinct volcanoes. The ripple left upon the shore, the cracks formed by the sun’s heat upon the muddy bottom of a dried-up pool, the very imprint of the drops of a passing rain-shower, have all been accurately preserved, and yield their evidence as to geographical conditions widely different from those which exist where such markings are now found.
But it is mainly by the remains of plants and animals imbedded in the rocks that the geologist is guided in unravelling the chronological succession of geological changes. He has found that a certain order of appearance characterizes these organic remains, that each great group of rocks is marked by its own special types of life, and that these types can be recognized, and the rocks in which they occur can be correlated even in distant countries, and where no other means of comparison would be possible. At one moment he has to deal with the bones of some large mammal scattered through a deposit of superficial gravel, at another time with the minute foraminifers and ostracods of an upraised sea-bottom. Corals and crinoids crowded and crushed into a massive limestone where they lived and died, ferns and terrestrial plants matted together into a bed of coal where they originally grew, the scattered shells of a submarine sand-bank, the snails and lizards which lived and died within a hollow tree, the insects which have been imprisoned within the exuding resin of old forests, the footprints of birds and quadrupeds, the trails of worms left upon former shores—these, and innumerable other pieces of evidence, enable the geologist to realize in some measure what the faunas and floras of successive periods have been, and what geographical changes the site of every land has undergone.
It is evident that to deal successfully with these varied materials, a considerable acquaintance with different branches of science is needful. Especially necessary is a tolerably wide knowledge of the processes now at work in changing the surface of the earth, and of at least those forms of plant and animal life whose remains are apt to be preserved in geological deposits, or which in their structure and habitat enable us to realize what their forerunuers were. It has often been insisted upon that the present is the key to the past; and in a wide sense this assertion is eminently true. Only in proportion as we understand the present, where everything is open on all sides to the fullest investigation, can we expect to decipher the past, where so much is obscure, imperfectly preserved, or not preserved at all. A study of the existing economy of nature ought thus to be the foundation of the geologist’s training.
While, however, the present condition of things is thus employed, we must obviously be on our guard against the danger of unconsciously assuming that the phase of nature’s operations which we now witness has been the same in all past time, that geological changes have taken place in former ages in the manner and on the scale which we behold to-day, and that at the present time all the great geological processes, which have produced changes in the past eras of the earth’s history, are still existent and active. Of course we may assume this uniformity of action, and use the assumption as a working hypothesis. But it ought not to be allowed any firmer footing, nor on any account be suffered to blind us to the obvious truth that the few centuries wherein man has been observing nature form much too brief an interval, by which to measure the intensity of geological action in all past time. For aught we can tell the present is an era of quietude and slow change, compared with some of the eras which have preceded it. Nor can we be sure that, when we have explored every geological process now in progress, we have exhausted all the causes of change which, even iu comparatively recent times, have been at work.
In dealing with the Geological Record, as the accessible solid part of the globe is called, we cannot too vividly realize that at the best it forms but an imperfect chronicle. Geological history cannot be compiled from a full and continuous series of documents. From the very nature of its origin the record is necessarily fragmentary, and it has been further mutilated and obscured by the revolutions of successive ages. And even where the chronicle of events is continuous, it is of very unequal value in different places. In one case, for example, it may present us with an unbroken succession of deposits many thousands of feet in thickness, from which, however, only a few meagre facts as to geological history can be gleaned. In another instance it brings before us, within the compass of a few yards, the evidence of a most varied and complicated series of changes in physical geography, as well as an abundant and interesting suite of organic remains. These and other characteristics of the geological record will become more apparent and intelligible as we proceed in the study of the science.
In the systematic treatment of the subject the following arrangement will here be followed :—
1. The Cosmical Aspects of Geology.— Under this head we may consider the evidence supplied by astronomy and physics regarding the form and motions of the earth, the composition of the sun and planets, and the probable history of the solar system.
2. Geognosy,—an Inquiry into the Materials of the Earth’s Substance.— In this division we deal with the parts of the earth, its envelopes of air and water, its solid crust, and the probable condition of its interior. Especially, we have to study the more important minerals of the crust, and the chief rocks of which that crust is built up. In this way we lay a foundation of knowledge regarding the nature of the materials constituting the mass of the globe, and may next proceed to investigate the processes by which these materials are produced and altered.
3. Dynamical Geology embraces an investigation of the various agencies whereby the rocks of the earth’s crust are formed and metamorphosed, and by which changes are effected upon the distribution of sea and land, and upon the forms of terrestrial surfaces. Such an inquiry necessitates a careful study of the existing geological economy of nature, and forms a fitting introduction to the investigation of the geological changes of former periods. This and the previous section include most of what is embraced under Physical Geography; and for the reason stated under that heading the subject will here be treated more in detail than is usual in geological treatises.
4. Structural Geology, or the Architecture of the Earth.— We now advance to consider how the various materials composing the crust of the earth have been arranged. We learn that some have been formed in beds or strata on the floor of the sea, that others have been built up by the slow aggregation of organic forms, that others have been poured out in a molten condition or in showers of loose dust from subterranean sources. We further find that, though originally laid down in almost horizontal beds, the rocks have subsequently been crumpled, contorted, and dislocated, that they have been incessantly worn down, and have often been depressed and buried beneath later accumulations.
5. Palaeontological Geology.— This branch of the subject deals with the organic forms which are found preserved in the crust of the earth. It includes such questions as the relations between extinct and living types, the laws which appear to have governed the distribution of life in time and in space, the relative importance of different genera of animals in geological inquiry, the nature and use of the evidence from orgauic remains regarding former conditions of physical geography. This subject will be more properly discussed in the article Palaeontology, and will therefore be only cursorily treated in the following pages.
6. Stratigraphical Geology. — This section might be called geological history. It works out the chronological succession of the great formations of the earth’s crust, and endeavours to trace the sequence of events of which they contain the record. More particularly it determines the order of succession of the various plants and animals which in past time have peopled the earth, and thus ascertains what has been the grand march of life upon the planet.
7. Physiographical Geology, starting from the basis of fact laid down by stratigraphical geology regarding former geographical changes, embraces an inquiry into the origin and history of the features of the earth’s surface—continental ridges and ocean basins, plains, valleys, and mountains. It explains the causes on which local differences of scenery depend, and shows under what very different circumstances, and at what widely separated intervals, the hills and mountains, even of a single country, have been produced.
PART I.—COSMICAL ASPECTS OF GEOLOGY.
Before geology had attained to the position of an inductive science, it was customary to begin all investigations into the history of the earth by propounding or adopting some more or less fanciful hypothesis in explanation of the origin of our planet, or even of the universe. Such preliminary notions were looked upon as essential to a right understanding of the manner in which the materials of the globe had been put together. To the illustrious James Hutton (1785) geologists are indebted for strenuously upholding the doctrine that it is no part of the province of geology to discuss the origin of things. He taught them that in the materials from which geological evidence is to be compiled there can be found “no traces of a beginning, no prospect of an end.” In England, mainly to the influence of the school which he founded, and to the subsequent rise of the Geological Society (1807), which resolved to collect facts instead of fighting over hypotheses, is due the disappearance of the crude and unscientific cosmologies by which the writings of the earlier geologists were distinguished.
But there can now be little doubt that in the reaction against those visionary and often grotesque speculations, geologists were carried too far in an opposite direction. In allowing themselves to believe that geology had nothing to do with questions of cosmogony, they gradually grew up in the conviction that such questions could never be other than mere speculation, interesting or amusing as a theme for the employment of the fancy, but hardly coming within the domain of sober and inductive science. Nor would they soon have been awakened out of this belief by anything in their own science. It is still true that in the data with which they are accustomed to deal, as comprising the sum of geological evidence, there can be found no trace of a beginning. The oldest rocks which have been discovered on any part of the globe have probably been derived from other rocks older than themselves. Geology by itself has not yet revealed, and is little likely ever to reveal, a trace of the first solid crust of our globe. If then geological history is to be compiled from direct evidence furnished by the rocks of the earth, it cannot begin at the beginning of things, but must be content to date its first chapter from the earliest period of which any record has been preserved among the rocks.
Nevertheless, though geology in its usual restricted sense has been, and must ever be, unable to reveal the earliest history of our planet, it no longer ignores, as mere speculation, what is attempted in this subject by its sister sciences. Astronomy, physics, and chemistry have in late years all contributed to cast much light on the earlier stages of the earth’s existence, previous to the beginning of what is commonly regarded as geological history. But whatever extends our knowledge of the former conditions of our globe may be legitimately claimed as part of the domain of geology. If this branch of inquiry therefore is to continue worthy of its name as the science of the earth, it must take cognizance of these recent contributions from other sciences. It must no longer be content to begin its annals with the records of the oldest rocks, but must endeavour to grope its way through the ages which preceded the formation of any rocks. Thanks to the results achieved with the telescope, the spectroscope, and the chemical laboratory, the story of these earliest ages of our earth is every year becoming more definite and intelligible.
Relations of the Earth in the Solar System.
Before entering upon the study of the structure and history of the earth, we may with advantage consider the general relations of our planet to the solar system, especially in view of its origin and history. It is now regarded as in the highest degree probable that all the members of that system have had a common origin. The investigations of recent yeare have revived and given a new form and meaning to the well-known nebular hypothesis, in which Laplace sketched the progress of the system from the state of an original nebula to its existing condition of a central incandescent sun with surrounding cool planetary bodies. He supposed that the nebula, originally diffused at least as far as the furthest member of the system, began to condense towards the centre, and that in so doing it threw off or left behind successive rings which on disruption and further condensation assumed the form of planets, sometimes with a further formation of rings, which in the case of Saturn remain, though in other planets they have broken up and united into satellites.
According to this view we should expect that the matter composing the various members of the solar system should be everywhere nearly the same. The fact of condensation round centres, however, indicates at least differences of density throughout the nebula. Mr Lockyer has, indeed, suggested that the materials composing the nebula arranged themselves according to their respective densities, the lightest occupying the exterior and the heaviest the interior of the mass. And if we compare the densities of the various planets, they certainly seem to support this suggestion. These densities are shown in the following table, that of the earth being taken as the unit :— [table]
There is not indeed a strict progression in the diminution of density, but the fact remains that, while the planets near the sun are about twice as heavy as they would be if they consisted of such a substance as granite, towards the outer limits of the system they are composed of matter as light as cork. Again, in some cases, a similar relation has been observed between the densities of the satellites and their primaries. The moon, for example, has a density little more than half that of the earth. The first satellite of Jupiter is less dense, though the other three are found to be more dense than the planet. Further, in the condition of the earth itself, a very light gaseous atmosphere forms the outer portion, beneath which lies a heavier layer of water, while within these two envelopes the materials forming the solid substance of the planet are so arranged that the outer layer or crust has only about half the density of the whole globe. Mr Lockyer finds in the sun itself evidence of the same tendency towards a stratified arrangement in accordance with relative densities, as will be immediately further alluded to.
There seems therefore to be much probability in the hypothesis that, in the gradual condensation of the original nebula, each successive mass left behind represented the density of its parent layer, and consisted of progressively heavier matter. The remoter planets, with their low density and vast absorbing atmospheres, may be supposed to consist of metalloids like the outer parts of the sun’s atmosphere, while the interior planets are no doubt mainly metallic. The rupture of each planetary ring would, it is conceived, raise the temperature of the resultant nebulous planet to such a height as to allow the vapours to rearrange themselves by degrees in successive layers, or rather shells, according to density. And when the planet gave off a satellite, that body would, it might be expected, have the composition and density of the outer layers of its primary.
For many years the only evidence available as to the actual composition of other heavenly bodies than our own earth was furnished by the aerolites, meteorites, or falling stars, which from time to time have entered our atmosphere from planetary space, and have descended upon the surface of the globe. Subjected to chemical analysis these foreign bodies show considerable diversities of composition; but in no case have they yet yielded a trace of any element not already recognized among terrestrial materials. Upwards of twenty of our elements have been detected in aerolites, sometimes in the free state, sometimes combined with each other. More than half of them are metals, including iron, nickel, manganese, calcium, sodium, and potassium. There occur also carbon, silicon, phosphorus, sulphur, oxygen, nitrogen, and hydrogen. In some of their combinations these elements, as found in the meteoric stones, differ from their mode of occurrence in the accessible parts of the earth. Iron, for example, occurs as native metal, alloyed with a variable proportion (6 to 10 per cent.) of metallic nickel. But in other respects they closely resemble some of the familiar materials of the earth’s rocky crust. Thus we have such minerals as pyrite, apatite, olivine, augite, hornblende, and labradorite. No more reliable proof could be desired that some at least of the other members of the solar system are formed of the same materials as compose the earth.
But in recent years a far more precise and generally applicable method of research into the composition of the heavenly bodies has been found in the spectroscope. By means of this instrument, the light emitted from self-luminous bodies can be analysed in such a way as to show what elements are present in their intensely hot luminous vapour. When the light of a burning metal is examined with a properly-arranged prism, it is seen to give a dark band or spectrum which is traversed by certain vertical bright lines. This is termed a radiation-spectrum. Each element appears to have its own characteristic arrangement of lines, which retain the same relative position, intensity, and colours. Moreover, gases and the vapours of solid bodies are found to intercept those rays of light which they themselves emit. The spectrum of burning sodium, for example, shows two bright yellow lines. If therefore white light from some other source passes through the vapour of sodium, these two bright lines become dark lines, that portion of the light being exactly cut off which would have been given out by the sodium itself. This is called an absorp)tion-spectrum.
By this method of examination it has been ascertained that many of the elements of which our earth is composed exist in the state of incandescent vapour in the atmosphere of the sun. Among these are some of our most familiar metals—iron, zinc, copper, nickel, with sodium, magnesium, barium, calcium, and vast quantities of free hydrogen. Moreover, as Mr Lockyer has pointed out, these elements appear to succeed each other in relation to their respective densities. Thus the coronal atmosphere which, as seen in total eclipses, extends to so prodigious a distance beyond the orb of the sun, consists mainly of sub-incandescent hydrogen and another element which may be new. Beneath this external vaporous envelope lies the chromosphere where the vapours of incandescent hydrogen, calcium, and magnesium can be detected. Further inward the spot-zone shows the presence of sodium, titanium, &c.; while still lower, a layer (the reversing layer) of intensely hot vapours, lying probably next to the inner brilliant photosphere gives spectroscopic evidence of the existence of incandescent iron, manganese, cobalt, nickel, copper, and other well-known terrestrial metals.
The spectrosope has likewise been successfully applied by Mr Huggins and others to the observation of the fixed stars and nebulae, with the result of establishing a similarity of elements between our own system and other bodies in sidereal space. In the radiation spectra of nebulae Mr Huggins finds the hydrogen lines very prominent; and he conceives that they maybe glowing masses of that element. Sir William Thomson and Professor Tait have suggested, on the other hand, that they are more probably clouds of stones in rapid motion, perhaps in an atmosphere of hydrogen. Among the fixed stars absorption spectra have been recognized, pointing to a structure resembling that of our sun, viz., a solid or liquid incandescent nucleus, surrounded with an atmosphere of glowing vapour. According to Mr Lockyer, those stars or nebulae which have the highest temperature have the simplest spectra, and in proportion as they cool their materials become more and more differentiated into what we call elements. He remarks that the most brilliant or hottest stars show in their spectra only the lines of gases, as hydrogen. Cooler stars, like our sun, give indications of the presence, in addition, of the more stable metals—magnesium, sodium, calcium, iron. A still lower temperature he regards as marked by the appearance of the other metals, metalloids, and compounds, so that the older a star or planet is the more will it lose free hydrogen, till, when it comes to the condition of our earth, all its free hydrogen will have disappeared. According to this view the atoms of all the elements existed originally in the nebula dissociated from each other by reason of the intense heat-As the nebula gravitated towards its nucleus and cooled, the atoms came together, and the elements appeared in a certain order, beginning with hydrogen, and passing on through the metals and metalloids into compounds such as we find on our globe. The sun would thus be a star considerably advanced in the process of differentiation or association of its atoms. It contains, so far as we know, no metalloids or compounds, while stars like Sirius show the presence only of hydrogen, with but a feeble proportion of metallic vapours; and on the other hand, the red stars indicate by their spectra that their metallic vapours have entered into combination, whence it is inferred that their temperature is lower than that of our sun.
Further confirmation of these views as to the order of planetary evolution is furnished by the form and structure of the earth. Reference has already been made to the fact that the outer crust of our planet possesses only about half the density of the whole mass. It consists largely of metalloids—oxygen, silicon, carbon, sulphur, chlorine. On the other hand, lavas and mineral veins, which are believed to have been supplied from some considerable depth, contain abundance of metallic ingredients.
The form of the globe likewise points to a former fluid condition. As the result of computations from ten measured arcs of the meridian made by different observers between the latitudes of Sweden and the Cape of Good Hope, Bessel obtained the following data for the dimensions of the earth :— [table]
The equatorial circumference is thus a little less than 25,000 miles, and the difference between the polar and equatorial diameters (nearly 26⅛ miles) amounts to about π ⅜∙θth of the equatorial diameter. More recently, however, it has been shown that the oblate spheroid indicated by these measurements is not a symmetrical body, the equatorial circumference being an ellipse instead of a circle. The diameter of which the vertices touch the surface of the globe in longitudes 14° 23' E. and 194° 23' E. of Greenwich is nearly two miles longer than that at right angles to it.
In obedience to the influence of rotation on its axis, our planet would tend to assume exactly such a flattening at the poles as it has been proved to possess. This was discovered and demonstrated by Newton, and the amount of the ellipticity was actually calculated by him, long before any measurement had confirmed such a conclusion.
The tendency of modern research is thus to give probability to the conception that not only in our own solar system, but throughout the regions of space, there has been a common plan of evolution, and that the matter diffused through space in stars, nebulae, and systems is substantially the same as that with which we are familiar. Hence the study of the structure and probable history of the sun and the other heavenly bodies comes to possess an evident geological interest, seeing that it may yet enable us to carry back the story of our planet far beyond the domain of ordinary geological evidence, and upon data not less reliable than those furnished by the rocks of the earth’s crust.
II. The Movements of the Earth in their Geological Relations.
We are here concerned only with those aspects of the earth’s motions which materially influence the progress of geological phenomena.
1. Rotation.— In obedience to the impulse communicated to it at its original separation, the earth rotates on its axis. This movement is completed in about 24 hours, and to it is due the succession of day and night. So far as observation has yet gone, this movement is uniform, though recent calculations of the influence of the tides in retarding rotation tend to show that a very slow diminution of the angular velocity is in progress. Thiβ velocity varies relatively in different places, according to their position on the surface of the planet. At each pole there can be no velocity, but from these two points towards the equator there is a continually increasing rapidity of motion, till at the equator it is equal to a rate of 507 yards in a second.
To the rotation of the earth are due certain remarkable influences upon currents of air, which circulate either towards the equator or towards the poles. Currents which move from polar latitudes travel from parts of the earth’s surface where the velocity of rotation is small to others where it is great. Hence they lag behind, and their course is bent more and more westward. An air current quitting the north polar or north temperate regions as a north wind is deflected out of its course and becomes a north-east wind. On the opposite side of the globe a similar current setting out straight for the equator is changed into a south-east wind. This is the reason why the well-known trade-winds have their characteristic westward deflexion. On the other hand, a current setting out northwards or southwards from the equator passes into regions having a less velocity of rotation than it possesses itself, and hence it travels on in advance and is gradually deflected eastward. The aerial currents blowing steadily across the surface of the ocean produce currents in its waters which have a westward tendency communicated to them indirectly from the effect of rotation. A certain deflexion is said to be experienced by such rivers as flow in a meridional direction, like the Volga. Those which flow polewards are asserted to press upon their eastern rather than their western banks, while those which run in the opposite direction are stated to be thrown more against the western than the eastern. The reality of this action may be doubted.
2. Revolution.— Besides turning on its axis the globe performs a movement round the sun, termed revolution. This movement is accomplished in rather more than 365 days. It determines for us the length of our year, which is, in fact, merely the time required for one complete revolution. The path or orbit followed by the earth round the sun is not a perfect circle but an ellipse, with the sun in one of the foci, the mean distance of the earth from the sun being 92,400,000 miles. By slow secular variations the form of the orbit alternately approaches and recedes from that of a circle. At the nearest possible approach between the two bodies, owing to change in the ellipticity of the orbit, the earth is 14,368,200 miles nearer the sun than when at its greatest possible distance. These maxima and minima of distance occur at vast intervals of time. The last considerable eccentricity took place about 200,000 years ago, and the previous one more than half a million of years earlier. Since the amount of heat received by the earth from the sun is inversely as the square of the distance, eccentricity must have had in past time much effect upon the climate of the earth, as will be pointed out further on (section 7, p. 218).
3. Precession of the Equinoxes.— If the axis of the earth were perpendicular to the plane of its orbit, there would be equal day and night all the year round. But it is really inclined to that plane at an angle of 23∣ o . Hence our hemisphere is alternately presented to and turned away from the sun, and in this way brings us the familiar alternation of the seasons—the long days of summer and the short days of winter. Again, were the earth a perfect sphere of uniform density throughout, the position of its axis of rotation would not change. But owing to the protuberance along the equatorial regions, the attraction chiefly of the sun and moon tends to pull the axis aside, or to make it describe a conical movement like that of the axis of a top round the vertical. Hence each pole points successively to different stars. This movement, called the precession of the equinoxes, in combination with other planetary movements, completes its cycle in 21,000 years. At present the winter in our northern hemisphere coincides with the earth’s approach to the sun, or peπM⅛, In 10,500 years hence it will take place when the earth is at the farthest part of its orbit from the sun, or in aphelion. This movement acquires great importance when considered in connexion with the secular variations in the eccentricity of the orbit (see section 7).
4. Change in the Obliquity of the Ecliptic.— The angle at which the axis of the earth is inclined to the plane of its orbit does not remain strictly constant. It oscillates through long periods of time to the extent of about a degree and a half, or perhaps a little more, on either side of the mean. According to Dr Croll, this oscillation must have considerably affected former conditions of climate on the earth, since, when the obliquity is at its maximum, the polar regions receive about eight and a half days more of heat than they do at present—that is, about as much heat as lat. 76° enjoys at this day. This movement must have augmented the geological effects of precession, to which reference has just been made, and which are described in section 7.
5. Stability of the Earth's Axis.— That the axis of the earth’s rotation has successively shifted, and consequently that the poles have wandered to different points on the surface of the globe, has been maintained by geologists as the only possible explanation of certain remarkable conditions of climate, which can be proved to have formerly obtained within the Arctic Circle. Even as far north as lat. 81° 45' abundant remains of a vegetation indicative of a warm climate, and including a bed of coal 25 to 30 feet thick, have been found in situ. It is contended that where these plants lived the ground could not have been permanently frozen or covered for most of the year with thick snow. In explanation of the difficulty, it has been suggested that the north pole did not occupy its present position, and that the locality where the plants occur lay in more southerly latitudes. Without at present entering on the discussion of the question whether the geological evidence requires necessarily so important a geographical change, let us consider how far a shifting of the axis of rotation has been a possible cause of change during that section of geological time for which there are records among the stratified rocks.
From the time of Laplace astronomers have strenuously denied the possibility of any sensible change in the position of the axis of rotation. It has been urged that, since the planet acquired its present oblate spheroidal form, nothing but an utterly incredible amount of deformation could overcome the greater centrifugal force of the equatorial protuberance. It is certain, however, that the axis of rotation does not strictly coincide with the principal axis of inertia. Though the angular difference between them must always have been small, we can, without having recourse to any extra-mundane influence, recognize two causes which, whether or not they may suffice to produce any change in the position of the main axis of inertia, undoubtedly tend to do so. In the first place a widespread upheaval or depression of certain portions of the surface to a considerable vertical amount might shift that axis. In the second place an analogous result might arise from the denudation of continental masses of land and the consequent filling up of sea-basins. Sir William Thomson freely concedes the physical possibility of such changes. “We may not merely admit,” he says, “but assert as highly probable, that the axis of maximum inertia and axis of rotation, always very near one another, may have been in ancient times very far from their present geographical position, and may have gradually shifted through 10, 20, 30, 40, or more degrees, without at any time any perceptible sudden disturbance of either land or water.” But though, in the earlier ages of the planet’s history, stupendous deformations may have occurred, and the axis of rotation may have often shifted, it is only the alterations which can possibly have occurred during the accumulation of the stratified rocks that need to be taken into account in connexion with former changes of climate. If it can be shown therefore that the geographical revolutions necessary to shift the axis are incredibly stupendous in amount, improbable in their distribution, and completely at variance with geological evidence, we may reasonably withhold our belief from this alleged cause of the changes of elimate during geological history.
It has been estimated by Sir William Thomson “that an elevation of 600 feet, over a tract of the earth’s surface 1000 miles square and 10 miles in thickness, would only alter the position of the principal axis by one-third of a second, or 34 feet.” Mr George Darwin has shown that on the supposition of the earth’s complete rigidity no redistribution of matter in new continents could ever shift the pole from its primitive position more than 3°, but that, if its degree of rigidity is consistent with a periodical readjustment to a new form of equilibrium, the pole may have wandered some 10° or 15° from its primitive position, or have made a smaller excursion and returned to near its old place. In order, however, that these maximum effects should be produced, it would be necessary that each elevated area should have an area of depression corresponding in size and diametrically opposite to it, that they should lie on the same complete meridian, and that they should both be situated in lat. 45°. With all those coincident favourable circumstances, an effective elevation of π ⅜θ∙ of the earth’s surface to the extent of 10,000 feet would sjιift the pole 11⅜'i a similar elevation of - 5 ⅛ would move it 1° 46J'; of ⅛> 3° 17'; and of ⅜, 8° 4⅜'. Mr Darwin admits these to be superior limits to what is possible, and that, on the supposition of intumescence or contraction under the regions in question, the deflexion of the pole might be reduced to a quite insignificant amount.
Under the most favourable conditions, therefore, the possible amount of deviation of the pole from its first position would appear to have been too small to have seriously influenced the climates of the globe within geological history. If we grant that these changes were cumulative, and that the superior limit of deflexion was reached only after a long series of concurrent elevations and depressions, we must suppose that no movements took place elsewhere to counteract the effect of those about lat. 45° in the two hemispheres. But this is hardly credible. A glance at a geographical globe suffices to show how large a mass of land exists now both to the north and south of that latitude, especially in the northern hemisphere, and that the deepest parts of the ocean are not antipodal to the greatest heights of the land. These features of the earth’s surface are of old standing. There seems, indeed, to be no geological evidence in favour of any such geographical changes as could have produced even the comparatively small displacement of the axis considered possible by Mr Darwin.
In an ingenious suggestion Dr John Evans contended that, even without any sensible change in the position of the axis of rotation of the nucleus of the globe, there might be very considerable changes of latitude due to disturbance of the equilibrium of the shell by the upheaval or removal of masses of land between the equator and the poles, and to the consequent sliding of the shell over the nucleus until the equilibrium was restored. This hypothesis starts on the assumption of a thin crust enclosing a liquid or viscous interior—an assumption which, as will be shown in subsequent pages, is negatived by considerations in physics. The Rev. O. Fisher has suggested that the almost universal traces of present or former volcanic action, the evidence from the compressed strata in mountain regions that the crust of the earth must have a capacity for slipping towards certain lines, the great amount of horizontal compression of strata which can be proved to have been accomplished, and the secular changes of climate—notably the former warm climate near the north pole—furnish grounds for inquiry “whether a fluid substratum over a rigid nucleus would not be compatible with mechanical considerations, and whether, under those circumstances, changes in latitude would not result from unequal thickening of the crust.”
6. Changes of the Earth's Centre of Gravity.— Though no known geological operation seems to have been capable of producing an effective change in the position of the axis of rotation of the earth, there may have been variations in the posir tion of its centre of gravity. Any change of that kind must affect the ocean, which of course adjusts itself in relation to the earth’s centre of gravity. The enormous accumulation of ice at one pole during the maximum of eccentricity will displace the centre of gravity, and, as the result of this change will raise the level of the ocean in the glacial hemisphere, Dr Croll has estimated that, if the present mass of ice in the southern hemisphere is taken at 1000 feet thick extending down to lat. 60°, the transference of this mass to the northern hemisphere would raise the level of the sea 80 feet at the north pole. Other methods of calculation give different results. Mr Heath puts the rise at 128 feet; Archdeacon Pratt makes it more; while the Rev. O. Fisher gives it at 409 feet. More recently, in returning to this question, Dr Croll remarks “that the removal of two miles of ice from the Antarctic continent [and at present the mass of ice there is probably thicker than that] would displace the centre of gravity 190 feet, and the formation of a mass of ice equal to the one-half of this, on the Arctic regions, would carry the centre of gravity 95 feet farther, giving in all a total displacement of 285 feet, thus producing a rise of level at the north pole of 285 feet, and in the latitude of Edinburgh of 234 feet.” A very considerable additional displacement would arise from the increment of water to the mass of the ocean by the melting of the ice. Supposing half of the two miles of Antarctic ice to be replaced by an ice-cap of similar extent and one mile thick in the northern hemisphere, the other half being melted into water and increasing the mass of the ocean, Dr Croll estimates that from this source an extra 200 feet of rise would take place in the general ocean level, so that there would be a rise of 485 feet at the north pole, and 434 feet in the latitude of Edinburgh. There must thus have been an alternate submergence and emergence of the low polar lands due to the alternate shifting of the centre of gravity.
7. Influence of the Earth’s Movements upon Climate.— Although the treatment of this subject involves a reference to questions which must be discussed in their proper place in subsequent parts of this article, it will be most appropriately inserted here as a distinct and most important branch of the astronomical relations of geology. In later pages it will be shown that the climate of the earth has undergone many great vicissitudes during geological history,—for example, that a warm and genial temperature has once prevailed in arctic latitudes, while at another time snow and ice extended far down into the heart of Europe. Of ibis latter change, which took place within a comparatively recent geological period, the traces still remain remarkably fresh, and have excited great interest and discussion. It is known as the Glacial Period or Ice Age. But we now know that other similar periods of great cold probably preceded it at widely separated intervals.
Various theories have been proposed in explanation of such striking variations in climate. Some of these have appealed to a change in the position of the earth’s axis relatively to the mass of the planet (p. 216). Others have been based on the. notion that the earth may have passed through hot and cold regions of space. Others, again, have called in the effects of terrestrial changes, such as the distribution of land and sea, on the assumption that elevation of land about the poles must cool the temperature of the globe, while elevation round the equator would raise it. But as the changes of temperature have affected vast areas of the earth’s surface, while there is a total absence of all proof of any such enormous vicissitudes in physical geography as would be required, and since there is accumulating proof in favour of periodic alternations of climate, there is a growing conviction that no mere local changes could have sufficed, but that secular variations in climate must be assigned to some general and probably recurring cause.
By degrees geologists accustomed themselves to the belief that the cold of the glacial period was not due to mere terrestrial changée, but was to be explained somehow as the result of cosmical causes. Among the various suggestions, one deserves careful consideration—change in the eccen tricity of the earth’s orbit. Sir John Herschel pointed out many years ago that the direct effect of a high condition of eccentricity is to produce an unusually cold winter followed by a correspondingly hot summer on the hemisphere whose winter occurs in aphelion, while an equable condition of climate will at the same time prevail on the opposite hemisphere. But both hemispheres must receive precisely the same amount of solar heat, because the deficiency of heat resulting from the sun’s greater distance during one part of the year is exactly compensated by the greater length of that season. Sir John Herschel even considered that the direct effects of eccentricity must thus be nearly neutralized. As a like verdict was afterwards given by Arago, Humboldt, and others, geologists were satisfied that no important change of climate could be attributed to change of eccentricity.
It is to the luminous memoirs of Dr James Croll that geology is indebted for the first fruitful suggestion in this matter, and for the subsequent elaborate development of the whole subject of the physical causes on which climate depends. He has been so good as to draw up for this article the following summary of his views (taken chiefly from his paper in the Phil. Mag. for February 1870). The reader will find the subject fully worked out in Dr Croll’s work, Climate and Time, 1875.
“Assuming the mean distance of the sun to be 92,400,000 miles, then when the eccentricity is at its superior limit, ’ O7775, the distance of the sun from the earth, when the latter is in the aphelion of its orbit, is no less than 99,584,100 miles, and when in the perihelion it is only 85,215,900 miles. The earth is, therefore, 14,368,200 miles farther from the sun in the former than in the latter position. The direct heat of the sun being inversely as the square of the distance, it follows that the amount of heat received by the earth in these two positions will be as 19 to 26. The present eccentricity being Ό168, the earth’s distance during our northern winter is 90,847,680 miles. Suppose now that, from the precession of the equinoxes, winter in our northern hemisphere should happen when the earth is in the aphelion of its orbit, at the time that the orbit is at its greatest eccentricity; the earth would then be 8,736,420 miles farther from the sun in winter than it is at present. The direct heat of the sun would therefore, during winter, be one-fifth less and during summer one-fifth greater than now. This enormous difference would necessarily affect the climate to a very great extent. Were the winters under these circumstances to occur when the earth was in the perihelion of its orbit, the earth would then be 14,368,200 miles nearer the sun in winter than in summer. In this case the difference between winter and summer in our latitudes would be almost annihilated. But as the winters in the one hemisphere correspond with the summers in the other, it follows that while the one hemisphere would be enduring the greatest extremes of summer heat and winter cold, the other would be enjoying perpetual summer.
“It is quite tnιe that whatever may be the eccentricity of the earth's orbit, the two hemispheres must receive equal quantities of heat pci annum; for proximity to the sun is exactly compensated by the effect of swifter motion. The total amount of heat received from the sun between the two equinoxes is therefore the same in both halves of the year, whatever the eccentricity of the earth’s orbit may be. For example, whatever extra heat the southern hemisphere may at present receive per day from the sun during its summer months, owing to greater proximity to the sun, is exactly compensated by a corresponding loss arising from the shortness of the season; and, on the other hand, whatever deficiency of heat we in the northern hemisphere may at present have per day during our summer half-year, in consequence of the earth’s distance from the sun, is also exactly compensated by a corresponding length of season.
“It is well known, however, that those simple changes in the sun’s summer and winter distances would not alone produce a glacial epoch, and that physicists, confining their attention to the purely astronomical effects, were perfectly correct in affirming that no increase of eccentricity of the earth’s orbit could account for that epoch. But the important fact was overlooked that, although the glacial epoch could not result directly from an increase of eccentricity, it might nevertheless do so indirectly from physical agents that were brought into operation ∙as a result of an increase of eccentricity. The following is an outline of what these physical agents were, how they were brought into operation, and the way in which they may have led to the glacial epoch.
“With the eccentricity at its superior limit and the winter occurring in the aphelion, the earth would, as we have seen, be 8,736,420 miles farther from the sun during that season than at present. The reduction in the amount of heat received from the sun, owing to his increased distance, would lower the midwinter temperature to an enormous extent. In temperate regions the greater portion of the moisture of the air is at present precipitated in the form of rain, and the very small portion which falls as snow disappears in the course of a few weeks at most. But in the circumstances under consideration, the mean winter temperature would be lowered so much below the freezing-point that what now falls as rain during that season would then fall as snow. This is not all; the winters would then not only be colder than now, but they would also be much longer. At present the winters are nearly eight days shorter than the summers; but with the eccentricity at its superior limit and the winter solstice in aphelion, the length of the winters would exceed that of the summers by no fewer than thirty-six days. The lowering of the temperature and the lengthening of the winter would both tend to the same effect, viz., to increase the amount of snow accumulated during the winter; for, other things being equal, the larger the snow-accumulating period the greater the accumulation. It may be remarked, however, that the absolute quantity of heat received during winter is not affected by the decrease in the sun’s heat, for the additional length of the season compensates for this decrease. As regards the absolute amount of heat received, increase of the sun’s distance and lengthening of the winter are compensatory, but not so in regard to the amouut of snow accumulated. The consequence of this state of things would be that, at the commencement of the short summer, the ground would be covered with the winter’s accumulation of snow. Again, the presence of so much snow would lower the summer temperature, and prevent to a great extent the melting of the snow.
“There are three separate ways whereby accumulated masses of snow and ice tend to lower the summer temperature, viz. :—
“First, By means of direct radiation. No matter what the intensity of the sun’s rays may be, the temperature of snow and ice can never rise above 32°. Hence the presence of snow and ice tends by direct radiation to lower the temperature of all surrounding bodies to 32°. In Greenland, a country covered with snow and ice, the pitch has been seen to melt on the side of a ship exposed to the direct rays of the sun, while at the same time the surrounding air was far below the freezing-point; a thermometer exposed to the direct radiation of the sun has been observed to stand above 100°, while the air surrounding the instrument was actually 12° below the freezing-point. A similar experience has been recorded by travellers on the snow-fields of the Alps. These results, surprising as they no doubt appear, are what we ought to expect under the circumstances. Perfectly dry air seems to be nearly incapable of absorbing radiant heat. The entire radiation passes through it almost without any sensible absorption. Consequently the pitch on the side of the ship may be melted or the bulb of the thermometer raised to a high temperature by the direct rays of the sun, while the surrounding air remains intensely cold. The air is cooled by contact with the snow-covered ground, but is not heated by the radiation from the sun.
“When the air is charged with aqueous vapour, a similar cooling effect also takes place, but in a slightly different way. Air charged with aqueous vapour is a good absorber of radiant heat, but it can only absorb those rays which agree with it in period. It so happens that rays from snow and ice are, of all others, those which it absorbs best. The humid air will absorb the total radiation from the snow and ice, but it will allow the greater part of, if not nearly all, the sun’s rays to pass unabsorbed. But during the day, when the sun is shining, the radiation from the snow and ice to the air is negative; that is, the snow and ice cool the air by radiation. The result is, the air is cooled by radiation from the snow and ice (or rather, we should say, to the snow and ice) more rapidly than it is heated by the sun; and, as a consequence, in a country like Greenland, covered with an icy mantle, the temperature of the air, even during summer, seldom rises above the freezing-point. Snow is a good reflector, but as simple reflection does not change the character of the rays they would not be absorbed by the air, but would pass into stellar space. Were it not for the ice, the summers of North Greenland, owing to the continuance of the sun above the horizon, would be as warm as those of England; but, instead of this, the Greenland summers are colder than our winters. Cover India with an ice sheet, and its summers would be colder than those of England.
. * “Second, Another cause of the cooling effect is that the rays which fall on snow and ice are to a great extent reflected back into space. But those that are not reflected, but absorbed, do not raise the temperature, for they disappear in the mechanical work of melting the ice. For whatsoever may be the intensity of the sun’s heat the surface of the ground will be kept at 32° so long as the snow and ice remain unmelted.
“Third, Snow and ice lower the temperature by chilling the air and condensing the vapour into thick fogs. The great strength of the sun’s rays during summer, due to his nearness at that season, would, in the first place, tend to produce an increased amount of evaporation. But the presence of snow-clad mountains and an icy sea would chill the atmosphere and condense the vapour into thick fogs. The thick fogs and cloudy sky would effectually prevent the sun’s rays from reaching the earth, and the snow, in consequence, would remain unmelted during the entire summer. In fact, we have this very condition of things exemplified in some of the islands of the Southern Ocean at the present day. Sandwich Land, λvhich is in the same parallel of latitude as the north of Scotland, is covered with ice and snow the entire summer; and in the island of South Georgia, which is in the same parallel as the centre of England, the perpetual snow descends to the very sea-beach. Captain Sir James Ross found the perpetual snow at the sea-level at Admiralty Inlet, South Shetland, in lat. 64°; and while near this place the thermometer in the very middle of summer fell at night to 23° F. The reduction of the sun’s heat and lengthening of the winter, which would take place when the eccentricity is near to its superior limit and the winter in aphelion, would in this country produce a state of things perhaps as bad as, if not worse than, that which at present exists in South Georgia and South Shetland.
“The cause which above all others must tend to produce great changes of climate, is the deflexion of great ocean currents. A high condition of eccentricity tends, we have seen, to produce an accumulation of snow and ice on the hemisphere whose winters occur in aphelion. The accumulation of snow in turn tends to lower the summer temperature, cut off the sun’s rays, and retard the melting of the snow. In short, it tends to produce on that hemisphere a state of glaciation. Exactly opposite effects take place on the other hemisphere, which has its winter in perihelion. There the shortness of the winters, combined with the high temperature arising from the nearness of the sun, tends to prevent the accumulation of snow. The general result is that the one hemisphere is cooled and the other heated. This state of things now brings into play the agencies which lead to the deflexion of the Gulf-stream and other great ocean currents.
“Owing to the great difference between the temperature of the equator and the poles, there is a constant flow of air from the poles to the equator. It is to this that the trade-winds owe their existence. Now, as the strength of these winds will, ⅛s a general rule, depend upon the difference of temperature that may exist between the equator and higher latitudes, it follows that , the trades on the cold hemisphere will be stronger than those on the warm. When the polar and temperate regions of the one hemisphere are covered to a large extent with snow and ice, the air, as we have just seen, is kept almost at the freezing-point during both summer and winter. The trades on that hemisphere will, of necessity, be exceedingly powerful; while on the other hemisphere, where there is comparatively little snow or ice, and the air is warm, the trades will consequently be weak. Suppose now the northern hemisphere to be the cold one. The north-east trade winds of this hemisphere will far exceed in strength the south-east trade winds of the southern hemisphere. The median line between the trades will consequently lie to a very considerable distance to the south of the equator. We have a good example of this at the present day. The difference of temperature between the two hemispheres at present is but trifling to what it would be in the case under consideration; yet we find that the south-east trades of the Atlantic blow with greater force than the north-east trades, sometimes extending to 10° or 15° N. lat., whereas the north-east trades seldom blow south of the equator. The effect of the northern trades blowing across the equator to a great distance will be to impel the warm water of the tropics over into the Southern Ocean. But this is not all; not only would the median line of the trades be shifted southwards, but the great equatorial currents of the globe would also be shifted southwards.
‘ ‘ Let us now consider how this would affect the Gulf-stream. The South American continent is shaped somewhat in the form of a triangle, with one of its angular comers, called Cape St Roque, pointing eastwards. The equatorial current of the Atlantic impinges against this corner; but as the greater portion of the current lies a little to the north of the corner, it flows westwards into the Gulf of Mexico and forms the Gulf-stream. A considerable portion of the water, however, strikes the land to the south of the cape, and is deflected along the shore of Brazil into the Southern Ocean, forming what is known as the Brazilian current. Now, it is obvious that the shifting of the equatorial current of the Atlantic only a few degrees to the south of its present position—a thing which would certainly take place under the conditions which we have been detailing— would turn the entire current into the Brazilian branch, and instead of flowing chiefly into the Gulf of Mexico, as at present, it would all flow into the Southern Ocean, and the Gulf-stream would consequently be stopped. The stoppage of the Gulf-stream, combined with all those causes which we have just been considering, would place Europe under a glacial condition, while at the same time the temperature of the Southern Ocean would, in consequence of the enormous quantity of warm water received, have its temperature (already high from other causes) raised enormously. And what holds true in regard to the currents of the Atlantic holds also true, though perhaps not to the same extent, of the currents of the Pacific.
“If the breadth of the Gulf-stream be taken at 50 miles, its depth at 1000 feet, its mean velocity at 2 statute miles an hour, the temperature of the water when it leaves the Gulf at 65°, and the return current at 40° F., then, as has been shown in Climate and Time, chapter ii., the quantity of heat conveyed into the Atlantic by this stream is equal to one-fourth of all the heat received from the sun by that ocean from the Tropic of Cancer to the Arctic Circle. From principles discussed at considerable length in the chapter referred to, it is shown that, but for the Gulf-stream and other currents, London would have a mean annual tempeχ∙ature 40° lower than at present.
“But there is still another cause which must be noticed :—a strong undercurrent of air from the north implies an equally strong upper current to the north. Now if the effect of the undercurrent would be to impel the warm water at the equator to the south, the effect of the upper current would be to carry the aqueous vapour formed at the equator to the north; the upper current, on reaching the snow and ice of temperate regions, would deposit its moisture in the form of snow; so that it is probable that, noλvithstanding the great cold of the glacial epoch, the quantity of snow falling in the northern region would be enormous. This would be particularly the case during summer, when the earth would be in the perihelion and the heat at the equator great. The equator would be the furnace where evaporation would take place, and the snow and ice of temperate regions would act as a condenser.
“The foregoing considerations, as well as many others which might be stated, lead to the conclusion that, in order to raise the mean temperature of the globe, water should be placed along the equator, and not land, as was contended by Sir Charles Lyell and others. For if land be placed at the equator, the possibility of conveying the sun’s heat from the equatorial regions by means of ocean currents is prevented.”
Inter-Glacial Periods.— Allusion has already been made to the fact that there is accumulating evidence to show that changes of climate have been recurrent, and that this alternation or periodicity goes far to prove them to be due to some general or cosmical cause. Dr Croll has ingeniously shown that every long cold period in each hemisphere must have been interrupted by several shorter warm periods, and “when the one hemisphere,” he says, “is under glaciation, the other is enjoying a warm and equable climate. But, owing to the precession of the equinoxes, the condition of things on the two hemispheres must be reversed every 10,000 years or so. When the solstice passes the aphelion, a contrary process commences; the snow and ice gradually begin to diminish on the cold hemisphere and to make their appearance on the other hemisphere. The glaciated hemisphere turns by degrees warmer, and the warm hemisphere colder, and this continues to go on for a period of ten or twelve thousand years, until the winter solstice reaches the perihelion. By this time the conditions of the two hemispheres have been reversed; the formerly glaciated hemisphere has now become the warm one, and the warm hemisphere the glaciated. The transference of the ice from the one hemisphere to the other continues as long as the eccentricity remains at a high value. It is probable that, during the warm inter-glacial periods, Greenland and the Arctic regions would be comparatively free from snow and ice, and enjoying a temperate and equable climate.”
PART II.—GEOGNOSY:
AN INVESTIGATION OF THE MATERIALS OF THE EARTH’S SUBSTANCE.
Before we enter upon any discussion of the geological changes which our planet has undergone, it is needful first of all to study the materials of which the planet consists. It is from the evidence furnished by the nature and arrangement of these materials that geological history must be compiled.
Viewed in a broad way then, the earth may be considered as consisting of (1) two envelopes,—an outer one of gas completely surrounding the planet, and an inner one of water covering about three-fourths of the globe; and (2) a globe cool and solid on its surface but possessing a high internal temperature.
I. The Envelopes.
1. The Atmosphere.— The gaseous envelope to which the name of atmosphere is given extends at least to a distance of 40 or 45 miles from the earth’s surface, perhaps in a state of extreme tenuity to a much greater height. But its thickness must necessarily vary with latitude and changes in atmospheric pressure; the layer of air lying over the poles is not so deep as that which surrounds the equator.
Geologically considered, the atmosphere presents itself as an agent of change by virtue of its composition and the chemical reactions which it effects, its varying temperature and consequent influence in expanding and contracting rocks, and its movements.
Many speculations have been made regarding the chemical composition of the atmosphere during former geological periods. There can indeed be no doubt that it must originally have differed very greatly from its present condition. The oxygen which now forms fully a half of the outer crust of the earth was originally doubtless part of the atmosphere. So, too, the vast beds of coal found all over the world, in geological formations of many different ages, represent so much carbonic acid once present in the air. The chlorides in the sea likewise were probably carried down out of the atmosphere in the primitive condensation of the aqueous vapour. It has often been suggested that during the Carboniferous period the atmosphere must have been warmer and with more aqueous vapour and carbonic acid in its composition than at the present day, to admit of so luxuriant a flora as that from which the coal seams were formed. There seems, however, to be at present no method of arriving at any certainty on this subject.
As now existing, the atmosphere is considered to be normally a mechanical mixture of nearly 4 volumes of nitrogen and 1 of oxygen, with a minute proportion of carbonic acid, and still smaller quantities of other substances. Expressed in a tabular form this composition is as follows :— [table]
These quantities are liable to some variation according to locality. On the sea, for example, the proportion of carbonic acid is said to average about 0Ό3. In the air of streets and houses the proportion of oxygen diminishes, while that of carbonic acid increases, According to the minute researches of Dr Angus Smith, very pure air should contain not less than 20 - 99 of oxygen, with 0Ό30 of carbonic acid; but he found impure air in Manchester to have only 20 ’ 2l of oxygen, while the proportion of carbonic acid in that city during fog was ascertained to rise sometimes to 0Ό679, and in the pit of the theatre to the very large amount of 0 ’ 2734. Small as the percentage of carbonic acid in ordinary air may seem, yet the total amount of this gas in the whole atmosphere probably exceeds what would be disengaged if all the vegetable and animal matter on the earth’s surface were burnt.
The other substances present in much more minute quantities are gases, vapours, and solid particles. Of these by much the most important is the vapour of water, which is always present, but in very variable amount according to temperature, ranging from about 4 to a maximum of 16 grains in 1000 grains of air. It is this vapour which condenses into dew, rain, hail, and snow. In assuming a visible form, and descending through the atmosphere, it takes up a minute quantity of air, and of the different substances which the air may contain. Being caught by the rain, and held in solution or suspension, these substances can be best examined by analysing rain-water. In this way ammonia, nitric, sulphurous, and sulphuric acids, chlorides, various salts, solid carbon, inorganic dust, and organic matter have been detected. M. J. J. Pierre found as the result of his analysis that in the neighbourhood of Caen, in France, a hectare of land receives annually from the atmosphere, by means of rain— [table]
To these ingredients must be added traces of ammonia, various salts, and organic substances, besides others still undetermined. The powerful oxidizing agent ozone is present in variable but always minute quantities in the air.
The comparatively small but by no means unimportant proportions of these various components of the atmosphere are much more liable than the more essential gases to great variations. Chloride of sodium, for instance, is, as might be expected, particularly abundant in the air bordering the sea. Nitric acid, ammonia, and sulphuric acid appear in the air of towns most conspicuously. The organic substances present in the air are sometimes living germs, such as probably often lead to the propagation of disease, and sometimes mere fine particles of dust derived from the bodies of living or dead organisms.
2. The Oceans.— About three-fourths of the surface of the globe (or about 144,712,000 square miles) is covered by the irregular sheet of water known as the sea. Within the last ten years much new light has been thrown upon the depths, temperatures, and biological conditions of the ocean-basins, more particularly by the “Lightning,” “Porcupine,” and “Challenger” expeditions fitted out by the British Government. It has been ascertained that few parts of the Atlantic Ocean exceed 3000 fathoms, the deepest sounding obtained there being one taken about 100 miles north from the island of St Thomas, which gave 3875 fathoms, or rather les3 than 4⅜ miles. The Atlantic appears to have an average depth in its more open parts of from 2000 to 3000 fathoms or from about 2 to 3^ miles. In the Pacific Ocean the “Challenger” got soundings of 3950 and 4475
fathoms, or about 4⅛ and rather more than 5 miles. But these appear to mark exceptionally abyssal depressions, the average depth being, as in the Atlantic, between 2000 and 3000 fathoms. We may therefore assume, as probably not far from the truth, that the average depth of the ocean is about 2500 fathoms, or nearly 3 miles.
The water of the oceans is distinguished from the ordinary terrestrial waters by a higher specific gravity, and the presence of so large a proportion of saline ingredients as to impart a strongly salt taste. The average density of seawater is about 1Ό26, but it varies slightly in different parts even of the same ocean. According to the recent observations of Mr J. Y. Buchanan during the “Challenger” expedition, some of the heaviest sea-water occurs in the pathway of the trade-winds of the North Atlantic, where evaporation must be comparatively rapid, a density of l - 02781 being registered. Where, however, large rivers enter the sea, or where there is much melting ice, the density diminishes; Mr Buchanan found among the broken ice of the Antarctic Ocean that it had sunk to 1O2418.
The greater density of sea-water depends of course upon the salts which it contains in solution. There seems no reason to doubt that these salts are, in the main, parts of the original constitution of the sea, and thus that the sea has always been salt. It is also probable that, as in the case of the atmosphere, the composition of the ocean water has in former geological periods been very different from what it is now, and that it has acquired its present character only after many ages of slow change, and the abstraction of much mineral matter originally contained in it. There is evidence indeed among the geological formations that large quantities of lime, silica, chlorides, and sulphates have in the course of time been removed from the sea.
But it is evident also that, whatever may have been the original composition of the oceans, they have for a vast section of geological time been constantly receiving mineral matter in solution from the land. Every spring, brook, and river removes various salts from the rocks over which it moves, and these substances, thus dissolved, eventually find their way into the sea. Consequently sea-water ought to contain more or less traceable proportions of every substance which the terrestrial waters can remove from the land, in short, of probably every element present in the outer shell of the globe, for there seems to be no constituent of this earth which may not, under certain circumstances, be held in solution in water. Moreover, unless there be some counteracting process to remove these mineral ingredients, the ocean water ought to be growing, insensibly perhaps, but still assuredly, salter, for the supply of saline matter from the land is incessant. It has been ascertained indeed, with some approach to certainty, that the salinity of the Baltic and Mediterranean is gradually increasing.
The average proportion of saline constituents in the water of the great oceans far from land is about three and a half parts in every hundred of water. But in enclosed seas, receiving much fresh water, it is greatly reduced, while in those where evaporation predominates it is correspondingly augmented. Thus the Baltic water contains from one-seventh to nearly a half of the ordinary proportion in ocean water, while the Mediterranean contains sometimes one-sixth more than that proportion. The mineral constituents include the following average ratios of salts:—
[table]
Besides these chief ingredients, sea-water has yielded minute traces of iodine, fluorine, silica, phosphoric acid, carbonate of lime and magnesia, silver, lead, copper, arsenic. Doubtless more perfect analysis will greatly increase this list.
In addition to its salts sea-water always contains dissolved atmospheric gases. From the researches conducted during the voyage of the “Bonité” in the Atlantic and Indian Oceans it was estimated that the gases in 100 volumes of sea-water ranged from 1 ’ 85 to 3Ό4, or from two to three per cent. From observations made during the “Porcupine” cruise of 1868 it was inferred that the proportion of oxygen was greatest (25 ’ 1 per cent.) in the surface water, and least (19 ’ 5) in the bottom water, while that of carbonic acid was least at the top (20 - 7) and greatest (27 ’ 9) at the bottom, and that the action of the waves was partially to eliminate the latter gas and to increase the amount of oxygen. More recently, however, during the voyage of the “Challenger,” Mr J. *Y. Buchauan ascertained that the proportion of carbonic acid was always nearly the same for similar temperatures, the amount in the Atlantic surface water, between 20° and 25° C., being 0Ό466 gramme per litre, and in the surface Pacific water 0Ό268. He points out the curious fact that, according to his analyses, sea-water contains sometimes at least thirty times as much carbonic acid as an equal bulk of fresh water would do, and he traces the greater power of absorption to the presence of the sulphates.
II. The Solid Globe.
1. General Considerations.— Within the atmospheric and oceanic envelopes lies the inner solid globe. Reference has already been made to the comparative density of the planet among the other members of the solar system. In all speculation about the history of the earth, the density of the whole mass of the planet as compared with water— the standard to which the specific gravities of terrestrial bodies are referred—is a question of prime importance. Various methods have been employed for determining the earth’s density. The deflexion of the plumb-line on either side of a mountain of known structure and density, the time of oscillation of the pendulum at great heights, at the sea-level, and in deep mines, the comparative force of gravitation as measured by the torsion balance—each of these processes has been tried with the following various results :—
Plumb-line experiments on Schiehallien (Maskelyne and
Playfair) gave as the mean density of the earth 4’713
Do. on Arthurs Seat, Edinburgh, (James) 5 - 316
Pendulum experiments on Mont Cenis (Carlini and Giulio).. 4’950 Do. in Harton coal-pit, Newcastle (Airy) 6 ’ 565
Torsion balance experiments (Cavendish) 5 ’ 480
Do. do. (Baily) 5 ’ 660
Though these observations are somewhat discrepant, we may feel satisfied that the globe has a mean density neither much more nor much less than 5 5; that is to say, it is five and a half times heavier than one of the same dimensions formed of pure water. Now the average density of the materials which compose the accessible portions of the earth is between 2 , 5 and 3; so that the mean density of the whole globe is about twice as much as that of its outer part. We might therefore infer that the inside consists of much heavier materials than the outside, and consequently that the mass of the planet must contain at least two dissimilar portions—an exterior lighter crust or rind, and an interior heavier nucleus. But the effect of pressure must necessarily increase the specific gravity of the interior as will be alluded to further on.
2. The Crust. — It was formerly a prevalent belief that the exterior and interior of the globe differed from each other to such an extent that, while the outer parts were cool aud solid, the vastly more enormous inner part being intensely hot was more or less completely fluid. Hence the term “crust” was applied to the external rind in the usual sense of that word. This crust was variously computed to be 10, 15, 20 or more miles in thickness. For reasons which will be afterwards given, the idea of internal liquidity has been opposed by eminent physicists and is now abandoned by most geologists. The term “crust,” however, continues to be used as a convenient word to denote the cool, upper, or outer layer of the earth’s mass, accessible to human observation. It is in the structure and history of this crust that the main subjects of geological investigation are contained. It will therefore be fully treated of in the following parts of this article.
There are, however, some general views as to its composition and the arrangement of its materials, which may appropriately find a place in this preliminary section. Evidently our direct acquaintance with the chemical constitution of the globe must be limited to that of the crust, though by inference we may eventually reach highly probable conclusions regarding the constitution of the interior. Chemical research has discovered that sixty-four simple or as yet indecomposable bodies, called elements, in various proportions and compounds, constitute the accessible part of the crust. Of these, however, the great majority are comparatively of rare occurrence. The crust, so far as we can examine it, is mainly built up of about sixteen elements, which may be arranged in the two following groups, the most abundant bodies being placed first in each list :— [table]
By far the most abundant and important of these elements is oxygen. It forms about 23 per cent, by weight of air, 88 ’ 88 per cent, of water, and about a half of all the rocks which compose the visible portion or “crust” of the globe. Another metalloid, silicon, comes next in abundance. It is always united with oxygen, forming the mineral silica which, either alone or in combination with various metallic bases as silicates, constitutes a half of all the known mass of the globe. Of the remaining metalloids carbon and sulphur sometimes occur in the free state, but usually in combination with oxygen or some base or metal. Chlorine and fluorine are found associated with metallic bases. Hydrogen is properly a metal, and occurs chiefly in combination with oxygen as the oxide, water. Phosphorus occurs with oxygen principally in phosphate of lime.
Of the metals by far the most important in the architecture of the exterior of the earth is aluminium. In conjunction with oxygen and silicon it forms the basis of most crystalline rocks. Calcium, magnesium, potassium, and sodium, combined with oxygen, enter largely into the composition of rocks. Iron is the great colouring material in nature, most of the yellow, brown, red, and green hues of rocks being due to some of its combinations. The sixteen elements mentioned in the foregoing lists form about ninety-nine parts of the earth’s crust; the other elements constitute only about a hundredth part, though they include gold, silver, copper, tin, lead, and the other useful metals, iron excepted.
It is clear then that, so far as accessible to our observation, the outer portion of our planet consists mainly of metalloids, and its metallic constituents have in great part entered into combination with oxygen, so that the atmosphere contains the residue of that gas which has not yet united itself to terrestrial compounds. In a broad view of the arrangement of the chemical elements in the external crust, the suggestive speculation of Durocher deserves attention. He regarded all rocks as referable to two layers or magmas co-existing in the earth’s crust the one beneath the other, according to their specific gravities. The upper or outer layer, which he termed the acid or siliceous magma, contains an excess of silica, and has a mean density of 2 ’ 65. The lower or inner layer, which he called the basic magma, has from six to eight times more of the earthy bases and iron oxides, with a mean density of 2 ’ 96. To the former he assigned the early plutonic rocks, granite, felsite, <kc., with the more recent trachytes; to the latter he relegated all the heavy lavas, basalts, diorites, &c. The ratio of silica is 7 in the acid magma to 5 in the basic. Though the proportion of this acid or of the earthy and metallic bases cannot be regarded as any certain evidence of the geological date of rocks, nor of their probable depth of origin, it is nevertheless a fact that (with many important exceptions) the eruptive rocks of the older geological periods are very generally super-silicated and of lower specific gravity, while those of later time are very frequently poor in silica but rich in the earthy bases and in iron and manganese, with a consequent higher specific gravity. The latter, according to Durocher, have been forced up from a lower zone through the lighter siliceous crust.
3. The Interior or Huclezts.— Though we cannot hope ever to have direct acquaintance with more than the mere outside skin of our planet, we may be led to infer the irregular distribution of materials within the crust from the present distribution of land and water, and the observed differences in the amount of deflexion of the plumb-line near the sea and near mountain-chains. The fact that the southern hemisphere is almost wholly covered with water appears explicable only on the assumption of an excess of density in the mass of that portion of the planet. The existence of such a vast sheet of water as that of the Pacific Ocean is to be accounted for, says Archdeacon Pratt, by the presence of “some excess of matter in the solid parts of the earth between the Pacific Ocean and the earth’s centre, which retains the water in its place, otherwise the ocean would flow away to the other parts of the earth.” The same writer points out that a deflexion of the plumb-line towards the sea, which has in a number of cases been observed, indicates that “the density of the crust beneath the mountains must be less than that below the plains, and still less than that below the ocean-bed.” Apart therefore from the depressions of the earth’s surface in which the oceans lie, we must regard the internal density, whether of crust or nucleus, to be somewhat irregularly arranged,—there being an excess of heavy materials in the water hemisphere and beneath the ocean-beds as compared with the continental masses.
In our ignorance regarding the chemical constitution of the nucleus of our planet, an argument has sometimes been based upon the known fact that the specific gravity of that nucleus is about double that of the crust. This has been held by some writers to prove that the interior must consist of much heavier material, and is therefore probably metallic. But in so reasoning they forget that the effect of pressure ought to make the density of the nucleus much higher, even if the interior consisted of matter no heavier than the crust. In fact, we might argue for the probable comparative lightness of the substance composing the nucleus. That the total density of the planet does not greatly exceed its observed amount seems only explicable on the supposition that some antagonistic force counteracts the effects of pressure. The only force we can suppose capable of so acting is heat. But how and to what extent this counterbalancing takes place is still unknown.
If we regard the question from another point of view, however, the idea of a metallic nucleus seems not improbable. When the materials of the globe existed in a fluid condition, as they are usually supposed to have done, they would doubtless arrange themselves in accordance with their relative specific gravities. The denser elements would sink towards the centre, the lighter would remain outside. That this distribution has certainly taken place to some extent is evident from the structure of the envelopes and crust. It is what might be expected if the constitution of the globe resembles on a small scale the larger planetary system of which it forms a part. The existence of a metallic interior has always been inferred from the metalliferous veins which traverse the crust, and which are commonly supposed to have been filled from below.
Admitting the possibility or even probability of a metallic nucleus, in spite of the comparatively low density of the globe as a whole, we might speculate further as to the arrangement of the denser internal materials. The late Mr David Forbes suggested that the planet might be supposed to consist of three layers of uniform densities, enclosed one within the other, the density increasing towards the centre in arithmetical progression. Allowing 2 , 5 as the specific gravity of the crust or outer layer, he assigned 12Ό or thereabouts as that of the middle layer, and supposed that the inner nucleus might possess one averaging 20 ’ 0. Materials do not yet exist for any satisfactory conclusions on this subject.
In the evidence obtainable as to the former history of the earth, no fact is of more importance than the existence of a high temperature beneath the crust, which has now been placed beyond all doubt. This feature of the planet’s organization is made clear by the following proofs :—
(1.) Volcanoes.— In many regions of the earth’s surface openings exist from which steam and hot vapours, ashes and streams of molten rock are from time to time emitted. The abundance of these openings seems inexplicable by any mere local causes, but must be regarded as indicative of a very high internal temperature. If to the still active vents of eruption we add those which have formerly been the channels of communication between the interior and the surface, there are probably few large regions of the globe where proofs of volcanic action cannot be found. Everywhere we meet with masses of molten rock which have risen from below as if from some general reservoir.
(2.) Hot Springs.— Where volcanic eruptions have ceased, evidence of a high internal temperature is still often to be found in springs of hot water which continue for centuries to maintain their heat. Thermal springs, however, are not confined to volcanic districts. They sometimes rise even in regions many hundreds of miles distant from any active volcanic vent. The hot springs of Bath (temp. 120° Fahr.) and Buxton (temp. 82° Fahr.) in England are fully 900 miles from the Icelandic volcanoes on the one side, and 1100 miles from those of Italy and Sicily on the other.
(3.) Borings, Wells, and Mines.— The influence of the seasonal changes of temperature extends downward from the surface to a depth which varies according to latitude, to the thermal conductivity of the soils and rocks, and perhaps to other causes. The cold of winter and the heat of summer may be regarded as following each other in successive waves downward, until they disappear along a limit at which the temperature remains constant. This zone of invariable temperature is commonly believed to lie somewhere between 60 and 80 feet down in temperate regions. At Yakutsk in eastern Siberia (lat. 62° N.), however, the soil is permanently frozen to a depth of about 700 feet. In Java, on the other hand, a constant temperature is said to be met with at a depth of only 2 or 3 feet.
It is a remarkable fact, now verified by observation all over the world, that below the limit of the influence of ordinary seasonal changes the temperature, so far as we yet know, is nowhere found to diminish downwards. It always rises; and its rate of increment never falls much below the average. The only exceptional cases occur under circumstances not difficult of explanation. On the one hand, the neighbourhood of hot-springs, of large masses of lava, or of other manifestations of volcanic activity, may raise the subterranean temperature much above its normal condition; and this augmentation may not disappear for many thousand years after the volcanic activity has wholly ceased, since the coolingdown of a subterranean mass of lava would necessarily be a very slow process. On the other hand, the spread of a thick mass of snow and ice over any considerable area of the earth’s surface, and its continuance there for several thousand years, would so depress the subterranean isothermals that for many centuries afterwards there might be a fall of temperature for a certain distance downwards. At the present day, in at least the more northerly parts of the northern hemisphere, there are such evidences of a former more rigorous climate, as in the well sinking at Yakutsk already referred to. Sir William Thomson has calculated that any considerable area of the earth’s surface covered for several thousand years by snow or ice, and retaining, after the disappearance of that frozen covering, an average surface temperature of 13° C., “would during 900 years show a decreasing temperature for some depth down from the surface, and 3600 years after the clearing away of the ice would still show residual effect of the ancient cold, in a half rate of augmentation of temperature downwards in the upper strata, gradually increasing to the whole normal rate, which would be sensibly reached at a depth of 600 metres.” But beneath the limit to which the influence of the changes of the seasons extends, observations in most parts of the globe show that the temperature invariably rises as we penetrate towards the interior of the earth. According to present knowledge the average rate of increase amounts to Γ Fahr, for every 50 or 60 feet of descent, and this rise is found whether the boring be made at the sea-level or on elevated ground. The subjoined table give3 the results of temperature observations at widely separated localties: — [table]
(4.) I regularities in the Downward Increment of Heat.— While these examples prove a progressive increase of temperature, they show also that this rate of increase is not strictly uniform. The more detailed observations which have been made in recent years have brought to light the important fact that considerable variations in the rate of increase take place even in the same bore. If, for instance, we examine the temperatures obtained at different depths in the Rose Bridge colliery shaft cited in the foregoing list, we find them to read as in the following columns :— [table]
At La Chapelle, in an important well made for the water-supply of Paris, observations have been taken of the temperature at different depths, as shown in the subjoined table:— [table]
In drawing attention to the temperature-observations at the Rose Bridge colliery—the deepest mine in Great Britain—Professor Everett points out that, assuming the surface temperature to be 49° Fahr., in the first 558 yards the rate of rise of temperature is l 0 for 57'7 feet; in the next 257 yards it is 1° in 48 , 2 feet; in the portion between 605 and 671 yards—a distance of only 198 feet—it is 1° in 33 feet; in the lowest portion of 432 feet it is 1° in 54 feet. When such irregularities occur in the same vertical shaft, it is not surprising that the average should vary so much in different places.
There can be little doubt that one main cause of these variations is to be sought in the different thermal conductivities of the rocks of the earth’s crust. The first accurate measurements of the conducting powers of rocks were made by the late Professor J. D. Forbes at Edinburgh (1837-1845). He selected three sites for his thermometers, one in “trap-rock” (a porphyrite of Lower Carboniferous age), one in loose sand, and one in sandstone, each instrument being sunk to a depth of 24 French feet from the surface. He found that the wave of summer heat reached the first instrument on 4th January, the second on 25th December, and the third on 3d November, the trap-rock ' being by far the worst conductor, and the solid sandstone by far the best.
The British Association has recently appointed a committee to investigate this subject in greater detail. Already some important determinations have been made by it regarding the absolute conductivity of various rocks. As a rule the lighter and more porous rocks offer the greatest resistance to the passage of heat, while the more dense and crystalline offer the least resistance. The resistance of opaque white quartz is expressed by the number 114, that of basalt by 273, while that of cannel coal stands very much higher at 1538, or more than thirteen times that of quartz.
It is evident also that, from the texture and structure of most rocks, the conductivity must vary in different directions through the same mass, heat being more easily conducted along than across the “grain,” the bedding, and the other numerous divisional surfaces. Experiments have been made to determine these variations in a number of rocks. Thus, the conductivity in a direction transverse to the divisional planes being taken as unity, the conductivity parallel with these planes was found in a variety of magnesian schist to be 4Ό28. In certain slates and schistose rocks from central France the ratio varied from 1: 2 ’ 56 to 1: 3 ’ 952. Hence in such fissile rocks as slate and mica-schist heat may travel four times more easily along the lines of cleavage or foliation than across them.
In reasoning upon the discrepancies in the rate of increase of subterranean temperatures, we must also bear in mind that certain kinds of rock are more liable than 0ther3 to be charged with water, and that, in almost every boring or shaft, one or more horizons of such water-bearing rocks are met with. The effect of this interstitial water is to diminish thermal resistance. Dry red brick has its resistance lowered from 680 to 405 by being thoroughly soaked in water, its conductivity being thus increased 68 per cent. A piece of sandstone has its conductivity heightened to the extent of 8 per cent, by being wetted.
Mr Mallet has contended that the variations in the amount of increase in subterranean temperature are too great to permit us to believe them to be due merely to differences in the transmission of the general internal heat, and that they point to local accessions of heat arising from transformation of the mechanical work of compression, which is due to the constant cooling and contraction of the globe. But it may be replied that these variations are not greater than, from the known divergences in the conductivities of rocks, they might fairly be expected to be.
(5.) Probable Condition of the Earth’s Interior.— Various theories (mostly fanciful) have been propounded on this subject. There are only three which merit serious consideration. (1.) One of these supposes the planet to consist of a solid crust and a molten interior. (2.) The second holds that, with the exception of local vesicular spaces, the globe is solid and rigid to the centre. (3.) The third contends that, while the mass of the globe is solid, there lies a liquid substratum beneath the crust.
1. The arguments in favour of internal liquidity may be summed up as follows, (a.) The ascertained rise of temperature inwards from the surface is such that, at a very moderate depth, the ordinary melting point of even the most refractory substances would be reached. At 20 miles the temperature, if it increases progressively, as it does in the depths accessible to observation, must be about 1760° Fahr.; at 50 miles it must be 4600°, or far higher than the fusingpoint even of so stubborn a metal as platinum, which melts at 3080° Fahr, (ά.) All over the world volcanoes exist from which steam and torrents of molten lava are from time to time erupted. Abundant as are the active volcanic vents, they form but a small proportion of the whole which have been in operation since early geological time. It has been inferred therefore that these numerous funnels of communication with the heated interior could not have existed and poured forth such a vast amount of molten rock, unless they drew their supplies from an immense internal molten nucleus, (c.) When the products of volcanic action from different and widely-separated regions are compared and analysed, they are found to exhibit a remarkable uniformity of character. Lavas from Vesuvius, from Hecla, from the Andes, from Japan, and from New Zealand present such an agreement in essential particulars as, it is contended, can only be accounted for on the supposition that they have all emanated from one vast common source. (fl.) The abundant earthquake shocks which affect large areas of the globe are maintained to be inexplicable unless on the supposition of the existence of a thin and somewhat flexible crust. These arguments, it will be observed, are only of the nature of inferences drawn from observations of the present constitution of the globe. They are based on geological data, and have been frequently urged by geologists as supporting the only view of the nature of the earth’s interior compatible with geological evidence.
2. The arguments against the internal fluidity of the earth are based on physical and astronomical considerations çf the greatest importance. They may be arranged as follows :—
(a.) Argument from precession and nutation.—The problem of the internal condition of the globe was attacked as far back as the year 1839 by the late Mr Hopkins of Cambridge, who endeavoured to calculate how far the planetary motions of precession and nutation would be influenced by the solidity or liquidity of the earth’s interior. He found that the precessional and nutational movements could not possibly be as they are if the planet consisted of a central ocean of molten rock surrounded with a crust of 20 or 30 miles in thickness, that the least possible thickness of crust consistent with the existing movements was from 800 to 1000 miles, and that the whole might even be solid to the centre, with the exception of comparatively small vesicular spaces filled with melted rock.
M. Delaunay, in a paper on The Hypothesis of the Interior Fluidity of the Globe , threw doubt on Hopkins’s views, and suggested that, if the interior were a mass of sufficient viscosity, it might behave as if it were a solid, and thus the phenomenon of precession and nutation might not be affected. Sir William Thomson, who had already arrived at the conclusion that the interior of the globe must be solid, and acquiesced generally in Hopkins’s conclusions, pointed out that M. Delaunay had not worked out the problem mathematically, otherwise he could not have failed to see that the hypothesis of a viscous and quasi-rigid interior “breaks down when tested by a simple calculation of the amount of tangential force required to give to any globular portion of the interior mass the precessional and nutational motions which, with other physical, astronomers, he attributes to the earth as a whole.” Sir William, in making this calculation, holds that it demonstrates the earth’s crust down to depths of hundreds of kilometres to be capable of resisting such a tangential stress (amounting to nearly τ ⅛th of a gramme weight per square centimetre) as would with great rapidity draw out of shape any plastic •substance which could properly be termed a viscous fluid. “An angular distortion of 8" is produced in a cube of glass by a distorting stress of about ten grammes weight per square centimetre. We may therefore safely conclude that the rigidity of the earth’s interior substance could not be less than a millionth of the rigidity of glass without very sensibly augmenting the lunar nineteen-yearly nutation.”
In Hopkins’s hypothesis he assumed the crust to be infinitely rigid and unyielding, which is not true of any material substance. Sir William Thomson has recently returned to the problem, in the light of his own researches in vortex-motion. He now finds that, while the argument against a thin crust aud vast liquid interior is still invincible, the phenomena of precession and nutation do not decisively settle the question of internal fluidity, though the solar semi-annual and lunar fortnightly nutations absolutely disprove the existence of a thin rigid shell full of liquid. If the inner surface of the crust or shell were rigorously spherical, the interior mass of supposed liquid could experience no processional or nutational influence, except in so far as, if heterogeneous in composition, it might suffer from external attraction due to non-sphericity of its surfaces of equal density. But “a very slight deviation of the inner surface of the shell from perfect sphericity would suffice, in virtue of the quasi-rigidity due to vortexmotion, to hold back the shell from taking sensibly more precession than it would give to the liquid, and to cause the liquid (homogeneous or heterogeneous) and the shell to have sensibly the same processional motion as if the whole constituted one rigid body.”
The assumption of a comparatively thin crust requires that the crust shall have such perfect rigidity as is possessed by no known substance. The tide-producing force of the moon and sun exerts such a strain upon the substance of the globe that it seems in the highest degree improbable that the planet could maintain its shape as it does unless the supposed crust were at least 2000 or 2500 miles in thickness. That the solid mass of the earth must yield to this strain is certain, though the amount of deformation is so slight as to have hitherto escaped all attempts to detect it. Had the rigidity been even that of glass or of steel, the deformation would probably have been by this time detected, and the actual phenomena of precession and nutation, as well as of the tides, would then have been very sensibly diminished. The conclusion is thus reached that the mass of the earth “is on the whole more rigid certainly than a continuous solid globe of glass of the same diameter.”
(δ.) Argument from the tides.—The phenomena of the oceanic tides are only explicable on the theory that the earth is either solid to the centre, or possesses so thick a crust (2500 miles or more) as to give to the planet practical solidity. Sir William Thomson remarks that, “were the crust of continuous steel, and 500 kilometres thick, it would yield very nearly as much as if it were india-rubber to the deforming influences of centrifugal force, and of the sun’s and moon’s attractions.” It would yield, indeed, so freely to these attractions “that it would simply carry the waters of the ocean up and down with it, and there would be no sensible tidal rise and fall of water relatively to land.” Mr George H. Darwin has recently investigated mathematically the bodily tides of viscous and semi-elastic spheroids, and the character of the ocean tides on a yielding nucleus. His results tend to increase the force of Sir William Thomson’s argument, since they show that “no very considerable portion of the interior of the earth can even distantly approach the fluid condition,” the effective rigidity of the* whole globe being very great.
(c.) Argument from relative densities of melted and solid rock.—The two preceding arguments must be considered decisive against the hypothesis of a thin shell or crust covering a nucleus of molten matter. It has been further urged, however, as an objection to this hypothesis, that cold solid rock is necessarily more dense than’ hot melted rock, and that even if a thin crust were formed over the central molten globe it would immediately break up and the fragments would sink towards the centre. Undoubtedly this would happen were the material of the earth’s mass of the same density throughout. But, as has been already pointed out, the specific gravity of the interior is at least twice as much as that of the visible parts of the crust. If this difference be due, not merely to the effect of pressure, but to the presence in the interior of intensely heated metallic substances, we cannot suppose that solidified portions of such rocks as granite and the various lavas could ever have sunk into the centre of the earth, so as to build up there the honey-combed cavernous mass which might have served as a nucleus in the ultimate solidification of the whole planet. From the considerations above advanced we have seen that the earth’s central mass may be plausibly conjectured to be metallic. Into this dense central mass the comparatively light crust could not sink, though its earliest formed portions would no doubt descend until they reached a stratum with specific gravity agreeing with their own, or until they were again melted.
3. The ingenious suggestion of Mr Fisher, already cited (ante, p. 217), in favour of the existence of a possible fluid or viscous substratum between the flexible outer shell and an inner rigid nucleus, is made with the view of reconciling the requirements of physics with those facts in geology which seem to demand the existence of a mobile mass of intensely hot matter at no great depth beneath the surface. Whether it does so must be left for physicists to decide. But, on geological grounds, it may be questioned whether such a fluid substratum is needed. We must bear in mind that the land of the globe, regarding the geological structure of which alone we know anything, covers but a small part of the whole surface of the planet; that the existing continents seem from earliest times to have specially suffered from the reaction between the heated interior and the cooled exterior, forming, as it were, lines of relief from the strain of compression; and that along such lines, if the substance of the interior be everywhere just about the melting point, relief from pressure by corrugation would cause liquefaction of the matter so relieved, and its ascent towards the surface; so that evidences of volcanic action on the terrestrial ridges might be expected to occur, and to be referable to all ages. Mr Fisher assumes the contraction of rock in cooling to be - 000007 linear for one degree Fahr.; and he argues that, as this amount would not account for the observed contraction in the crust, we must have recourse to some additional explanation, such as the escape of steam and vapours from volcanic orifices. The validity of the assertion that the amount of horizontal compression of the superficial strata is greater than the cooling of a solid earth can account for may be questioned. The violently contorted rocks indicative of great horizontal compression occur chiefly along the crests of the great terrestrial ridges where the maximum effects of corrugation were to be looked for. To the argument from climate it may be replied on the other hand, with great plausibility, that secular changes may be accounted for by the effect of the variations in the eccentricity of the earth’s orbit combined with the precession of the equinoxes, as already described.
(6.) Age of the Darth and Measures of Geological Time.— The age of our planet is a problem which may be attacked either from the geological or physical side.
1. The geological argument rests chiefly upon the observed rates at which geological changes are being effected at the present time, and is open to the obvious preliminary objection that it assumes the existing rate of change as the measure of past revolutions,—an assumption which may be entirely erroneous, for the present may be-a period when all geological events march forward more slowly than they used to do. The argument proceeds on data partly of a physical and partly of au organic kind, (a) The physical evidence is derived from such facts as the observed rates at which the surface of a country is being lowered by rain and streams, and new sedimentary deposits are formed. These· facts will be more particularly dwelt upon in later portions of this article. If we assume that the land has been worn away, and that stratified deposits have been laid down nearly at the same rate as at present, then we must admit that the stratified portion of the crust of the earth must represent a very vast period of time. Dr Croll puts this period at not less, but possibly much more, than 60 million years. (6) On the other hand, human experience, so far as it goes, warrants the belief that changes in the organic world proceed with extreme slowness. Yet in the stratified rocks of the earth’s crust we have abundant proof that the whole fauna and flora of the earth’s surface have passed through numerous cycles of revolution,—species, genera, families,appearing and disappearing many times in succession. On any supposition it must be admitted that these vicissitudes in the organic world can only have been effected with the lapse of vast periods of time, though no reliable standard seems to be available whereby these periods are to be measured. The argument from geological evidence is strongly in favour of an interval of probably not much less than 100 million years since the earliest form of life appeared upon the earth, and the oldest stratified rocks began to be laid down.
2. The argument from physics as to the age of our planet is based by Sir William Thomson upon three kinds of evidence:—(1) the internal heat and rate of cooling of the earth; (2) the tidal retardation of the earth’s rotation; and (3) the origin and age of the sun’s heat.
(1.) Sir William Thomson, applying Fourier’s theory of thermal conductivity, pointed out some years ago (1862) that in the known rate of increase of temperature downward and beneath the surface, and the rate of loss of heat from the earth, we have a limit to the antiquity of the planet. He showed, from the data available at the time, that the superficial consolidation of the globe could not have occurred less than 20 million years ago, or the underground heat would have been greater than it is; nor more than 400 million years ago, otherwise the underground temperature would have shown no sensible increase downwards. He admitted that very wide limits were necessary. In more recently discussing the subject, he inclines rather towards the lower than the higher antiquity, but concludes that the limit, from a consideration of all the evidence, must be placed within some such period of past time as 100 millions of years.
(2.) The argument from tidal retardation proceeds on the admitted fact that, owing to the friction of the tidewave, the rotation of the earth is retarded, and is therefore much slower now than it must have been at one time. Sir William Thomson contends that had the globe become solid some ten thousand million years ago, or indeed any high antiquity beyond 100 million years, the centrifugal force due to the more rapid rotation must have given the planet a very much greater polar flattening than it actually possesses. He admits, however, that, though 100 million years ago that force must have been about 3 per cent, greater than now, yet “nothing we know regarding the figure of the earth and the disposition of land and water would justify us in saying that a body consolidated when there was more centrifugal force by 3 per cent, than now might not now be in all respects like the earth, so far as we know it at present.” Professor Tait, in repeating this argument, concludes that, taken in connexion with the previous one, “it probably reduces the possible period which can be allowed to geologists to something less than ten millions of years.” He does not state, however, on what grounds he so reduces the available period, nor does he notice the objection urged by Dr Croll that, granting the gradual submergence of the polar lands owing to the slackened speed of rotation, the subaerial denudation of the rising equatorial land might well keep pace with the effects of the oceanic subsidence, so that we cannot infer from the present form of the earth what may have been its precise amount of polar compression at the time of solidification.
(3.) The third argument, based upon the age of the sun’s heat, is confessedly less reliable than the two previous ones. It proceeds upon calculations as to the amount of heat which would be available by the falling together of masses from space, which gave rise by their impact to our sun. The vagueness of the data on which this argument rests may be inferred from the fact that in one passage Professor Tait places the limit of time during which the sun has been illuminating the earth as, “on the very highest computation, not more than about 15 or 20 millions of years,” while, in another sentence of the same volume, he admits that, “by calculations in which there is no possibility of large error, this hypothesis [of the origin of the sun’s heat by the falling together of masses of matter] is thoroughly competent to explain 100 millions of years solar radiation at the present rate, perhaps more.” One hundred millions of years is probably amply sufficient for all the requirements of geology.
III. Composition of the Earth’s Crust.
MINERALS AND ROCKS.
The visible and accessible portion of the earth is formed of minerals and rocks. A mineral may be classified as an inorganic body distinguished by a more or less definite chemical composition, and usually a characteristic geometrical form. A rock is an aggregate mass, sometimes of one, more commonly of two or more minerals. Upwards of 800 species of minerals and a vast number of varieties have been described. A very large proportion of these occur but rarely, and, though interesting and important to the mineralogist, do not demand the special attention of the geologist. While almost every mineral may be made to yield data of more or less geological significance, only those which enter into the composition of rock masses, or which are of frequent occurrence as accessories there, require to be familiarly known by the student of geology.
1. Rock-Forming Minerals.
The following are the more important minerals which enter into the composition of rocks :—
Quartz (SiO 2 ) occurs either crystallized as rock-crystal, or nop-crystalline as calcedony. In the former condition it is an essential constituent of granite, felsite, and many other igneous rocks, as well as of sandstone and numerous aqueous rocks. The non-crystallized or colloid quartz is chiefly met with in cavities and fissures of rock where it has been slowly deposited from aqueous solution. Numerous varieties of calcedony occur, as agate, carnelian, jasper, flint, chert, Lydian-stone, &c.
Felspars (silicates of alumina, with potash, soda, or lime) constitute the most abundant group of rock-forming minerals. For the purposes of the petrographer they are conveniently divided into two series—(1) the Monoclinic or Orthoclase felspars (with cleavage angles of 90°), containing from 4 to 16 per cent of potash and usually more or less soda, sometimes as much as 10 per cent., and (2) the Triclinic or Plagioclase felspars (with oblique cleavage angles, or less than 90°), including a soda group with 8 to 12 per cent, of soda, and a lime group with 6 to 20 per cent, of lime. The felspars form a large part of most igneous rocks. By their decay they form clay, and in that condition enter largely into the composition of the argillaceous stratified rocks, such as shale, mudstone, slate, &c.
Hornblende is a meta-silicate of magnesium, with lime, iron, or manganese, and frequently alumina. The white non-aluminous varieties (tremolite, actinolite, anthophyllite, asbestos) chiefly occur as constituents of such metamorphic rocks as crystalline limestone, gneiss, &c. The black or dark green aluminous varieties enter as essential constituents into the composition of many rocks, as diorite and hornblende slate.
Augite (resembling hornblende in composition) is divisible into two groups. The pale non-aluminous varieties (diopside, sahlite, coccolite, &c. ) occur under conditions like those of the pale hornblendes. The dark aluminous or common augite is abundant as an ingredient of some igneous rocks, as basalt. Allied to augite are diallage (important as a constituent of diallage-rock or gabbro), hypersthene, and bronzite. Uralite is the name of a mineral of frequent occurrence among Paheozoic rocks, having the external form of augite and the cleavage of hornblende.
Olivine (an ortho-silicate of magnesium, with part of the magnesium replaced by iron or manganese) is a conspicuous ingredient among the basalt rocks. It appears also to have been the original magnesian constituent of many rocks now altered into serpentine.
Nepheline (a silicate of alumina and soda with a little potash) takes the place of felspar in some lavas. It likewise occurs among the ejected blocks of Somma, and, in the form of elaeolite, among the ancient crystalline rocks of Norway.
Leucite (K 2 Al 2 Si 4 O 12 ) is a characteristic ingredient of many Tertiary and recent lavas. It has not been met with among any of the Palaeozoic or Secondary igneous rocks, nor ever in association with quartz.
Haüyne and Nosean are two minerals allied to garnet, found in some Tertiary lavas.
Mica.— Under this general term are included several species of minerals distinguished by their basal cleavage into thin laminae and by their splendent or silvery lustre. The non-magnesian micas include muscovite or potash-mica, the most abundant of all, and lepidolite or lithia-mica; of the magnesian micas the most important is biotite. Muscovite enters into tne composition of granite, gneiss, mica-schist, micaceous sandstone, and many other rocks. Biotite is likewise abundantly distributed among the older crystalline rocks. Lepidomelane is a black mica often found in fine-grained granites. Other species are margarodite—an abundant constituent of many unctuous schists formerly called talc-schists, and haughtonite— which, according to Heddle, is the common mica of the granites in the Scottish Highlands.
Garnet (an aluminous ortho-silicate with lime, magnesia, iron, or manganese) occurs in rhombic dodecahedrons or allied forms, and also massive in many metamorphic rocks, as mica-schist, eclogite, &c.
Epidote (a variable silicate of lime, alumina, iron, or manganese) occurs in yellow or greenish translu·ent crystals or crystalline masses in many of the older crystalline rocks, though seldom as an abundant constituent. It is probably always an alteration-product.
Tourmaline, in its common black variety, schorl, forms with quartz the rock known as schorl-rock, and occurs in some granites, gneisses, schists, and other crystalline rocks.
Zircon (silicate of zirconium) is found as a constituent of zirconsyenite, and more sparingly in other crystalline rocks.
The hydrous silicates have resulted from the alteration of the anhydrous forms. As constituents of rocks they may be grouped into two series:—(1) the aluminous, including the zeolites, and (2) the magnesian, embracing talc, chlorite, serpentine, and their allies.
Zeolites form a numerous genus of minerals distinguished usually by their boiling up before the blowpipe, owing to the escape of their water of crystallization, by their frequent pearly lustre, inferior lιardness, and their occurrence in cavities and veins where they have been deposited from solution. They are found as abundant secondary products in many amygdaloids, also in altered limestones and other metamorphic rocks.
Serpentine (SiO 2 ,44Ί4; MgO,42'97; H 2 O,12∙89) is a dull impure, usually green, granular to compact, more rarely foliated, mineral, with a hardness of 3 to 4 or even sometimes 5. Like the other hydrous magnesian silicates it has a soapy or greasy feel. It occurs abundantly in many altered rocks as a pseudomorph after some of the anhydrous magnesian silicates, also as a massive rock forming huge beds often associated with metamorphosed limestones.
Chlorite is a general term including several minerals which agree in possessing a greenish colour, soapy feel, hardness of only 2 to 2 '5, and specific gravity of 2 ’ 65 to 2 ’ 85. It occurs in chlorite slate and in many rocks as an alteration-product.
Talc (SiO 2 ,59 to 63; MgO,30 to 33; H 2 O, from a trace up to 7 per cent.) occurs in hexagonal plates or scales, cleaving readily into flexible non-elastic laminae, but most commonly granular and massive, white to pale leek or apple-green, with marked pearly lustre on cleavage-planes. It is met with in tale-slate, also frequently in crystalline rocks as a result of the alteration of hornblende, augite, or other anhydrous magnesian silicate.
Dclcssite and Saponite are soff green hydrous magnesian silicates found as products of altération in basalt-rocks.
Carbon occurs chiefly as beds in the form of coal, lignite, peat, &c. Graphite, however, is often met with in black or steel-grey splendent scales and granular masses in metamorphosed rocks. Anthracite also takes sometimes the form of black glancing grains or of a diffused fine black dust through certain palaeozoic formations.
Carbonates play an important part both as individual minerals and as rock-masses. The three most important are calcite, dolomite, and siderite.
Calcite (carbonate of lime) is one of the most abundant minerals. It occurs crystallized as a secondary product in most rocks which have undergone decomposition, especially where they contain silicates into tne composition of which lime enters. It is also found massive as limestone, forming beds having sometimes an aggregate thickness of many hundred feet and an extent of thousands of square miles.
Dolomite (carbonate of lime and magnesia) is likewise both a product of alteration and an original formation. In the former condition it is met with crystallized as bitter-spar in many metamorphic rocks as well as in veins and cavities of unaltered formations. It occurs also as an amorphous granular substance, sometimes replacing calcite, and sometimes in vast beds or masses of original deposit.
Siderite, Chalybite, or Spathic Iron (carbonate of iron) occurs both crystallized and massive. In the crystallized form it is comparatively unimportant as a constituent of rocks, being then found chiefly in veins and cavities where other alteration-products have been deposited. But in its massive condition it is found mixed with clay and other impurities, and forming beds and nodules which are among the most important ores of iron.
Sulphur, though seldom occuning in large masses, is widely diffused as an accessory ingredient of rocks. It occurs crystallized or finely granular in mineral veins, in nodules of limestone, and other concretions, and in beds of limestone and marl. It also takes the form of a crust in the sublimations of volcanic vents. Its frequent association in Tertiary strata with the remains of lacustrine shells, insects, and plants shows that it has in these cases been formed at ordinary temperatures from aqueous solutions.
Sulphides, combinations of sulphur with the metals, iron, copper, lead, zinc, and a few others, have a wide distribution among rocks. Where aggregated into masses they form mineral veins. It is the iron sulphides which deserve chiefly the attention of the petrographer. They occur in two varieties— pyrite, crystallizing in isometric forms, and marcasite, in rhombic forms. The former has a remarkably extensive diffusion throughout rocks of all ages, usually as minute crystals and thin streaks, but often in concretions and more massive veins. Marcasite also is abundantly distributed though less so than pyrite. From its greater liability to oxidation the strata through which it is diffused are apt to yield rapidly to the action of the weather, sulphuric acid and different alum compounds being produced.
Sulphates.— The most generally occurring sulphates in rocks are gypsum and barytes. Gypsum (hydrous sulphate of lime) in minute monoclinic prisms and macles may be obtained by the evaporation of sea-water, and in larger crystals of the same form it is found in many stratified formations. It likewise occurs as a secondary product in laminar or fibrous veins through rocks of igneous origin. Beds of gypsum, resulting from aqueous deposition, frequently appear interstratified with rock-salt and the associated products of evaporation. The anhydrous sulphate, anhydrite, likewise occurs among rock-salt deposits, but has a much more limited diffusion than gypsum. Barytes (sulphate of baryta) almost always occurs in veins or threads running through rocks. It is a common vein-stone in association with metallic ores.
Halite or Rock-salt (chloride of sodium) is more widely diffused than was formerly supposed. Microscopic research has shown its presence in the form of cubes in the minute cavities in the quartz of granite and other rocks. It occurs as scattered crystals, generally replaced by clay or some other substance, in many stratified formations. Its chief habitat, however, is in the various saliferous deposits where it takes the form of solid beds of salt.
Fluorite or Fluor-spar (fluoride of lime) is essentially a vein-stone, associated with metallic ores, especially with sulphides of lead and zinc. It occurs also in scattered cubes through various crystalline rocks, such as granite, gneiss, porphyrite.
Apatite (phosphate of lime, with fluorine and often chlorine) has been shown by microscopic investigation to have a very wide distribution among crystalline rocks. It occurs in fine needles or stouter hexagonal prisms in a large number of crystalline rocks, as granite, quartz-trachyte, syenite, diorite, basalt, and many others. It also occurs massive as beds among the more ancient geological formations.
Iron oxides.— These are abundantly distributed through rocks of all ages. Haematite (peroxide of iron) occurs crystallized in veins through crystalline rocks, also massive and earthy in beds, and sometimes in minute scales (rubin-glimmer) disseminated through the minerals of many crystalline rocks. Magnetite (Fe 3 O 4 ) has an extensive diffusion in the form of minute octohedra or grains through crystalline rocks. In some of these rocks indeed, as in basalt, it plays the part of a chief constituent. It also occurs in many metamorphic rocks both scattered in detached crystals and segregated into veins or beds. Titanoferrite or titaniferous iron is likewise found as a plentiful ingredient in many crystalline rocks, particularly among the older basalts and dolerites. Hydrous iron oxide or limonite is diffused through almost all rocks. It is the usual brown or yellow colouring substance of minerals, and may be looked for wherever rocks containing iron have been exposed to the weather. It occurs also mixed with clay and other impurities in beds, as in the bog-iron-ore of lakes and marshes.
2. Rocks.
I. General Characters.
A rock may be defined as a mass of mineral matter, composed of one, more usually of several, kinds of minerals,—having, as a rule, no definite external form, and liable to vary considerably in chemical composition. The crust of the earth is built up of rocks, including under this term, not only hard solid masses like granite and limestone, but even all loose incoherent deposits such as mud, soil, peat, and blown sand.
Rocks may be distinguished by external and internal characters.
i. External Characters.
1. Structure, or the manrter in which the component particles have been built up into the mineral masses called rocks, is the fundamental character. Viewed broadly, there are two leading types of structure among rocks—crystalline or massive, and fragmental.
(a.) Crystalline—consisting of a network of interlaced crystals and crystalline particles. Sometimes those crystals are large (half an inch or more in length), as in many granites, when the texture is called coarse or macrocrystalline; in other cases they are so minute as not to be discernible with the naked eye, when the texture is microcrystalline or compact. While the crystalline structure is particularly characteristic of rocks which have crystallized from igneous fusion, it is not altogether peculiar to them. It may be produced by chemical deposit from aqueous solutions, or it may be developed in rocks previously granular by chemical infiltration and metamorphism.
Under the head of crystalline it is usual to include the glassy or vitreous structure. Rocks possessing this character are natural glasses produced by igneous fusion, such as obsidian and pitchstone. In most of these rocks, however, the process of devitrification may be observed; the glass has evidently become more and more stony as it cooled, by the appearance in it of small spherules, or hairs, or crystals, until in some cases it has become entirely lithoid. These stages are best studied with the microscope, and belong to the internal rather than the external characters.
When larger crystals than those of the compact base are scattered through a rock, the texture is said to ^porphyritic. Many rocks, when in a melted condition, have had a cellular texture given to them by their imprisoned steam, like the open, cavernous texture of ill-baked bread. Several varieties of this texture are distinguished,—as vesicular, when there are comparatively few and small holes; scoriaceous, when the cavities occupy about as much space as the solid part, and are of very unequal sizes and forms ; ρumiceous, when the cells are much more numerous than the solid portion, and when, consequently, a piece of the rock may even float in water; amygdaloidal, when by subsequent infiltration the cells have been filled up with concretions of calcite, calcedony, zeolite, &c., which, from the elongated flattened form of the cells, are frequently almond-shaped.
Foliated rocks have their crystalline ingredients arranged in more or less defined layers, which usually inosculate. Schistose rocks are those where the foliated arrangement has been so produced that the rock splits into rude rough laminae or plates.
Most of the crystalline rocks have resulted from igneous fusion. Some, like limestone, have been formed as deposits in water. The foliated rocks are generally believed to have acquired their peculiar character from the re-crystallization of their ingredients along original divisional planes, such as the lines of deposit.
(δ.) Fragmental or Clastic.—These are all derivative from previously formed masses. They vary in texture from coarse masses consisting of accumulated blocks, several feet or even yards in length, to such fine aggregates as only show their secondary origin by microscopic investigation. They are said to be conglomeratic when they consist of beds of rounded water-worn pebbles like compacted gravel; agglomeratic, when the blocks are large, rounded, or subangular, and tumultuously thrown together; brecciated, when the fragments are angular and not water-worn. .Most clastic rocks are bedded, that is, arranged in beds or layers. Each bed may consist of many thin layers or laminae, which, when they enable the rock to split up into thin leaves, give what is called a shaly or fissile structure. Many fragmental rocks show a concretionary structure. When the concretions are like the roe of a fish, and of a calcareous nature, they form the oolitic structure; when of larger size, like peas, they give the pisolitic structure. There is often also a crystalline structure developed in rocks originally quite fragmental; many limestones, for example, made up originally of water-worn fragments of shells, corals, <fcc., slowly acquire a crystalline character from the action of percolating and slightly acidulous water. The action of rain on the exposed parts of a recent coral reef produces this change in the dead coral.
2. Colour.— This character varies so much even in thesame rock, according to the freshness of the surface examined, that it possesses but a subordinate value as a means of discriminating rocks. Nevertheless, when cautiously used, it may be made to afford valuable indications as to the probable nature and composition of rocks. It is in this respect always desirable to compare a freshly-broken with a weathered piece of the rock. White indicates usually the absence or comparatively small amount of the metallic oxides, especially iron. It may either be the original colour of the rock, as in chalk and calc-sinter, or may be developed· by weathering, as the white crust on flints and on many porphyries. Black seldom occurs on a weathered surface of rock. Its existence may be due either to the presence of carbon, when weathering will not change it much, or to some iron-oxide (magnetite chiefly), or some silicate rich in iron (as hornblende and augite). Many rocks (basalts and dolerites particularly) which look quite black on a fresh surface, become red, brown, or yellow on exposure. Yellow* as a dull earthy colouring matter, almost always indicates the presence of hydrated peroxide of iron. Bright, metallic, gold-like yellow is usually that of iron-sulphide. Brown occurs as the original colour in some carbonaceous rocks (lignite), and ferruginous beds (bog-iron-ore, clay-ironstone,&c.). It very generally, on weathered surfaces, points to the oxidation and hydration of minerals containing iron. Bed, in the vast majority of cases, is due to the presence of granular peroxide of iron. This mineral gives dark blood-red to pale flesh-red tints. As it is liable, however, to hydration, these hues are often mixed with brown and yellow. Green, as the prevailing tint of rocks, occurs among metamorphic schists, when its presence is usually due to some of the hydrous magnesian silicates (chlorite, talc, serpentine). It occurs also among the igneous rocks, especially those of older geological formations, where some of the hornblende, olivine, or other similar silicates have been altered. Among the sedimentary rocks it is principally due to the proto-silicate of iron in glauconite. Carbonate of copper colours some rocks a bright emerald or verdigris green. The mottled character so common among many stratified r0ck3 is frequently traceable to unequal weathering, some portions being more oxidized than others; while some, on the other hand, become deoxidized from the reducing action of decaying organic matter. To the latter cause may be attributed the circular green spots so often found among red strata.
3. Lustre, as an external character of rocks, does not possess the value which it has among minerals. In most rocks the granular texture prevents the appearance of any distinct lustre. Where a rockshows a completely vitreous lustre itwill usually be found to consist of a volcanic glass. A splendent soιni-metallic lustre may often be observed upon the foliation planes of schistose rocks and upon the laminae of micaceous sandstones. As this silvery lustre is almost invariably due to the presence of mica, it is commonly called distinctively micaceous. A metallic lustre is met with sometimes in 0ed3 of anthracite; more usually its occurrence among rocks indicates the presence of metallic oxides or sulphides.
4. Hardness and Frangihility.—A rock which can easily bo scratched with the nail is almost always much decomposed, though some chloritic and talcose schists are soft enough to be thus affected. Compact rocks which can easily be scratched with the knife, and are apparently not decomposed, may be limestones, or other fragmental masses. Crystalline rocks, aβ a rule, cannot be scratched with the knife unless considerable force be used. The ease with which a rock may be broken is the measure of its frangibility. Most rocks break most easily in one direction; attention to this point will sometimes throw light upon their internal structure.
5. Fracture is the surface produced when a rock is split or broken, and depends for its character upon the texture of the mass. Finely granular compact rocks are apt to break with a splintery fracture where wedge-shaped plates adhere by their thicker ends to, and lie parallel with, the general surface. When the rock breaks off into concave and convex rounded shell-like surfaces, the fracture is said to be conchoidal, as may be seen in obsidian and other vitreous rocks, and in exceedingly compact limestones. The fracture may also be foliated, slaty, or shady, according to the structure of the rock. Many black, opaque, compact rocks are translucent on the thin edges of fracture, and afford there, with the aid of a lens, a glimpse of their internal composition.
6. Feel.— Practice enables a geologist to discriminate some rocks by the feel of their weathered or fresh surfaces. The hydrous magnesian silicates, as already mentioned, have a marked soapy or greasy feeling under the fingers. Some micaceous schists, with margarodite or an allied mica, likewise exhibit the same character.
7. Smell.— Many rocks when freshly broken emit distinctive odours. Those containing volatile hydrocarbons give sometimes an appreciable bituminous odour, as is the case with some of the dolerites, which in central Scotland have been intruded through coal-seams and carbonaceous shales. Limestones have often a fetid odour; rocks full of decomposing sulphides are apt to give a sulphurous odour; those which are highly siliceous yield, on being struck, an em- ρyreumatic odour. It is very characteristic of argillaceous rocks to emit a strong earthy smell when breathed upon.
8. Specific gravity is an important character among rocks as among minerals. It varies from 0 ’ 6 among the hydrocarbon compounds to 3 ’ 1 among the basalts. As already stated, the average specific gravity of the rocks of the earth’s crust may be taken to be about 2'5, or from that to 3Ό.
9. Magnetism is a distinguishing feature of many igneous and some metamorphic rocks. In some cases it exists in such development as powerfully to affect the magnetic needle, so that observations with that instrument among rocks of this character are deceptive. But even when much more sparingly present, the existence of magnetic iron in a rock may be shown by reducing the rock to powder in an agate mortar, washing carefully the triturated powder, and drying the heavy residue, from which grains of magnetite may be extracted with a magnet. This may be done with any basalt ii. Internal Characters.
These are revealed chiefly by the microscope and chemical analysis. By the former we learn what are the component minerals of a rock, how they are built up into its mass, and what changes they have undergone. By the latter we are taught the chemical constitution of rocks, and are enabled to bring into close relations rocks which have externally no resemblance to each other, or, on the other hand, to show that rocks externally similar are chemically very distinct.
1. Microscopic Examination.— This method of inquiry has made great advances during the last 20 years, especially from the labours of German petrographers. Slices are cut from the rocks to be examined, and after being polished on one side with great care, are cemented by that side with Canada balsam to glass, and are then ground down until they attain the requisite transparency. In this way the minutest features in the structure of a rock can be leisurely studied. By the application of polarized light to these thin slices a marvellously delicate method of petrographical analysis is afforded.
Among the igneous rocks three leading types of microscopic structure have been established, chiefly through the researches of Professor Zirkel of Leipsic :—(1.) Purely-crystalline.—Granite (fig. 1) is a good example, consisting, as it does, entirely of crystals interlaced with each other. (2.) Halfcrystalline.—In this division, which embraces most of the eruptive masses, the rocks consist of a non-crystalline amorphous matrix with crystals scattered through it. This matrix may be either (a) entirely glassy, (figs. 2 and 3); (ό) partly devitrified through separation of peculiar little granules and needles which are not “microlites” of the component parts of the rock;
(c) an aggregation of such little granules, needles, and hairs, between which no glass, or almost none, appears (microcrystallitic); or ∖ d) microfelsitic, nearly related to the two previous groups, and consisting of an amorphous mass marked usually with indefinite or half-effaced granules and filaments. (3.) Non-crystalline. —Rocks of this class are much less common than those of the other two. In their most typical condition they consist entirely of a non·
crystallized or amorphous (n⅛Λi individualisirtef) substance, sometimes glassy and sometimes microfelsitic.
Rocks really formed of compacted sediment become sometimes so close-grained that their origin may not be apparent to the naked eye. Their truly derivative character is well brought out by the microscope. In fig. 4, for example, the structure of a piece of fine greywacke is shown. It will be observed that the component particles are not crystals, but broken and more or less rounded fragments of different minerals. The larger white pieces are quartz, the darker portions consist of granules of slate, felspar, and other substances, with a little siliceous ferruginous cement. Many exceedingly compact and even flinty argillaceous rocks are in this way shown by microscopic examination to be formed of water-worn particles.
Rocks which have been so affected by subsequent changes as to acquire a new crystalline character, and to receive the name Metamorphic, exhibit many characteristic features of structure under the microscope. Limestones, for example, which have been altered into saccharoid marble are found to consist of crystalline grains of calcite, showing the characteristic cleavage of that mineral (fig. 5). The foliated rocks (schists) show a curious blending of the characters of igneous and sedimentary rocks. Thus they have often a distinctly granular structure, resembling that of sedimentary deposits, with, at the same time, an arrangement of the micaceous folia reminding us of the fluid structure of igneous rocks. In fig. 6, for instance, the quartz-grains are to be observed in layers separated by folia of mica which curve and twist like the microlites in an obsidian (compare fig. 2).
Much light has been cast on the origin and history of igneous rocks by microscopic investigation. It is easy, for example, to see in what order the several mineral components have crystallized out of the original glass. Thus in basalt the magnetite has appeared before the augite, in which it has been abundantly enclosed. Again, the movement of the still liquid or viscid rock, when many of its crystals had already been produced, is beautifully shown by the “fluid structure” (fig. 2), where minor crystals and particles are drawn into curving lines which bend round the large crystals, and also by the frequent fracture of the larger crystals and the insertion of portions of the general ground-mass of the rock between the separated pieces. That intensely saline water was present during the formation of many crystalline rocks is proved by the presence in their crystals of minute cavities filled with fluid and containing cubes of halite (common salt). Liquid carbonic acid ha3 been observed in such cavities.
Most interesting and important information is likewise afforded by the microscope regarding the subsequent changes which rocks have undergone through the influence chiefly of percolating water. Every gradation of alteration from the fresh mineral to its complete pseudomorph may be observed. In this way many serpentines have been shown to have been originally olivine rocks. It can be seen, too, how certain minerals decay, and to what products their alteration gives rise, even when the general mass of the rock looks to the eye still tolerably fresh. There can be little doubt that the application of microscopic analysis is destined to throw much light upon both the formation and the subsequent history of the sedimentary rocks. These have not yet been so sedulously explored as the igneous groups. The obscure subject of metamorphism is especially open to elucidation by the microscopic method.
2. Chemical Analysis.— This method of investigation must supplement the work of the microscope. A mere chemical analysis gives the ultimate chemical constitution of a rock, but may afford little clue to its mineral structure, which can only be thoroughly examined by means of the microscopic method. On the other hand, many rocks do not allow of satisfactory determination of their constitution by means of the microscope. For these chemical analysis is, of course, indispensable. But our knowledge of no rock can be considered complete until the rock has been subjected to both processes of investigation.
II. Classification and Description.
A precise yet convenient classification of rocks is still required. We may adopt chemical characters as the basis of arrangement, and group rocks according as they may be sulphates, carbonates, silicates, &c.; but in so doing wo place together rocks which, from a geological point of view, have no real affinity. Again, we may select mineralogical composition as the groundwork of the classification; but in this case also great violence may be done in the geological relationships of the rocks. In many respects the long established geological arrangement according to manner of production is a useful one—igneous, aqueous, and metamorphic rocks. There is, of course, the obvious objection to it that it starts upon a preconceived theory of the origin of the rocks, and this objection must be admitted to be serious. Every year, however, is diminishing its force by making us more certain of the mode of formation of different rocks; and, probably, some modification of it will in the end be very generally adopted. In the meantime the most eligible course seems to be to choose a scheme of arrangement which, confessedly imperfect and temporary, shall recognize at once the mineralogical, chemical, and geological relations of the rocks. With this object the following classification will be adopted here.
I. Crystalline and Glassy Non-Fragmental Rocl·s.
1. Simple Rocks (composed of one mineral substance), —chiefly of aqueous rocks formed from chemical precipitates.
2. Compound Rocks (composed of two or more minerals), including (a) Massive series, embracing the various igneous rocks, as granite and lava, and (δ) Schistose series, including all the crystalline schists and most of the so-called metamorphic rocks.
II. Fragmental or Clastic Rocks, including (a) Gravel and Sand Rocks, (δ) Clay Rocks, (c) Plant-formed Rocks, (d) Rocks formed from animal remains, (e) Volcanic Agglomerates and Tuffs.
L Crystalline and Glassy or Non-Fragmental Rocks.
The great majority of these rocks are original formations; that is, they have not been palpably derived from the destruction of pre-existing rocks, as in the case of the fragmental series. They include all chemical precipitates, whether these possess a distinctly crystalline or a dull granular texture, all rocks which have consolidated from igneous fusion, and all the schistose and metamorphic rocks which, whatever may have been their original character, now possess a crystalline or foliated structure.
1. Simple Rocks. Limestone is a mass of carbonate of lime, either nearly pure or mixed with clay or other impurity. Few rocks vary more in texture and composition. It may be a hard flinty close-grained mass, breaking with a splintery or conchoidal fracture; or a crystalline rock built up of fine crystals of calcite and resembling loaf sugar in colour and texture (fig. 5); or a dull earthy friable chalk-like deposit; or a compact massive finely-granular rock resembling a close-grained sandstone or freestone. The colours, too, vary extensively, the most common being shades of blue-grey and creamcolour passing into white. Some limestones are highly siliceous, the calcareous matter having been accompanied with silica in the act of deposition; others are argillaceous, sandy, ferruginous, dolomitic, or bituminous. To some of these varieties particular names have been assigned:— Oolite, a granular limestone built up of small roe-like grains, each of which consists of concentric coats of lime; Pisolite, an oolitic or pisolitic limestone where the grains are as large as peas; Travertine (calcareous tufa), the material deposited by calcareous springs, usually white or yellowish, varying in texture from a soft chalk-like substance or marl to a compact building-stone; Stalactite, the calcareous pendant deposit formed on the roofs of caverns, vaults, bridges, &c. The water from which the hanging lime-icicles are derived drips to the floor, and on further evaporation there gives rise to the crust-like deposit known as stalagmite. Hydraulic limestone contains sufficient silica (and usually alumina) that, when it is burnt and subsequently mixed with water, a compound containing silicate of calcium is formed, which has the property of “setting” or hardening under water. Limestones containing perhaps as much as 25 per cent, of silica, alumina, iron, &c., which in themselves would be unsuitable for many of the ordinary purposes for which limestones are used, can be used for making hydraulic mortar. These limestones occur sometimes in beds like those in the Lias of Lyme Regis, sometimes in nodules like those of Sheppey, from which Roman cement is made. Cement-stone is the name given to many pale dull ferruginous limestones, which contain an admixture of clay, and some of which can be profitably used for making hydraulic mortar or cement. Fetid limestone (stinkstein, swincstone) gives off a fetid smell, like that of sulphuretted hydrogen gas, when struck with a hammer. In some cases, as in that at North Berwick, the rock seems to have been deposited by volcanic springs containing decomposable sulphides as well as lime. In other instances the odour may be connected with the decomposition of organic matter. In some quarries in the Carboniferous Limestone of Ireland, as mentioned by Mr Jukes, the freshly broken rock may be smelt at a distance of a hundred yards when the men are at work, and occasionally the stench becomes so strong that the workmen are sickened by it and require to leave off work for a time. Cornstone is an arenaceous or siliceous limestone particularly characteristic of some of the Palaeozoic red sandstone formations. Rottenstone is a decomposed siliceous limestone from which most or all of the lime has been removed, leaving a siliceous skeleton of the rock. A similar decomposition takes place in some ferruginous limestone with the result of leaving a yellow skeleton of ochre.
Marble is limestone which has acquired a granular crystalline structure. Ordinary statuary marble is a familiar example of this rock. It is white, fine-grained, composed of minute crystalline granules of calcite, and resembles loaf-sugar, whence the term ‘‘ saccharoid” often applied to it (fig. 5). Fine silvery scales of mica or talc may often be noticed even in the purest marble. Some limestones associated with gneiss and schist are peculiarly rich in minerals,—tremolite, actinolite, anthophyllite, zoisite, and many other species occurring there, often in great abundance. Many varieties of colour and texture occur among these limestones, as may be seen in the numerous kinds of ornamental marble.
Dolomite (Magnesian Limestone) is a massive formation of the carbonates of lime and magnesia, commonly associated with gypsum, rock-salt, and other results of the evaporation of saturated saline waters. It is dull granular to finely crystalline in texture, sometimes full of cavities lined with crystals of dolomite, sometimes aggregated into botryoidal, mammillated, and other concretionary forms. Dolomite also occurs as the result of a chemical transformation of ordinary limestone, carbonate of magnesia replacing carbonate of lime. This process, known as dolomitization, was largely insisted on by Von Buch and has been discussed by Bischof. The metamorphic variety of the rock is generally quite crystalline, resembling saccharoid marble in texture, and sometimes even in colour, though yellowish tints are apt to predominate. As a proof of the dolomitization or conversion of limestone into dolomite the fact may be noticed that fossil shells, and other organisms consisting originally of calcite, have been altered into dolomite. On a small scale a similar change may be observed in a limestone where it is traversed by some igneous dyke. Even along the vertical joints of limestone where no igneous matter has penetrated, and where percolating water has probably been the only agent of change, the limestone is changed for some distance on either side into a dull yellow dolomite, locally termed “dunstone.”
Gypsum occurs as a rock in the form of beds and concretions as well as in strings and veins. It is associated with red strata, often with dolomite, rock-salt, and anhydrite.
Rock-salt, massive chloride of sodium, occurs in beds sometimes 60 to 90 feet in thickness, nιdely crystalline, usually stained red from an admixture of red sediment, like that of the red clays and sandstones among which the salt-beds occur.
Ironstone.— Besides the iron ores met with in veins associated with other accompaniments of metalliferous lodes, there are many which were doubtless formed as chemical precipitates on the floors of lakes and other sheets of water. Some of these deposits (either peroxide or carbonate of the protoxide of iron) arise from precipitation in water or on moist ground where organic matter, especially of vegetable origin, has decomposed. The hard crust of hydrous peroxide of iron which forms under wet or boggy soil (moorband pan, bog-iron-ore) is an example of such a deposit now in course of formation. Where the peroxide has been reduced and become carbonate, it occurs in beds or nodules usually mixed with a variable proportion of clay (clay ironstone), and sometimes with a good deal of carbonaceous matter from associated vegetation (blackband ironstone). Clay iron-ore is one of the most valuable ores of the metal, and occurs largely in beds and nodules in the Carboniferous system, as well as in parts of the Jurassic series in Britain. In some of the oldest geological formations extensive beds occur of haematite and magnetite.
Serpentine.— This mineral occurs massive, forming large bands of rock. In some places it may have been an original deposit from oceanic water, comparable to the glauconite found filling the chambers of Foraminifera, and occurring extensively both in old geological formations and on the present ocean-floor. The serpentines associated with the gneisses and other crystalline rocks have had this origin assigned to them by Sterry Hunt. There can be· no doubt, however, that many, probably most, serpentines are the results of the alteration of pre-existing rocks. Tscheπnak pointed out that much serpentine has been produced by the hydration and alteration of olivine, and this view has been confirmed by illustrations from all parts of the world. In many serpentines the forms of the original crystals of olivine may still be detected. Hence the difficulty in understanding how there could be intrusive masses of serpentine—a hydrated magnesian silicate—is now removed, for we see that the original olivine-rocks may have been intruded as-molten masses which would preserve their external characters as eruptive rocks though undergoing an internal conversion into serpentine. With many Palaeozoic limestones, and more particularly with the crystalline beds which occur among the schistose rocks, serpentine is frequently associated. Some of this may represent the result of an alteration of dolomite, though, as above stated, it may with more probability be connected with some original oceanic deposit of a magnesian silicate.
2. Compound Rocks.
Nearly the whole of the rocks in this division consist of two or more minerals. A few examples occur, however, where, at least in some parts of their mass, the rocks are formed of only one mineral. Strictly speaking, these exceptions should be placed among the simple rocks. But they are so closely linked with the compound masses that to separate them would do much more violence to geological continuity than any harm likely to arise from the present arrangement. Besides, a rock which, like obsidian, may appear quite simple at one place may, at a short distance, show the presence of other minerals entitling it to a place among the compound rocks.
(a.) Massive Rocks.— This important division consists almost entirely of rocks which have resulted from igneous fusion. Considered from a chemical point of view, these rocks may be described as mixtures in different proportions of silicates of alumina, magnesia, lime, potash, and soda, usually with magnetic iron and phosphate of lime, and, in a great group of rocks, with an excess of silicic acid, existing as free quartz. Taking this last feature as a basis of arrangement, some petrographers have proposed to divide the igneous rocks into an acid group, including such rocks as granite, quartz-porphyry, and quartz-trachyte, where the percentage of silica ranges from 60 to 75, and a basic group, typified by such rocks as leucite-lava and basalt, where the proportion of silica is only about 50 per cent.
In the vast majority of igneous rocks the chief silicate is a felspar,—the number of rocks where the felspar is represented by another silicate (as leucite or nepheline) being comparatively few and unimportant. As the felspars group themselves into two great series, the monoclinic or orthoclase, and the triclinic or plagioclase, the former with, on the whole, a preponderance of silica, and as the minerals occur under tolerably distinct and definite conditions, it has been proposed to divide the felspar-bearing massive rocks into two series,—(1) the orthoclase rocks, having orthoclase as their chief silicate, and often with free silica in excess, and (2) the plagioclase rocks, where the chief silicate is some species of triclinic felspar. The former series corresponds generally to the acid group above mentioned, while the plagioclase rocks are on the whole decidedly basic. It has been objected to this arrangement that the so-called plagioclase felspars are in reality very distinct minerals, with proportions of silica, ranging from 43 to 69 per cent.; soda from 0 to 12; and lime from 0 to 20. But in the state of minute subdivision in which the minerals occur in many igneous rocks, it is often scarcely possible to determine the species of felspar.
Without attempting here any formal classification, according either to relative proportion of silica or to the distinguishing felspar, it may be sufficient to arrange the following description of the massive rocks in a continuous series, with the most typical acid or orthoclase rocks at the beginning, and the basic felsparless rocks at the end.
Granite is a thoroughly crystalline-granular admixture of felspar, mica, and quartz. The felspar is chiefly orthoclase, but striated triclinic felspars (as oligoclase and albite) may often be observed in smaller quantity. The mica in most granites seems to be the potash or muscovite variety, usually of a white silvery aspect; sometimes it is dark brown or black, and belongs to biotite (magnesian mica) or lepidomelane. Dr Heddle finds the common mica of the granites in the Scottish Highlands to be a new variety, which he has called haughtonite. The quartz may be observed to form a kind of paste or magma wrapping round the other ingredients (see fig. 1). It is only in cavities of the granite that the component minerals occur as independent well-formed crystals, and there too the accessory minerals are chiefly found, such as beryl, topaz, tourmaline, &c.
Microscopic examination of granite by Sorby and other later observers has shown that the quartz is full of cavities containing liquid, sometimes in such numbers as to amount to a thousand millions in a cubic inch. The liquid in these cavities appears usually to be water-containing chlorides of soda and potash, with sulphates of potash, soda, and lime.
Granites vary in texture from extremely coarse crystalline masses, with crystals an inch or more in length, to fine granular rocks which pass into elvanite or felsite. They are sometimes porphyritic, with large scattered orthoclase crystals. The variety known as graphic granite is distinguished by the way in which the quartz is crystallized through the felspar in imperfect or hemihedral shells arranged with their longer axes generally parallel, so as to produce on cross fracture the appearance of Hebrew characters.
The mean of eleven analyses of granites made by Dr Haughton gave the following average composition Silica, 72 Ό7; alumina, 14'81; peroxide'of iron, 2'22; potash, 511; soda, 279; lime, 1’63; magnesia, 0’33; loss by ignition, 1'09; total, 100 Ό5,—with a mean specific gravity of 2 ’ 66.
Granite occurs (1) as an eruptive rock, forming huge bosses, which, rise through other formations both stratified and unstratified, and sending out veins into the surrounding and overlying rocks, which usually show evidence of much alteration as they approach the granite; (2) connected with true volcanic rocks, and forming probably the lower portions of masses which flowed out at the surface as lavas; and (3) in the heart of mountain chains and elsewhere, so intimately connected with metamorphic rocks as to suggest that it is itself a final stage of the metamorphism of rocks. Granite is thus a decidedly plutonic rock; that is, it has consolidated at some depth beneath the surface, and in this respect differs from the superficial volcanic rocks, such as lavas, which have flowed out above ground from volcanic orifices.
Quartz-porphyry, Felsite-porphyry, Felstone, Felsite.— These names are given to different conditions under which orthoclase and quartz unite to form a massive eruptive rock. When the quartz forms well-marked grains, blebs, or crystals, the rock is a quartz-porphyry; when the felspar and quartz are so intimately mixed as to appear to the eye as a homogeneous matrix, the rock is called by one of the other epithets.
The base of the rock, whether in the form of quartz-porphyry or felstone, is usually exceedingly compact or even flinty in texture. Under the microscope it shows the microfelsitic character, the true nature and origin of which is still the subject of much discussion among petrographers. When the quartz occurs in forms visible to the naked eye it usually appears as round or irregular grains, varying in size from mere specks up to pieces as large as a pea or larger. Less frequently it occurs definitely crystallized, and sometimes in perfect doubly terminated pyramids.
Besides the othoclase of the diffused matrix, this mineral occurs in crystalline fragments and crystals, which sometimes reach to the length of an inch or more, so as to give a markedly porphyritic character to the rock. Triclinic felspars usually occur, though perhaps not so commonly as in granite. Mica and hornblende are among the most frequent of the minerals which accompany the two essential constituents, while apatite, magnetite, and pyrite are not infrequent accessories.
The colours of quartz-porphyry and felstone depend chiefly upon those of the felspar,—flesh-red, reddish-brown, purple, yellow, bluish or slate-grey, and even white, being in different places characteristic. The presence of much mica or hornblende gives dark grey, brown, or greenish tints. It will be observed in this, as in other rocks containing much felspar, that the colour, besides depending on the hue of that mineral, is greatly regulated by the nature and stage of decomposition. A rock weathering externally with a pale yellow or white crust may be found to be quite dark in the central undecayed portion.
The flesh-red quartz-porphyry of Dobritz, near Meissen, in Saxony, was found by Rentzsch to have the following chemical composition:—Silica, 76'92; alumina, 12’89; potash, 4'27; soda, 0’68; lime, 0’68; magnesia, 0*98; oxide of iron, 1’15; water, 1’97; total, 99‘54,—specific gravity, 2 ’ 49.
Besides the differences of colour already referred to, minor varieties in composition are produced by the relative abundance and size of the felspar crystals, and by the presence of mica {micaceous quartzporphyry, micaceous felstone, or felsite'), hornblende {homblendic quartz-porphyry or felstone), or other accessory ingredient. The variety called minette consists of a felsite base with crystals of orthoclase and dark mica, and may be regarded as a micaceous felstone, bearing the same relation to the acid felspar-rocks (felstones) that mica-porphyrite does to the more basic forms (porphyrites). When the base is very compact, and the felspar-crystals well defined and of a different colour from the base, the rock sometimes takes a good polish, and may be used with effect as an ornamental stone. In ordinary language such a stone is classed with the “marbles,” under the name of “porphyry.”
Closely related to the quartz-porphyries and felstones, of which, indeed, it can be regarded only as a variety, comes the rock known as elvan or elvanite. This is a Cornish term for a crystalline-granular mixture of quartz and orthoclase, forming veins which proceed from granite, or occur only in its neighbourhood and are evidently associated with it. It forms an immediate stage between granite and quartz-porphyry.
Quartz-porphyry and the other varieties included under this species occur (1) with plutonic rocks, as eruptive bosses or veins, often associated with granite, from which, indeed, the elvanite, as just stated, may be seen to proceed directly; of frequent occurrence also as ve⅛s and irregularly intruded masses amoDg highly convoluted rocks, especially when these have been more or less metamorphosed; (2) in the chimneys of old volcanic orifices, forming there the “neck” or plug by which the vent is filled up; and (3) as truly volcanic rocks which have been erupted at the surface in the form of flows of lava, either {a) submarine, as in the felstones of Wales, associated with the marine Lower Silurian rocks, or (⅛) subaerial, as probably in the quartz-porphyry of Arran, and perhaps in the series of the “green-slates and porphyries " of the Silurian system in Cumberland, which Professor Ramsay has conjectured to be the products of a subaerial volcano.
Pitchstone {Retinite) is a glassy rock, having a pitch-like or resinous lustre, and a black or dark-green colour ranging through shades of green, brown, and yellow to nearly white. It is essentially an orthoclase rock, and may be regarded as the natural glass of many of the more granular or crystalline orthoclase rocks, such as the quartzporphyries or felsites. Examined microscopically, it is found to consist of glass in which are diffused, in greater or less abundance, hair-like microlites, angular or irregular grains, or more definitely formed crystals. The pitchstone of Corriemlls, in the island of Arran, presents abundant green, feathery, and dendritic microlites of a pyroxenic character (see fig. 3).
According to Durocher the mean composition of pitchstone is— silica, 70’6; alumina, 15’0; potash, 1*6; soda, 2'4; lime, 1*2; magnesia, 0 ‘6; oxides of iron and manganese, 2 - 6; loss by ignition, 6'0. The mean specific gravity is given as 2’34.
Pitchstone is found either as intrusive dykes, veins, or bosses, probably in close connexion with former volcanic activity, or in sheets which, like the porphyritic pitchstone of the Isle of Eigg, flowed out at the surface as lava-streams.
Bipartie {Rhyolite, Quartz-trachyte) is an orthoclase rock containing an excess of silica which usually appears in distinct grains or in doubly terminated pyramids. The orthoclase, which is of the variety termed sanidine, is sparingly accompanied with triclinic felspar. Other frequent ingredients are magnesia-mica, hornblende, augite, apatite, and magnetite. Considerable diversity exists in the texture of this rock. Some varieties are coarse and granitoid in character, and are regarded by some petrographers as the equivalents in Tertiary times of the granite of older geological periods. From this crystalline aspect intermediate varieties may be obtained like the quartz-porphyries, passing by degrees into more or less distinctly vitreous rocks. Throughout all these gradations, however, a characteristic ground mass can be seen under the microscope having a glassy, enamel-like, or porcellanous character. An analysis by Vom Rath of a rhyolite from the Euganean Hills gave—silica, 76Ό3; alumina, 13’32; soda, 5‘29; potash, 3’83; protoxide of iron, 174; magnesia, 0'30; lime, 0’85; loss, 0'32; total, 101'68,— specific gravity, 2 ‘553. Liparite is a volcanic rock of late geological date occurring in the form of erupted lavas.
Obsidian is a volcanic glass representing the vitreous condition of a highly silicated sanidine-rock, such as liparite. It resembles bottle glass, having a perfect conchoidal fracture, and breaking into sharp splinters, semi-transparent or translucent at the edges. The colours of the rock are black, brown, or greyish-green, rarely yellow, blue, or red, but not infrequently streaked or banded with paler and darker lines. When a thin slice of obsidian is prepared for the microscrope it is found to be very pale yellow, grey, or nearly colourless. On being magnified it shows that the usual dark colours are almost always produced by the presence of minute crystals, needles, and black hair-like bodies. In rare examples the obsidian appears as a perfect glass without any foreign admixture. The minute crystals and hair-like bodies sometimes so increase in abundance as to make the rock lose the aspect of a glass and assume that of a dull flint-like or enamel-like stone. This devitrification can only be properly studied with the microscope. Again little granules {sρherιdites) of a dull grey enamel (pearlstone) appear, and in some parts of the rock so abundantly as to alter its character and convert it from obsidian into pearlstone. The average chemical composition of the rock is—silica, 71’0; alumina, 13*8; potash, 4Ό; soda, 5'2; lime, IT; magnesia, 0*6; oxides of iron and manganese, 3'7; loss, 0'6; total, 100‘0,—mean specific gravity, 2’40. Obsidian occurs as a product of the volcanoes of late geological periods.
Pearlstone {Perlite) is another vitreous condition of sanidine lava. As its name denotes, it consists of vitreous or enamel-like globules, occasionally assuming polygonal forms by mutual pressure. These globules sometimes constitute the entire rock, their outer portions shading off into each other so as to form a compact mass; in other cases they are separated by and cemented in a compact glass or enamel. They consist of successive very thin shells, which, in a transverse section, are Seen as concentric rings, usually full of the same kind of hair-like crystallites and crystals as in obsidian. Occasionally there are found among them true spherulites where the internal structure is radiating fibrous. When such spherulites Occupy the main mass they give rise to sphendite-rock. Pumice is a general term for the cellular and filamentous or froth-like parts of lavas. In the great majority of cases it is a form of the obsidians, showing under the microscope the usually vitreous characters, and possessing a specific gravity of 2’0 to 2'53, though, owing to its porous nature, it possesses great buoyancy and readily floats on water. At Hawaii, however, some of the pyroxenic or olivine lavas give rise to a pumiceous froth which has the usual outward characters of ordinary pumice.
The rocks enumerated up to this point are all orthoclase-rocks, and markedly siliceous, frequently showing their excess of silica in the form of quartz grains or crystals. In the succeeding group free quartz is not found as a marked constituent, although occasionally it occurs in some quantity. In this series syenite may be regarded as the equivalent of granite in the quartzose series, orthoclase porphyry of quartz-porphyry and felstone, and trachyte of liparite.
Syenite.— According to the modem nomenclature, this name, which was formerly given in England to a granite with hornblende replacing mica, is now restricted to a rock consisting essentially of a mixture of orthoclase and hornblende, to which plagioclase, quartz, and mica are occasionally added. The name syenite, first used by Pliny in reference to the rock of Syene, was introduced by Werner as a scientific designation, and applied to the rock of the Plauenseher-Gnιnd, Dresden. Werner afterwards, however, made that rock a greenstone. The base of all syenites like that of granites is crystalline, without a traee of any amorphous substance between the crystals, such as most igneous rocks contain. Hence the texture is of that crystalline kind commonly known as granitic.
The typical syenite of the Plauenscher-Grund, formerly described as a coarse-grained mixture of flesh-coloured orthoclase and black hornblende, containing no quartz, and with no indication of plagioclase, was regarded as a normal orthoclase-hornblende rock. Microscopical research has, however, shown that well-striated triclinic felspar, as well as quartz, oecurs in it. Its composition is shown by the following analysis:—silica, 59'83; alumina, 16’85; protoxide of iron, 7 *01; lime, 4'43; magnesia, 2'61; potash, 6’57; soda, 2 - 44; water, &c., 1'29; total, 101’03. The average specific gravity of syenite is from 2 75 to 2’90.
Syenite occurs under conditions similar to those in which granite is found; it has been erupted in large irregular masses, especially among metamorphic rocks, as well as in smaller bosses and veins.
Orthoclase Porphyry {Quartzless Porphyry) is an orthoclase rock containing no quartz, or a very sparing admixture of that mineral, but with a little plagioclase, and not unfrequently with some hornblende and dark biotite. It contains from 55 to 65 per cent, of silica. It differs thus from quartz-porphyry and felstone in its smaller proportion of silica, but the distinction is one which, except by chemical or microscopical analyses, must often be difficult to establish between the fine compact felstones and the orthoclase porphyries, especially when the latter contain free quartz. This rock is sometimes termed syenite-porphyry, since it is associated with syenite much in the same way that elvanite is with granite. It is like syenite a plutonic rock, and occurs in veins, dykes, and intrusive sheets. Probably, however, many of the so-called “felstones” which occur as lavas, contemporaneously ejected with the older Palaeozoic formations, are really orthoclase-porphyries.
Trachyte, a term originally applied to a large series of modern volcanic rocks possessing a characteristic roughness (τραχυs) under the finger, is now restricted to rocks consisting essentially of sanidine, with more or less triclinic felspar, usually with hornblende, biotite, or augite, and sometimes with magnetite and apatite. In microscopic structure the rock is distinguishable from the quartz-trachytes or liparites by the absence or feeble development of any microfelsitic ground-mass, and in general by the presence of a porphyritic base, consisting either of a pure glass or of one with devitrification products. The average composition of trachyte may be stated thus:—silica, 60Ό; alumina, 17‘0; protoxide of iron, 8Ό; magnesia, 1Ό; lime, 3’5; soda, 4Ό; potash, 5'0; loss by ignition, 1 ’ 5. Average specific gravity, 2’65. Trachyte is a volcanic rock of Tertiary and post-Tertiary date.
Phonolite {Clinkstone), a term suggested by the metallic ringing sound emitted by the compact varieties when struck, is applied to a mixture of sanidine felspar and nepheline with hornblende and usually nosean. An average specimen contained silica, 57 7; alumina, 20 ’ 6; potash, 6Ό; soda, 7’0; lime, 1’5; magnesia, 0’5; oxides of iron and manganese, 3‘5; loss by ignition, 3'2 per cent. The specific gravity may be taken as about 2 ’ 58. Phonolite is sometimes found splitting into thin slabs which can be used for roofing purposes. Occasionally it assumes a porphyritic texture from the presence of large crystals of sanidine or of hornblende. When the rock is partly decomposed and takes a somewhat porous texture, it resembles trachyte in appearance.
Like trachyte, phonolite is a thoroughly volcanic rock and of late geological date. It occurs sometimes filling the pipes of volcanic orifices, sometimes as sheets which have been poured out in the form of lava-streams, and sometimes as dykes and veins.
In the rocks enumerated up to this point the essential felspar constituent is orthoclase; in the felspar rocks now to be described the corresponding ingredient is nearly always some triclinic form. In the volcanic rocks of this series there is usually some mineral of the hornblende or augite family present in such quantity as to give a green or even black colour to the mass.
Porphyrite may be used as the designation of rocks which consist essentially of some triclinic felspar, and show a glassy or partially devitrified ground-mass containing abundant crystals of plagioclase with magnetite or titaniferous iron, and sometimes hornblende, augite, or mica. These rocks include many varieties which have not yet been thoroughly examined. The texture varies from coarse crystalline-granular to exceedingly close-grained, and passes occasionally even into vitreous. Porphyrite is a volcanic rock very characteristic of the later palaeozoic formations, occurring there as interstratified lava-beds, and in eruptive sheets, dykes, veins, and irregular bosses. t Diorite {Greenstone in part) is a crystalline mixture of oligoclase or some allied felspar and hornblende with magnetic iron and apatite. Where free quartz occurs the rock is called quartz-diorite. The more compact dark varieties have been termed aphanite. The average chemical composition of this rock may be taken to be— silica, 53’2; alumina, 16 Ό; potash, 1*3; soda, 2'2; lime, 6 ’3; magnesia, 6 ’0; oxides of iron and manganese, 14 Ό; loss by ignition, 1Ό. The mean specific gravity is about 2’95. Diorite occurs as an eruptive rock under conditions similar to those of quartz-porphyry and syenite. It is found in palaeozoic volcanic regions, as in North Wales, in “neck'”-like masses which may mark the position of some of the volcanic orifices of eruption.
Propylite is a name given to certain Tertiaιy volcanic rocks consisting of a plagioclase felspar and hornblende in a fine-grained groundmass. They are subject to considerable alteration, the hornblende being converted into epidote. Some quartziferous propylites have been described by Zirkel from Clarence King’s Survey of the iQth Parallel, wherein the quartz abounds in liquid cavities containing briskly moving bubbles, and sometimes double enclosures with an interior of liquid carbonic acid.
Hornblende-Andesite is a rock of late geological date consisting of a plagioclase felspar and hornblende often with a little sanidine. The ground-mass is frequently quite crystalline, or shows a small proportion of a felsitic nature, with microlites and granules. When the rock contains free quartz it is called Dacite.
In the next series of rocks augite plays a similar part to that taken by hornblende in the foregoing species.
Diabase.— This name has been given to certain dark green or black eruptive rocks found in the older geological formations and consisting essentially of a triclinic or plagioclase felspar, augite, magnetic or titaniferous iron, sometimes olivine, and usually with more or less of a diffused greenish substance which has resulted from the alteration of the augite or olivine. The texture is sometimes quite crystalline; in other cases it shows a felsitic ground-mass. Except that the so-called diabases are confined to Palaeozoic rocks and the basalts to Tertiary and post-Tertiary formations, there seems no essential distinction between these two groups, though, of course, as the diabases are much older and have been far longer exposed to metamorphic processes, they are in general less fresh than the basalts.
Melaphyre is a term which has been so variously used that the sense in which it is taken must always be explained. Thus Seuft described it as a rock having an indistinctly mixed character, with colours ranging from dirty greenish-brown, or reddish-grey, or greenish black-brown, to a complete black; hard and tough when fresh, and then showing crystals of reddish-grey labradorite, with magnetic titaniferous iron, and usually with carbonates of lime and iron, and ferruginous chlorite (delessite), in crystalline grains; compact or earthy, or sometimes porphyritic or amygdaloidal. Naumann defines melaphyre as a close-grained rock, very often amygdaloidal, composed essentially of labradorite, with an undetermined silicate, some titaniferous iron, carbonates of lime and of iron, and sometimes crystals of augite, rubellan, and mica. Zirkel called it generally crypto-crystalline, sometimes porphyritic, very often amygdaloidal rock, consisting of a mixture of oligoclase and augite with magnetic iron. Lastly, Rosenbusch proposes to restrict it to those plagioclase-augite rocks which contain olivine, and possess a distinct porphyry ground-mass. There can be little .doubt that, like the so-called diabases, the melaphyres are merely older forms of the great basalt-family.
Augite-Andesite is the name given to certain dark eruptive rocks of Tertiary and post-Tertiary date which consist of a triclinic felspar (oligoclase, or some species rather richer in silica than labradorite) and augite, with sometimes sanidine, hornblende, biotite, magnetite, or apatite, and in some varieties quartz. The composition of an example from Santorin, erupted in the year 1866, was found to be as follows:—silica, 67‘35; alumina, 15’72; magnesia, 1’16; lime, 3'60; soda, 5'04; potash, 1*86; oxides of iron, 1’94; loss by ignition, 0 ’ 36; total, 101Ό6. Mean specific gravity, 2 ’ 75.
Basalt. — Under this term are included those widespread and important woleanie rocks which consist of a triclinic felspar, probably always labradorite, augite, olivine, and magnetic or titaniferous iron, with apatite, and sometimes sanidine or nepheline. The more coarsely crystalline varieties are known as dolerite, while those of intermediate texture have been termed ατm7τιcrife,the more close-grained black heavy kinds being distinctively basalts. The chemical composition of an average variety may be set down as silica, 45 Ό; alumina, 15 Ό; magnesia, 6'5; lime, 10 '5; soda, 3 5; potash, 1'5; oxides of iron and manganese, 15Ό; loss by ignition, 3'0. Mean specific gravity, 2'95.
Zirkel has described under the name of basalt certain rocks in which the part of the felspar is played by another mineral, in some cases nepheline, in others leucite.
Basalt occurs as dykes and veins, intrusive bosses and sheets, and successive contemporaneous flows. It often presents a columnar structure, as at Staffa and the Giant’s Causeway, whence the structure has been popularly termed basaltic.
Tachylite is a black volcanic glass often found in association with basalt, of which indeed it is merely the vitreous condition. Thus a basalt-dyke sometimes shows a thin crust of this pitch-like substance on its walls, where the molten rock was first suddenly cooled.
Palagonite is a volcanic glass related to basalt, and found in fragments in some tuffs.
Gabbro {Diallage-rock) is a compound of a triclinic felspar and diallage often with olivine, and also very generally with magnetic or titaniferous iron and apatite, more rarely with hornblende, biotite, or quartz. An average chemical composition is silica, 50 Ό; alumina 15Ό; magnesia, 7Ό; lime, 10'0; soda, 2’5; potash, 0’5; oxides of iron and manganese, 12’5; loss by ignition, 2 ’ 5. Mean specific gravity, 2 - 95.
A very few crystalline massive rocks occur without felspar as an essential constituent; but they are of comparatively little importance as rock-masses, though interesting in themselves and sometimes of considerable beauty.
Pikrite is a rock rich in olivine, usually more or less serpentinized, with augite, magnetite, or ilmenite, and a little brown biotite, hornblende, or apatite.
Eulysite is a mixture of olivine, augite, and red garnet.
Garnet-olivine-rock is composed of olivine, diallage, and garnet.
Enstatite-olivine-rock consists of olivine and enstatite (bronzite or hypersthene) with magnetite or chromite.
Lherzolite is a mixture of olivine, pyroxene, picotite, and usually some magnetite.
Eclogite is a compound of garnet and omphacite, or smaragdite (hornblende).
Dτmite is a mixture of olivine and chromite, found with serpentine. Limburgite is composed of crystals of olivine, augite, and magnetite, in a base more or less vitreous.
(δ.) Schistose or Foliated Hocks.— These form an exceedingly well-defined characteristic series. They are distinguished from the massive rocks by the possession of an internal arrangement into more or less closely parallel layers or folia, consisting of materials which have assumed a crystalline character along these layers. The layers may be composed of only one mineral, but are usually of two or more, which occur either in distinct, often alternate, laminae or intermingled in the same layer. Moreover, the layers are not usually continuous for more than a short space. As a rule they are strikingly lenticular, thickening out and then dying away, and reappearing after an interval on the same or a different plane. They are likewise characteristically welded, as it were, into each other, the crystalline particles of one lamina being so intermingled with those of the layers above and below it that the whole coheres as a tough not easily fissile mass. Though arranged in distinct layers, a schistose rock is usually distinguished from an ordinary sedimentary one by the irregularity of its lamination, and by a remarkable and eminently distinctive puckering or crumpling of the folia. The vast plications which can be seen from a distance running up the face of a mountain are repeated on a smaller scale in hand specimens, and even down to such proportions as can only be seen with a microscope. As already stated, the origin of these rocks has been the subject of much discussion. That they are metamorphosed sediments, and not original chemical precipitates, is the general opinion of geologists. See part iv.
A foliated rock showing this characteristic irregular fissility in a marked degree is termed a “schist.” This word, placed after the distinguishing mineral of the rock, is used as the name of the rock, as mica-schist, chlorite-schist, hornblende-schist. If the mass loses its fissile tendency owing to the felting together of the component mineral into a tough coherent whole, the word rock is usually substituted for schist, as in hornblende-rock, actinolite-rock, and so on. There are thus three kinds of fissility among rocks: —(1) that of original deposit, as in shale,—this is termed lamination; (2) that of cleavage, as in clay slate; (3) that of foliation, as in the schists. There is a fourth kind of divisional planes, that of joints, sometimes so closely placed together as almost to rival the others, as will be pointed out in part iv.
Mica-schist (Mica-slate) is a schistose aggregate of quartz and mica. The relative proportions of the two minerals vary widely even in the same mass of rock. Each is arranged in lenticular wavy laminae. The quartz shows greater inconstancy in the number and thickness of its folia. Frequently a layer of this mineral may often be seen to swell out to a thickness of an inch or more, and, dwindling rapidly down to a mere thread, disappear. The quartz may often be observed to retain a granular character like that of quartz-rock, no doubt indicative of its originally sedimentary origin (see fig. 6). The mica lies in thin plates, sometimes so dovetailed into each other as to form long continuous irregular crumpled folia, separating the quartz layers, and often in the form of thin spangles and membranes nιnning in the quartz. Among the accessory minerals, garnet, felspar, and hornblende are not infrequent. Mica-schist forms extensive regions in Norway, Scotland, the Alps, and other parts of Europe, in connexion with other members of the schistose family of rocks. It is also found encircling granite masses in Scotland and Ireland as a metamorphic zone a mile or so broad, which shades away into the unaltered strata of greywacke or slate outside.
Though the possession of a fissile structure, showing abundant divisional surfaces covered with glistening mica, is characteristic of mica-schist, we must distinguish between this structure and that of many micaceous sandstones which can be split into thin seams each splendent with the sheen of its mica-flakes. A little examination will show that in the latter case the mica has not crystallized in situ, but exists merely in the foπn of detached worn scales, which, though lying on the same general plain, are not welded into each other as in a schist; also that the quartz does not exist in folia but in rounded separate grains.
Gneiss is a crystalline schistose aggregate of the same minerals as in granite—felspar, quartz, and mica. The relative proportions of these minerals, and the manner in which they are grouped with each other, give rise to numerous varieties of the rock. As a rule the folia are coarser and the schistose character less perfect than in mica-schist. Sometimes the quartz lies in tolerably pure bands a foot or even more in thickness with plates of mica scattered through it. These quartz layers may be replaced by a crystalline mixture of quartz and felspar, or the felspar will take the form of independent lenticular folia, while the mica runs abundantly through the rock, and by its own ready cleavage imparts a fissile structure to the whole. Sometimes the mica is mostly or wholly replaced by hornblende, in other cases by talc (protogine-gneiss). Like mica-schist, gneiss occupies a large space in regions where the older geological formations come to the surface. Varieties of it are also found in the metamorphic zone encircling some masses of granite. So coarse is the texture of many gneisses that they cannot, in hand-specimens nor even in large blocks, be certainly discriminated from granite. In such cases it is only by examination in the field and the detection of clear evidence of foliated structure that their tree character can be determined.
An interesting and important variety is met with in some regions of gneiss and schist, viz., conglomerate bands in which pebbles of quartz and other materials from less than an inch to more than a foot in diameter are imbedded in a foliated matrix. Examples of this kind are found in the pass of the Tête Noir between Martigny and Charnouni, in N. W. Ireland, in the islands of Bute and Islay, and in different parts of Argyllshire. These enclosures are not to be distinguished from the ordinary water-worn blocks of true conglomerates; but the original matrix which encloses them has been so altered as to acquire a micaceous foliated structure, and to wrap the pebbles round as with a kind of glaze. These facts are of considerable value in regard to the theory of the origin of the crystalline schists.
Granulite (Leptynite) is a crystalline schistose aggregate of orthoclase and quartz, with some garnet and kyanite.
Chlorite-schist (or Chlorite-slate) is a schistose aggregate of green chlorite, often with some quartz, felspar, mica, or talc. The more massive forms (lapis ollaris, potstone) can be cut as building stone, or for the manufacture of articles for domestic use.
Talc-schist is a schistose aggregate of whitish-green or yellowish talc often combined with felspar or quartz. Dr Heddle has recently shown that many so-called talc-schists contain no talc, but owe their unctuous character to a variety of mica (rnargarodite).
Hornblende-schist is a schistose mass of black or dark-green hornblende, but often interleaved with felspar, quartz, or mica. When the schistose character disappears, the mass becomes a hornblende rock (amphibolite). When the variety actinolite occurs instead of common hornblende it forms actinolite-schist.
Numerous other varieties of schists have been described, but they occupy very subordinate places among the foliated rocks. The following analyses show the chemical composition of the more important of those which have been enumerated :— [table]
Λs the disappearance of the schistose structure produces a crystalline amorphous compound like that of a massive or ordinary igneous rock, we are brought at last round again to rocks which we cannot distinguish from those to which elsewhere an igneous origin is assigned. In gneiss, for example, the same minerals occur which form granite, and possess a crystalline character. Any process, such as irregular internal motion of the mass, which could destroy the schistose structure and produce a thoroughly granite-like texture, would give rise to a rock which, whatever its previous history might have been, could not be distinguished from granite. That such internal transformations have taken place among the crystalline gneissose masses can hardly be doubted. And thus, at the one end of the schistose series, we may have ordinary unaltered sediment; at the other, after many intermediate stages, a thoroughly crystalline amorphous rock like granite or syenite.
∏. Fragmental (Clastic) Rocks,
This great series embraces all rocks of a secondary or derivative origin; in other words, all formed of particles which had previously existed on the surface of the earth in another form, and the accumulation and consolidation of which gave rise to new compounds. Some of these rocks have been produced by the mechanical action of running water, such as gravel, sand, and mud; others have arisen from the gathering together of the remains of once living plants or animals; others have been formed by the consolidation of the loose debris thrown out by volcanoes,
(a.) Gravel and Sand Kocks.— Ordinary gravel and sand are produced by the action of running water on every seacoast and river-course. These sedimentary materials, being mere mechanical formations, vary indefinitelyin composition, according to the nature of the source from which they are derived. As a rule they consist of the detritus of siliceous rocks, these being among the most durable materials. Quartz, in particular, enters largely into the composition of sandy and gravelly detritus. Fragmentary materials tend to group themselves according to their size and relative density. Hence they are apt to occur in layers, and to show the characteristic stratified arrangement of sedimentary rocks. They may enclose the remains of any plants or animals entombed on the same sea-floor, river-bed, or lake-bottom.
Blown sand is sand which has been produced by previous waveaction, and is blown into long ridges or dunes by prevailing winds. It varies in composition as ordinary sandstones do, being sometimes entirely siliceous, sometimes calcareous where derived from triturated shells or other calcareous organisms. Layers of finer and coarser particles often alternate as in water-formed sandstone. Grasses and other plants bind the surface of the shifting sand, but are apt to be covered by fresh encroachments of the loose material, and then by their decay they give rise to dark peaty layers in the sand. Calcareous blown sand is compacted into hard stone by the action of rain-water which alternately dissolves a little of the lime and re-deposits it on evaporation as a thin crust cementing the grains of sand together.
Cliff-debris consists of angular rubbish disengaged by frost and ordinary atmospheric waste from the face of clins, crags, and steep slopes. It slides down the declivities of hilly regions, and accumulates at the base of slopes and precipices, until washed away by rain or by brooks. It naturally depends for its composition upon the nature of the solid rocks from which it is derived. The material constituting glacier moraines is of this kind.
Rain-wash is a loam or earth which accumulates on the lower parts of slopes or at their base, and is due to the gradual descent of the finest particles of disintegrated rocks by the transporting action of lain. Brick-earth is the name given in the south-east of England to thick masses of such loam which are extensively used for making bricks.
Subsoil is the broken-up part of the rocks immediately under the soil. Its character of course is determined by that of the rock out of which it is formed by subaerial disintegration.
Soil is the product of the subaerial decomposition of rocks and of the decay of plants and animals. Primarily the character of the soil is determined by that of the subsoil, of which indeed it is merely a further disintegration. The formation of soil is treated in part iii., pages 265, 269.
Conglomerate (Puddingstone) is a name given to any rock formed of consolidated gravel or shingle. The component pebbles are rounded and water-worn. They may consist of any kind of rock, though usually of some hard and durable sort, such as quartz or quartz-rock. A special name may be given according to the nature of the pebbles, as quartz-conglomerate, limestone-conglomerate, graniteconglomerate, &c. The paste or cementing matrix may consist of a hardened sand or clay, and may be siliceous, calcareous, argillaceous, or ferruginous. In ⅛e coarser conglomerates, where the blocks may exceed 6 feet in length, there is often very little indication of stratification. Except where the flatter stones show by their general parallelism the rude lines of deposit, it may be only when the mass of conglomerate is taken as a whole, in its relation to the rocks below and above it, that its claim to be considered a stratified rock will be conceded.
Breccia is a rock in which the stones are angular and not rounded, and usually with less trace of stratification than in conglomerate. Intermediate stages between this rock and the preceding, where the stones are partly angular and partly subangular and rounded, are known as lmecciated conglomerate. Sandstone is a rock formed of consolidated sand. The component grains are for the most part of quartz—a most durable mineral, which must here be regarded as the residue left after all the more decomposable minerals of the original rocks have been carried away in solution or in suspension as fine mud. The colours of sandstones arise, not so much from that of the quartz, which is commonly white or grey, as from the film or crust which often coats the grains and holds them together as a cement. As already stated iron is the great colouring ingredient of rocks. In sandstones it gives rise to red, brown, yellow, and green hues, according to its degree of oxidation and hydration. In ordinary red sandstones, for example, each grain of sand is coated with red earthy haematite. In yellow sandstone the oxide has become hydrous in the form of limonite.
There is as much variety of composition among sandstones as among conglomerates. Though they consist for the most part of siliceous grains, they include others of clay, felspar, mica, or other mineral; and these may increase in number so as to give a special character to the rock. Thus sandstones may be argillaceous, felspathic, micaceous, calcareous, &c. By an increase in the argillaceous constituents, a sandstone may pass into one of the clay-rocks, just as modern sand on the sea-floor shades imperceptibly into mud. On the other hand, by an augmentation in the size of the grains a sandstone may become a grit, or a pebbly or conglomeratic sandstone, and pass into a fine conglomerate. À piece of fine-grained sandstone seen under the microscope looks like a coarse conglomerate, so that the difference between the two rocks is little more than one of relative size.
Among the varieties of sandstones may be mentioned Flagstone, a thin-bedded sandstone capable of being split into slabs or flags; Freestone, a sandstone which can be cut freely in any direction (the term is popularly applied to some limestones and other rocks); and Buhrstone, a highly siliceous, exceedingly compact, though cellular, rock (with Chara seeds, &c.), found alternating with unaltered Tertiary strata in the Paris basin, and forming from its hardness and roughness an excellent material for the grindstones of flour-mills.
Greywadce is a compact aggregate of rounded or subangular grains of quartz, slate, felspar, or other minerals or rocks cemented by a paste which is usually siliceous but may be argillaceous, felspathic, or calcareous. Grey, as its name denotes, is the prevailing colour; but it passes into brown, brownish-purple, and sometimes, where anthracite occurs, into black. The rock is distinguished from ordinary sandstone by its darker hue, its hardness, the variety of its component grains, and above all by the compact cement in which the grains are imbedded. In many varieties so pervaded is the rock by the siliceous paste that it possesses great toughness, and its grains seem to graduate into each other as well as into the surrounding matrix. Such rocks when fine-grained can hardly, at first sight or with the unaided eye, be distinguished from some compact igneous rocks, though a microscopic examination at once reveals their fragmental character. In other cases, where the greywacke has been formed mainly out of the debris of granite, quartz-po∏>hyry, or other felspathic masses, the grains consist so largely of felspar” and the paste also is so felspathic, that the rock might be mistaken for some close-grained granular porphyry. Greywacke occurs extensively among the Palaeozoic formations in beds alternating with shales and conglomerates. It represents the sand of the Palaeozoic sea-floor, retaining often its ripple-marks and sun-cracks. The metamorphism it has undergone has generally not been great, and for the most part is limited to induration, partly by pressure and partly by permeation of a siliceous cement.
Quartz-rock (Quartzite) is a close-grained granular aggregate of quartz cemented by a highly siliceous matrix. Originally it consisted of a tolerably pure quartz-sand, which has been metamorphosed by pressure and the transfusion of a siliceous cement into an exceedingly hard mass. This cement was probably produced by the solvent action of heated water upon the quartz grains, which very generally seem to shade off into each other, or into the intervening silica. It is owing no doubt to the purely siliceous character of the grains that the blending of these with the surrounding cement is more intimate than in greywacke, so much so that the rock often assumes an almost flinty homogeneous texture. That quartz-rock as here described is an original sedimentary rock and not a chemical deposit is shown, not only by its granular texture, but by the exact resemblance of all its leading features to ordinary sandstone—false-bedding, alternation of coarser and finer layers, worm-burrows, and fucoid-casts. It occurs in the form of large masses interstratified with limestones, slates, and schists. It is also met with locally as an altered form of sandstone, where this rock is traversed by igneous dykes and indurated into quartz-rock for a distance of a few inches or feet from the intrusive mass. Bands of highly silicated sandstones, having the lustrous aspect, fine grain, and great hardness of quartz-rock, occur among the unaltered shales and other strata of the Carboniferous system. In such cases, the supposition of any general metamorphism being inadmissible, we must suppose either that these quartzose bands have been indurated, for example, by the passage through them of thermal silicated water, or that the quartz-rock is there an original formation.
(ό.) Clay-rocks.— Thes3 are composed of the finer argillaceous sediments or muds derived from the waste of previously formed rocks. Perfectly pure clay, hydrated silicate of alumina, may be seen where some granites and other felspar-bearing rocks decompose. But, as a rule, the clay is mixed with various impurities.
Pipe-clay is white, nearly pure, and free from iron. Fire-clay is a deposit largely found in connexion with coal-seams, contains little iron, and is nearly free from lime and alkalies. Some of the most typical fire-clays are . those long used at Stourbridge, Worcestershire, for the manufacture of pottery. The best glasshouse pot-clay, that is, the most refractory, and therefore used for the construction of pots which have to stand the intense heat of a glass-house, has the following composition:—silica, 73'82; alumina, 15'88; protoxide of iron, 2’95; lime, trace; magnesia, trace; alkalies, ’ 90; sulphuric acid, trace; chlorine, trace; water, 6 ’ 45; specific gravity, 2 '51. A very siliceous close-grained or flinty variety, termed Gannister, occurs in the Lower Coal-measures of the north of England, and is now largely ground down as a material for the hearths of iron furnaces. Brick-clay is properly rather an industrial than a geological term, since it is applied to any clay, loam, or earth, from which bricks or coarse pottery are made. It is an impure clay, containing a good deal of iron, with other ingredients. An analysis gave the following composition of a brick-clay :—silica, 49 - 44; alumina, 34 ’ 26; sesquioxide of iron, 7∙74; lime, 1 ’ 48; magnesia, 5 ’ 14; water, 1 ’ 94.
Mudstone is a fine, usually more or less sandy, argillaceous rock, having no fissile character, and of somewhat greater hardness than any form of clay. The term Clay-rock has been applied by some writers to an indurated clay requiring to be ground and mixed with water before it acquires plasticity.
When clay has been deposited intermittently so as to assume a thinly stratified or fissile structure, it receives the general name of Shale. Under this term are included all laminated and indurated clays which are capable of being split along t⅛e lines of deposit into hard leaves. They present almost endless varieties of texture and composition, passing on the one haud into clays, on the other into flagstones and sandstones, or again, through calcareous gradations into limestone, or through ferruginous varieties into clayironstone, and tlιrough bituminous kinds into coal. An important variety, known as Oil-shale, and containing so much bituminous matter that it is now extensively used as a source for the manufacture of solid paraffin and mineral oils is described in the next section.
Flinty-slatc {Lydian-stone, Hornstone) is siliceous shale or mudstone which has been indurated into an exceedingly compact flinty mass, breaking with a conchoidal or splintery fracture, and usually of dark colours, black, brown, and red, more rarely white.
Clay-slate is a compact close-grained, very hard, fissile argillaceous rock, dull lead-blue, grey, green, red, purple, or black in colour, splitting into thin leaves which are not those of original deposit but those produced by a superinduced cleavage. In this case the rock has been affected by great lateral pressure, whereby its particles have been forced to adjust themselves with their longer axes perpendicular to the direction of pressure. This rearrangement has imparted to the rock a fissility wholly independent of original lamination. The possession of this cleavage is the distinctive character of a true slate.
(c.) Rocks formed of the Debris of Plants.— These have sometimes been produced by the decay and entombment of vegetation 'on the spot where it grew, sometimes by the drifting of the plants to a distance and their consolidation there. In the latter case, they may be mingled with inorganic sediment, so as to pass into carbonaceous shale.
Beat is vegetable matter, more or less decomposed and chemically altered, found in boggy places and elsewhere where marshy plants grow and decay. It varies from a pale yellow or brown fibrous substance, like turf or compressed hay, in which the plant remains are abundant and conspicuous, to a compact dark-brown or black material, resembling black clay when wet and some varieties of lignite when dried. The nature and proportions of the constituent elements of peat, after being dried at 100° C., are illustrated by the analysis of an Irish example which gave—carbon, 60'48; hydrogen, 6’10; oxygen, 32’55; nitrogen, 0*88; while the ash was 3 - 30.
There is always a large proportion 'of water which cannot be driven off even by drying the peat. In the manufacture of compressed peat for fuel this constituent, which of course greatly lessens the value of the peat as compared -with an equal weight of coal, is driven off to a great extent by chopping the peat into fine pieces, and thereby exposing a large surface to evaporation. The ash varies in amount from less than 1’00 to more than 65 per cent., and consists of sand, clay, ferric oxide, sulphurie acid, and minute proportions of lime, soda, potash, and magnesia.
Lignite, is compressed and chemically altered vegetable matter, often retaining a lamellar or ligneous texture, and stems with woody fibre crossing each other in all directions. It varies from pale brown or yellow to deep brown or black. Some shade of brown is the usual colour, whence the name brown coal, by which it is often known. It occurs in beds chiefly among the Tertiary strata, under conditions similar to those in which coal is found in older formations. It may be regarded as a stage in the alteration and mineralization of vegetable matter intermediate between peat and true coal.
Coal, the most completely mineralized form of vegetable matter, occurs as a black (sometimes dark-brown), brittle, usually lustrous substance, intercalated in beds between strata of sandstone, shale, fireclay, &c., in geological formations of Palaeozoic, Secondary, and Tertiary age. The word coal is rather a popular than a scienific term, as it is indiscriminately applied to any mineral substance capable of being used as fuel. Strictly employed it ought only to be used with reference to beds of fossilized vegetation, the result either of the growth of plants on the spot or of the drifting of them thither.
The following analyses show the chemical constituents in some of the principal varieties of coal :— [table]
Diatom-earth is a siliceous deposit formed chiefly of the frustules of diatoms. It is laid down both in salt and in fresh water. Wide tracts of it are now being deposited on the bed of the South Pacific. In Virginia, United States, an extensive tract occurs·covered with diatom-earth to a depth of 40 feet. It is used as a polislιing or tripoli powder.
Oil shale {Brandschicfer) is shale containing such a proportion of hydrocarbons as to be capable of yielding mineral oil on slow distillation. This substance occurs as ordinary shales do, in layers or beds, interstratified with other aqueous deposits, as in the Scottish coal-fields. It is in a geological sense true shale, and owes its peculiarity to the quantity of vegetable (or animal) matter which lias been preserved among its inorganic constituents. It consists of fissile argillaceous layers, highly impregnated with bituminous matter, passing on one side into common shale, on the other into cannel or parrot coal. The richer varieties yield from 30 to 40 gallons of crude oil to the ton of shale. They may be distinguished from non-bituminous or feebly bituminous shales (throughout the shale districts of Scotland) by the peculiarity that a thin paring eurls up in front of the knife, and shows a brown lustrous streak. Some of the 6hales in the Lothians are crowded with the valves of ostracod crustaceans, besides scales, corprolites, &c., of ganoid fishes {Ρala ’ o ∣ ι-iscus, Amblyptenιs, Mcgalichthys, &c.); and it is possible that the bituminous matter may in some cases have resulted from animal organisms, though the abundance of plant remains indicates that it is probably in most cases of vegetable origin. Under the name “pyroschists” Sterry Hunt classes the clays or shales (of all geological ages) which are hydrocarbonaceous, and yield by distillation volatile hydrocarbons, inflammable gas, &c.
(<7.) Rocks formed of Animal Remains.— These may be formed on land, as in bone caves, but most abundantly under water, as on the bottom of lakes and of the sea. They may be calcareous, siliceous, or phosphatic.
Limestone.— Besides the limestones resulting from the deposition of chemical precipitates of carbonate of lime, there is another important series derived from the remains of organisms, either by growth on the spot, or by accumulation as mechanical sediment. Limestone so originating has often been so altered that it cannot always be distinguished from that which has been chemically produced, especially when it has been exposed to the action of percolating acidulated water, for in that case a crystalline texture is gradually superinduced, by which the original organic structures in the mass are wholly or in great part obliterated. Limestone composed of the remains of living organisms forms thin layers and massive beds. In some instances, as in that of the English and Irish Mountain Limestone, it occurs in masses several thousand feet thick, which extend for hundreds of square miles, and form the rock out of which picturesque valleys, gorges, hills, and table-lands have been excavated. Limestone may be either of fresh water or of marine origin. Some of the more common and important varieties may be here enumerated :—
Coral-rock is limestone formed by the continuous growth of coralbuilding polyps. This substance affords an excellent illustration of the way in which organic structure may be effaced from a limestone entirely formed from the remains of once living animals. Though thé skeletons of the reef-building corals remain distinct on the upper surface, those of their predecessors beneath them are gradually obliterated by the passage through them of percolating water dissolving and redepositing carbonate of lime. This same action may be observed among the stalactites of a damp vault, in which, though the successive rings of growth are preserved, a crystalline divergent structure is superinduced, which traverses these rings from the centre outward. We can thus understand how a mass of crystalline limestone may have been produced from one formed out of organic remains without the action of any subterranean heat, but merely by the permeation of water from the surface. Crinoidal {Encrinite) Limestone is a rock composed in great part of joints of encrinites, with Foraminifera, corals, and mollusks. It varies in colour from white or pale grey, through shades of bluish-grey (sometimes yellow or brown, less commonly red) to a dark-grey or even black colour. It is abundant among Palaeozoic formations, being especially characteristic of the lower part of the Carboniferous system. Chalk as a lithological term is applied to a white soft rock, meagre to the touch, soiling the fingers, formed of a fine calcareous flour derived from the remains of Foraminifera, echinoderms, mollusks, and other marine organisms. It occurs in massive beds, and covers a great part of the south-east and east of England. In Ireland and elsewhere it assumes a firmer grain and various colours, so as to pass into some of the numerous varieties of compact white limestone. Shell-Marl, a soft white, earthy, or crumbly deposit, is formed in lakes and ponds by the accumulation of the remains of shells and Entomostraca on the bottom. When such calcareous deposits become solid compact stone they are known as fresh-water {lacustrine) limestone. These are generally of a smooth texture, and either dull white or pale grey, their fracture only slightly conchoidal, rarely splintery. Ooze is a mud of organic origin found covering vast areas of the floor of the Atlantic and other oceans. Some of it is calcareous and formed wholly or mostly of the remains of Foraminifera, particularly of forms of the genus Globigerina ; hence this deposit has been termed foraminiferal or globigerina ooze. Sometimes it is mainly siliceous, consisting of the remains of Radiolaria (Radiolarian ooze) or of diatoms (Diatom ooze). These deposits are further referred to in the section of this article which treats of the geological aspects of the ocean. Shell-sand is a sand composed in great measure or wholly of comminuted shells, found commonly on a low shelving coast exposed to prevalent on-shore winds. This deposit when thrown above the reach of the waves and often wetted by rain, or by trickling runnels of water, is apt to become consolidated into a mass, owing to the solution and redeposit of lime round the grains of shell.
Flint and Chert are siliceous rocks (which, though not strictly fragmental, may be conveniently placed here) found in nodules and layers in limestones of many different geological ages. Flint is a dark horny substance, breaking with a splintery to conchoidal fracture. It is particularly abundant in the chalk formation. Chert is an impure flint, containing more clay or lime with the silica. These substances seem in some cases to have had a directly organic origin, having been secreted from sea-water by the living organisms; in other cases, where for example we find a calcareous shell, or echinus, or coral, converted into silica, it would seem that the substitution of silica for lime has been effected by a process of chemical pseudomorphism either after or during the formation of the limestone.
(e.) Volcanic Fragmental Rocks form an interesting group composed of the loose materials ejected from volcanic vents. In their typical condition they consist merely of consolidated volcanic debris, including bombs, scoriae, ejected blocks, sand, lapilli, and dust. It is evident, however, that, when these materials were deposited in water, there would necessarily be a limit beyond which they would not extend, and where they would be mingled with and would insensibly pass into ordinary non-volcanic sediment. Hence we may expect to find transitional varieties between rocks formed directly from the results of volcanic explosion and ordinary sedimentary deposits. Moreover, as these fragmental volcanic masses usually consist almost wholly of the detritus' of different lavas, which have been blown into fragments in the volcanic chimneys, we may expect to find, on the other hand, a passage from them into rocks derived from consolidated lava-beds by ordinary aqueous erosion. (See part iv.)
Volcanic Conglomerate is a τock composed mainly or wholly of rounded or subangular fragments ∙of any volcanic rocks in a paste derived chiefly or wholly from the same materials, usually exhibiting a stratified arrangement, and often found intercalated between successive sheets of lava. In most cases conglomerates of this kind have been formed by the accumulation of materials ejected from volcanic vents; occasionally, as just remarked, they may have resulted from the aqueous erosion of previously solidified lavas, or from a combination of both these processes. There does not appear at present to be any satisfactory method of always determining the exact mode of formation, except that well-rounded and smoothed stones will almost certainly indicate long-continued water-action rather than trituration in a volcanic vent.
The volcanic conglomerates may receive different names according to the nature of the component fragments: thus we have basalt-conglomerates, where these fragments are wholly or mainly of basalt, trachyte-conglomerates, porph yrite-conglomerates, phonolite-conglomerates, &c.
Volcanic Breccia resembles volcanic conglomerates, except that the stones are angular. This angularity indicates an absence of aqueous erosion, and, under the circumstances in which it is found, usually points to volcanic explosions. There is a great variety of breccias, as basalt-breccia, diabase-breccia, &c.
Volcanic Agglomerate is the name given to a tumultuous assemblage of blocks of all sizes up to masses several yards in diameter. It is met with in the “necks” or pipes of old volcanic orifices. The stones and paste are commonly of one or more volcanic rocks, such as basalt or porphyrite, but they include also fragments of the surrounding rocks, whatever these may be, through which the volcanic orifice has been drilled. As a rule agglomerate is devoid of stratification; but sometimes it includes portions which have a more or less distinct arrangement in beds of coarser and finer detritus, often placed on end or inclined in different directions at high angles.
Volcanic 7 ’ w∕.-This general term may be made to include all the finer kinds of volcanic detritus, ranging on the one hand through coarse gravelly deposits into conglomerates, and on the other into exceedingly compact fine-grained rocks formed of the finest and most impalpable kind of volcanic dust. Some tuffs are full of microlites or imperfect forms of crystallization derived from the lava which has blown into dust. Others are formed of small rounded or angular grains of different lavas with fragments of various rocks through which the volcanic funnels have been drilled. Minutely cellular grains, as if derived from the ebullition of very fluid glassy lava like palagonite, constitute much of the tuff in some of the volcanic necks of Carboniferous age in central Scotland. Some tuffs have consolidated under water, others on dry land. As a rule they are distinctly stratified. Near the original vents of eruption they commonly present rapid alternations of finer and coarser detritus indicative of successive phases of volcanic activity.
The tuffs may be subdivided according to the nature of the lava from the disintegration of which they have been formed. Thus we ∖ aαvc felsite-tuffs, trachyte-tuffs, basalt-tuffs, pumice-tuffs, porphyritetuffs, palagonite-tuffs. Some varieties have received special names. Trass {Duckstein, Tuffstein) is a compact yellow pumiceous tuff which has filled up some of the valleys of the Eifel region and is largely quarried as an hydraulic mortar. Peperino is an Italian tuff of late geological date, full of separate crystals of augite and other minerals.
PART III.—DYNAMICAL GEOLOGY.
Under this section is included the investigation of those processes of change which are at present in progress upon the earth, whereby modifications are made on the structure and composition of the crust, on the relations between the interior and the surface, as shown by volcanoes, earthquakes, and other terrestrial disturbances, on the distribution of oceans and continents, on the outlines of the land, on the form and depth of the sea-bottom, on climate, and on the races of plants and animals by which the earth is tenanted. It brings before us, in short, the whole range of activities which it is the province of geology to study, aud leads us to precise notions regarding their relations to each other, and the results which they achieve. A knowledge of this branch of the subject is thus the essential groundwork of a true and fruitful acquaintance with the principles of geology, seeing that it necessitates a study of the present order of nature, and thus provides a key for the interpretation of the past.
The whole range of operations included within the scope of inquiry in this branch of the science may be regarded as a vast cycle of change, into which we may break at any point, and round which we may travel, only to find ourselves' brought back to our starting-point. It is a matter of comparatively small moment at what part of the cycle we begin our inquiries. We shall always find that the changes we see in action have resulted from some that preceded, and give place to others which follow them.
At an early time in the earth’s history, anterior to any of the periods of which a record remains in the visible rocks, the chief sources of geological action probably lay within the earth itself. The planet still retained a great store of its initial heat, and in all likelihood was the theatre of great chemical changes, giving rise, perhaps, to manifestations of volcanic energy somewhat like those which have so marvellously roughened the surface of the moon. As the outer layers of the globe cooled, and the disturbances due to internal heat and chemical action became less marked, the influence of the sun, which must always have operated, would then stand out more clearly, giving rise to that wide circle of superficial changes wherein variations of temperature and the circulation of air and water over the surface of the earth come into play.
In the pursuit of his inquiries into the past history and into the present régime of the earth, the geologist, must needs keep his mind ever open to the reception of evidence for kinds and especially for degrees of action which he had not before imagined. Human experience has been too short to allow him to assume that all the causes aud modes of geological change have been definitively ascertained. On the earth itself there may remain for future discovery evidence of former operations by heat, magnetism, chemical change, or otherwise, which may explain many of the phenomena with which geology has to deal. Of the influences, so many and profound, which the sun exerts upon our planet, we can as yet only dimly perceive a little. Nor can we tell what other cosmical influences may have lent their aid in the evolution of geological changes.
In the present state of our knowledge, all the geological energy upon and within the earth must ultimately be traced back to our parent sun. There is, however, a certain propriety and convenience in distinguishing between that part of it which is due to the survival of some of the original energy of the planet, and that part which arises from the present supply of energy received day by day from the sun. In the former case we have to deal with the interior of the earth and its reaction upon the surface; in the latter we deal with the surface of the earth, and to some extent with its reaction on the interior. This distinction allows of a broad treatment of the subject under two divisions :—
I. Hypogene or Plutonic Action— the changes within the earth caused by original internal heat and by chemical action.
II. Epigene or Surface Action— the changes produced on the superficial parts of the earth, chiefly by the circulation of air and water set in motion by the sun’s heat.
DIVISION I—HYPOGENE ACTION. An Inquiry into the Geological Changes in Progress beneath the Surface of the Earth.
In the discussion of this branch of the subject we must carry iu our minds the conception of a globe still intensely hot in its interior, radiating heat into space, and consequently contracting in bulk. Portions of molten rocks from inside are from time to time poured out at the surface. Sudden shocks are generated by which destructive earthquakes are propagated to and along the surface. Wide geographical areas are pushed up or allowed to sink down. In the midst of these movements very remarkable changes are produced upon the rocks of the crust; they are shattered, fractured, squeezed, crumpled, rendered crystalline, and even fused.
Section. I.—Volcanoes and Volcanic Action.
The term volcanic action (vulcanism or vulcanicity) embraces all the phenomena connected with the expulsion of heated materials from the iuterior of the earth to the surface. Among these phenomena there are some of an evanescent character, while others leave permanent proofs of their existence. It is naturally to the latter that the geologist gives the chief attention, for it is by their means that he can trace the former phases of volcanic activity in regions where, for many ages, there have been no volcanic eruptions. In the operations of existing volcanoes he can observe only the superficial manifestations of volcanic action. But, examining the rocks of the earth’s crust, he discovers that in the lapse of ages, amid the many terrestrial revolutions which geology reveals, the very roots of former volcanoes have been laid bare, displaying subterranean phases of vulcanism which could not be studied in any modern volcano. Hence an acquaintance only with active volcanoes will not give us a complete knowledge of volcanic action. It must be supplemented and enlarged by an investigation of the traces of former volcanoes preserved in the crust of the earth.
The openings by which the heated materials from the interior reach the surface include volcanoes (with their accompanying orifices), hot-springs, and gas-springs. A volcano may be defined as a conical eminence, composed wholly or mainly of materials which have been ejected from below, and which have accumulated at the surface round the vent of eruption. As a rule it presents at its summit a cup-shaped cavity termed the crater, at the bottom of which is the top of the main funnel or pipe whereby the communication is maintained with the heated interior. A volcano, when of small size, may consist merely of one diminutive cone; when of the largest dimensions, it forms a huge mountain, with many subsidiary cones and many lateral fissures or pipes, from which the heated volcanic products are given out.
Volcanoes may break through any kind of geological formation. In Auvergne, in the Miocene period, they burst through the granitic and gneissose plateau of central France. In Lower Old Red Sandstone times they pierced contorted Silurian rocks in central Scotland. In late Tertiary and post-Tertiary ages they found their way through soft marine strata, and formed the huge piles of Etna, Somma, and Vesuvius. On the banks of the Rhine, at Bonn and elsewhere, they have penetrated some of the older alluvia of that river. In many instances, also, newer volcanoes have appeared on the sites of older ones. In Scotland the Carboniferous volcanoes have risen on the sites of those of the Old Red Sandstone, those of the Permian period have broken out among the earlier Carboniferous eruptions, while the Miocene lavas have been injected into all these older volcanic masses, Again, the newer puys of Auvergne were sometimes erupted through much older and already greatly denuded basalt-streams. Somma and Vesuvius have arisen out of the great Neapolitan plain of marine tuff. In central Italy, also, newer cones have been thrown up upon the great Roman plain of more ancient volcanic debris.
It is usual to class volcanoes as active, dormant, and extinct. This arrangement, however, often presents considerable difficulty in its application. An active volcano cannot of course be mistaken, for even when not in actual eruption it shows, by its abundant evolution of steam and hot vapours, that it might break out into activity at any moment. But it is in many cases impossible to decide whether a volcano should be called extinct or only dormant. The volcanoes of Silurian age in Wales, of Carboniferous age in Ireland, of Permian age in the Hartz, of Miocene age in the Hebrides, are certainly all extinct. But the Miocene volcanoes of Iceland are still represented there by Skaptar-Jökull, Hecla, and their neighbours. Somma, in the first century of the Christian era, would have, been naturally regarded as an extinct volcano. Its fires had never been known to have been kindled within human tradition; its vast crater was a wilderness of wild vines and brushwood, haunted, no doubt, by wolf and wild-boar. Yet in a few days, in the autumn of the year 79 the half of the crater walls was blown out by a terrific series of explosions, the present Vesuvius was then formed within the limits of the earlier crater, and since that time volcanic action has been intermittently exhibited up to the present day. Some of the intervals of quietude, however, have been so considerable that the mountain might then again have been claimed as an extinct volcano. Thus, in the 131 years between 1500 and 1631, so completely had eruptions ceased that the crater had once more become choked with copsewood. A few pools and springs of very salt and hot water remained as memorials of the former condition of the mountain. But this period of quiescence closed with the eruption of 1631, —the most powerful of all the known explosions of Vesuvius, except the great one of 79.
In the island of Ischia, Mont Epomeo was last in eruption in the year 1302, its previous outburst having taken place, it is believed, about 17 centuries before that date. From the craters of the Eifel, Auvergne, the Vivarais, and central Italy, though many of them look as if they had only recently been formed, no eruption has been known to come during the times of human history or tradition. From these examples it is clear that no real distinction can be drawn between dormant and extinct volcanoes. Also, from the fact above mentioned, that volcanoes have often appeared on areas occupied by those of previous geological periods, it is equally evident that volcanic action is apt to show itself again and again even at vast intervals within the same regions and over the same sites.
§ 1. Volcanic Products.
The materials erupted from volcanic vents may be classed as (1) gases and vapours, (2) lavas, (3) fragmentary substances. A brief summary under each of these heads may be given here; the mode and order of appearance of the several products will be stated in § 2.
1. Gases and Vapours.— These play an important part in volcanic activity; they show themselves in the earliest stages of a volcano’s history, and continue to appear for centuries after all the other evidences of subterranean action have ceased to be manifested. By much the most abundant of them all is steam. It has been estimated to form - 1 ζ *(^⅛-ths of the whole cloud which hangs over an active volcano. In great eruptions it rises in prodigious quantities, and is rapidly condensed into a heavy rainfall. M. Fouqué calculated that during 100 days Etna had ejected vapour enough to form, if condensed, 2,100,000 cubic metres of water. But even from volcanoes which, like the Solfatara near Naples, have been dormant for many centuries it sometimes still rises without intermission and in considerable volume. Jets of vapour rush out from clefts in the sides and bottom of a crater with a noise like that made by the steam blown off by a locomotive. The number of these funnels is often so large, and the amount of vapour so abundant, that only now and then, when the wind blows the dense cloud aside, can a momentary glimpse be had of a part of the bottom of the crater; while at the same time the rush and roar of the escaping steam remind one of the din of some vast factory. Aqueous vapour rises likewise from rents on the outside of the volcanic cone. It issues so copiously from the molten lava which pours down the slopes of the cone that the stream of rock is almost concealed from view by the cloud; and it continues to escape from fissures of the Java, far below the point from which the molten matter proceeded, for a long time after the rock has solidified and come to rest. So saturated, as it were, are many crystalline lavas with the vapour of water that Mr Scrope even suggested that they derive their mobility from this cause.
Probably in no case is the steam mere pure vapour of water. It is associated with other vapours and gases disengaged from the potent chemical laboratory underneath. Of these probably the most abundant is sulphuretted-hydrogen, readily traceable by its characteristic odour. Its liability to decomposition leads to the deposition of a yellow crust of sulphur about the orifices from which it rises; perhaps, also, in great part to the alteration of this gas must we attribute the sulphuric acid so frequently observed at volcanic vents. Another gas especially abundant at Vesuvius, but not detected in all volcanoes, is hydrochloric acid, which comes away with the steam, and gives it its pungent suffocating fumes. Carbonic acid appears in most volcanoes, partly as one of their usual products, but chiefly given off during the feebler conditions of activity and in the last stages of vulcanism. After an eruption of Vesuvius this gas has sometimes been discharged so copiously that hundreds of hares, pheasants, and partridges have been suffocated by it. In the ancient volcanic regions of the Eifel and Auvergne, it still rises in prodigious quantities. Bischof estimates that the quantity of carbonic acid evolved in the Brohl Thai amounts to five millions of cubic feet, or 300 tons of gas in one day. Nitrogen, derived doubtless from the decomposition of atmospheric air dissolved in the water which penetrates into the volcanic foci, has been frequently detected among the gaseous emanations.
With these gases and vapours are associated many substances which, sublimed by the volcanic heat, appear as deposits along crevices and surfaces wherein they reach the air and are cooled. Besides sulphur, which has been already mentioned, there are several chlorides (particularly that of sodium, and less abundantly those of iron, copper, and lead), sal-ammoniac, specular iron, oxide of copper (tenorite), boracic acid, and other substances. Sodium chloride sometimes appears so abundantly that wide spaces of a volcanic cone, as well as of the newly erupted lava, are crusted with salt, which can even be profitably removed by the inhabitants of the district. Considerable quantities of these chlorides may thus be buried between successive sheets of lava, and in long subsequent times may give rise to mineral springs, as has been suggested with reference to the saline waters which issue from volcanic rocks of Old Red Sandstone and Carboniferous age in Scotland. The ironchloride forms a bright yellow and reddish crust on the crater walls, as well as on loose stones on the slopes of the cone. Specular iron forms abundantly as thin lamellae in the fissures of Vesuvian lavas. Tenorite may be seen at the edge of the crater of Vesuvius, condensing into fine filaments which are blown away by the wind. An order of succession has been observed to take place in the appearance of the different volcanic gaseous discharges.
In connexion with the aqueous vapour of volcanoes, reference may be made here to the abundant discharges of water which accompany volcanic explosions. Three sources of this water may be assigned :—(1) from the melting of snow by a rapid accession of temperature previous to or during an eruption,—this takes place from time to time on Etna, and still more markedly in Iceland and among the snowy ranges of the Andes; (2) from the condensation of the vast clouds of steam which are discharged during an eruption,—this undoubtedly is the chief source of the destructive torrents so frequently observed to form part of the phenomena of a great volcanic explosion; and (3) from the disruption of reservoirs of water filling subterranean cavities, or of lakes occupying crater-basins,—this has several times been observed among the South American volcanoes, where immense quantities of dead fish, which inhabited the water, have been swept down with the escaping torrents. The volcano of Agua, in Guatemala, has never been known to discharge anything but water. In the large crater of Idjèn, one of the volcanoes of Java, lay a hot steaming lake of acid water. In the beginning of the year 1817 an eruption took place, by which the water was discharged with frightful destruction down the slopes of the mountain. After the explosion the basin filled again with water, but its temperature was no longer high.
In many cases the water rapidly collects volcanic dust as it rushes down, and soon becomes a pasty mud, or it issues at first in this condition from the volcanic reservoirs, from which, after violent detonations, it is discharged. Hence arise what are termed mud-lavas or aqueous lavas, which in many respects behave like true igneous lavas. This mud eventually consolidates into one of the numerous forms of tuff,—a rock which, as has been already stated (p. 239), varies greatly in the amount of its coherence, in its composition, and in its internal arrangement. Obviously, unless where subsequently altered, it can possess none of the crystalline structure of true lavas. As a rule it betrays its aqueous origin by more or less distinct evidence of stratification, by the multifarious pebbles, stones, blocks of rock, tree-trunks, branches, shells, bones, skeletons, which it has swept along in its course and preserved within its mass. Sections of this compacted tuff may be seen at Herculaneum. The trass of the Brohl Thai and other valleys in the Eifel district, referred to on p. 239, is another example of an ancient volcanic mud.
2. Lavas.— The term lava is applied generally to all the molten rocks which flow out from volcanoes. The behaviour of the lava as it issues and flows down the volcanic cones will be described in the next sub-section. It will be sufficient to refer here to some of the leading peculiarities of the lavas viewed as volcanic products. (1.) Their specific gravity ranges between 2 , 37 and 3 ’ 22. (2.)
The heavier varieties contain much magnetic or titaniferous iron, with augite and olivine, their composition being basic, and their proportion of silica averaging about 45 or 50 per cent. In this group come the basalts, dolerites, nepheline-lavas, and leucite-lavas. The lighter varieties contain commonly a minor proportion of metallic bases, but are rich in silica, their percentage of that acid ranging "between 60 and 80. They are thus not basic but acid rocks. Among their more important species, trachyte, rhyolite, obsidian, pitchstone, and pumice may be enumerated. (3.) They differ much in texture, (a) Some are entirely crystalline, consisting of nothing but an interlaced mass of crystals and crystalline particles, as in some dolerites, (δ) Some show more or less of a half-glassy or stony matrix, in which the constituent crystals are imbedded; this is the most common arrangement. (c) Others are entirely vitreous, such crystals or -crystalline particles as occur in them being quite subordinate, and, so to speak, accidental enclosures in the main glassy mass. Obsidian or volcanic glass is the type of this group, (d) They further differ in the extent to which their mass has been affected by the interstitial steam at the time of their eruption. Some show no outward signs of any influence from that cause; in others, however, the expansion of the imprisoned steam has been such as to expand the still molten stone into an open cellular texture, somewhat like that of open ill-baked bread. Such a vesicular arrangement very commonly appears on the upper surface of a lava current. (4.) They vary greatly in colour and general external aspect. The heavy basic lavas are usually darkgrey, or almost black, though, on exposure to the weather, they usually acquire a brown tint from the oxidation and hydration of their iron. Their surface is commonly rough and rugged, until it has been sufficiently decomposed by the atmosphere to crumble into excellent soil which, under favourable circumstances, supports a luxuriant vegetation. The less dense lavas, such as phonolites and trachytes, are frequently paler in colour, sometimes pale yellow or buff, and decompose into light soils; but the obsidians present rugged black sheets of rock roughened with ridges and heaps of froth-like pumice.
3. Fragmentary Materials.— Under this title we include all the substances which, driven up into the air by volcanic explosions, fall in solid form to the ground—the dust, ashes, sand, cinders, and blocks of every kind which are projected from a volcanic orifice. These materials must obviously differ greatly in composition, texture, and appearance, even during a single eruption, and still more in successive explosions of the same volcano. For the sake of convenience separate names are applied to some of the more distinct varieties. Of these the more important are the following. (1.) Ashes and Sand.— In many eruptions vast quantities of an exceedingly fine light grey powder are ejected. As this substance greatly resembles what is left after a piece of wood or coal is burnt in an open fire, it has been popularly termed ash, and this name has been adopted by geologists. If, however, by the word ash the result of combustion is implied, its employment to denote any product of volcanic action must be regretted as apt to convey a wrong impression. The fine ashlike dust ejected by a volcano is merely lava in an extremely fine state of comminution. So minute are the particles that they find their way readily through the finest chinks of a closed room, and settle down upon floor and furniture as ordinary dust does when a house is shut up for a short time. From this finest form of material gradations may be traced through what is termed volcanic sand into the coarse varieties of ejected matter. In composition the ash and sand vary necessarily with the nature of the lava from which they are derived. (2.) When the fragments range from the size of a pea to that of a walnut they are called lapilli. These are pieces of lava, round, subangular, or angular in shape, and having the same indefinite range of composition as the finer dust, As a rule, the coarse fragments fall nearest the focus of eruption. Sometimes they are solid fragments of lava, but more usually they have a cellular texture, while sometimes they are so light and porous as to float readily on water, and, when ejected near the sea, to cover its surface over a large area. (3.) Volcanic Blocks are large pieces of stone, often angular in shape. In some cases they appear to be fragments loosened from already solidified rocks in the chμnney of the volcano. Hence we find among them pieces of older tuffs, and of lavas recognizably belonging to early eruptions. But in other and numerous instances they are not volcanic rocks at all, but belong to earlier formations through which the volcanic chimney has been drilled. Blocks of a coarsely crystalline granitoid lava have been particularly observed both on Etna and Vesuvius. In the year 1870 a mass of that kind weighing several tons was to be seen lying at the foot of Vesuvius, within the entrance to the Atrio del Cavallo. Similar blocks occur among the Carboniferous volcanic pipes of central Scotland. In the older tuffs of Somma, blocks of altered limestone form the chief repositories of the Vesuvian minerals. (4.) Volcanic Bombs and Slags.— These have originally formed portions of the column of lava ascending the pipe of the volcano, and have been detached and hurled into the air by the successive explosions of steam. In the case of a bomb, we have a round, elliptical, or pear-shaped and often discoidal mass of lava, from a few inches to several feet in diameter. Sometimes it is tolerably solid throughout, more usually vesicular. Not unfrequently it consists of a hollow interior, with a shell which is vesicular towards the centre and becomes more close-grained towards the outside. There can be no doubt that, when torn by eructations of steam from the surface of the boiling lava, the material of these bombs was in as thoroughly molten a condition as the rest of the mass. From the rotatory motion imparted by its ejection, it took a circular form, and in proportion to its rapidity of rotation and fluidity would be the amount of its “flattening at the poles.” The centrifugal force within would allow the expansion of the interstitial vapour, while the outer surface would rapidly cool and solidify; hence the solid crust, and the porous or even cavernous interior. Such bombs, varying from the size of an apple to that of a man’s body, were found by Mr Darwin abundantly strewed over the ground in the island of Ascension. They have been found likewise in the tuff of Lower Carboniferous date at Dunbar, in Scotland. When the ejected fragment of lava has a rough irregular form, and a porous structure like the clinker of au iron-furnace, it is known as a slag.
The fragmentary materials erupted by a volcano and deposited around it acquire by degrees more or less consolidation, partly from the mere pressure of the higher upon the lower strata, partly from the influence of infiltrating water. It has been already stated (part ii., p. 239) that different names are applied to the rocks thus formed. The coarse, tumultuous, unstratified accumulation of volcanic debris within a crater or funnel is called agglomerate. When the debris, though still coarse, is more rounded, and is arranged in a stratified form, it is a volcanic conglomerate. The finer-grained varieties, formed of dust and lapilli, are included in the general designation of tuffs. These are usually pale-yellowish, greyish, or brownish, sometimes black rocks, granular, porous, and often incoherent in texture. Organic remains sometimes occur in tuff. Where the volcanic debris has accumulated over the floor of a lake, or of the sea, the entombing and preserving of shells and other organic objects must continually take place. But even in the tuffs of a volcanic cone traces of animals and plant3 may be preserved. Professor Guiscardi of Naples has found about 100 species of marine shells of living species in the old tuffs of Vesuvius. Marine shells have been picked up within the crater of Monte Nuovo, and have been frequently observed in the old or marine tuff of that district. The showers of ash which fall on the outer slopes of a volcano, or the sheets of mud which sometimes are spread out there, cover over and often preserve the land-shells, insects, and vegetation living on the area at the time. The older tuffs of Vesuvius have yielded many remains of the shrubs and trees which at successive periods have clothed the flanks of the mountain.
§ 2. Volcanic Action.
We have now to consider the circumstances under which the various solid, liquid, and gaseous products of a volcano are emitted. Volcanic action may be either constant or periodic. Stromboli, in the Mediterranean, so far as we know, has been uninterruptedly emitting hot stones, steam, and lava, from the earliest period of history. Among the Moluccas the volcano Sioa, and in the Friendly Islands that of Tofua, have never ceased to be in eruption since their first discovery. The lofty cone of Sangay, among the Andes of Quito, is always giving off hot vapours; Cotopaxi, too, is ever constantly active. But, though examples of unceasing action may thus be cited from widely different quarters of the globe, they are nevertheless exceptional. The general rule is that a volcano breaks out from time to time with greater or less fury, and after longer or shorter intervals of quiescence.
To what particular cause or series of causes any special eruption may be due is a question to which at present no definite answer can be given. An attempt has been made to show that the explosions of a volcano are to some extent regulated by the conditions of atmospheric pressure over the area at the time. In the case of a volcanic funnel like Stromboli, where, as Mr Scrope pointed out, the expansive subterranean force within, and the repressive effect of atmospheric pressure without, just balance each other, any serious disturbance of that pressure might be expected to make itself evident by a change in the condition of the volcano. Accordingly, it has long been remarked by the fishermen of the Lipari Islands that in stormy weather there is at Stromboli a more copious discharge of steam and stones than in fine weather. They make use of the cone as a weather-glass, the increase of its activity indicating a falling, and the diminution a rising barometer. In like manner Etna, according to S. von Waltershausen, is most active in the winter months. When we remember the connexion now indubitably established between a more copious discharge of fire-damp in mines and a lowering of atmospheric pressure, we may be prepared to find a similar influence affecting the escape of vapours from the upper surface of the lava column of a volcano; for it must not be forgotten that it is not so much to the lava itself as to the expansive vapours accompanying it that the manifestations of volcanic activity are due. Among the Vesuvian eruptions since the middle of the 17th century, the number which took place in winter and spring was to that of those which broke out in summer and autumn as 7 to 4. But there may be other causes besides atmospheric pressure concerned in these differences; the preponderance of rain during the winter and spring may be one of these. At present we must wait for further data.
Kluge has sought to trace a connexion between the years of maximum and minimum sun-spots and those of greatest and feeblest volcanic activity, and has constructed lists to show that years which have been specially characterized by terrestrial eruptions have coincided with those marked by few sun-spots and diminished magnetic disturbance. Such a connexion cannot be regarded as having yet been satis·, factorily established. Again, the same author has called attention to the frequency and vigour of volcanic explosions about the middle of August, at or near the time of the yearly meteoric shower. But in this case, likewise, the cited examples can hardly yet be looked upon as more than coincidences.
The case of Kilauea, in Hawaii, seems to show a regular system of eruptive periods. Dana has pointed out that outbreaks of lava have taken place from that volcano at intervals of from eight to nine years, this being the time required to fill the crater up to the point of outbreak, or to a depth of 400 or 500 feet. But the great eruption of 1868 did not occur until after an interval of 18 years. The same author suggests that the missing eruption may have been submarine.
The approach of an eruption is not always indicated by any premonitory symptoms, for many tremendous explosions are recorded to have taken place in different parts of the world without any perceptible warning. Much in this respect would appear to depend upon the condition of liquidity of the lava, and the amount of resistance offered by it to the passage of the escaping vapours through its mass. In Hawaii, where the lavas are remarkably liquid, vast outpourings of lava have taken place quietly without earthquakes during the present century. But even there the great eruption of 1868 was accompanied by tremendous earthquakes.
The eruptions of Vesuvius are often preceded by a failure or diminution of the wells and springs in the district. But more frequent indications of an approaching outburst are conveyed by sympathetic movements of the ground beneath. Rumblings and groanings from a subterranean source are heard; slight tremors succeed, increasing in frequency and violence till they become distinct earthquake shocks. The vapours from the crater rise more abundantly into the air. All this time the lava column in the pipe or funnel of the volcano has been slowly ascending, forced upward and kept in perpetual agitation by the passage of the elastic vapours through its mass. If a long previous interval of quiescence has elapsed, there may be much solidified lava towards the top of the vent which will restrain the ascent of the still molten portion underneath. A vast pressure is thus exercised on the sides of the cone. Should these be too weak to resist, they will open in one or more rents, and the liquid lava will issue from the outer slope of the mountain; or the energies of the volcano will be directed towards clearing the obstruction in the chief throat, until, with tremendous explosions, and the rise of a vast cloud of dust and fragments, the bottom and sides of the crater are finally blown out, and the top of the cone disappears. The lava may now escape from the lowest part of the lip of the crater, while, at the same time, immense numbers of red-hot bombs, scoriae, and stones are shot up into the air, most of them falling back into the crater, but many descending upon the outer slopes of the cone, and some even upon the country beyond the base of the mountain. The lava rushes down at first like one or more rivers of melted iron, but, as it cools, its rate of motion lessens. Clouds of steam rise from its surface, as well as from the central crater. Indeed, every successive paroxysmal convulsion of the mountain is marked, even at a distance, by the rise of huge ball-like wreaths or clouds of steam, mixed with dust and stones, forming a vast column which towers sometimes a couple of miles above the summit of the cone. By degrees these diminish in frequency and intensity. The lava ceases to flow, the showers of stones and dust dwindle down, and after a time, which may vary from hours to days or months, even in the régime of the same mountain, the volcano becomes once more tranquil.
Let us now consider some of the aspects of this action which have more particular geological interest from the permanent changes with which they are connected, or from the way in which they enable us to detect and realize conditions of volcanic energy in former periods.
Fissures.— During the convulsions which culminate in the formation of a volcano, as well as in the subsequent progress of the mountain so formed, the ground at and around the focus of action is liable to be rent open by fissures. These tend to diverge from the focus; but around the vent where the rocks have been most exposed to concussion the fissures sometimes intersect each other in all directions. In the great eruption of Etna, in the year 1669, a series of six parallel fissures opened on the side of the mountain. One of these, with a breadth of two yards, ran for a distance of 12 miles, in a somewhat winding course, to within a mile of the top of the cone. Similar fissures, but on a smaller scale, have often been observed on Vesuvius; and they are recorded from many other volcanoes.
Two obvious causes may be assigned for the production of fissures :—(1) the enormous expansive force of the imprisoned vapours acting upon the walls of the funnel and convulsing the cone by successive explosions; and (2) the hydrostatic pressure of the lava-column in the funnel, amounting to about 125 lb per square inch, or 8 tons on the square foot, for each 100 feet of depth. Both of these causes may act simultaneously.
Into the rents thus formed the molten lava naturally finds its way. It is indeed forced into them, and solidifies there like iron in a mould. The cliff's of many an old crater show how marvellously they have been injected by such veins or dykes of lava. Those of Somma, and the Vai del Bue on Etna, have long been known. The dykes project now from the softer tuffs like great walls of masonry. Such wedges of solid rock driven into the cone must widen its dimensions, and at the same time from their length and ramifications must bind the substance of the cone together, and thus strengthen it against the effects of future convulsions. We can understand, therefore, how there should be for a time an alternation in the character of the eruptions of a volcano, depending in great measure upon the relation between the height of the cone on the one hand and the strength of its sides on the other. When the sides have been well braced together by interlacing dykes, and further thickened by the spread of volcanic materials all over their slopes, they may resist the effects of explosion and of the pressure of the ascending lava-column. In this· case the volcano may find relief only from its summit, and if the lava flows forth it will do so from the top of the cone. As the cone increases in elevation, however, the pressure from within upon its sides augments. Eventually egressis once more established on the flanks by means of fissures, and a new series of lava-streams is poured out over tha lower slopes.
Though lava very commonly issues from the lateral fissures on a volcanic cone, it may sometimes approach the surface without actually flowing out. The great fissure on Etna in 1669, for example, was visible even from a distance by the long line of vivid light which rose from the incandescent lava within. Again, it frequently happens that minor volcanic cones are thrown up on the line of a fissure. This may arise either from the congelation of the lava round the point of emission, or from the accumulation of ejected scoriae round the fissure-vent. Of the former structure examples occur in Hawaii, where the lava is remarkably liquid, and rapidly hardens into tears or drops, like wax down the sides of a candle. Where in viscid lavas the steam tears off and throws up many scoriae and bombs, a cone of such loose materials will naturally form round the orifice by which the lava escapes from the flank of the mountain.
Explosions.— Although volcanic materials may be erupted to a large extent without the appearance of visible fissures, they cannot in such cases reach the surface without some explosive action, and the consequent displacement and removal of previously existing rock. Vents are thus blown out of the solid crust, the volcanic energy being, as it were, concentrated on a given point, which we may suppose must usually be the weakest in the structure of that part of the crust. The operation has often been observed in volcanoes already formed, and has even been witnessed on ground previously unoccupied by a volcanic vent. The history of the cone of Vesuvius brings before us a long series of such explosions, beginning with that of 79—the greatest which has occurred within the times of human history—and coming down to the present day. Even now, in spite of all the lava and ashes poured out during the last eighteen centuries, it is easy to see how stupendous must have been that earliest explosion, by which the southern half of the ancient crater was blown out. At every successive important eruption, a similar but minor operation takes place within the present cone. The hardened cake of lava forming the floor is burst open, and with it there usually disappears much of the upper part of the cone, and sometimes, as in 1872, a large segment of the crater-wall. In the year 1538 a new volcano, Monte Nuovo, was formed in 24 hours on the margin of the Bay of Naples. A cavity was drilled by successive explosions, and such quantities of stones, scoriae, and ashes were thrown out from it as to form a hill 440 English feet above the sea-level, and more than a mile and a half in circumference. Most of the fragments now to be seen on the slopes of the cone and inside its beautifully perfect crater are of various volcanic rocks, many of them being black scoriae; but pieces of Roman pottery, together with fragments of the older underlying tuff, and some marine shells, have been obtained; and these doubtless formed part of the soil and subsoil dislocated and ejected during the explosions.
It is not necessary, and it does not always happen, that any actual solid or liquid volcanic rock is ejected even when explosions of considerable violence take place, whereby the rocks through which the funnel rises are much shattered. Thus among the cones of the extinct volcanic tract of the Eifel, some occur consisting entirely, or nearly so, of comminuted debris of the Devonian greywacke and slate which form the prevailing rocks there, and through which the various volcanic vents have been opened. Evidently in these cases elastic vapours only forced their way to the surface; and we see what probably often takes place in the early stages of a volcano’s history, though the fragments of the underlying disrupted rocks are in most instances buried and lost under the far more abundant subsequent volcanic materials. Sections of ancient volcanic necks or pipes sometimes afford an excellent opportunity of observing that these orifices were originally opened by the blowing out of the solid crμst and not by the formation of fissures. Many examples occur in Scotland among volcanic rocks of Old Red Sandstone, Carboniferous, and Permian age. The orifices are there filled with fragmentary materials wherein portions of the surrounding and underlying rocks form a noticeable proportion.
Showers of Dust and Stones.— A communication having been opened, either by fissuring or explosion, between the heated interior and the surface, fragmentary materials seldom fail to be ejected from it. These may consist at first mainly of the rocks through which the orifice has been opened, as has just been explained. But if eruptive energy continues, they soon appear in larger quantities, and consist of thoroughly volcanic substances. In a great eruption vast numbers of red-hot stones are shot up into the air, and fall back partly into the crater and partly on the outer slopes of the cone. But instances are known where large stones, ejected obliquely, have described huge parabolic curves in the air, and fallen at a great distance. Stones 8 Bö in weight occur among the ashes which buried Pompeii. The volcano of Antuco in Chili is said to send stones flying to a distance of 36 miles, and Cotopaxi is reported to have hurled a 200-ton block 9 miles.
But in many great eruptions, besides a constant shower •of stones and scoriae, a vast column of exceedingly fine dust rises out of the crater, sometimes to a height of more than a mile, and then spreads outwards like a sheet of cloud. So dense sometimes is this dust-cloud that the sun is obscured, and for days together the darkness of night reigns for miles around the volcano. In 1822 this was the case at Vesuvius, the ashes not only falling thickly on the villages round the base of the mountain, but travelling as far as Ascoli, which is 56 Italian miles distant from the volcano on one side, and as Casano, 105 miles on the other. But probably the most stupendous outpouring of volcanic ashes on record was that which took place, after a quiescence of 26 years, from the volcano Coseguina, in Nicaragua, during the early part of the year 1835. On that occasion utter darkness prevailed Over a circle of 35 miles radius, the ashes falling so thickly that, even 8 leagues from the mountain, they covered the ground to a depth of about 10 feet. It was estimated that the rain of dust and sand fell over an area at least 270 geographical miles in diameter. Some of the finer materials, thrown so high as to come within the influence of an upper air-current, were borne away eastward, and fell four days afterwards at Kingston, in Jamaica—a distance of 700 miles.
An inquiry into the origin of these showers of fragmentary materials brings vividly before us some of the essential features of volcanic action. We find that bombs, slags, and lapilli may be thrown up in comparatively tranquil states of a volcano, but that the showers of fine dust are discharged with violence, and only appear when the volcano becomes more energetic. Thus, at the constantly, but quietly, active volcano of Stromboli, the column of lava in the pipe may be watched slowly rising and falling with a slow rhythmical movement. At every rise the surface of the lava swells up into blisters several feet in diameter, which by and by burst with a sharp explosion that makes the walls of the «rater vibrate. A cloud of steam rushes out, carrying with it hundreds of fragments of the glowing lava, sometimes to a height of 1200 feet. It is by the ascent of steam through its mass that a column of lava is kept boiling at the bottom of a crater, and by the explosion of successive larger bubbles of steam that the various bombs, slags, and fragments of lava are torn off and tossed into the air. It has often been noticed at Vesuvius that, after each great concussion, a huge ball-like cloud of steam rushes up from the crater. Doubtless it is the sudden escape of that steam which causes the explosion. The violence of the explosion will depend greatly upon the viscidity of the lava, and the consequent resistance offered to the upward passage of the steam. Explosions and accompanying scoriae are abundant at Vesuvius, where the lavas are comparatively viscid; they are almost unknown at Kilauea, where the lava is remarkably liquid.
The steam, collecting into larger or smaller vesicles, works its way upward through the substance of the molten lava. As the elasticity of this compressed vapour overcomes the pressure of the overlying lava, it escapes at the surface, and there the lava is thus kept in ebullition. But this comparatively quiet operation, which may be watched within the craters of many active volcanoes, does not produce clouds of fine dust. The friction of the millions of stones ascending and descending through the air in the dark column above the crater, though it must doubtless cause much dust and sand, can give rise to but an insignificant proportion of what is actually reduced to the condition of extreme subdivision necessary to produce widespread darkness and a thick far-reaching deposit of ashes. The explanation now accepted calls in the explosive action of steam as the immediate cause of the trituration. A sudden and powerful explosion of steam, it is maintained, will blow the top of the lava column into dust, like water shot out of a gun. We must remember that the aqueous vapour by which many lavas are so largely impregnated must exist interstitially far down in the lava-column, under an enormous pressure, and at a white heat. The sudden ascent of lava so constituted will relieve the pressure rapidly without sensibly affecting the temperature of the mass. Consequently the white-hot steam will at length explode, and reduce the molten mass containing it to the finest powder.
Evidently no part of the operations of a volcano has greater geological significance than the ejection of such enormous quantities of fragmentary matter. In the first place, the fall of these loose materials round the orifice of discharge is one main cause of the growth of the volcanic cone. The heavier fragments gather around the vent, and there too the thickest accumulation of finer dust takes place. Hence, though successive explosions may blow out the upper part of the crater-walls, and prevent the mountain from growing so rapidly in height, every eruption must add to the diameter of the cone. In the second place, as every shower of dust and sand adds to the height of the ground on which it falls, thick volcanic accumulations may be formed far beyond the base of the mountain. In these are entombed trees and other kinds of vegetation, together with the bodies of many animals, as well as the works of man. Hence new geological formations arise which, in their component materials, not only bear witness to the volcanic eruptions which produced them, but preserve a record of the land-surfaces over which they spread. In the third place, besides the distance to which the fragments may be hurled by volcanic explosions, or to which they may be diffused by the ordinary aerial movements, we have to take into account the vast spaces across which the finer dust is sometimes borne by upper currents in the atmosphere. An instance has already been cited where ashes from Coseguina fell 700 miles away, having been carried all that long distance by a high counter-current of air, moving apparently at the rate of about 7 miles an hour in an opposite direction to that of the wind which blew at the surface. On several occasions ashes from one of the Icelandic volcanoes have fallen so thickly between the Orkney and Shetland Islands that vessels passing there have had the unwonted deposit shovelled off their decks in the morning. In the year 1783, during an eruption of Skaptar-Jökull, so vast an amount of fine dust was ejected that the atmosphere over Iceland continued loaded with it for months afterwards. It fell in such quantity over parts of Caithness—a distance of 600 miles—as to destroy the crops; that year is still spoken of by the inhabitants as the year of “the ashie.” Traces of the same deposit were observed even as far as Holland. Hence it is evident that volcanic deposits may be formed in regions many hundreds of miles distant from any active volcano. A single tlιiu layer of volcanic detritus in a group of sedimentary strata would thus not of itself prove the existence of contemporaneous volcanic action in its neighbourhood. It might be held to have been wind-borne from a volcano in a distant and separate region.
Lava-streams.— A microscopic examination of their intimate structure shows that lavas have been truly molten rocks. They usually consist fundamentally of a glass through which are diffused, in greater or less abundance, various microlites and crystals. Their degree of liquidity, at the time of emission, seems to depend on the extent to which the rock remains in the condition of glass, viscidity increasing with the development of the microlites and crystals out of the glassy menstruum in which, no doubt, originally their component molecules were diffused. The fluidity may also be governed in no small degree by the amount of vapour existing interstitially in the molten mass. Mr Scrope indeed contended that aqueous vapour was the main cause of the mobility of such crystalline lavas as those of Vesuvius. But even where the lava pours forth with a liquidity like that of melted iron, it speedily assumes a more viscid motion, as the process of devitrification advances and the rock is exposed to the chilling effects of radiation and of contact with air and soil. An interesting fact, admirably shown by the microscope, but often easily observable with the naked eye, is that in lava still liquid and mobile well-defined crystals make their appearance. These sometimes are broken during the continued movement of the surrounding mass, the separated fragments becoming involved in the general glassy base or portions of that base, are injected into the fractures of the crystals. Well-defined crystals of leucite may be seen in specimens of Vesuvian lava, which has been ladled out from a white-hot stream, impressed with a stamp, and thus suddenly congealed. On the other hand, the obsidians have solidified in the condition of complete glass, often without any trace of devitrification. The green pyroxenic lava of Hawaii exhibits so extreme a degree of fluidity that, during its ebullition in pools of the crater, jets not more than a quarter of an inch in diameter are tossed up, and, falling back on one another, make “a column of hardened tears of lava,” while, in other places, the jets thrown up and blown aside by the wind give rise to long threads of glass which lie thickly together like mown grass, and are known by the natives under the name of Pele’s Hair, after one of their divinities.
It would be of the highest interest and importance to know accurately the temperature with which a lava stream issues. The difficulty of making any direct observation at the point of outflow has hitherto been insuperable. Measurements have been taken at various distances below the point where the moving lava could be safely approached; but these are not satisfactory, seeing that the outer crust of the lava cools rapidly, and gives no measure of the temperature even a short way underneath. Experiments made by Scacchi and Sainte-Claire Deville on the Vesuvian lava erupted in 1855, by thrusting thin wires of silver, iron, and copper into the lava, indicated a temperature of scarcely 700° C. Earlier observations of a similar kind, made in 1819, when a silver wire 7 ⅛th inch in diameter at once melted in the Vesuvian lava of that year, gave a greatly higher temperature. Evidence of the high temperature of lava has been adduced from the alteration it has effected upon refractory substances in its progress, as where, at Torre del Greco, it overflowed the houses, and was afterwards found to have fused the fine edges of flints, to have decomposed brass into its component metals, the copper actually crystallizing, and to have melted silver, and even sublimed it into small octohedral crystals. But such facts, though full of interest and importance, give us no clue to the absolute initial temperature of the lava, which must be greatly higher than that of the stream after several miles of descent on the mountain slopee, and after some hours or days of cooling.
In spite of this very high temperature, however, the lava issues abundantly charged with aqueous vapour, to the expansion of which, as we have seen, its ebullition and expulsion are mainly due. As this vapour at once begins to escape when the lava issues into the air, it shows itself by a dense white cloud hanging over the moving mass. The lava streams of Vesuvius sometimes appear with as large and dense a steam cloud at their lower ends as that which escapes at the same time from the main crater. Even after the molten mass has flowed several miles, steam continues to rise abundantly both from its end and from numerous points along its surface.
From the wide extent of basalt dykes, such as those of Britain, some of which rise to the surface at a distance of 200 miles and upwards from the main volcanic regions of their time, it is evident that the molten lava may sometimes occupy a far greater superficial area underneath than the mere circumference of the actual pipe or of the volcanic cone. We must conceive of a vast reservoir of melted rock impregnated with superheated steam, and impelled upwards by the elastic force of the vapour. The lava may be regarded rather as the sign than as the cause of volcanic action. It is the pressure of the imprisoned vapour, and its struggles to get free, which produce the subterranean earthquakes, the explosions, and the outpouring of lava. As soon as the vapour finds relief, the terrestrial commotion calms down again, and the quiescence continues until another accumulation of vapour demands a repetition of the same phenomena.
It is evident that the vapour may succeed in effecting its escape without driving molten rock up to the surface. There may be tremendous explosions without an actual outcome of lava. But, in most cases, so intimately are vapours and lava commingled in the subterranean reservoirs that they rise together, and the explosions of the one lead to the outflow of the other. The first point at which the lava makes its appearance at the surface will largely depend upon the structure of the ground. Two causes have been assigned in a foregoing section (p. 244) for the fissuring of a volcanic cone. As the molten mass rises within the chimney of the volcano, continued explosions of vapour take place from its upper surface, the violence of which may be inferred from the vast clouds of steam, of ashes, and of stones which are hurled to so great a height into the air. These explosions must at the same time powerfully affect the sides of the funnel, exposed as these are to the enormous pressure exerted by the imprisoned vapour. We cannot therefore be surprised that, when a volcano experiences shocks of such intensity as to be felt over a radius 100 miles or more, its sides should at last give way, and large divergent fissures should be opened down its cone. Again, the hydrostatic pressure of the column of lava must have a potent influence. At a depth of 1000 feet below the top of the column the pressure exerted on each square foot of the surrounding walls must amount to more than 80 tons. We may well believe that such a force, acting upon the walls of a funnel already shattered by a succession of terrific explosions, will be apt to prove too great for their resistance. When this happens, the lava pours forth from the outside of the cone. So fissured is the cone sometimes that the lava issues freely from many points. A volcano so affected has been graphically described as “sweating fire.” More usually the lava issues only from one or two points. Should these lie well down on the cone, far below the summit of the lava-column, the lava, on its first escape, driven by hydrostatic pressure, will sometimes spout up high into the air—a fountain of molten rock. This was observed in 1794 on Vesuvius, and in 1832 on Etna.^ In the eruption of 1852 at Mauna Loa, an unbroken fountain of lava, from 200 to 700 feet in height and 1000 feet broad, burst out at the base of the cone. Similar “geysers” of molten rock have subsequently been noticed in the same region. Thus, in March and April 1868, four fiery fountains, throwing the Java to heights varying from 500 to 1000 feet, continued to play for several weeks.
In a lofty volcano, therefore, the chances are always rather against the lava rising to the lip of the crater and flowing out there. It does so now and then; but more frequently it escapes from some fissure or orifice in a weak part of the cone. In minor volcanoes, on the other hand, where the explosions are less violent, and where the thickness of the cone in proportion to the diameter of the funnel is often greater, the lava very commonly rises in the crater. Should the crater walls be too weak to resist the pressure of the molten mass, they will give way, and the lava will rush out from the breach. This is seen to have happened in several of the puys of Auvergne, so well figured and described by Mr Scrope. But if the crater be massive enough to withstand the pressure, the lava, if still impelled upward by the struggling vapour, will at last flow out from the lowest part of the rim.
It was at one time supposed that lava beds could not consolidate on such steep slopes as those of most volcanoes, and that their present inclined position was to be attributed to a central upheaval of each mountain. This idea formed the subject of the famous theory of elevation-craters {Erhe-bungskratere) of L. von Buch, E. de Beaumont, and other geologists. It was a matter of prime importance in the interpretation of volcanic action to have this question settled. To Constant Prevost belongs the merit of having completely exposed the fallacy of this theory. He pointed out that there was no more reason why lavas should not consolidate on steep slopes than that tears or drops of wax should not do so. Mr Poulett Scrope also showed conclusively that the steep slope of the lava-beds of a volcanic cone was original. Sir Charles Lyell and Mr Hartung subsequently obtained abundant additional evidence from the Canary Islands, Etna, and other volcanic districts, to disprove the elevation theory. Geologists are now agreed that thick sheets of lava, with all their characteristic features, can consolidate on slopes of even 35° and 40°. The lava in the Hawaii Islands has cooled rapidly on slopes of 25°, that from Vesuvius, in 1855, is here and there as steep as 30°. On the east side of Etna, a cascade of lava, which poured, in 1689, into the vast hollow of the Cava Grande, has an inclination varying from 18° to 48°, with an average thickness of 16 feet. On Mauna Loa some lava-flows are said to have congealed on slopes of 49°, 60°, and even 80°, though in these cases it could only be a layer of rock stiffening and adhering to the surface of the steep slope. Even when it consolidates on a steep slope, a stream of lava forms a sheet with parallel, upper, and under surfaces, a general uniformity of thickness, and often greater evenness of surface than where the angle of descent is low.
At its first appearance, where it issues from the mountain, the lava glows with a white heat, and flows with a motion which has been compared to that of honey or of melted iron. It soon becomes red, and, like a coal fallen from a hot fireplace, rapidly grows dull as it moves along, until it assumes a black, cindery aspect. At the same time the surface congeals, and soon becomes solid enough to support a heavy block of stone. Its aspect depends, not merely on the composition and fluidity of the lava, but on the point of egress, whether from the crater or from a fissure, on the form of the ground, the angle of slope, and the rapidity of flow. Lavas which have been kept in ebullition within the central chimney are apt to acquire a rough cellular texture. The surface of the moving stream breaks up into rough brown or black cinder-like slags, and irregular rugged cakes, which, with the onward motion, grind and grate against each other with a harsh metallic sound, sometimes rising into rugged mounds or getting seamed with rents and gashes, at the bottom of which the red-hot glowing lava may be seen. When lava escapes from a lateral fissure it may have no scoriae, but its surface will present froth-like, curving lines, as in the scum of a slowly flowing river, or will be arranged in curious ropy folds, as the layers have successively flowed over each other and congealed. These and many other fantastic coiled shapes were exhibited by the lava which flowed from the side of Vesuvius in 1858. A large area which has been flooded with lava is perhaps the most hideous and appalling scene of desolation anywhere to be found on the surface of the globe.
A lava stream at its point of escape from the side of a volcanic cone occupies a comparatively narrow breadth; but it usually spreads out as it descends, and moves more slowly. The sides of the moving mass look like huge embankments. or like some of the long mounds of “clinkers ,r one sees in a great manufacturing district. The advancing end of the mass is often much steeper, creeping onward like a great wall or rampart, down the face of which the rough blocks of hardened lava are ever rattling.
The rate of movement is regulated by the fluidity of the lava, by its volume, and by the form and inclination of the ground. Hence, as a rule, a lava-stream moves faster at first than afterwards, because it has not had time to stiffen, and its slope of descent is considerably Steeper than further down the mountain. One of the most fluid and swiftly flowing lava-streams ever observed on Vesuvius was that erupted on 12th August 1805. It is said to have rushed down a space of 3 Italian (3f English) miles in the first four minutes, but to have widened out and moved more slowly as it descended, yet finally to have reached Torre del Greco in three hours. A lava erupted by Mauna Loa in 1852 went as fast as an ordinary stage-coach, or 15 miles in two hours. Long after a current has been deeply crusted over with slags and rough slabs of lava it continues to creep slowly forward for weeks or even months.
It happens sometimes that, as the lava moves along, the pressure of the still molten mass inside bursts through the outer hardened and deeply seamed crust, and rushes out with, at first, a motion much more rapid than that of the main stream; but such an offshoot rapidly congeals and comes to rest, though sometimes not before doing much damage to vineyards, gardens, houses, or other property in its course. Any sudden change in the form or slope of the ground, too, will affect the flow of the lava. Thus, should the stream reach the edge of a steep defile or cliff, it will pour over it in a cataract of glowing molten rock, with clouds of steam, showers of fragments, and a noise utterly indescribable. Or if, on the other hand, the current should encounter a ridge or hill across its path, it will accumulate in front of it until it either finds egress round the side or actually overrides and entombs the obstacle. The hardened crust or shell within which the still fluid lava moves serves to keep the mass from spreading. We often find, however, that the lava has subsided here and there inside its crust, and has left curious cavernous spaces and tunnels. Into these, when the whole is cold, we may creep, and may find them sometimes festooned and hung with stalactites of lava.
As a rule a lava-stream shows three component layers. At its bottom lies a rough, slaggy mass, produced by the rapid cooling of the lava, and the breaking up and continued onward motion of the scoriform layer. The central and main portion of the stream consists of solid lava, often, however, with a more or less carious and vesicular texture. The upper part, as we have seen, is a mass of rough broken-up slags, scoriae, or clinkers. The proportions borne by these respective layers to each other vary continually. Some of the more fluid ropy lavas of Vesuvius have an inconstant and thin slaggy crust; others may be said to consist of little else than scoriae from top to bottom. These divergences in texture seem to depend largely upon the amount of interstitial steam imprisoned within the lava, and the conditions under which it can effect its escape. Throughout the whole mass, but more especially along its upper surface, •the steam under its diminished pressure expands, and pushing the molten rock aside, segregates into small bubbles or irregular cavities. Hence, when the lava solidifies, these steam-holes are seen to be sometimes so abundant that a detached portion of the rock containing them will float in water. They are often elongated in the direction of the motion of the lava-stream.
But, besides producing a general vesicular texture in the upper parts of the lava-stream, the aqueous vapour gives rise to much more striking features on the surface of the lava. If the outburst takes place from an orifice or fissure on the exterior of the volcanic cone, so vast an amount of steam will rush out there, with such boiling and explosion of the lava, that a cone of bombs, and slags, and irregular lumps of lava, will probably form round the spot—in fact a miniature or parasitic volcano, which will remain as a marked cone on its parent mountain long after the eruption which gave it birth has ceased. Moreover, even after such abundant discharge of steam, the lava-stream continues to exhale it, as it were, from every pore. Here and there on the surface of the moving mass a fissure opens, and a column of roaring hissing vapours rushes out from it, accompanied as before by an abundant discharge of lavafragments, or even by the rise and outflow of the lava from beneath. Some lava-streams are thus dotted over with small cones a few feet or yards in height. Besides the steam which, in condensing, makes its presence so conspicuous, many other vapours entangled in the pores of the lava escape from its fissures. The points at which vapours are copiously disengaged are termed fumarole. Among the exhalations, chlorides may be mentioned as particularly prominent; chloride of sodium frequently shows itself, not only in fissures, but even over the cooled crust of the lava, in small crystals, in tufts, or as a granular and even glassy incrustation. Chloride of iron is deposited as a yellow coating at the fumarole, where also bright emerald green films and scales of chloride of copper may be more rarely observed. Many chemical changes take place in the escape of the vapours through the lava. Thus specular-iron, probably the result of the mutual decomposition of steam and iron chloride, forms abundant scales, plates, and small crystals in the fumarole and vesicles of the lava. Salammoniac also appears in large quantity on many lavas, not merely in the fissures, but also on the upper surface of the current. This salt is not directly a volcanic product, but results from some decomposition, probably from that of the aqueous vapour, whereby a combination is formed with atmospheric nitrogen.
The hardened crust of a lava-stream is a bad conductor of heat. Consequently, when the surface of the mass has become cool enough to be walked upon, the red hot mass may be observed through the rents to lie only a few inches below. Many years therefore may elapse before the temperature of the whole mass has fallen to that of the surrounding soil. Eleven months after an eruption of Etna, Spallanzani could see that the lava was still red hot at the bottom of the fissures, and a stick thrust into one of them instantly took fire. The Vesuvian lava of 1785 was found by Breislak seven years afterwards to be still hot and steaming internally, though lichens had already taken root on its surface. The ropy lava erupted by Vesuvius in 1858 was observed in 1870 to be still so hot, even near its termination, that steam issued abundantly from its rents, many of which were too hot to allow the hand to Re held in them. Hoffmann records that the lava which flowed from Etna in 1787 was still steaming in 1830. But still more remarkable is the case of Jorullo, in Mexico, which sent out lava in 1759. Twenty-one years later a cigar could still be lighted at its fissures; after 44 years it was still visibly steaming; and even in 1846, that is, after 87 years of cooling, two vapour columns were still rising from it.
This extremely slow rate of cooling has justly been regarded as a point of high geological significance in regard to the secular cooling and probable internal temperature of our globe. Some geologists have argued indeed that, if so comparatively small a portion of molten matter as a lava stream can maintain a high temperature under a thin, cold crust for so many years, we may, from analogy, feel little hesitation in believing that the enormously vaster mass of the globe may, beneath its relatively thin crust, still continue in a molten condition within. More legitimate deductions, however, might be drawn, if we knew more accurately and precisely in each case the rate of loss of heat, and how it varies in different lava-streams. Sir William Thomson, for instance, has suggested that, by measuring the temperature of intrusive masses of igneous rock in coal-workings and elsewhere, and comparing it with that of other non-volcanic rocks in the same regions, we might obtain data for calculating the time which has elapsed since these igneous sheets were erupted.
In its descent a stream of lava may reach a water-course, and, by throwing itself as a great embankment across the stream, may pond back the water and form a lake. Such is the origin of the picturesque Lake Aidât in Auvergne. Or the molten current may usurp the channel of the stream, and completely bury the whole valley, as has happened again and again among the vast lava-fields of Iceland. No change in physiography is so rapid and so permanent as this. The channel which has required, doubtless, many thousands of years for the water laboriously to excavate, is sealed up in a few hours under 100 feet or more of stone, and a still longer interval may elapse before this newer pile is similarly eroded.
By suddenly overflowing a brook or pool of water, molten lava sometimes has its outer crust shattered to fragments by a sharp explosion of the generated steam, while the fluid mass within rushes out on all sides. Numerous instances have occurred where the lavas of Etna and Vesuvius have protruded into the sea. Thus a current from the latter mountain entered the Mediterranean at Torre del Greco in 1794, and pushed its way for 360 feet outwards, with a breadth of 1100 and a height of 15 feet. So quietly did it advance that Breislak could sail round it in a boat and observe its progress.
In passing from a fluid to a solid condition, and thus contracting, lava acquiree different structures. Lines of divisional planes or joints traverse it, especially perpendicular to the upper and under surfaces of the sheet. These lines at various irregular distances cross each other so as to divide the rock into rude prisms. Occasionally another series of joints at a right angle to these traverses the mass parallel with its bounding surfaces, and thus the rock acquires a kind of fissile or bedded appearance. The most characteristic structure, however, among volcanic rocks is the prismatic, or, as it is incorrectly termed, “basaltic.” Where this arrangement occurs, as it does so commonly in basalt, the mass is divided into tolerably regular pentagonal, hexagonal, or irregularly polygonal prisms or columns, set close together at a right angle to the main cooling surfaces. These prisms vary from 2 or 3 to 18 or more inches in diameter, and range up to 100 or even 150 feet in length.
Considerable discussion has arisen a≡ to the mode in which this columnar structure has been produced. The experiments of Mr Gregory Watt were supposed to explain it by the production of a number of spherical concretions in the cooling mass, and the gradual pressure of those soft balls into hexagonal columns, as the mass contracted in cooling. He melted a mass of basalt, and on allowing it to cool observed that, when a small portion was quickly chilled, it took the form of a kind of slag-like glass, not differing much in appearance from obsidian; a larger mass, more slowly cooled, returned to a stony state. He remarked that during this process small globules make their appearance, which increase in size by the successive formation of external concentric coats, like those of an onion. And he supposed, as each spheroid must be touched by six others, the whole, if exposed to the same pressure acting in every direction, must be squeezed into a series of hexagons. To account, however, for a long column of basalt, we should have to imagine a pile of balls standing exactly centrically one upon the other, an arrangement which seems hardly possible. The prismatic structure is a species of jointing, due to the contraction of the rock as a whole, and not to the production of any internal peculiarities of texture. The concretionary structure associated with the columnar arises from a common tendency to weather out into nodular forms, and may be observed even where the rock is not columnar. Prismatic forms have been superinduced upon rocks by a high temperature and subsequent cooling, as where coal an J sandstone have been invaded by basalt. They may likewise be observed to arise during the consolidation of a substance, as in the case of starch. In that substance the columnar structure is apt to radiate from certain centres, as may also be seen sometimes in basalt and other igneous rocks.
Mr Mallet has recently investigated this subject, and concludes that “all the salient phenomena of the prismatic and jointed structure of basalt can be accounted for upon the admitted laws of cooling, and contraction thereby, of melted rock possessing the known properties of basalt, the essential conditions being a very general homogeneity in the mass cooling, and that the cooling shall take place slowly, principally from one or more of its surfaces.”
In the more perfectly columnar basalts the columns are sometimes articulated, each prism being separable into vertebrae, with a cup and ball socket at each articulation. This peculiarity is traced by Mr Mallet to the contraction of each prism in its length and in its diameter, and to the consequent production of transverse joints, which, as the resultant of the two contracting strains, are oblique to the sides of the prism, but, as the obliquity lessens towards the centre, assume necessarily, when perfect, a cup-shape, the convex surface pointing in the same direction as that in which the prism has grown. This explanation, however, will hardly account for cases, which are not uncommon, where the convexity points the other way, or where it is sometimes in one direction, sometimes in the other. The remarkable spheroids which appear in many weathered igneous rocks besides basalts may probably be due to some of the conditions under which the original contractions took place. They are quite untraceable on a fresh fracture of the rock. It is only after some exposure to the weather that they begin to appear, and then they gradually crumble away by the successive formation and disappearance of external weathered crusts or coats, which fall off into sand and clay. Almost all augitic or hornblendic rocks, even granite, exhibit the tendency to decompose into rounded spheroidal blocks.
By the outpouring of lava two important kinds of geological change are produced. In the first place, the surface of a country is thereby materially changed. Stream-courses, lakes, ravines, valleys, in short all the minor features of a landscape, may be completely overwhelmed under a sheet of lava, 100 feet or more in thickness. The drainage of the district is thus effectually altered, and all the numerous changes which flow from the operations of running water over the land are arrested and made to begin again in new channels. In the second place, considerable alterations may likewise be caused by the effects of the heat and vapours of the lava upon the subjacent or contiguous ground. Instances have been observed in which the lava has actually melted down opposing rocks, or masses of slags, on its own surface. Interesting observations, already referred to, have been made at Torre del Greco under the lava stream which overflowed part of that town in 1794. It was found that the window-panes of the houses had been devitrified into a white, translucent, stony substance, that pieces of limestone had acquired an open, sandy, granular texture, without loss of carbonic acid, and that iron, brass, lead, copper, and silver objects had been greatly altered, some of the metals being actually sublimed. We can understand therefore that, retaining its heat for so long a time, a mass of lava may induce many crystalline structures, rearrangements, or decompositions in the rocks over which it comes to rest, and proceeds slowly to cool. This is a question of considerable importance in relation to the behaviour of ancient lavas which have been intruded among rocks beneath the surface, and have subsequently been exposed, as will be referred to in the sequel.
But, on the other hand, the exceedingly trifling change produced even by a massive sheet of lava has often been remarked with astonishment. On the flank of Vesuvius we may see vines and trees still flourishing on little islets of the older land-surface completely surrounded by a flood of lava. Professor Dana has given an instructive account of the descent of a lava-stream from Kilauea in June 1840. Islet-like spaces of forest were left in the midst of the lava, many of the trees being still alive. Where the lava flowed round the trees the stumps were usually consumed, and cylindrical holes or casts remained in the lava, either empty or filled with charcoal. In many cases the fallen crown of the tree lay near, and so little damaged that the epiphytic plants on it began to grow again. Yet so fluid was the Java that it hung in pendent stalactites from the branches, which nevertheless, though clasped round by the molten rock, had barely their bark scorched. Again, for nearly 100 yeaι∙3 there has lain on the flank of Etna a large sheet of ice which, originally in the form of a thick mass of snow, was overflowed by the molten flood, and has thereby been protected from the evaporation and thaw which would certainly have dissipated it long ago, had it been exposed to the air. The heat of the lava has not sufficed to melt it. There seems reason to suspect, however, that in other cases snow and ice have been melted in large quantities by overflowing lava. The great floods of water which rushed down the flank of Etna, after an eruption of the mountain in the spring of 1755, have been thus explained.
One further aspect of a lava-stream may be noticed here—the effect of time upon its surface. While all kinds of lava must, in the end, crumble down under the influence of atmospheric waste and, where other conditions permit, become coated with soil and support some kind of vegetation, yet extraordinary differences may be observed in the facility with which different lava-streams yield to this change, even on the flank of the same mountain. Every one who ascends the slopes of Vesuvius remarks this fact. After a little practice it is not difficult there to trace the limits of certain lavas even from a distance, in some cases by their verdure, in others by their barrenness. Five hundred years have not sufficed to clothe with green the still naked surface of the Catanian lava of 1381; while some of the lavas of the present century have long given footing to bushes of furze. Some of the younger lavas of Auvergne, which certainly flowed in times anterior to those of history, are still singularly bare and rugged. Yet, on the whole, where lava is directly exposed to the atmosphere, without receiving protection from occasional showers of volcanic ash, or being liable to be washed bare by heavy torrents of rain, its surface decays in a few years sufficiently to afford soil for a few plants in the crevices. When these have taken root they help to increase the disintegration. At last, as a more or less continuous covering of vegetation spreads over the rock, the traces of its volcanic origin one by one fade away from its surface. Some of the Vesuvian lavas of the present century already support vineyards.
Torrents of Water and Hud. —-We have seen that large quantities of water accompany many volcanic eruptions. In some cases, where ancient crater-lakes or internal reservoirs have been shaken by repeated detonations, and finally disrupted, the mud which has thus been produced issues at once from the mountain. Such “mud-lavas,” on account of their liquidity and 3wiftness of motion, are more dreaded for their destructiveness than even the true melted lavas. On the other hand, rain or melted snow, rushing down the cone and taking up loose volcanic dust, is converted into a kind of mud that grows more and more pasty as it descends. The mere sudden rush of such large bodies of water down the steep declivity of a volcanic cone cannot fail to effect much geological change. Deep trenches are cut out of the loose volcanic slopes, and sometimes large areas of woodland are swept away, the debris being strewn over the plains below.
.During the great Vesuvian eruption of 1622 a torrent of this kind poured down upon the villages of Ottajano and Massa, overthrowing walls, filling up streets, and even burying houses with their inhabitants. It was by similar streams from the same volcano that some of the Roman cities on its flanks were overwhelmed in the first century. Many of the volcanoes of Central and South America discharge large quantities of mud directly from their craters. Thus in the year 1691 Imbaburu, one of the Andes of Quito, emitted floods of mud, so largely charged with dead fish that pestilential fevers arose from the subsequent effluvia. Seven years later (1698), during an explosion of another of the same range of lofty mountains, Carguairazo (14,706 feet), the summit of the cone is said to have fallen in, while torrents of mud, containing immense numbers of the fish Pymelodus Gyclopum i poured forth and covered the ground over a space of four square leagues. The carbonaceous mud (locally called rn,oyd) emitted by the Quito volcanoes sometimes escapes from lateral fissures, sometimes from the craters. Its organic contents, and notably its siluroid fish, which are the same as those found living in the streams above ground, prove that the water is derived from the surface, and accumulates in craters or underground cavities until discharged by volcanic action. Similar but even more stupendous and destructive outpourings have taken place from the volcanoes of Java, where wide tracts of luxuriant vegetation have at different times been buried under masses of dark grey mud, sometimes 100 feet thick, with a rough hillocky surface from which the top of a submerged palm-tree might have been seen protruding.
Between the destructive effects of mere water-torrents and that of these mud-floods there is, of course, the notable difference that, whereas in the former case a portion of the surface is swept away, in the latter, while sometimes considerable demolition of the surface takes place at first, the main result is the burying of the ground under a new tumultuous deposit by which the surface is greatly changed, not only as regards its temporary aspect, but in its more permanent features, such as the position and form of its water-courses.
Hud-volcanoes.— Though probably seldom if ever strictly volcanic in the proper sense of that term, certain remarkable orifices of eruption may be noticed here to which the names of mud-volcanoes, salses, air-volcanoes, and macalubas have been applied. These are conical hills formed by the accumulation of fine and usually saline mud, which, with various gases, is continuously or intermittently given out from the orifice or crater in the centre. They occur in groups, each hillock being sometimes less than a yard in height, but ranging up to elevations of 100 feet, or even sometimes, as in the plains of the lower Indus, to 400 feet. Like true volcanoes, they have their periods of repose, when either no discharge take≡ place at all, or mud oozes out tranquilly from the crater, and their epochs of activity, when large volumes of gas, and sometimes columns of flame, rush out with considerable violence and explosion, and throw up mud and stones to a height of several hundred feet.
The gases play much the same part therefore in these phenomena that steam does in those of true volcanoes. They consist of carbonic acid gas, carburetted hydrogen, sulphuretted hydrogen, and nitrogen. The mud is usually cold. In the water occur various saline ingredients, among which common salt generally appears. Naphtha is likewise frequently present. Large pieces of stone, differing from those in the neighbourhood, have been observed among the ejections, indicative doubtless of a somewhat deeper source than in ordinary cases. Heavy rains may wash down the minor mud cones and spread out the material over the ground, but gas-bubbles again appear through the sheet of mud, and by degrees a new series of mounds is once more thrown up.
There can be little doubt that these phenomena are to be traced to chemical changes in progress underneath. Dr Daubeny explained them in Sicily by the slow combustion of beds of sulphur. The frequent occurrence of naphtha and of inflammable gas points, in other cases, to the disengagement of hydrocarbons from subterranean strata.
Mud volcanoes occur in Iceland, in Sicily (Macaluba), in many districts of northern Italy, at Tamar and Kertch, at Baku on the Caspian, over an area of about 1000 square miles near the mouth of the Indus, and in other parts of the globe.
Gaseous Discharges.— Some of these belong to true volcanic phenomena, others are closely associated with the mud-volcanoes. To the former class we may assign the copious emanations of carbonic acid which so frequently take place in districts where volcanic activity has been long dormant or extinct. The gas either comes out directly from fissures of the rock, or rises dissolved in the water of springs. The old volcanic districts of Europe furnish many examples. Thus on the shoree of the Laacher See—an ancient crater lake of the Eifel—carbonic acid gas issues from numerous openings called moffette, round which dead insects, and occasionally mice and birds may be found. In the’ same region occur hundreds of springs more or less charged with the gas. The famous Valley of Death in Java contains one of the most remarkable gas-springs in the world. It is a deep, bosky hollow, from one small space on the bottom of which carbonic acid issues so copiously as to form the lower stratum of the atmosphere. Tigers, deer, and wild-boar, enticed by the shelter of the spot, descend and are speedily suffocated. Many of their skeletons, together with those of man himself, have been observed.
In the second class of gas-springs we may group the emanations of carburetted hydrogen, which, when they take fire, are known as Fire-wells. They occur in many of the districts where mud-volcanoes appear, as in northern Italy, on the Caspian, in Mesopotomia, in southern Kurdistan, and in many parts of the United States. It has been observed that they rise especially in regions where beds of rock-salt lie underneath, and as that rock has been ascertained often to contain compressed carburetted hydrogen, the solution of the rock by subterranean -water, and the consequent liberation of the gas, has been offered as an explanation of these fire-wells.
Geysers.— In various regions where volcanic action still continues, or where it has long been dormant, there occur eruptive fountains of hot water and steam, to which the general name of geyser3 is given, from the well-known examples in Iceland, which were the first to be seen and described. Besides the Great and Little Geysers, the Strokkr, and other minor springs of hot water in Iceland, other, perhaps still more striking, examples have in recent years been brought to light in that tract of the western territories of the United States set aside as the “Yellowstone National Park,” and good illustrations are also found in New Zealand. A geyser possesses a vertical pipe in the ground, terminating at the surface in a basin which is formed of siliceous sinter, and may rise some feet or yards above the general level. At more or less regular intervals rumblings and sharp detonations occur underneath, followed by an agitation of the water in the basin, and then by the violent expulsion of a column of water and steam to a considerable height in the air. The hot water contains silica in solution, which, on cooling and evaporating, is deposited at the surface; and thus the geyser builds up its basin, sometimes raising it into a long, solitary, finger-like pillar.
Bunsen and Descloiseaux spent some days experimenting at the Icelandic geysers, and ascertained that in the Great Geyser, while the surface temperature is about 212° Fahr, that of the lower portions of the tube is much higher—a thermometer giving as high a reading as 266° Fahr. The water there must consequently be 48° above the normal boiling-point, but is kept in the fluid state by the pressure of the overlying column. At the basin, however, the water cools quickly. After an explosion it accumulates there, and eventually begins to boil. The pressure on the column below being thuβ relieved, a portion of the superheated water flashes into steam, and as the change passes down the pipe, the whole column of water and steam rushes out with great violence. The water thereafter gradually collects again in the pipe, and after an interval of some hours the operation is renewed. The experiments made by Bunsen proved the cause of the eruption to lie in the high temperature of a portion of the pipe. He hung stones by strings to different depths in the funnel of the geyser, and found that only those in the higher part were cast out by the rush of water, sometimes to a height of 100 feet, while at the same time the water at the bottom was hardly disturbed at all. 1
These observations give an additional interest and importance to the phenomena of geysers in relation to those of volcanic action. They show that the eruptive force is steam; that the water column, even at a comparatively small depth, has a temperature considerably above 212°; that this high temperature is local; and that the eruptions of steam and water take place periodically, and with such vigour as to eject large stones to a height of 100 feet.
§ 3. Structure of Volcanoes.
It is now admitted that a volcano is due to the accumulation of material round the vent of eruption, and not to any blister-like expansion of the ground. The structure of a volcanic cone necessarily depends in great measure upon the nature of the substances ejected. The following are the more important and interesting types of this kind of structure :—
(1.) Cones of Non-volcanic Materials.— These are due to the discharge of steam or other aeriform product through the solid crust without the emission of any true ashes or lava. The materials ejected from the cavity are wholly, or almost wholly, parts of the surrounding rocks through which the volcanic pipe has been drilled. Some of the cones surrounding the crater-lakes or maare of the Eifel consist chiefly of fragments of the underlying Devonian slates.
(2.) Tuff-Cones i Cinder-Cones.— Successive eruptions of fine dust and stones, often rendered pasty by getting mixed with the water so copiously condensed during an eruption, form a cone in which the materials are solidified by pressure into tuff. Sometimes the cones are made up only of loose cinders, like Monte Nuovo in the Bay of Baiae. Cones consisting entirely of loose volcanic materials often arise on the flanks or round the roots of a great volcano, as happens to a small extent on Vesuvius, and on a larger scale upon Etna. They likewise occur by themselves apart from any lava-producing volcano, though usually they afford indications that columns of lava have risen in their funnels, and even now and then that this lava has reached the surface. Admirable examples are furnished by the cones of the Phlegraean fields near Naples. Ancient cones of a similar character occur among the Carboniferous rocks of Scotland. The materials of the cone are arranged in more or less regular beds which dip away from the funnel, their inclination corresponding with that of the cone. Inside the crater they slope steeply inward towards the crater-bottom.
(3.) Mud-Cones or Salses are formed by the accumulation and consolidation of mud round the vents of mud-volcanoes. They sometimes reach a height of 400 feet.
(4.) Lava-Cones.— These are comparatively rare, since, in most cases, the emission of lava is accompanied by the discharge of ashes. Owing to its liquidity, the lava flows off quickly, and the cones have very gentle slopes. The most remarkable examples are those in the Hawaii Islands described by Professor Dana. They attain a great height, but so small is their angle of inclination, that they may be described as only gently-31oping mounds, and their craters have been compared to vast open quarries on a hill or moor.
1 Bunsen, Ann. der Chemie und Pharmacie. Ixii. (1847), p. 1.
(5.) Cones of Tuff and Lava.— These are by far the most frequent. They may be taken, indeed, as the typical form of volcanoes. Around the central vent the successive showers of scoriae and ashes gather, with occasional streams of lava, which from time to time descend different sides of the growing cone. The component strata have thus the usual inclination outwards, though, when seen in section along the crater-walls, they look approximately horizontal. By continued explosions fissures are formed in the sides of the cone. These are injected with lava, producing a network of dykes which strengthen the cone, or they serve as channels of escape for lava, and give rise to lateral eruptions and minor or parasitic cones. As the main cone increases in diameter by the accumulation of fragmentary and molten ejections, it rises also in height, until at last few or no eruptions take place from its summit, but only from openings on its flanks.
(6.) Submarine Volcanoes.— It is not only on the surface of the land that volcanic action shows itself. It takes place likewise under the sea, and as the geological records of the earth’s past history are chiefly marine formations, the characteristics of submarine volcanic action have no small interest to the geologist. Unfortunately, the phenomena of recent volcanic eruptions under the sea are for the most part inaccessible. Here and there, as among the islands of the Greek Archipelago and at Tahiti, elevation of the seabed has taken place, and brought to the surface beds of lava, which had been erupted and had consolidated under water. There does not appear to be any appreciable difference either in external aspect or in internal structure between such submarine lavas and those erupted on the land. Some of them are highly scoriaceous. There is no reason indeed why slaggy lava and loose scoriae should not accumulate under the pressure of a deep column of the ocean. At the Hawaii Islands, on 25th February 1877, masses of pumice, during a submarine volcanic explosion, were ejected to the surface, one of which struck the bottom of a boat with considerable violence and then floated. At the same time, when we reflect to what a considerable extent the bottom of the great ocean basins is dotted over with volcanic cones, rising often solitary from profound depths, we can understand how large a proportion of the actual eruptions may take place under the sea. The foundations of these volcanic islands doubtless consist of submarine lavas and fragmentary materials, which, in each case, continued to accumulate to a height of two or three miles, until the pile reached the surface of the water and the phenomena became subaerial. The immense abundance and wide diffusion of volcanic detritus over the bottom of the Pacific and Atlantic Oceans, even at distances remote from land, as has been made known by the voyage of the “Challenger,” may indicate the prevalence and persistence of submarine volcanic action, though, at the same time, it must be admitted that an extensive diffusion of volcanic debris from the islands is effected by winds and ocean-currents.
§ 4. Geographical Distribution of Volcanoes.
The chief facts under this head may thus be summarized. (1.) Volcanoes occur along the margins of the ocean basins, particularly along lines of dominant mountain ranges. The vast hollow of the Pacific is girdled with a wide ring of volcanic foci. (2.) They rise as a striking feature in the heart of the ocean basins. Most of the oceanic islands are volcanic. Even the coral islands have in all likelihood been built upon the tops of submarine volcanic cones. (3.) Volcanoes are thus situated, as a rule, close to the sea. When they occur inland they sometimes appear in the neighbourhood of a large sheet of water. Yet so many instances have been observed where volcanoes have appeared at great distances from any sheet of water that the proximity of a lake or of the sea cannot be regarded as necessary for the evolution of volcanic phenomena. (4.) The dominant arrangement of volcanoes is in lines along subterranean lines of weakness, as in the chain of the Andes, the Aleutian Islands, and the Malay Archipelago. (5.) Where the linear arrangement does not hold, it gives place to one in groups, as in Italy, Iceland, and the volcanic islands of the great oceans.
§ 5. Distribution of Volcanic Action in Time.
Besides the existence of what are called extinct volcanoes, the geologist can adduce proofs of the former presence of active volcanoes in many countries where cones and craters and all ordinary aspects of volcanic mountains have long disappeared. Sheets of lava, beds of tuff, dykes, and necks representing the sites of volcanic vents have been recognized abundantly. These manifestations of volcanic action, moreover, have as wide a range in geological time as they have in geographical area. Every great geological period, back at least as far as the Lower Silurian, has had its volcanoes. In Britain, for instance, there were active volcanic vents in the Lower Silurian period, whence the lavas and tuffs of Snowdon, Aran Mowddwy, and Cader Idris were ejected. The Lower Old Red Sandstone epoch was one of prolonged volcanic activity in central Scotland. The earlier half of the Carboniferous period likewise witnessed the outburst of innumerable small volcanoes over the same region. During the Permian period a few scattered vents existed in the south-west of Scotland, and in the time of the New Red Sandstone some similar points of eruption appeared in the south of England. The older Tertiary ages were distinguished by the outpouring of the enormous basaltic plateaus of Antrim and the Inner Hebrides.
Thus it can be shown that, within the same comparatively limited geographical space, volcanic action has been rife at intervals during a long succession of geological ages. The existing active volcanoes of Iceland rise from amid Tertiary lavas and tuffs, which form part of a great volcanic ridge, extending down through the Faroe Islands into the west of Britain. Volcanic action, which now manifests itself so conspicuously along certain lines, seems to have continued in that linear development for protracted periods of time. The actual vents have changed, dying in one place and breaking out in another, yet keeping on the whole along the same lines.
§ 6. Causes of Volcanic Action.
Volcanoes depend, of course, upon the internal heat of the planet as their prime source of energy. But the modus operandi whereby that internal heat manifests itself in volcanic action is a problem by no means of easy solution. Were this action merely an expression of the intensity of the heat, we might expect it to have manifested itself in a far more powerful manner in former periods, and to exhibit a regularity and continuity commensurate with the exceedingly slow diminution of the earth’s temperature. But there is no geological evidence in favour of greater volcanic intensity in ancient times than in more recent periods; on the contrary, it may be doubted whether any of the Palaeozoic volcanoes equalled in magnitude those of the Tertiary period, or whether any of the latter ever produced such stupendous changes as have been effected by modern volcanoes still active. On the other hand, no feature of volcanic action is more conspicuous than its spasmodic fitfulness.
The mere presence of a high internal temperature, therefore, would probably not of itself produce the phenomena of volcanoes, at least in the present condition of the planet. There can be no doubt that one essential exciting cause of volcanic action is the descent of water from the surface. It has already been nointed out how invariably steam plays a chief part in volcanic eruptions, how it issues in vast clouds from the crater, and continues to rise copiously from the lava even after the molten rock has travelled for some miles, and has assumed a solid surface. The quantity of water which descends into the interior must be enormous. The floor of the sea, the beds of rivers and lakes, are all leaky. Of the annual rain which sinks beneath the surface of the land, we cannot tell what proportion is detained and prevented from rising again in springs. Not only does this subterranean water percolate down cracks and joints, it infiltrates through the very pores of the rocks, and can do so even against the pressure of steam on the further side.
Accordingly, there has arisen a very prevalent belief among geologists, that it is to the enormous expansive force of perhaps white-hot water finding access to, and imprisoned in, some of the heated empty spaces at the roots of volcanoes that the explosions of a crater and the subsequent rise of a lava-column are due. It has been supposed that, somewhat like the reservoirs in which the hot water and steam accumulate under the Icelandic geysers, these volcanic spaces receive a constant influx of water from the surface, which cannot escape by other channels, but is in great part converted into vapour or retained in the fluid state at an enormously high temperature and under vast pressure. In the course of time, the materials filling up the chimney are unable to withstand the upward expansion of this imprisoned vapour and water, so that, after some premonitory rumblings, the whole opposing mass is blown out, and the vapour rushes up in the well-known masses of cloud. Meanwhile, the removal of the overlying column relieves some of the pressure from the water-charged lava, which therefore begins to rise in the funnel until it forces its way through some weak part of the cone, or pours over the top of the crater. After a time the vapour is expended, the energy of the volcano ceases, and there comes a variable period of repose, until a renewal of the same phenomena brings on another eruption. By such successive paroxysms it is supposed that the form of the internal reservoirs and tunnels become changed; new spaces for the accumulation of superheated water are formed, whence in time new volcanic vents issue, while the old ones gradually die out.
As physical considerations negative the idea of a comparatively thin crust surmounting a molten interior whence volcanic energy might be derived, geologists have found themselves involved in great perplexity to explain volcanic phenomena for the production of which a source of no great depth would seem to be necessary. They have supposed the existence of pools or lakes of liquid lava lying beneath the crust, and at an inconsiderable depth from the surface. They have sometimes appealed to the influence of the contraction of the earth’s mass, assuming that the contraction is greater in the outer than in the inner portions, and that the effect of this must be to squeeze out some of the internal molten matter through weak parts of the crust. Cordier, for example, calculated that a contraction of only a single millimetre (about τ⅛th of an inch) would suffice to force out to the surface lava enough for 500 eruptions, allowing 1 cubic kilometre (about 1300 million cubic yards) for each eruption.
The influence of contraction as the grand source of volcanic energy has recently been insisted upon on quite different grounds by Mr Mallet, who has developed the theory that all the present manifestations of hypogene action are due directly to the more rapid contraction of the hotter internal mass of the earth, and the consequent crushing in of the outer cooler shell. He points to the admitted difficulties in the way of connecting volcanic phenomena with the existence of internal lakes of liquid matter, or of a central ocean of molten rock. Observations made by him, on the effects of the earthquake shocks accompanying the volcanic eruptions of Vesuvius and of Etna, showed that the focus of disturbance could not be more than a few miles deep,—that, in relation to the general mass of the globe, it was quite superficial, and could not possibly have lain under a crust of 800 miles or upwards in thickness. The occurrence of volcanoes in lines, and especially along some of the great mountain-chains of the planet, is likewise dwelt upon by him as a fact not satisfactorily explicable on any previous hypothesis of volcanic energy. But he contends that all these difficulties disappear when once the simple idea of cooling and contraction is adequately realized. “The secular cooling of the globe,” he remarks, “is always going on, though in a very slowly descending ratio. Contraction is therefore constantly providing a store of energy to be expended in crushing parts of the crust, and through that providing for the volcanic heat. But the crushing itself does not take place with uniformity; it necessarily acts∕>er saltτιm after accumulated pressure has reached the necessary amount at a given point, where some of the pressed mass, unequally pressed as we must assume it, gives way, and is succeeded perhaps by a time of repose, or by the transfer of the crushing action elsewhere to some weaker point. Hence, though the magazine of volcanic energy is being constantly and steadily replenished by secular cooling, the effects are intermittent.” He offers an experimental proof of the sufficiency of the store of heat produced by this internal crushing to cause all the phenomena of existing volcanoes. The slight comparative depth of the volcanic foci, their linear arrangement, and their occurrence along lines of dominant elevation become, he contends, intelligible under this hypothesis. For, since the crushing in of the crust may occur at any depth, the volcanic sources may vary in depth indefinitely; and as the crushing will take place chiefly along lines of weakness in the crust, it is precisely in such lines that crumpled mountain-ridges and volcanic funnels should appear. Moreover, by this explanation it is sought to harmonize the discordant observations regarding the variations in the rate of increase of temperature downward within the earth. In some parts of the crust the crushing must be much greater than in other parts; and since the heat “is directly proportionate to the local tangential pressure which produces the crushing and the resistance thereto,” it may vary indefinitely up to actual fusion. So long as the crushed rock remains out of reach of a sufficient access of subterranean water, there would, of course, be no disturbance. But if, through the weaker parts, water enough should descend and be absorbed by the intensely hot crushed mass, it would be raised to a very high temperature, and, on sufficient diminution of pressure, would flash into steam and produce the commotion of a volcanic eruption.
This ingenious theory requires the operation of sudden and violent movements, or at least that the heat generated by the crushing should be more than can be immediately conducted away through the crust. Were the crushing slow and equable, the heat developed by it would doubtless be tranquilly dissipated through the crust, the temperature of which might not be sensibly affected in the process, or not to such an extent as to cause any appreciable molecular rearrangement of the particles of the rock. Moreover, as the action is general throughout the whole mass of the outer shell, there does not seem in the theory any valid reason why volcanic action should be limited to particular lines, and why it should continue so long persistently on these lines. The existence of weak parts of the crust is postulated; but the successive crushings and consequent fusions might be expected to strengthen these weaker parts and to make volcanic conditions less easy.
It cannot indeed be denied that there exists the most convincing geological evidence in favour of the secular contraction of the globe, that during the process masses of sedimentary strata, many thousands of feet in thickness, have been crumpled aud crushed, and that the crumpling has often been accompanied by such an amount of heat and evolution of chemical activity as to produce a movement and rearrangement of the elements of the rocks,—this change sometimes advancing to the point of actual fusion. There is reason to believe that some at least of these periods of intense terrestrial disturbance have been followed by periods of prolonged volcanic action in the disturbed areas. Mr Mallet’s theory is thus, to some extent, supported by independent geological testimony. The existence, however, of large reservoirs of fused rock, at a comparatively small depth beneath the surface, may be conceived as probable, apart altogether from the effects of the subsidence of the outer shell upon the inner nucleus. The connexion of volcanoes with lines of elevation, and consequent weakness in the earth’s crust, is precisely what might have been anticipated on the view that the nucleus, though practically solid, is at such a temperature and pressure that any diminution of the pressure, by corrugation of the crust or otherwise, will cause the subjacent portion of the nucleus to melt. It is along the lines of elevation that the pressure must be relieved, and it is there that the consequent melting will take place. On these lines of tension and weakness, therefore, the conditions for volcanic excitement must be best developed. Water is there able soonest to reach the intensely-heated materials underneath the crust, and to give rise to the volcanic explosions. The periodicity of eruptions will thus depend upon the length of time required for the storing up of sufficient steam, and on the amount of resistance in the crust to be overcome. In some cases the intervals of activity, like those of the geysers, return with considerable regularity. In other cases, the shattering of the crust, or the upwelling of vast masses of lava, or the closing of subterranean passages for the descending water, or other causes may vary the conditions so much, from time to time, that the eruptions may follow each other at very unequal periods, and with very discrepant energy. Each great outburst exhausts for a while the vigour of the volcano, and an interval is needed for the renewed accumulation of vapour.
Section II.—Earthquakes.
The phenomena of earthquake-motion having been discussed in the article Earthquakes, we shall notice here only those which have a marked geological importance from the way in which they affect the crust or surface of the earth, briefly describing the effects of earthquakes upon the surface of the land, and upon terrestrial and oceanic waters, the permanent changes of level occasioned by them, their distribution and geological relations, and their causes.
1. Effects upon the Soil and General Surface of a Country.— The earth-wave or wave of shock underneath a country may traverse a wide region and affect it violently at the time without leaving any trace of its passage. Loose objects, however, are apt to be displaced. Thus blocks of rock, already disengaged from their parent masses, may be rolled down into the valleys below. Large landslips are thus produced, and these may give rise to very considerable subsequent changes in the drainage of the localities where they take place. It has often been observed that the soil is rent by the passage of the earthquake. Fissures appear, varying in size from mere cracks, like those due to desiccation, up to deep and wide chasms. Where these cracks are numerous, and where, consequently, the ground has been much disturbed, permanent modifications of the landscape may be produced. Trees are thrown down and buried, wholly or in part, in the rents. These superficial effects may, in a few years, be effaced by the gradual levelling power of the atmosphere. Where, however, the chasms are wide and deep enough to intercept any rivulets, or to serve as channels for heavy rain-torrents, they are sometimes further excavated, so as to become gradually enlarged into ravines and valleys. As a rule, each rent is only a few yards long. Sometimes it may extend for half a mile or even more. In the earthquake which shook the South Island of New Zealand in 1848, a fissure was formed averaging 18 inches in width and traceable for a distance of 60 miles parallel to the adjacent axis of the mountain-chain. The subsequent earthquake of 1855, in the same region, gave rise to a fracture which could be traced along the base of a line of cliff for a distance of about 90 miles.
Remarkable circular cavities are sometimes formed in the ground during the passage of the earth-wave. In many cases these holes serve as funnels of escape for an abundant discharge of water, so that when the disturbance ceases they appear as pools. They are believed to be caused by the sudden collapse of subterranean water-channels and the consequent forcible ejection of the water to the surface.
2. Effects upon Terrestrial Waters.— Springs are temporarily affected by earthquake movements, becoming greater or smaller in volume, sometimes muddy or dis∙ coloured, and sometimes increasing in temperature. Brooks and rivers have been observed to flow with an interrupted course, increasing or diminishing in size, stopping in their flow so as to leave their channels dry, and rolling forward with increased rapidity. Lakes are still more sensitive to the tremors of the ground beneath. Their waters occasionally rise and fall for several hours, even at a distance of many hundred miles from the centre of disturbance. Thus, on the day of the great Lisbon earthquake, many of the lakes of central and north-western Europe were so affected as to maintain a succession of waves rising to a height of 2 or 3 feet above their usual level. Cases, how· ever, have been observed where, owing to excessive subterranean movement, lakes have been emptied of their contents and their beds left permanently dry. On the other hand, areas of dry ground have been depressed, and have become the sites of new lakes.
Some of the most important changes in the fresh water of a region, however, are produced by the fall of masses of rock and earth. Landslips, by damming up a stream, may so arrest its water a3 to form a lake. The barrier, if of sufficient strength, may be permanent, and the lake will then remain. Owing, however, to the usually loose, incoherent character of its materials, the dam thrown across the pathway of a stream runs a great risk of being undermined by the percolating water. When this is the case, a sudden giving way of the barrier will allow the confined water to rush with great violence down the valley, and produce perhaps tenfold more havoc there than may have been caused by the original earthquake. When the landslip is of sufficient dimensions to divert the stream from its previous course, the new channel thus taken may become permanent, and a valley may be cut out or widened.
3. Effects upon the Sea.— The great sea-wave propagated outward from the centre of a sub-oceanic earthquake, and reaching the land after the earth-wave has arrived there, gives rise to much destruction along the maritime parts of the disturbed region. As it approaches the shore, the littoral waters retreat seawards, sucked up, as it were, by the advancing wall of water, which, reaching a height of sometimes 60 feet, rushes over the bare beach and sweeps inland, carrying with it everything which it can dislodge and bear away. Loose blocks of rock are thus lifted to a considerable distance from their former position, and left at a higher level. Deposits of sand, gravel, and other superficial accumulations are torn up and swept away, while the surface of the country, as far as the limit reached by the wave, is strewn with debris. If the district has been already shattered by the passage of the earth-wave, the advent of the great sea-wave augments and completes the devastation.
4. Permanent Changes of Level.— It has been observed, after the passage of an earthquake, that the level of the disturbed country has been changed. Thus after the terrible earthquake of 19th November 1822 the coast of Chili for a long distance was found to have risen from 3 to 4 feet, so that along the shore the littoral shells were exposed adhering still to the rocks amid multitudes of dead fish. The same coast-line has since been further upraised by subsequent earthquake shocks. On the other hand, many instances have been observed where the effect of the earthquake has been to depress permanently the disturbed ground. For example, by the Bengal earthquake of 1762 an area of 60 square miles on the coast, near Chittagong, suddenly went down beneath the sea, leaving only the top of the higher eminences above water. The succession of earthquakes which in the years 1811 and 1812 devastated the basin of the Mississippi gave rise to widespread depressions of the ground, over some of which the river spread so as to form new lakes, with the tops of the trees still standing above the surface of the water.
5. Distribution and Geological Delations of Earthquakes.— While no large space of the earth’s surface seems to be free from at least some degree of earthquakemovement, there are regions more especially liable to the visitation. As a rule, earthquakes are most frequent in volcanic districts, the explosions of a volcano being very generally preceded or accompanied by tremors of greater or less intensity. In the Old World the great belt of earthquake disturbance stretches in an east and west direction, along that tract of remarkable depressions and elevations lying between the Alps and the mountains of northern Africa, and spreading eastward so as to enclose the basins of the Mediterranean, Black Sea, Caspian, and Sea of Aral, and to.rise into the great mountain-ridges of Central Asia. In this zone lie numerous volcanic vents, both active and extinct or dormant, from the Azores on the west to the basaltic plateaus of India on the east. The Pacific Ocean is surrounded with a vast ring of volcanic vents, and its borders are likewise subject to frequent earthquake shocks. Some of the most terrible earthquakes within human experience have been those which have affected the western seaboard of South America.
6. Causes of Earthquakes.— An earthquake shock has been, defined by Mr Mallett as the transit of a wave of elastic compression through the crust and surface of the earth, generated by some sudden impulse within the crust. The passage of such a wave has been imitated experimentally, and some of its characteristic features have been illustrated by accidental explosions at powder-works. But though the phenomena point to some sudden and violent blow inflicted upon the solid crust, it is impossible to do more than speculate on the probable nature of this blow. In some cases it may arise from the sudden flashing into steam of water in the spheroidal state, from the sudden condensation of steam, from the explosions of a volcanic orifice, from the falling in of the roof of a subterranean cavity, or from the sudden snap of subterranean rocks subjected to prolonged and intense strain. But we are still in ignorance as to the actual immediate cause of any earthquake in. regions remote from active volcanoes. So much, at least, is certain, that the shock must arise from some.sudden and violent impulse, whereby a wave or undulation is propagated in all directions through the solid substance of the crust.
Section III.—Secular Upheaval and Depι∙ession.
Besides the sudden movements due to earthquake-shocks, the crust of the earth undergoes in many places oscillations of an extremely quiet and uniform character, sometimes of an elevatory, sometimes of a subsiding nature. So tranquil are these changes that they produce from day to day no appreciable alteration in the aspect of the ground affected. Only after the lapse of several generations, and by means of careful measurements, can they really be proved. Indeed, in the interior of a country nothing but a series of accurate levellings from some unchanged datum-line might detect the change of level, unless the effects of this terrestrial movement showed themselves in altering the drainage. It is only along the sea-coast that a ready measure is afforded of any such movement. In popular language it is usual to speak of the sea as rising or sinking relatively to the land. But so long as the volume of the ocean remains the same, the general sea-level can neither rise nor fall, unless by some movement of the solid globe underneath it. And, as we cannot conceive of any possible augmentation of the oceanic waters, nor of any diminution save what may be due to the extremely slow process of abstraction by the hydration of minerals, or absorption into the earth’s interior, we are compelled to regard the sea-level as practically a constant datum-line, any deviation from which, in the apparent heights of sea and land, must be due to movement of the land and not of the sea.
There are indeed certain cosmical causes which may affect the relative levels of sea and land. Thus the accumulation of large masses of snow and ice as an ice-cap at one of the poles would, as has been above pointed out (ante, p. 217), tend to displace the earth’s centre of gravity, and as a consequence to raise the level of the ocean in the hemisphere so affected, and to diminish it in a corresponding measure elsewhere. The return of the ice into the state of water would produce an opposite effect. Dr Croll has also drawn attention to the fact that, as a consequence of the diminution of centrifugal force owing to the retardation of the earth’s rotation caused by the tidal wave, the sea-level must have a tendency to subside at the equator and rise at the poles. A larger amount of land need not ultimately be laid bare at the equator, for the change of level resulting from this cause would be so slow that the general degradation of the surface of the land might keep pace with it, and diminish the terrestrial area as much as.the retreat of the ocean tended to increase it. Dr Croll has further pointed out that the waste of the equatorial land, and the deposition of the detritus in higher latitudes, must still further counteract the effects of retardation and the consequent change of ocean-level.
Such widespread general causes of change must produce equally far-reaching effects. But in examining the changes of level between land and sea, we find them to be eminently local and variable in character, pointing to some local and unequally acting cause,—so that, while admitting these cosmical and widespread influences to be part of the general system of geological change, we must yet hold the sealevel, for all practical purposes, to be invariable, any apparent oscillations of that level upon the land being due to terrestrial movements.
§ 1. Movements of Upheaval.
Various maritime tract3 of the land have been ascertained to have undergone in recent time3, or to be still undergoing, a gradual elevation above the sea. Thus, the coast of Siberia, for 600 miles to the east of the river Lena, the western tracts of South America, and the Scandinavian peninsula, with the exception of a small area at its southern apex, have been proved to have been recently upheaved. The proofs of this change of level chiefly to be relied on are the following:—
(1.) The position of rocks covered with barnacles or other littoral adherent animals, or pierced by lithodomous shells. A single stone with these creatures on its surface would not necessarily prove anything, for it might be cast up by a storm; but a line of large boulders, which had evidently not been moved since the cirripedes and molluscs lived upon them, and still more a solid cliff with these marks of littoral or sub-littoral life upon its base, now raised above highwater mark, would be sufficient to demonstrate a rise of land. The amount of the upheaval might be pretty accurately determined by measuring the vertical distance between the upper edge of the barnacle zone upon the upraised rock, and the limit of the same zone on the present shore.
(2.) A line of sea-caves, now standing at a distance above high water-mark beyond the reach of the sea, would afford evidence of recent uprise, since caves of this kind are only hollowed out by the waves between tide-marks.
(3.) One of the most striking proofs of upheaval is furnished by what are termed raised beaches. A beach is the space between tide-marks, where the sea is constantly busy producing sand and gravel, mingling with them the remains of shells and other organisms, sometimes piling the deposits up, sometimes sweeping them away out into opener water. The terrace or platform thus formed is a well-marked feature of coast-lines skirting tidal seas. When the land rises with sufficient rapidity to carry up the line of littoral deposits above the reach of the waves, the flat terrace thus elevated is known as a raised beach. The former high-water mark then lies inland, and while its sea-worn caves are in time hung with ferns and mosses, it furnishes itself an admirable platform, on which meadows, fields, and gardens, roads, houses, villages, and towns spring up, while a new beach is made below the margin of the uplifted one. Raised beaches abound round many parts of the coast-line of Britain. Some excellent examples occur in Cornwall and Devon. The coast-line on both sides of Scotland is fringed with raised beaches, sometimes four or five occurring above each other, at heights of 25, 40, 60, 75 and 100 feet above the present high-water mark. Each terrace marks a former lower level of the land with regard to the sea, and probably a lengthened stay of the land at that level, while the intervals between them represent the vertical amount of each successive uplift of the land, and show that the land in its upward movement did notremain long enough at intermediate points for the formation of terraces. A succession of raised beaches, rising above the present sea-level, may therefore be taken as pointing to a former prolonged upheaval of the country, interrupted by long pauses, during which the general level did not materially change.
(4.) Any stratum of rock containing marine organisms, which have manifestly lived and died where their remains now lie, must be held to prove upheaval of the land. In this way it can be shown that most of the solid land now visible to us has once been under the sea. Even high on the peaks of the cliffs and the flanks of the Himalaya mountains, undoubted marine shells occur in the solid rocks.
(5.) In countries which have been long settled by a human population, it is sometimes possible to prove, or at least to render probable, the fact of recent uprise of the land by reference to tradition, to local names, and to works of human conetrurtinn. Piers anil hαrbonra, if now found to stand above the upper limit of high-water, furnish indeed indisputable evidence of a rise of land since their erection.
§ 2. Movements of Subsidence.
It is more difficult to trace the downward movement of the land, for the evidence of each successive sea-margin is carried down and washed away or covered up. Nevertheless, the fact of subsidence can be satisfactorily established by the following kinds of proof:—
(1.) The results of mere erosion by the sea and those of actual depression of the level of the land cannot always be distinguished without some care. The encroachment of the sea upon the land, involving, it may be, the disappearance of successive fields, roads, houses, villages, and even whole parishes, does not necessarily indicate a sinking of the land. Such destruction of the coast-line may, indeed, be in progress without any actual change of level. Should the sea, however, rise to the level of roads and buildings which it never used to touch, should former half-tide rocks cease to show even at low water, and should rocks, previously above the reach of the highest tide, be turned first into shore reefs, then into skerries and islets, we infer that the coast-line is sinking. Such kind of evidence is found in Scania, the most southerly part of Sweden. Streets, built of course above high-water mark, now lie below it, with older streets lying beneath them, so that the subsidence is of some antiquity. A stone, the position of which had been exactly determined by Linnaeus in 1749, was found after 87 years to be 100 feet nearer the water’s edge. The west coast of Greenland, for a space of more than 600 miles, is perceptibly sinking. It has there been noticed that, over ancient buildings on low shores, as well as over entire islets, the sea has risen. The Moravian settlers have been more than once driven to shift their boat-poles inland, some of the old poles remaining visible under water.
(2.) As the land is brought down within reach of the waves, its characteristic surface-features are, of course, apt to be effaced, so that the submerged area which passes down beneath the sea may retain little or no evidence of its having been a land-surface. It will be covered, as a rule, with sea-worn sand or silt. Hence, no doubt, the reason why, among the marine strata which form so large a part of the stratified portion of the earth’s crust, and where there are many proofs of depression, actual traces of landsurfaces are comparatively rare. It is only under very favourable circumstances, as, for instance, where the area is sheltered from prevalent winds and waves, and where, therefore, the surface of the land can sink tranquilly under the sea, that fragments of that surface may be completely preserved under overlying marine accumulations. It is in such places that “submerged forests” occur. These are stumps or roots of trees still in their positions of growth in their native soil. Beds of peat, full of tree-stumps, hazelnuts, branches, leaves, and other indications of a terrestrial surface, are often found in similar situations. Sir Henry de la Beche has described, round the shores of Devon, Cornwall, and western Somerset, a vegetable accumulation, consisting of plants of the same species as those which now grow freely on the adjoining land, and occurring as a bed at the mouths of valleys, at the bottoms of sheltered bays, and in front of and under low tracts of land, the seaward side of which dips beneath the present level of the sea. Over this submerged land-surface sand and silt containing estuarine shells have generally been deposited, whence we may infer that in the submergence the valleys first became estuaries, and then sea-bays. If now, in the course of ages,, a series of such submerged forests should be formed one over the other, and if, finally, they should, by upheaval of the sea-bottom, be once more laid dry, so as to be capable of examination by boring, well-sinking, or otherwise, they would prove a former long-continued depression, with intervals of rest. In such a case, the intervals of pause would be marked by the buried forests, and the progress of the depression by the strata of sind and mud lying between them. In short, the evidence would be strictly on a parallel with that furnished by a succession of raised beaches as to a former protracted elevation with long pauses.
(3.) An interesting kind of proof of an extensive depression of the north-west of Europe is furnished by the deep fjords or sea-loch3 by which that region is indented. A fjord is a long, narrow, and often singularly deep inlet of the sea, which terminates inland at the mouth of a glen or valley. The word is Norwegian, and in Norway fjords are characteristically developed. The English word “firth,” however, is the same, and the western coasts of the British Isles furnish many excellent examples of fjords. In Scotland they are usually called lochs, as Loch Hourn, Loch Nevis, Loch Fyne, Gareloch; in Ireland they are sometimes known by the name of loughs, as Lough Foyle, but more commonly by that of bays, as Dingle Bay, Bantry Bay. There can be little doubt that, though now filled with salt water, fjords have been originally land valleys. The long inlet was first excavated as a land-valley or glen. This valley exactly corresponds in form and character with the hollow of the fjord, and must be regarded as merely its inland prolongation. That the glens have been excavated by subaerial agents is a conclusion borne out by a great weight of evidence. If, therefore, we admit the subaerial origin of the glen, we must also grant a similar origin to its seaward prolongation. Every fjord will thus mark the site of a submerged valley. This inference is confirmed by the fact that fjords do not, as a rule, occur singly. Like the glens on the land they lie in groups; so that when they are found intersecting a long line of coast like that of the west of Norway, or the west of Scotland, we conclude that the land has there sunk down so as to permit the sea to run far up and fill the submerged glens.
(4.) Evidence of widespread depression over the area of the Pacific Ocean is furnished by the numerous atolls or coral islands scattered throughout that vast expanse of water. Mr Darwin ascertained that the reef-building corals do not live at a greater depth than about 15 or 20 fathoms. Yet reefs and circular islets of coral rise with nearly perpendicular sides from a depth of 2000 feet and upwards, until they reach the surface of the sea. As the corals could not have begun to grow upward from such vast depths, Mr Darwin first suggested that the sites of these coral reefs had undergone a progressive subsidence, the rate of upward growth of the reefs keeping pace, on the whole, with the depression. A fringing reef would first be formed fronting the land within the limit of the 20 fathom line. Growing upward until it reached the surface of the water, it would be exposed to the dash of the waves, which would break off pieces of the coral and heap them upon the reef. In this way islets would be formed which, by successive accumulations of materials thrown up by the breakers or brought by winds, would remain permanently above water. On these islets palms and other plants, whose seeds might be drifted from the adjoining land, would take root and flourish. Inside the reef there would be a shallow channel of water, communicating, through gaps in the reef, with the main ocean outside. Fringing reefs of this character are of common occurrence at the present time. In the case of a continent they front its coast for a long distance, but they may entirely surround an island. If the site of a fringing reef undergoes depression ata rate sufficiently slow to allow the corals to keep pace with it, the reef will grow upward as the bottom sinks downward. The lagoon channel inside will become deeper and wider, while, at the same time, the depth of the water outside will increase. In this way a barrier reef will be formed. Continued slow depression must continually diminish the area of the land enclosed within one of these rings of coral-reef, while the reef itself retains much the same size and position. At last the final peak of the original island disappears under the lagoon, and an atoll or true coral island is formed. Should any more rapid or sudden downward movement take place, it might carry the atoll down beneath the surface, as seems to have happened at the Great Chagos bank in the Indian Ocean, which is a submerged atoll. It has recently been suggested that barrier reefs do not necessarily prove subsidence, seeing that they may grow outward from the land upon the top of a talus of their own debris broken down by the waves, and may thus appear to consist of solid coral which had grown upward from the bottom during depression, although only the upper layer, 20 fathoms or thereabouts in thickness, is composed of solid, unbroken, coral growth. The explanation may doubtless account for some barrier-reefs, and for the way in which the steep seaward face of all such reefs is formed and maintained. But it does not elucidate the existence of submerged atolls, the presence of gaps in atolls answering to gaps in the fringing reefs opposite to the mouths of rivers; and the difficulty of supposing that, in a coral archipelago, there should have been scores of submerged peaks so nearly of the same height as to rise within 20 fathoms of the surface, and yet so seldom actually to tower above it. According to the simple and luminous theory of Mr Darwin, every stage in the progress of the changes is open to observation, from the incipient fringing reef to the completed and submerged atoll. Every observed fact fits in harmoniously with the others, and we reach the impressive conclusion that a vast area of the Pacific Ocean, fully 6000 geographical miles from east to west, has undergone a recent subsidence, and may be slowly sinking still.
It by no means follows, however, as some writers have imagined, that the present Pacific Ocean occupies the site of a vast submerged continent. All the coral islands seem to have been built on volcanic peaks. Wherever any non-calcareous rock appears it is of volcanic origin. We must therefore conceive of these oceanic islands as detached volcanic eminences rising out of a wide area of subsidence, and doubtless as deriving their existence from the results of that subterranean movement.
§ 3. Causes of Secular Upheaval and Depression.
These movements, without question, we must again trace back to consequences of the original internal heat of the earth. There are various ways in which the heat may have acted. Thus a considerable accession of heat expands rocks, and, on the other hand, a loss of heat causes them to contract. We may suppose therefore that, during the subterranean changes, a great extent of the crust underneath a tract of land may have its temperature slowly raised. The effect of this increment would be to cause a slow uprise of the ground above. The gradual transference of the heat to another quarter might produce a steady subsidence. Such variations in subterranean temperature, however, could give rise at the most to but very insignificant elevations or depressions.
A far more important and generally effective cause is to be sought in the secular contraction of the globe. If our planet has been steadily losing heat by radiation into space, it must have progressively diminished in volume. The cooling implies contraction. According to Mr Mallet, the diameter of the earth is less by at least 189 miles since the time when the planet was a mass of liquid. But the contraction has not manifested itself uniformly over the whole surface of the planet. The crust varies much in structure, in thermal resistance, and in the position of its isogeothermal lines. As the hotter nucleus contracts more rapidly by cooling than the cooled and hardened crust, the latter must sink down by its own weight, and in so doing requires to accommodate itself to a continually diminishing diameter. The descent of the crust gives rise to enormous tangential pressures. The rocks are crushed, crumpled, and broken in many places. Subsidence must have been the general rule, but every subsidence would doubtless be accompanied with upheavals of a more limited kind. The direction of these upheaved tracts would largely depend upon the original structure of the crust. It would be apt to occur in lines which, once taken as lines of weakness or relief from the intense strain, would probably be made use of again and again at successive paroxysms or more tranquil periods of contraction. Mr Mallet, in the paper already cited, has ingeniously connected these movements with the linear direction of mountain chains, volcanic vents, and earthquake shocks.
Though the origin of the present surface configuration of the land will be more conveniently discussed in a subsequent part of this article, after the structure of the earth’s crust has been described, a few words may be inserted here, where some allusion to the subject might be looked for. Mountains may arise from three causes—(1) from the corrugation of the earth’s crust due to the effects of secular contraction; (2) from the accumulation of materials poured out of volcanic orifices; and (3) from the isolation of elevated masses of ground, owing to the removal, by denudation, of the materials originally connecting them, and to the îonsequeût formation of valleys. Mountains formed in the volcanic way are almost always conical, and are either solitary, as Etna, or occur in linear groups, like the volcanoes of Java. Those formed by denudation are of minor dimensions, and rather deserve the name of hills. Mountain-chains, on the other hand, which are the dominant features of the earth’s surface, though they may have lines of volcanic vents along their crests, are not formed essentially of volcanic materials, but of the sedimentary and crystalline rocks of the crust which have been ridged up into vast folds. If the continental lands may be compared to great undulations of the solid surface of the globe, the mountain-chains may be likened to the breaking crests of such wave-like undulations. In their internal structure mountain-chains bear witness to the intense crumpling of the rocks during the process of upheaval. As a consequence of the uprise of two or more parallel ranges of mountain, lines of longitudinal valley must be produced. But no sooner is a mass of land raised above the sea than it is exposed to the attacks of air, rain, frost, springs, glaciers, or other meteoric agents of disintegration. Its surface is then worn down, the flow of water down its slopes cuts out gulleys, ravines, and valleys, so that eventually a very rugged surface is produced, on which, probably, no portion of the original surface of upheaval may remain, but where new lines of minor ridge and valley may appear as the combined result of internal geological structure and atmospheric denudation. The reader, however, is referred to part vii. of this article for a fuller treatment of this interesting subject.
During the movements by which mountain masses have been upheaved, the stratified rocks have been so compressed as to occupy, in many cases, but a small proportion of the horizontal extent over which they originally extended. They have adjusted themselves to this diminished area by undergoing intense plication, and thus acquiring a much greater vertical depth. On the other hand, they have been abundantly fractured, some portions of their mass being pushed up, others being let down, so that the crust is traversed with a kind of complicated network of fissures. The discussion of these features of geological structure, however, must likewise be postponed to part iv., where the facts regarding their occurrence will be described.
Section IV.—Hypogene Causes of Changes in the Texture, Structure, and Composition of Rocks.
The phenomena of hypogene action hitherto under consideration have related almost wholly to the effects produced at the surface. It is evident, however, that these phenomena must be accompanied by very considerable changes in the rocks which form the earth’s outer crust. These rocks, as just stated, have been subjected to enormous pressure; they have been contorted, crumpled, and folded back upon themselves, as if thousands of feet of solid limestones, sandstones, and shales had been merely a few layers of carpet; they have been shattered and fractured everywhere; they have in one place been pushed far above their original position, in another depressed far beneath it: so great has been the compression which they have undergone that their component particles have in many places been rearranged, and even crystallized. They have here and there actually been reduced to fusion, and have been abundantly invaded by masses of molten rock from below.
In the present section of this article we shall consider chiefly the nature of the agencies by which such changes have been effected; the results achieved, in so far as they constitute part of the architecture or structure of the earth’s crust, will be discussed in part iv. At the outset, it is evident that we can hardly hope to detect many of these processes of subterranean change actually in progress and watch their effects. The very vastness of some of them places them beyond our direct reach, and we can only reason regarding them from the changes which we see them to have produced. But a good number are of a kind which we can in some measure imitate in our laboratories and furnaces. It is not requisite, therefore, to speculate wholly in the dark on this subject. Since the original and classic researches of Sir James Hall, the founder of experimental geology, great progress has been made in the investigation of hypogene processes by experiment. The conditions of nature have been imitated as closely as possible, and varied in different ways, with the result of giving us an increasingly clear insight into the physics and chemistry of subterranean geological changes. The succeeding portions of this section of the article will therefore be chiefly devoted to an illustration of the nature of hypogene action, in so far as that can be inferred from the results of actual experiment. The subject maybe conveniently treated under three heads— (1) the effects of heat; (2) the effects of pressure and contraction; (3) the influence of water in effecting changes in minerals and rocks.
§ 1. Effects of Heat.
The importance of heat among the transformations of the rocks has been fully admitted by geologists, since it used to be the watchword of the Huttonian or Vulcanist school at the end of last century. Two sources of subterranean heat may have at different times and in different degrees co-operated in the production of hypogene changes—(1) the internal heat of the globe, of which some account has already been given, and (2) the heat due to the transformation of mechanical energy in the crumpling, fracturing, and crushing of the rocks of the crust, as these have been from time to time compelled to adjust themselves to the diminishing diameter of the more rapidly cooling and contracting interior. From which of these two sources, or from what combination of them, the heat was derived by which any given change in the rocks was produced, is a problem which admits in the meantime of only very partial solution. Leaving this question for the present, let us examine the effects which are clearly referable to the influence of heat. It is evident that the conditions under which these effects are produced must vary almost infinitely. We have to consider (1) the temperature, from the lowest at which any change is possible up to that of complete fusion; (2) the nature of the rock operated upon, some materials being much more susceptible of change from heat than others;
(3) the pressure under which the heat acts, the potency of this agency being much increased with increase of pressure;
(4) the presence of water, whereby chemical changes take place which would not be possible in dry heat.
In an interesting series of experiments the illustrious De Saussure (1779) fused some of the rocks of Switzerland and France, and inferred from them, contrary to the opinion previously expressed by Desmarest, 1 that basalt and lava have not been produced from granite, but from hornstone (pierre de corne), varieties of “schorl,” calcareous clays, marls, and micaceous earfhs, and the cellular varieties from different kinds of slate. 2 He observed, however, that the artificial products obtained by fusion were glassy and enamel-like, and did not always recall volcanic rocks, though some exactly resembled porous lavas.
Sir James Hall, about the year 1790, began an important investigation, in which he succeeded in reducing various ancient and modern volcanic rocks to the condition of glass, and in restoring them, by slow cooling, to a stony state. Since that time many other researches of a more complicated kind havg been undertaken, especially by Delesse, Daubrée, Deville, Bunsen, Bischof, H. and W. Rogers. By these observations it has been abundantly proved that all rocks undergo molecular changes when exposed to high temperature, that when the heat is sufficiently raised they become fluid, that if the glas3 thus obtained is rapidly cooled it remains vitreous, aud that, if allowed to cool slowly, a more or less distinct crystallization sets in, the glass is devitrified, and a lithoid product is the result.
Illustrations of the influence of different degrees of heat upon rocks of different kinds may often be very instructively observed at lime-kilns, especially those roughly-built kilns or pits which may still be met with in outlying districts. Some of the stones lining such cavities will be found with no sensible change, others show a somewhat cellular texture, others have acquired a rudely prismatic structure, while some have had their surfaces fused into a rough glaze or enamel. The bricks or stones used for lining furnaces present similar illustrations, the columnar or prismatic structure being occasionally very perfectly developed in these materials. Mere contact with a highly-heated surface, and subsequent gradual cooling, have often produced this prismatic arrangement in rocks of the most diverse kinds. Thus, in Scotland, beds of sandstone and seams of coal have acquired the most perfect prismatic structure from the intrusion of basalt dykes or sheets through them. To these and other effects of intruded igneous rocks reference will be made in the sequel. In these cases the alteration is merely local, and has obviously been produced by contact with a highly heated surface. But, besides such minor effects due merely to contact, others of a more general kind affect large masses of rock or whole districts of country.
When rocks are exposed to temperatures as high as their melting-points, they fuse into glass which, in the great majority of cases, is of a bottle-green or black colour, the depth of the tint depending mainly on the proportion of iron. In this respect they resemble the natural glasses— pitchstones and 0bsidιan3. They almost always contain minute cells or bubbles, arising probably from the disengagement of water or of oxygen during the fusion. But after the most thorough fusion which has been found possible, minute granules usually appear in the solidified glass. Sometimes these consist of specks of quartz, which is especially apt to remain unmelted when the rock is very siliceous, or of other minerals of the original rock. But a microscopic investigation of fused rocks shows that, even in what seems to be a tolerably homogeneous glass, there are abundant minute hair-like, feathered, needle-shaped, or irregularly-aggregated bodies diffused through the glassy paste. These crystallites are in some cases colourless silicates, in others they are opaque metallic oxides, particularly oxides of iron. They precisely resemble the crystallites which are observed in many volcanic rocks, such as obsidian, pitchstone, and basalt. They may be obtained even from the fusion of a granitic or granitoid rock, as in the well-known case of the Mount Sorrel syenite near Leicester, which, being fused and slowly cooled, yielded abundant crystallites, including exquisitely grouped octohedra of magnetite.
According to the observations of Delesse, volcanic rocks, when reduced to a molten condition, attack briskly the sides of the Hessian crucibles in which they are contained, and even eat them through. This is an interesting fact, for it helps to explain how some intrusive igneous rocks have come to occupy positions previously filled by sedimentary strata, and why, under such circumstances, the composition of the same mass of rock should be found to vary considerably from place to place. 3
It would appear that, in passing from a crystalline into a vitreous condition, rocks undergo a diminution of density, which, on the whole, is greater the more silica and alkali are present, and is less as the proportion of iron, lime, and alumina increases. According to Delesse, granites, quartziferous porphyries, and such highly silicated rocks l03e from 8 to 11 per cent, of their density when they are reduced to the condition of glass, basalts lose from 3 to 5 per cent., and lavas, including the vitreous varieties, from 0 to 4 per cent. This diminution of density (and consequent increase of volume) may account for minor oscillations of the ground so often observed in volcanic regions. If we suppose a subterranean mass of lava 500 feet thick to pass from the fluid to the crystalline condition, this might cause a subsidence of the ground above to the extent of about 20 or 25 feet. The transition of a similar mass of rock from the solid to the fluid condition would cause an uprise to a like amount.
So far as experiments have yet been conducted, it has been found impossible to obtain from a piece of fused rock a crystalline substance exactly resembling the original mass. Externally it may appear quite stony, but its internal structure, as revealed by the microscope, shows it to be essentially a slag or glass, and not a truly crystalline rock. There is another fundamental difference between the natural and artificial products. When a compound containing substances of different fusibilities is artificially melted, and allowed thereafter to cool in such a way that the various ingredients may separate from each other, they appear in their order of fusibility, the most refractory coming first, and the most fusible being the last to take a solid form. But in rocks which have crystallized naturally from a fluid condition, it is often to be observed that the component minerals have been far from obeying what might have been supposed to be their invariable law. Thus, in all parts of the world, granite presents the very striking fact that its quartz, which we call an infusible mineral, has actually solidified after the more fusible felspar. In the Vesuvian lavas the 1 Mem. Aeαd. Seien., 1771, p. 273.
2 De Saussure, Voyages dans les Alps, edit. 1803, tome i. p. 178.
3 Bulk Soe. Géol. France, 2d ser., iv. 1382.
difficultly fusible leucite may be seen to have enclosed crystals already formed of the fusible augite. In many ancient crystalline rocks the pyroxenic constituents, which offer a less resistance to fusion, have assumed a crystalline form before the more refractory triclinic felspars. From these facts it is clear that, in the fusion of rocks and in their subsequent consolidation, there must have been conditions under which the normal order of appearance of the minerals was disturbed or reversed. Yet another fact may be mentioned to show further the difference between the kind of fusion which has frequently obtained in nature and that of the ordinary operations of a glass-work or iron-furnace. As far back as the year 1846 Scheerer showed that there exist in granite various minerals which could not have consolidated save at a comparatively low temperature. He instanced especially several gadolinites, orthites, and allanites, which cannot endure a higher temperature than a dull-red heat without altering their physical characters; and he concluded that granite, though it may have possessed a high temperature, cannot have solidified from simple igneous fusion.
We may conclude, therefore, that the manner in which rocks have been melted within the crust is not that mere simple fusion which we can accomplish artificially, but that it has involved conditions which have not been successfully imitated in any laboratory or furnace. Two obvious differences must occur to the reader between the natural and artificial operations. In the first place, rocks which Lave undoubtedly once been in a fluid or at least pasty condition, and which have been injected as veins and dykes into previously consolidated masses, contain water imprisoned within their component crystals. This is not water which has been subsequently introduced. It is contained in minute cells, which it usually does not now completely fill, but which it no doubt did occupy completely at the time and temperature at which the rock was consolidated. We have seen (ante, p. 250) how abundant are the discharges of watervapour from volcanic fires, how the molten lava-streams issue from their vents, saturated, as it were, with steam, and how the steam continues to rise from them long after they have congealed and come to rest. In the solid crystals of lava which were erupted only recently, as well as in those of early geological periods, the presence of water in minute cavities may be readily detected. It is in the quartz of such rocks, and still more in that of granite, that the detection of water-cavities is most easily made. The quartz of granite is usually full of them. “A thousand millions,” says Mr J. Clifton Ward, “might easily be contained within a cubic inch of quartz, and sometimes the contained water must make up at least 5 per cent, of the whole volume of the containing quartz.” Thus microscopic investigation confirms the conclusion arrived at by Scheerer in the memoir already cited, that at the time of its eruption granite must have been a kind of pasty mass containing a considerable proportion of water. It is common now to speak of the “aquo-igneous” origin of some eruptive rocks, and to treat their production as a part of what 13 termed the “hydro-thermal” operations of geology. We may conclude that, while some rocks, like obsidian and pitchstone, which so closely resemble artificial glasses, may have been derived from a simple igneous fusion such as can be imitated in a furnace (though even in these the presence and influence of water may be traced), the great majority of rocks have had a more complex origin, and in a great number of cases can be proved to have been mingled with more or less water while they were still fluid.
In the second place, there can be no question that, in the great hypogene laboratory of nature, rocks have been softened and fused under enormous pressure. Besides the pressure due to their varying depth from the s rface, they must have been subject to the enormous expansion of the superheated water or vapour which filled all their cavities. Mr Sorby has ingeniously estimated the probable pressure under which granite consolidated by taking the ratio between the size of the liquid cavities in the quartz and that of the contained bubble or vacuity. Assuming the temperature of consolidation to have been 680° Fahr, (360 c Cent.), or a dull-red heat, he inferred that in many cases the pressure under which the granite consolidates must have been equal to that of an overlying mass of rock 50,000 feet, or more than 9 miles, in thickness. It is not probable that any such thick overlying mass ever did cover -the granite; the pressure, even if it be allowed to have been so great, m ast have been due partly to other causes, such as the compression due to secular contraction.
It would appear therefore that perfect anhydrous fusion, or the reduction of a rock to the state of a completely homogeneous glass, has been a comparatively rare process in nature, or at least that such glasses, if originally formed, have in the vast majority of cases undergone?devitrification and crystallization, until the glassy base has been reduced to a smaller or larger fraction of the total mass of the rock, or has entirely passed into a stony condition. In many volcanic rocks the original vitreous base or ground-mass can be readily observed with the microscope between the definitely-formed crystals. Crystallites, or arrested stages in the crystallization of iron oxides and of silicates, can often be detected in these rocks, more especially where it is evident that they must have cooled with comparative rapidity, as where they have been thrust into narrow fissures to form dykes. But in rocks such as granite, where no glass exists, but where the mineral constituents are all crystalline, no trace of the crystallites occurs. Doubtless such differences point to original distinctions in the kind and degree of fusion of the rocks. It seems reasonable to suppose that those rocks which show a glassy ground-mass, and the presence of crystallites, have been fused under conditions more nearly resembling those of the simple igneous fusion of experiment.
It has long been known that many mineral substances can be obtained in a crystalline form from the condensation of vapours. This process, called sublimation, may be the result of the mere cooling and reappearance of bodies which have been vaporized by heat and solidify on cooling, or from the solution of these bodies in other vapours or gases, or from the reaction of different vapours upon each other. These operations are of common occurrence at volcanic vents, and in the crevices of recently erupted and still hot lava-streams. They have been successfully imitated by experiment. In the early researches of Sir James Hall on the effects of heat modified by compression, he obtained by sublimation “transparent and well-defined crystals,” lining the unoccupied portion of a hermetically-sealed iron tube, in which he had placed and exposed to a high temperature some fragments of limestone (Trans. Roy. Sοc. Edin., vi. 110). Numerous experiments have been made by Messrs Delesse, Daubrée, and others, in the production of minerals by sublimation. Thus many of the metallic sulphides found in mineral veins have been produced by exposing to a comparatively low temperature (between that of boiling water and a dull-red heat) tubes containiug metallic chlorides and sulphide of hydrogen. By varying the materials employed, corundum, quartz, apatite, and other minerals have been obtained. It is not difficult, therefore, to understand how, in the crevices of lava-streams and volcanic cones, as well as in some mineral veins, sulphides and oxides of iron and other minerals may have been formed by the ascent of heated vapours. Superheated steam is endowed with a remarkable power of dissolving that intractable substance, silica; artificially heated to the tern-perature of the melting point of cast-iron, it rapidly attacks silica, and deposits the mineral in snow-white crystals as it cools.
§ 2. Effects of Pressure and Contraction.
Besides the influence of pressure in raising the melting point of subterranean rocks, and in permitting water to remain fluid among them at temperatures far above the boiling-point, even at a red or perhaps a white heat, we have to consider the effects produced by the same cause upon rocks already solidified. The simplest and most obvious result of pressure upon such rocks is their consolidation, as where a mass of loose sand is gradually compacted into a more or less coherent stone, or where a layer of vegetation is compressed into peat, lignite, or coal. In many cases the cohesion of a sedimentary rock is due merely to the pressure of the superincumbent strata. But it usually happens that some cementing material has contributed to bind the component particles together. Of these natural cements the most frequent are peroxide of iron, silica, and carbonate of lime.
Pressure equally distributed over a rock presenting everywhere nearly the same amount of resistance will promote consolidation, but may produce no further internal change. If, however, the pressure becomes extremely unequal, or if the rock subjected to it can find escape from the influence in one or more directions, there will be a disturbance or rearrangement of the particles, which by this means are made to move upon each other. Five consequences of these movements may be noticed here.
(1.) Cleavage.— When a mass of rock, owing to subsidence or any other cause, is subjected to powerful lateral compression, its innate particles, which in all rocks have almost invariably a longer and shorter axis, tend, under the intense strain, to rearrange themselves in the line of least resistance, that is, with their long axes perpendicular to the direction of the pressure. The result of this readjustment is that the rock affected by it acquires a facility for splitting along the lines in which its component particles have placed themselves. Fine-grained argillaceous rocks show most characteristically this internal change; but in coarse materials it becomes less conspicuous, or even disappears. Rocks which have been thus acted on, and have acquired this superinduced fissility, are said to be cleaved, and the fissile structure is termed cleavage. This has been proved experimentally by Sorby, who produced perfect cleavage in pipeclay through which scales of oxide of iron had previously been mixed. Dr Tyndall superinduced cleavage on bees-wax and other substances by subjecting them to severe pressure. Cleavage among rocks occurs on a great scale in countries where the strata have been greatly plicated, that is, where they now occupy much less horizontal surface than they once did, and consequently where, in accommodating themselves to their diminished area, they have had to undergo much powerful lateral compression. The structure of districts with cleaved rocks is described in part iv.
(2.) Further evidence of the compression to which rocks have been subjected is furnished by the way in which contiguous pebbles in a conglomerate may be found to have been squeezed into each other, and even sometimes to have been elongated in a certain general direction. It is doubtless the coarseness of the grain of such rocks which permits the effects of compression to be so readily seen. Similar effects must take place in fine-grained rocks, though they escape observation. Organic remains both of plants and animals may often be found to have undergone considerable distortion from this cause. M. Daubrée has imitated experimentally the indentations produced by the contiguous portions of conglomerate pebbles.
(3.) The ingenious experiments of Μ. Tresca on the flow of solids have proved that, even at ordinary atmospheric temperatures, solid resisting bodies like lead, cast-iron, and ice, may be so compressed as to undergo an internal motion of their parts which is closely analogous to that of fluids. Thus, a solid jet of lead has been produced by placing a piece of the metal in a cavity between the jaws of a powerful compressing machine. Iron, in like manner, has been forced to flow in the solid state into cavities and take their shape. On cutting sections of the metals so compressed, their particles or crystals are found to have ranged themselves in lines of flow which follow the contour of the space into which they have been squeezed. Such experiments are of considerable geological interest, for they show that in certain circumstances, under great pressure, the unequally mixed particles of rocks within the earth’s crust may have been forced to move upon each other, and thus to acquire a “fluid-structure” resembling that which is seen in rocks which have possessed true liquidity. No large sheet of rock can be expected, however, to have undergone this internal change; the effects could only be produced exceptionally at places where there was an escape from the pressure as, for instance, along the sides of fissures, or in other cavities of rocks. The explanation cannot be applied to the case of rocks like schists, which display a kind of rude foliation or fluid-structure over areas many thousands of square miles in extent.
(4.) Plication.— Reference has already been made to the fact that, owing to the more rapid contraction of the inner portion of the globe, the outer layer or crust is from time to time forced to adjust itself to this change by subsiding. As a consequence of the subsidence, the descending area requires to occupy less horizontal space, and must therefore suffer powerful lateral compression. The rocks are thus crumpled up, as, in the classic experiment of Sir James Hall, folds of cloth are folded when a weight is placed upon them and they are squeezed from either side. The mere subsidence of such a curved surface as that of our globe must thus necessarily produce much lateral compression. Mr J. M. Wilson has calculated that, if a tract of the earth’s surface, 345 miles in breadth, be depressed one mile, it will undergo compression to the extent of 121 yards; at two miles the compression will be 189 yards; at eight miles 598 yards. The observed amount of compression in districts of contorted rocks, however, far exceeds these figures. Another cause of the compression and contortion of rocks is the injection into them of igneous masses from below, but this is probably a minor source of disturbance. The character of plicated rocks is described in part iv. p. 300.
(5.) Faults.— Closely connected with the disturbances which have produced contortions come those by which the crust of the earth has been fractured. But in this case the movement is one of elevation rather than of subsidence; for, instead of having to occupy a diminished diameter, the rocks get more room by being pushed up, and as they cannot occupy the additional space by any elastic expansion of their mass, they can only accommodate themselves to the new position by a series of dislocations. Some portions will be pushed up farther than others, and this will happen more particularly to those which have a broad base. These will rise more than those with narrow bottoms, or the latter will seem to sink relatively to the former. Each broad-bottomed segment will thus be bounded by two sides sloping towards the upper part of the block. This is found to be almost invariably the case in nature. A fault or dislocation is nearly always inclined from the vertical, and the side to which the inclination rises, and from which it “hades,” is the upthrow side. The details of these features of geological structure are discussed in part iv., section v.
§ 3. Influence of Water.
In the great geological contest fought at the beginning of the century between the Neptunists and the Plutonists, the two great battle-cries were, on the one side, Water, on the other, Fire. The progress of the science since that time has shown that each of the parties had truth on its side, and had seized one aspect of the problems touching the origin of rocks. If subterranean heat has played a large part in the construction of the materials of the earth’s crust, water, on the other hand, has performed a hardly less important share of the task. They have often co-operated together, and in such a way that the result must be regarded as their joint achievement, wherein the respective share of each can hardly be exactly apportioned. In the following brief résumé of this subject we shall consider the changes produced by pure water, by water charged with substances in solution, and by water raised above ordinary temperatures.
By numerous observations it has been proved that all rocks within the accessible portion of the earth’s crust contain interstitial water, or, as it is sometimes called, quarry-water (eαu-de-carriere). This is not chemically combined with their mineral constituents, but merely retained in their pores. Most of it evaporates when the stone is taken out of the parent rock and freely exposed to the atmosphere. The absorbent powers of rocks vary greatly, and chiefly in proportion to their degree of porosity. Gypsum absorbs from about 0 ’ 50 to 1 ’ 50 per cent, of water by weight; granite, about 0 ’ 37 per cent.; quartz from a vein in granite, 0 ’ 08; chalk, about 20Ό; plastic clay, from 19 ’ 5 to 24 ’ 5. These amounts may be increased by exhausting the air from the specimens and then immersing them in water.
The water enclosed within the interstices and crystals of igneous rocks may be either an original constituent, deriving its origin, like any of the component minerals, from molten reservoirs within the earth’s crust, or it may have descended from the surface to the incandescent rocks. Many facts may be adduced in support of the greater probability of the second view. Besides the general proximity of volcanic orifices to large sheets of water, we have abundant evidence of the actual descent of water from the surface, both through fissures, and also by permeation through the solid substance of rocks. All surface rocks contain water, and no mineral substance is strictly impervious to the passage of liquid. The well-known artificial colouring of agates proves that even the mineral substances apparently most homogeneous and impervious can be traversed by liquids. Μ. Daubrée has instituted a series of experiments to illustrate the power possessed by water of penetrating rocks, in virtue of their porosity and capillarity, even against a considerable counterpressure of vapour; and, without denying the presence of original water, he concludes that the interstitial water of igne0u3 rocks may all have been derived by descent from the surface.
The presence of interstitial water must affect the chemical constitution of rocks. It is now well understood that there is probably no terrestrial substance which, under proper conditions, is not to some extent soluble in water. By an interesting series of experiments, made many years ago by Messrs Rogers, it was ascertained that many of the ordinary mineral constituents of rocks could be dissolved to an appreciable extent even by pure water, and that the change was accelerated and augmented by the presence of carbonic acid. Silica, alkaliferous silicates, and iron 0xide3 can be taken up and held in solution by pure water, even at ordinary temperatures, in considerable quantities.
The mere presence of pure water therefore within the pores of subterranean rocks cannot but give rise to changes in the composition of these rocks. Some of the more soluble materials must be dissolved, and, as the water evaporates, must be redeposited in a new form.
But water in a natural state is never chemically pure. In its descent through the air it absorbs oxygen and carbonic acid, besides other impurities (see p. 267), and as it filters through the soil it abstracts more carbonic acid, as well as other results of decomposing organic matter. It is thu3 enabled to effect numerous decompositions of the rocks underneath. The nature of these changes may be inferred from the composition of spring water, to which reference will subsequently be made (p. 270). For the present it will be sufficient to remark that two important kinds of chemical decomposition must evidently arise from the action of such infiltrating water. (1.) The presence of the organic matter must exercise a reducing power on oxides. This will be more especially the case with those of iron, the nearly insoluble haematite being reduced to the protoxide, which, converted into carbonate, is readily removable in solution. There can be little doubt that by this means a vast amount of ferruginous matter is extracted from subterranean rocks and carried to the surface. (2.) The presence of carbonic acid enables the water to attack vigorously the mineral constituents of rocks. Alkaline carbonates, with carbonates of lime and magnesia, and protoxides of iron and manganese, are produced, and these substances borne onward in solution give rise to further reactions among the rocks through which they are carried. “In the decomposition of rocks,” says Bischof, “carbonic acid, bicarbonate of lime, and the alkaline carbonates bring about most of the decompositions and changes in the mineral kingdom.”
The microscopic study of rocks has thrown much light upon the mineralogical alterations in rocks due to the influence of percolating water. Even the most solid-looking, unweathered rocks, are found to have been affected by such metamorphism. Their hydrous magnesian silicates, for example, are partially or wholly converted into such hydrous forms as serpentine, chlorite, or delessite. The process of conversion may often be watched. It can be seen to have advanced along the fissures or cleavage-planes of the minerals leaving the intervening sections still fresh; or it may be observed to have proceeded in such a way that diffused alteration-products are dispersed in filaments or irregular patches through the base of the rock, or gathered together and even recrystallized in cavities; or the whole rock, as in many serpentines, has undergone an entire transformation. Much information regarding such internal alterations of rocks may be obtained from the study of pseudomorphs, that is, crystals having the external form of the mineral of which they originally consisted, with the internal structure and composition of the mineral which has replaced it. Serpentine representing olivine, clay taking the place of rock-salt, silica that of wood, and marcasite that of molluscan shell, are familiar examples. There is no reason to doubt that these changes may, in the course of ages, have been effected at ordinary temperatures by water descending from the surface of the ground.
But two other considerations require to be taken into account in the discussion of the internal transformations of rocks by subterranean water. (1.) In the first place, the water has often been at a high temperature. Mere descent into the crust of the earth will raise the temperature of the water until, if this descent be prolonged, a point far above 212° Fahr, may be reached. Experiments have shown that the chemical action of water is vastly increased by heat. Thus M. Daubrée exposed a glass tube containing about half its weight of water to a temperature of about 400° C. At the end of a week he found the tube so entirely changed into a white, opaque, powdery mass as to present not the least resemblance to glass. The remaining water was highly charged with an alkaline silicate containing 63 per cent, of soda and 37 per cent, of silica, with traces of potash and lime. The white solid substance was ascertained to be composed almost entirely of crystalline materials. These consisted partly of minute perfectly limpid bipyramidal crystals of quartz, but chiefly of very small acicular prisms of wollastonite. It was found, moreover, that the portion of the tube which had not been directly in contact with the water was as much altered as the rest, whence it was inferred that at these high temperatures and pressures the vapour of water acts chemically like the water itself. (2.) In the second place, the effect of pressure must be recognized as most important in enabling water, especially when heated, to dissolve and retain in solution a larger quantity of mineral matter than it could otherwise do. In M. Daubrée’s experiments just cited, the tubes were hermetically sealed and secured against fracture, so that the pressure of the greatly super-heated vapour had full effect. By this means, with alkaline water, he not only produced the two minerals above mentioned, but also felspar and diopside.
It is important to observe that the three conditions required for these changes—the presence of alkaline water, a high temperature, and considerable pressure—are precisely those which it can be affirmed must exist abundantly within the crust of the earth. We must admit the possibility of rocks originally at the surface being depressed so as to come within the influence of internal heat, and to contain within their pores abundant interstitial water more or less charged with alkaline carbonates. Rocks under these conditions, so far as we can judge, can hardly escape internal decomposition and recomposition. Mere descent to a great depth beneath, the surface will not necessarily result in metamorphism, as has been shown in the case of the Nova Scotian and of the South Welsh coal-field, where sandstones, shales, clays, and coal-seams can be proved to have been once depressed 14,000 to 17,000 feet below the sealevel, under an overlying mass of rock, and yet to have sustained no serious alteration. Perhaps the failure of change may be explicable on the supposition that these Carboniferous strata were comparatively dry. But where rocks possess sufficient interstitial water, and are depressed within the crust so as to be exposed to a considerable temperature and to great pressure, they must be metamorphosed,—the extent of the metamorphism depending partly upon the vigour of the attack made upon them by the water, partly on their own composition and proneness to chemical change, and partly upon the length of time during which the process is continued.
A metamorphosed rock must thus be one which has suffered a mineralogical rearrangement of its substance. It may or may not have been a crystalline rock originally. Any rock capable of alteration (and all rocks must be so in some degree) will, when subjected to the required conditions, become a metamorphic rock. The resulting structure, however, will, in most cases, bear witness to the original character of the mass. A sedimentary rock, for example, consisting of alternate layers of different texture and composition, will doubtless retain, even in its metamorphosed condition, traces of that fundamental structure. The water will travel more easily along certain layers than along others; some laminae will be more readily affected, or will give rise to a set of reactions different from those of contiguous layers. Hence the rearrangement and recrystallization due to metamorphism will take place along the predetermined lines of stratification, so long as these lines have hot been effaced or rendered inoperative by any other geological structure. It is doubtless to this cause that the foliated character of gneiss, mica-schist, and so many other metamorphic rocks is to be ascribed.
In the process of metamorphism, therefore, as well as in that of fusion, to which reference has already been made, the influence of water would seem to have been always conspicuous. Indeed, as will be shown in part iv., it is extremely difficult in many cases to draw a line between the results of metamorphism and igneous fusion, or to decide whether a rock should be called igneous or metamorphic. It has been pointed out above, for example, that in many rocks which have undoubtedly been in a fluid condition, as proved by their injected veins and dykes, the constituent minerals have not appeared in the order of their respective fusibilities. Scheerer, Élie de Beaumont, and Daubrée have shown how the presence of a comparatively small quantity of water in such rocks has contributed to suspend their solidification, and to promote the crystallization of their silicates at temperatures considerably below the point of fusion. In this way the solidification of quartz in granite after the crystallization of the silicates, which would be unintelligible on the supposition of mere dry fusion, becomes explicable. The phenomena of metamorphism in the architecture of the earth’s crust are discussed in part iv.
DIVISION II—EPIGENE OR SURFACE ACTION
It is on the surface of the globe and by the operation of agents working there that at present the chief amount of visible geological change is effected. In considering this branch of inquiry, we are not involved in the same preliminary difficulty regarding the very nature of the agencies as we found to be the case in the investigation of plutonic action. On the contrary, the surface agents are carrying on their work under our very eyes. We can watch it in all its stages, measure its progress, and mark in many ways how accurately it represents similar changes which for long ages previously must have been effected by the same means. But in the systematic treatment of this subject we encounter a difficulty of another kind. We discover that while the operations to be discussed are numerous and often complex, they are so interwoven into one great network that any separation of them under different subdivisions is sure to be more or less artificial, and to convey an erroneous impression. While, therefore, under the unavoidable necessity of making use of such a classification of subjects, we must bear always in mind that it is employed merely for convenience, and that in nature superficial geological action must be continually viewed as a whole, since the work of each agent has constant reference to that of the others, and is not properly intelligible unless that connexion be kept in view.
The movements of the air; the evaporation from laud and sea; the fall of rain, hail, and snow; the flow of rivers and glaciers; the tides, currents, and waves of the ocean; the growth and decay of organized existence, alike on land and in the depths of the sea;—in short, the whole circle of movement, which is continually in progress upon the surface of our planet, are the subjects now to be examined. It would be desirable to adopt some general term to embrace the whole of this range of inquiry. For this end the word epigene may be suggested as a convenient term, and antithetical to hypogene or subterranean action.
The simplest arrangement of this part of Geological Dynamics will be into three sections :—
I. Air. —The influence of the atmosphere in destroying aud forming rocks.
II. Water. —The geological functions of the circulation of water through the air and between sea and land, and the action of the sea.
III. Life. —The part taken by plants and animals in preserving, destroying, or reproducing geological formations.
The words destructive, reproductive, and conservative, employed in describing the operations of the epigene agents, do not necessarily imply that anything useful to man is destroyed, reproduced, or preserved. On the contrary, the destructive action of the atmosphere may turn barren rock into rich soil, while its reproductive effects sometimes turn rich land into barren desert. Again, the conservative influence of vegetation has sometimes for centuries retained as barren morass what might otherwise have become rich meadow or luxuriant woodland. The terms, therefore, are used in a strictly geological sense, to denote the removal and re-deposition of material, and its agency in preserving what lies beneath it.
Section I.—Air.
Its composition having been already treated of {ante, p. 220), we shall consider here (I) the motions, and (2) the geological action of the air, which arises partly from its composition, and partly from its movements.
I. Movements of the Air.
These are due to differences in the pressure or density of the atmosphere, the law being that the air always moves from where the pressure is high to where it is low. Atmospheric pressure is understood to be determined by two causes, temperature and aqueous vapour.
1. Temperature.— Warm air, being less dense than cold air, ascends, while the latter flows in to take its place. The unequal heating of the earth’s surface, by causing upward currents from the warmed portions, produces horizontal currents from the surrounding cooler regions inwards to the central ascending mass of heated air. To this cause the trade winds and the familiar land and sea breezes are due.
2. Aqueous Vapour.— In proportion as the quantity of watery vapour increases, the density of the air lessens. Consequently moist air tends to rise as warmed air does, with a corresponding but often very violent inflow of the drier and consequently heavier air from the surrounding tracts. The ascent of the moist air lessens the atmospheric pressure, which is indicated by the fall of the barometer. When the up-streaming vapour rises into the higher regions of the atmosphere, it expands and cools, condensing into visible form, and descending in copious showers to the earth. Unequal and rapid heating of the air, or accumulation of aqueous vapour in the air, and possibly some other influences not yet properly understood, give rise to extreme disturbances of pressure, and consequently to storms and hurricanes. For instance, the barometer sometimes indicates in tropical storms a fall of an inch and a half in an hour, showing that somewhere about a twentieth part of the whole mass of the atmosphere has in that short space of time been displaced over a certain area of the earth’s surface. No such sudden change can occur without resulting in the most destructive tempest or tornado. In Britain the tenth of an inch of barometric fall in an hour is regarded as a large amount, such as only accompanies great storms. When the pressure of the air at one place is shown by the barometer to differ from that at a neighbouring locality at the same time, the wind will be observed to move on the whole from the area of high to the area of low pressure; and if the difference be great or sudden, the movement of the air may rise to the force of a hurricane until the equilibrium of pressure is restored.
The meteorological conditions of the atmosphere do not belong to the scope of this article (see Atmosphere, Climate, Meteorology). The reader, however, may note as of interest from a geological point of view the ascertained velocity aud pressure exercised by the air in motion across the surface of the earth as expressed in the subjoined table :— [table]
II. Geological Influences of the Air.
The paramount importance of the atmosphere as the vehicle for the circulation of temperature and moisture over the globe, and consequently as powerfully influencing the distribution of chmate and the growth of plants and animals, must be fully recognized by the geologist. Attention will be confined at present to the direct changes produced on the surface of the earth by the air—(1) on land, and (2) on water.
1. Its Influence on Land.
I. Destructive Influences. —These are either (a) chemical or {β) mechanical, though in nature the two kinds of action are often inseparably interwoven.
(a) Under the denomination of chemical changes we include the oxidation of those minerals which can contain more oxygen, as in the peroxidation and precipitation of protosalts of iron; likewise the absorption of carbonic acid by rocks, and the production of alkaline and earthy carbonates and bicarbonates, which still further promote the process of decomposition. In the one case the active agent of change is the oxygen of the air, or rather of the aqueous vapour in the air, for perfectly dry air seems to have little or no oxidizing effect. A familiar illustration is afforded by the rust, or oxide, which forms on iron when exposed to moisture, though this iron may be kept long bright if allowed to remain screened from moist air. In the other case, the active agent is the carbonic acid of the air, though here again it appears to be requisite that moisture should intervene as the medium of introducing the acid to the substance which is to be altered by it. The occurrence of sulphuric and nitric acids in the air, especially noticeable in large towns, likewise leads to considerable corrosion of metallic surfaces, as well as of stones and lime. The mortar of walls may often be observed to be slowly swelling out and dropping off, owing to the conversion of the lime into sulphate. Great injury is likewise done from a similar cause to marble monuments in exposed graveyards.
As a rule, the changes effected by the air lead to many subsequent transformations. For example, the oxidation of the bisulphide of iron produces sulphuric acid, which decomposes silicates, carbonates, and other compounds with which it comes in contact. These changes, however, are more appropriately noticed under the head of rain (p. 267).
{β) Among the more recognizable mechanical changes of a destructive kind, brought about by the atmosphere, we may notice the following influences :—
1. Expansion and Contraction.— The effect of heat is to expand rocks, of cold to contract them. Strictly speaking, these results on the surface of the earth are due, not to the air, but to the heat-rays of the sun which reach the rocks through the air. In countries with a great annual range of temperature considerable difficulty is sometimes experienced in selecting building materials liable to be little affected by the alternate expansion and contraction which prevents the joints of masonry from remaining close and tight. In the United States, for example, with an annual thermometric range of more than 90° Fahr, this difficulty led to some experiments by Colonel Totten on the amount of expansion and contraction in different kinds of buildingstones, caused by variations of temperature. It was found that in fine-grained granite the rate of expansion was Ό00004825 for every degree Fahr, of increment of heat; in white crystalline marble it was Ό00005668; and in red sandstone Ό00009532, or about twice as much as in granite. If the daily variations in temperature are large, the effects are still more striking. In tropical climates with intensely hot days and extremely cold nights, the rapid nocturnal contraction produces sometimes a strain so great as to rival frost in its influence upon the surface of exposed rocks, disintegrating them into sand, or causing them to crack or peel off in skins or irregular pieces. Dr Livingstone found in Africa (12° S. lat., 34° E. long.) that surfaces of rock which during the day were heated up to 137° Fahr, cooled so rapidly by radiation at night that, unable to sustain the strain of contraction, they split and threw off sharp angular fragments from a few ounces to 100 or 200 ft> in weight.
2. Frosh— Though properly belonging to the subject of the geological behaviour of water to be afterwards described in more detail, the disintegrating action of frost may be noticed here. In freezing water expands, and thereby exerts an enormous strain upon any enclosed cavities or walls which may confine it. The consequence of this action is that in countries exposed to frost a continual disintegration of the surface of rocks goes on. This superficial decay combines with the chemical and mechanical operations of the atmosphere to produce considerable modifications in the forms of rocks and cliffs.
3. Wind.— By driving loose sand over rocks, prevalent winds produce on them a scratched and polished surface, as has happened with ancient monuments buried in the sands of the African deserts. It is said that at Cape Cod holes have even been drilled in window glass by the same agency. Cavities are now and then hollowed out of rocks by the gyration in them of little fragments of stone or grains of sand kept in motion by the wind. Hurricanes form important geological agents upon land in uprooting trees, and thus sometimes impeding the drainage of a country, and giving rise to the formation of peat mosses.
Weathering of Rocks.— Under the term “weathering” are included all the superficial changes which rocks undergo in consequence of the action of atmospheric processes upon them. The nature and rapidity of the disintegration depend partly on the one hand upon the climate, and partly on the other upon the composition, texture, and exposure of the rocks. In very dry countries, where the range of temperature is not extreme, weathering is reduced to a minimum. But even if the climate be dry, considerable disintegration may be caused, as has been already explained, by rapid changes of temperature between day and night. It is where moisture prevails, however, that weathering chiefly takes place. The nature of the changes will be more properly considered in the section which is devoted to the action of rain.
II. Reproductive Influences. —These arise partly from the result of the chemical and mechanical disintegration involved in weathering, and partly from the transporting power of winds and aerial currents. Under the former head is the formation of soil; under the latter may be noticed the production of sand-hills, the fall of dust-showers and coloured rain, and the transport of seeds.
Soil.— Of the detritus produced by the action of the air on rocks, and washed away by rains and streams, part remains on the land and forms soil. All soil may be considered as the result of the decomposition of rocks, mingled with decayed vegetable and animal matter. Were it not for the action of rain in washing the loose materials to a greater or less distance from their source, the soil of every locality ought to be merely the decayed upper surface of the rocks underneath. But wherever rain falls, the soil is moved from higher to lower levels. Hence in some cases a good soil is laid down upon rocks which of themselves would only produce a poor one. This action of rain in the formation of soil is further alluded to on p. 270, and the co-operative influence of plant and animal life on p. 289.
Sand-hills or Dunes.— Winds blowing continuously upon loose materials, such as sand, drive them onward, and pile them into irregular heaps and ridges, called “dunes.” This takes place more especially on windward coasts either of the sea or of large inland lakes, where the shores are sandy; but similar effects may be seen even in the heart of a continent, as in the sandy deserts of the Sahara and of Arabia. The dunes travel inland in parallel, irregular, and often confluent ridges, between which rain-water is sometimes arrested to form pools {étangs of the French coasts), where formations of peat occasionally take place. On the coast of Gascony the sea for 100 miles is so barred by sand-dunes that in all that distance only two outlets exist for the discharge of the drainage of the interior. As fast as one ridge is driven away from a beach another forms in its place, so that a series of huge sandy billows, as it were, is continually on the move from the sea margin towards the interior. A stream or river may temporarily arrest their progress, but eventually they push the obstacle aside or in front of them. In this way the river Adour, on the west coast of France, has had its mouth shifted two or three miles. Occasionally, as at the mouths of estuaries, the sand is blown across so as gradually to exclude the sea, and thus to aid the fluviatile deposits in adding to the breadth of the land. The coast of Norfolk is fringed with sand-hills 50 to 60 feet high. On parts of the coast of Cornwall, the sand consists mainly of fragments of shells and corallines, and through the action of rain becomes sometimes indurated into a compact stone by carbonate of lime or oxide of iron. Long tracts of blown sand are likewise found along many parts of the Scottish and Irish coast-lines.
On the western border of the European continent extensive sand-dunes exist. They extend for many leagues along the French coast, and thence, by Flanders and Holland, round to the shores of Courland and Pomerania. In Denmark they are said to cover an area of 260 square miles. On the coast of Holland they are sometimes, though rarely, 260 feet high,—a common average height being 50 to 60 feet. The breadth of this maritime belt of sand varies considerably. On the east coast of Scotland it ranges from a few yards to 3 miles; on the opposite side of the North Sea it attains on the Dutch coast sometimes to as much as 5 miles. The rate of progress of the dunes towards the interior depends upon the wind, the direction of the coast, and the nature of the ground over which they have to move. On the low and exposed shores of the Bay of Biscay, when not fixed by vegetation, they travel inland at arate of about 16⅛ feet; in Denmark at from 3 to 24 feet per annum. In the course of their march they envelop houses and fields; even whole parishes and districts once populous have been overwhelmed by them.
Along the margins of large lakes and inland seas many of the phenomena of an exposed sea-coast are repeated, and on no inferior scale. Among these must be included sand-dunes, such as occur at the south-eastern end of Lake Michigan and on the eastern borders of the Caspian Sea. The shifting of vast waves of sand by the wind is exemplified on the grandest scale in the sandy deserts of Africa, Arabia, and Central Asia. Such arid wastes of loose sand, situated far inland and far distant from any sheet of fresh water, suggest curious problems in physical geography. Their sites may have been at a comparatively recent geological period covered by the sea; or, lying in rainless climates and having their surfaces exposed to the disintegrating effects of great extremes of temperature, the tracts may have become sandy and barren through atmospheric disintegration. The desert of the Sahara furnishes a good illustration of a dried-up sea-bed. In the rainless tract to the east of the Red Sea lie the great sandy deserts and hills of Arabia, of which Mr Palgrave has given so graphic a narrative. Captain Sturt found vast deserts of sand in the interior of Australia, with long lines of dunes 200 feet high, united at the base and stretching in straight lines as far as the eye could reach. In the south-east of Europe great tracts of sandy desert occur in Poland, and run through the southern provinces of Russia.
Dust-showers, Blood-rain.— In tropical countries, where great droughts are succeeded by violent hurricanes, the dust or sand of dried lakes or river-beds is sometimes borne away into the upper regions of the atmosphere, where, meeting with strong aerial currents which transport it for hundreds and even thousands of miles, it may descend again to the surface, in the form of “red-fog,” “sea-dust,” or “sirocco-dust.” This transported material, usually of a brick-dust or cinnamon colour, is occasionally so abundant as to darken the air and obscure the sun, and to cover the decks, sails, and rigging of vessels which may even be hundreds of miles from land. Rain falling through such a dust-cloud mixes with it, and descends either on sea or land as what is popularly called “blood-rain.” This is frequent on the north-west of Africa, about the Cape Verd Islands, in the Mediterranean, and over the bordering countries. A microscopic examination of this dust by Ehrenberg led him to the belief that it contains numerous diatoms of South American species; and he inferred that a dust-cloud must be swimming in the atmosphere, carried forward by continuous currents of air in the region of the trade-winds and anti-trades, but suffering partial and periodical deviations. But much of the dust must come from the sandy plains and desiccated pools of the north of Africa. Daubrée recognized in 1865 some of the Sahara sand which fell in the Canary Islands. On the coast of Italy a film of sandy clay, identical with that in parts of the Libyan desert, is occasionally found on windows after rain. In the middle of last century an area of northern Italy, estimated at about 200 square leagues, was covered with a layer of dust which in some places reached a depth of one inch. Should the travelling dust encounter a cooler temperature, it may be brought to the ground by snow, as has happened in the north of Italy, and more notably in the east and south-east of Russia, where the snows are sometimes rendered dirty by the dust raised by the winds on the Caspian steppes. It is easy to see that a prolonged continuance of this action must give rise to widespread deposits of dust, mingled with the soil of the land, and with the silt and sand of lakes, rivers, and the sea; and that the minuter organisms of tropical regions may thus come to be preserved in the same formations with the terrestrial or marine organisms of temperate latitudes. Transportation of Seeds.— Besides the transport of dust and minute organisms for distances of many thousands of miles, the same agency may come into play also in the transport of living seeds, which, finally reaching a congenial climate and soil, may take root and spread. We are yet, however, very ignorant as to what extent this cause has actually operated in the establishment of any given local flora. With regard to the minute forms of vegetable life, indeed, there can be no doubt as to the efficacy of the wind to transport them across vast distances on the surface of the globe. L τ pwards of 300 species of diatoms have been found in the deposits left by dust-showers. Among the millions of organisms thus transported it is hardly conceivable that some should not fall into a fitting locality for their continued existence and the perpetuation of their species.
2. Influence of the Air on Water.
The action of the air upon water will be more fitly noticed in the section devoted to water (p. 285). It will be enough to notice here—
1. Ocean Currents.— The in-streaming of air from cooler latitudes towards the equator causes a drift of the sea-water in the same direction. Owing to the rotation of the earth, these aerial currents tend to take a more and more westerly trend as they approach the equator. This they communicate to the marine currents, which, likewise moving into regions having a greater velocity of rotation than their own, are all the more impelled in the same westerly direction. Hence the westerly belt or equatorial current, which flows across the great ocean. Owing to the position of the continents across its path, this great current cannot move uninterruptedly round the earth. It is split into branches which turn to right and left, and, bathing the shores of the land, carry some of the warmth of the tropics into more temperate latitudes.
2. Waves.— The impulse of the wind upon a surface of water throws that surface into pulsations which range in size from mere ripples to huge billows. Long-continued gales from the seaward upon an exposed coast indirectly effect much destruction, by the formidable battery of billows which they bring to bear upon the land. Wave-action is likewise seen in a marked manner when wind blows strongly across a broad inland sheet of water, such as Lake Superior. (See p. 279.)
3. Alteration of the Water-level.— When the wind blows freshly for a time down a lake or into a bay or arm of the sea, it drives the water before it, and keeps it temporarily at a higher level, at the further or windward side. In a tidal sea, such as that which surrounds Great Britain, and which sends abundant long arms into the land, this action can often be studied. It is no infrequent occurrence that a high tide and a gale should happen at the same time. Whenever that takes place, then at those bays or firths which look windward the high tide rises to a greater height than elsewhere. With this conjunction of wind and tide, considerable damage to property has sometimes been done by the flooding of warehouses and stores, while even a sensible destruction of cliffs and sweeping away of loose materials may be chronicled by the geologist. On the other hand, a wind from the opposite quarter will drive the water out of the inlet, and thus make the water-level lower than it should otherwise be.
Section II,—Water.
Of all the terrestrial agents by which the surface of the earth is geologically modified, by far the most importantis water. When following hypogene changes in a foregoing part of this article, we found how large a share is taken by water in the phenomena of volcanoes and other subterranean movements. When we returned to the surface of the earth and began to watch the operations of the atmosphere, we saw how impossible it is to consider these apart from the action of the aqueous vapour by which the atmosphere is pervaded. We must now study in detail the working of this wonderful geological agent itself.
The substance which we term water exists on the earth in three well-known forms — (1) gaseous, as invisible vapour; (2) liquid, as water; and (3) solid, as ice. The gaseous form has already been noticed in our inquiry into the geological characteristics of the air. It is in the air that this condition of the water-substance prevails. By the sun’s heat vast quantities of vapour are continually raised from the surface of the seas, rivers, lakes, snow-fields, and glaciers of the world. This vapour remains invisible until the air containing it is cooled down to below its dew-point, or point of saturation,—a result which follows upon the union or collision of two aerial currents of different temperatures, or the rise of the air into the upper cold regions of the atmosphere, where it is chilled by expansion, by radiation, and by contact with cold mountains. At first minute particles appear, which either remain in the liquid condition, or, if the temperature is sufficiently low, are at once frozen into ice. As these changes take place over considerable spaces of the sky, they give rise to the phenomena of clouds. Further condensation augments the size of the cloud-particles, and at last they fall to the surface of the earth, if still liquid, as rain; if solid, as snow or hail; and if partly solid and partly liquid, as sleet. As the vapour is largely raised from the ocean surface, so in great measure it falls back again directly into the ocean. A considerable proportion, however, descends upon the land, and it is this part of the condensed yapour which we have now to follow. Upon the higher elevations it falls as snow, and gathers there into snow-fields, which, by means of glaciers, send their drainage down towards the valleys and plains. Elsewhere it falls chiefly as rain, some of which sinks underground to gush forth again in springs, while the rest pours down the slopes of the land, feeding brooks and torrents, which, swollen further by the springs, gather into broader and yet broader rivers, whereby the drainage of the land is carried out to sea. Thence once more the vapour rises to reappear in clouds, and feed the innumerable water-channels by which the land is furrowed from mountain-top to sea-shore.
Here then is a vast system of circulation, ceaselessly renewed. And in that system there is not a drop of water which is not busy with its allotted task of changing the face of the earth. When the vapour ascends into the air it is almost chemically pure. But when, after being condensed into visible form, and working its way over or under the surface of the land, it once more enters the sea, it is no longer pure, but more or less loaded with material taken by it out of the air, rocks, or soils through which it has travelled. Day by day the process is advancing. So far as we can tell, it has never ceased since the first shower of rain fell upon the earth. We may well believe, therefore, that it must have worked marvels upon the surface of our planet in past time, and that it may effect vast transformations in the future. As a foundation for such a belief let us now inquire what it can be proved to be doing at the present time.
The subject of the geological operation of water upon the globe maybe conveniently studied under the following subdivisions :—
A. Terrestrial Waters. —Under this head are to be considered—(1) the liquid state, including rain, underground water, brooks, rivers, and lakes; and (2) the solid state—frost, river-ice, snow, hail, glaciers.
B. Oceanic Waters. —Including the influence of marine currents, tides, and waves, and the part taken by the sea in the general geological regime of the earth.
A. Terrestrial Waters.
I. IN THE LIQUID STATE.
§ 1. Pain.
Rain effects two kinds of changes upon the surface of the land. (1.) It acts chemically upon soils and stones, and sinking under ground continues, as we shall find, a great series of similar reactions there. (2.) It acts mechanically, by washing away loose materials, and thus powerfully affecting the contours of the land.
I. Chemical Action. —This depends mainly upon the nature and proportion of the substances abstracted by rain from the air in its descent to the earth. Rain always absorbs a little air, and as we have already seen (p. 220) air always contains carbonic acid as well as other ingredients, in addition to its nitrogen and oxygen. If rain be regarded as an agent washing the air and taking impurities out of it, we may the better realize how by means of these it is enabled to work many chemical changes which, were it to reach the earth as pure water, it could not accomplish.
Composition of Pain-Water.— Numerous analyses of rain-water show that it contains in solution about 25 cubic centimetres of gases per litre. An average proportional percentage is by measure—nitrogen, 66’4; oxygen, 31'2; carbonic acid, 2 - 4,—the oxygen being in greater proportion than in air, owing to its greater solubility in water. Common salt, ammonia, sulphates, nitric acid, inorganic dust, and organic matter are usually present in minute quantities in rain water. So far as we know at present, the three ingredients which are chiefly effective in the chemical reactions due to rain are the oxygen, carbonic acid, and organic matter. Permeability of all Pocks by Water.— Though minerals and rocks differ vastly in their degree of porousness, there is none known which is not in some degree permeable by water. Even such hard and apparently impenetrable substances as flint and agate are found to be permeable. For, in fact, rocks and minerals when examined with the microscope are seen to be made up of variously-shaped grains, crystals, or particles, and it is in the minute channels and interstices between these particles, or even through the particles themselves, that the water works its way. Evidently, the smaller the interstices the less easily will the water force a passage into or through the stone. This permeability, though well marked upon the surface of the land, becomes still more so underground, where the rocks are sometimes quite saturated with water.
Liability of all Pocks to alteration by Water.— There is probably no known substance which is not, under some condition, soluble in water containing carbonic acid or other natural reagent. Rain-water, descending with the gases, acids, and organic matter it has abstracted from the air and soil, effects a chemical disintegration of the rocks. This action was referred to in the description of the air as partly due to atmospheric moisture, but it is chiefly carried on by rain. And as rain is so widely and almost universally distributed over the globe, this chemical action must be of very general occurrence.
Nature of the Changes effected.— Confining our attention to its three chiefly active ingredients, we find that rain water reacts chemically upon rocks by—1. Oxidation.— The prominence of oxygen in rain-water, and its readiness to unite with any substance which can contain more of it, render this a marked feature of the passage of rain over rocks. A thin oxidized pellicle is formed on the surface, and this, if not at once washed off by the rain, sinks deeper until a crust is formed over the stone. As already remarked, this process is simply a rusting of those minerals which, like metallic iron, have no oxygen, or have not their full complement of it. 2. Deoxidation.— Organic matter having an affinity for more oxygen decomposes peroxides by depriving them of some part of their share of that element, and reducing them to protoxides. These changes are especially noticeable among the iron oxides so abundantly diffused among rocks. Hence rain-water, in sinking through soil and obtaining such organic matter, becomes thereby a reducing agent. 3. Solution.— This may take place either by the simple action of the water, as in the solution of rocksalt, or by the influence of the carbonic acid present in the rain. Of the latter (Carbonation) a familiar example is the corrosion of marble slabs down which rain has trickled for a time. The carbonic acid dissolves some of the lime, which as a bicarbonate is held in solution in the carbonated water, but is deposited again when the water loses its carbonic acid or evaporates. It is not merely carbonates, however, which are liable to this kind of destruction. Even silicates of lime, potash, and soda, combinations existing abundantly as constituents of rocks, are attacked; their silica is liberated, and their alkalies or alkaline earths, becoming carbonates, are removed in solution. 4. Hydration.— Some minerals, containing little or no water, and therefore called anhydrous, when exposed to the action of the atmosphere, absorb water, or become hydrous, and are then usually more prone to further change. Hence the rocks of which they form part become disintegrated.
Weathering.— The weathering of rocks is dependent upon two sets of conditions—(1) meteorological, as the range of temperature, abundance of moisture, height above the sea, and exposure, and (2) lithological,—the composition and texture of the rocks themselves. As regards the composition of rocks, those which consist of particles liable to little chemical change from the influence of moisture are best fitted to resist weathering, provided their particles have sufficient cohesion to withstand the mechanical processes of disintegration. Siliceous sandstones are excellent examples of this permanence. Consisting wholly or mainly of the durable mineral quartz, they are sometimes able so to withstand decay that buildings made of them still retain, after the lapse of centuries, the chisel-marks of the builders. Some rocks which yield with comparative rapidity to the chemical attacks of moisture show no marks of disintegration on their surface, which remains clean and fresh. This is particularly the case with limestones. The reason lies obviously in the fact that limestone when pure is wholly soluble in acidulous water. Rain falling on this rock removes some of it in solution, and will continue to do so until the rock is dissolved away. It is only where the limestone contains impurities that a weathered crust of more or less insoluble particles remains behind. Hence the relative purity of limestones may be roughly determined by comparing their weathered surfaces, where, if they contain much sand, the grains will be seen projecting from the calcareous matrix, and where should the rock be very ferruginous, the yellow hydrous peroxide or ochre will be found as a powdery crust. In limestones containing abundant encrinites, shells, or other organic remains, the weathered surface commonly presents the fossils standing out in relief. This seems to arise from the crystalline arrangement of the lime in the organic structures, whereby they are enabled to resist disintegration better than the general mechanically aggregated matrix of the rock. An experienced fossil collector will always search well those weathered surfaces, for he often finds there, delicately picked out by the weather, minute and frail fossils which are wholly invisible on a freshly broken surface of the stone. Many rocks weather with a thick crust or even decay inwards for many feet or yards. Basalt, for example, often shows a yellowish-brown ferruginous layer on its surface, formed by the conversion of its felspar into kaolin and the removal of its silicate of lime as carbonate, by the hydration of its olivine and augite and their conversion into serpentine, saponite, or some other hydrous magnesian silicate, and by the conversion of its magnetite into limonite. Granite sometimes shows in a most remarkable way the distance to which weathering can reach. It may often be dug into for a depth of 20 or 30 feet, the quartz crystals and veins retaining their original positions, while the felspar is completely kaolinized.
It is to the effects of weathering that the abundant fantastic shapes assumed by crags and other rocky masses are due. Most varieties of rock have their own characteristic modes of weathering, whereby they may be recognized even from a distance. To some of these features reference will be made in a subsequent section.
II. Mechanical Action. —When a rock has been so corroded by weathering that the cohesion of the particles on its exposed surface is destroyed, these particles are washed off by rain. This detritus is either held in suspension in the little runnels into which the rain-drops gather as they begin to flow over the land, or is pushed by them along the surface. In this way the rain carries off by mechanical movement what it has already loosened by chemical action.
III. Results of Rain-Action. —It is evident that the general result of the fall of rain upon a land-surface must be a disintegration and consequent lowering of that surface. At first we may be inclined to imagine that this waste must be so slow and slight as to be hardly appreciable. But a little observation will suffice to furnish many proofs of its existence and comparatively rapid progress in some places. We are familiar, for example, with the pitted channelled surface of the ground lying immediately under the drip of the eaves of a house. We know that the fragments of stone and gravel are left sticking up prominently because the earth around and above them has been washed away, and because, being hard, they resist the action of the falling drops and screen the earth below them. On a far larger scale we may notice the same kind of operation in districts of conglomerate, where the larger blocks, serving as a protection to the rock underneath, come to form as it were the capitals of slowly-deepening columns of rock. In the same way in certain valleys of the Alps a stony clay is cut by the rain into pillars, each of which is protected by, and indeed owes its existence to, a large block of stone which lay originally in the heart of the mass. These columns are of all heights, according to the positions in which the stones may have originally lain. There are instances, however, where the disintegration has been so complete ‘that only a few scattered fragments remain of a once extensive stratum, and where it may not be easy to realize that these fragments are not transported boulders. In Dorsetshire and Wiltshire, for example, the surface of the country is in some parts so thickly strewn with fragments of sandstone and conglomerate “that a person may almost leap from one stone to another without touching the ground. The stones are frequently of considerable size, many being four or five yards across, and about four feet thick.” They have been used for the huge blocks of which Stonehenge and other of the so-called druidical circles have been constructed, hence they have been termed Druid Stones. Other names are Sarsen Stones (supposed to indicate that their accumulation has been popularly ascribed to the Saracens), and Grey Wethers, from their resemblance in the distance to flocks of (wether) sheep. They are found lying abundantly on the chalk, suggestive at first of some former agent of transport by which they were brought from a distance. It is now, however, generally admitted that they are simply fragments of some of the sandy Tertiary strata which once covered the districts where they occur, and that while the softer portions of these strata have been carried away, the harder parts (their hardness perhaps increasing by exposure) have remained behind as Grey Wethers, and have subsequently suffered from the inevitable splitting and crumbling action of the weather.
But it is not from any single example, however striking, that the real importance of rain as a geological agent can be adequately realized. To form a true conception of this momentous action, we need to watch what takes place over a wide region. The whole land-surface over which rain falls is exposed to waste. As Hutton long ago insisted, the superficial covering of decayed rock or soil is constantly, though slowly, travelling outward and downward to the sea. In this ceaseless transport rain acts as the great carrying agent. The particles of rock loosened by atmospheric waste, by frost or the chemical disintegration of the rain itself, are washed off to form new soil. But they as well as the particles of the soil are step by step moved downward over the face of the land till they reach the nearest brook or river, whence their seaward progress may be rapid. A heavy rain discolours the water-courses of a country, because it loads them with the fine debris which it removes from the general surface of the land. In this way rain serves as the means whereby the work of the other disintegrating forces is made conducive to the general degradation of the land. The decomposed crust produced by weathering, which would otherwise accumulate over the solid rock and protect it from further decay, is removed by rain so as to expose a fresh surface to further decomposition. This decay is general and constant, but not uniform. In some places, from the nature of the rock, from the flatness of the ground, or from other causes, rain works under great difficulties. There the rate of waste must consequently be extremely slow. In other places, again, the rate may be rapid enough to be readily appreciable from year to year. A survey of this department of geological activity shows how the unequal wasting by rain has helped to produce the details of the present relief of the land, those tracts where the destruction has been greatest forming hollows and valleys, others, where it has been less, rising into ridges and hills.
Rain-action is not always merely destructive. Usually it is accompanied by reproductive effects, and, as already remarked, the mouldered rock which it washes off furnishes materials for the formation of soil. In favourable situations it has gathered together accumulations of loam and earth from neighbouring higher ground—the “brick-earth,” “head,” and “rain-wash” of the south of England—earthy deposits, sometimes full of angular stones, derived from the subaerial waste of the rocks of the neighbourhood.
§ 2. Underground Water.
When rain falls upon the land its further progress becomes twofold. The greater part of it sinks into the ground and apparently disappears; the rest flows off into runnels, brooks, and rivers, and in this way moves downward to the sea. It is most convenient to follow first the course of the subterranean water.
We have seen that all rocks are more or less porous. They are moreover traversed by abundant joints and cracks. Hence, from the bed of the ocean, from the bottoms of lakes and rivers, as well as from the surface of the land on which rain falls, water is continually filtering downward into the rocks beneath. To what depth this descent of the surface water may go is not known. It may reach as far as the intensely heated interior of the planet, for, as the researches of M. Daubrée have shown, capillary water has the capacity of penetrating rocks even against a high counter-pressure of vapour. The water at extreme depths may be under such pressure as to retain its liquid condition at a red or even at a white heat. Probably the depth to which the water descends varies indefinitely according, to the varying nature of the rocky crust. Some shallow mines are practically quite dry, while others of great depth require large pumping engines to keep them from being flooded by the water which pours into them from the surrounding rocks. As a rule, however, the upper layers of rock are fuller of moisture than those deeper down.
The water which in this way sinks into the earth is not permanently removed from the surface, though there may perhaps be a slight loss due to absorption and chemical alteration of the rocks. It accumulates underneath, until by the pressure of the descending column it is forced to find a passage through joints or fissures upward to the surface. The points at which it issues are termed springs. Iu most districts the rocks underneath are permeated with water below a certain limit which is termed the water-level. This line is not a strictly horizontal one like that of the surface of a lake. Moreover, it is liable to rise and fall according as the seasons are wet or dry. In some places it lies quite near, in others far below, the surface. A well is an artificial hole dug down below the water-level, into which the water percolates. Hence, when the water-level happens to be at a small depth the wells are shallow, when at a great depth they require to be deep.
Since the rocks underneath the surface vary greatly in porosity, some contain far more water than others. It often happens that, percolating along some porous bed, the subterranean water finds its way downward until it passes under some more impervious rock. Hindered in its progress, it accumulates in the porous bed, from which it may be able to find its way up to the surface again only by a tedious circuitous passage. If, however, a bore-hole be sunk through the upper impervious bed down to the water-charged stratum below, the water will eagerly avail itself of this artificial channel of escape, and will rise in the hole, or even rush up and gush out as ayei djeau above ground. Wells of this kind are now largely employed. They bear the name of Artesian, from the old province of Artois in France, where they have long been in use.
That the water really circulates underground, and passes not merely between the pores of the rocks but in crevices and tunnels which it has no doubt to a large extent opened for itself along numerous natural joints and fissures, is proved by the occasional rise of leaves, twigs, and even live fish, in the shaft of an Artesian well. Such evidences are particularly striking when found in districts without surface waters, and even perhaps with little or no rain. They have been met with, for instance, in sinking wells in some of the sandy deserts on the southern borders of Algeria. In these and similar cases it is clear that the water may, and sometimes does, travel for many leagues underground away from the district where it fell as rain or snow, or where it leaked from the bed of a river or lake.
The temperature of springs affords a convenient but not always quite reliable indication of the relative depth from which they have risen. Some springs are just one degree or less above the temperature of ice. Others in volcanic districts issue with the temperature of boiling water. Between these two extremes every degree may be registered. Very cold springs may be regarded as probably deriving their supply from cold or even snow-covered mountains. Certain exceptional cases, however, occur where ice forms in caverns (glacières) even in warm and comparatively low districts. Water issuing from these ice caves is of course cold. On the other hand, springs whose temperature is much higher than the mean temperature of the places at which they emerge must have descended far enough to be warmed by the internal heat of the earth. The hottest springs are found in volcanic districts. But even at a great distance from any active volcano, thermal springs, as they are called, appear with a temperature of 120° Fahr, (which is that of the Bath springs) or even more. These have probably risen from a great depth. If we could assume a progressive increase of 1° Fahr, of subterranean heat for every 60 feet of descent, the water at 120° issuing at a locality whose ordinary temperature is 50°, should have been down at least 4200 feet below the surface. But from what has been stated in a previous section (p. 224) regarding the irregular stratification of temperature within the earth’s crust, such estimates of the probable depth of the sources of springs are liable to various errors.
Apart from its vast importance in a social point of view, the underground circulation of water has a profound interest for the geologist, from the light which it affords as to the changes that rocks undergo, and the manner in which these changes are effected. For, like all the other geological agents, it does not move on its course without doing work. We have now to inquire what is the nature of that work. A convenient arrangement will be to group its study under two heads—(1) chemical action, and (2) mechanical action.
(1.) Every spring, even the most clear and sparkling, contains mineral matter in chemical solution, obtained from the rocks through which the water travels in its journey from the surface into the interior, and back to the surface again. The nature of the mineral ingredients depends partly upon the composition of the rocks traversed, partly upon the gases, acids, or other reagents which may have been present in the rain, or may have been obtained by the water in its subterranean journey, and partly upon the depth to which the water may have reached and the temperature to which it may have been raised.
We have already (ante, p. 267) considered the substances extracted by rain from the air and used by it in the disintegration of rocks. The same reagents are of course carried underneath the ground, when the rain-water sinks out of sight, and continue there the processes of decomposition and alteration which they are seen to effect at the surface. But other sources are open to the subterranean water for the augmentation of its chemical reagents. (1.) In descending through the soil the meteoric water encounters abundant organic matter, which abstracts its oxygen and replaces it by carbonic acid. This interchange probably in many cases far more than compensates for the expenditure of these gases employed in subaerial disintegration. In so far as the water carries down from the soil any oxidizable organic substance, its action must be to reduce the oxides it encounters among rocks. It is remarkable that ordinary vegetable soil possesses the power of removing from the water which permeates it potash, silica, phosphoric acid, ammonia, and organic matter, elements which had been already in great measure abstracted from it by living vegetation, and which are again taken up by the same organic agents. (2.) Carbonic acid gas is sometimes largely evolved within the earth’s crust, especially in regions of extinct or dormant volcanoes. Subterranean water coming in the way of this gas greedily dissolves it, and thereby obtains an enormously increased power of attacking even the most obdurate rocks. (3.) Whenever the water has its temperature considerably raised, its solvent capacity, especially for silica, is largely augmented. Hot springs often contain a large proportion of that substance in solution. (4.) The production of some of the compounds which are due to decompositions effected by the water, and are carried along with it in solution, increases its ability to accomplish further decompositions. Thus the alkaline carbonates, which are among the earliest products of the action of the water, enable it to dissolve silica and decompose silicates.
The study of these alterations belongs to the subject of Metamorphism, of which some account has already been given (ante, p. 262). Let us look at the results achieved by them, as shown in the composition of the water which issues from different springs. Considered from this point of view, springs may be treated as (1) common or ordinary springs, that is, those which contain only such average proportions of mineral matter as occur in ordinary potable water, and (2) mineral springs, or those where the proportion of foreign ingredients is large enough to give a marked character to the water. These two groups, however, merge insensibly into each other.
Common Springs.— The materials ordinarily present in common spring water are, besides atmospheric air and its gases, carbonate and sulphate of lime, common salt, with chlorides of calcium and magnesium, and sometimes organic matter. The amount of dissolved contents in ordinary drinking water does not exceed ’ 5 or at most 1Ό gramme per litre; the best waters contain even less.
Mineral Springs.— These may be roughly but conveniently classified according to the prevailing mineral substance contained in them, which may range in amount from 1 to 300 grammes per litre. Calcareous— containing so much lime that it is deposited as a white crust as the water evaporates. Spring water when saturated with carbonate of lime contains about 105 parts in 100,000. Springs of this kind are common in limestone countries. As the water flows away from its point of exit, it throws down a deposit of calcareous tufa or travertine, which, as it encrusts moss, twigs, and other objects, gives them the appearance of having been turned into stone, whence the springs are popularly termed “petrifying.” Enormous accumulations of this kind have been formed in some parts of Italy, where the rock so produced is extensively quarried as a building material.
Ferruginous or Chalybeate— containing a large proportion of iron in the total mineral ingredients. Such waters have an inky taste, and often deposit along their course a yellow, brown, or red ochry deposit, consisting mainly of hydrated peroxide of iron. They may be frequently observed in those districts where beds or veins of ironstone occur, or where the rocks contain much iron in combination.
Siliceous— depositing silica or flint. Although silica may be dissolved and retained in solution even in cold water, it is in the hot water issuing in volcanic countries that it occurs most abundantly, and where true siliceous springs exist. The geysers of Iceland, New Zealand, the Yellowstone region, and other districts are illustrations. When the heated water of these thermal springs cools and evaporates, the silica is deposited as siliceous sinter round their basins, or in picturesque mounds at the point of escape. One of the sinter beds in the geyser region of Iceland is said to be two leagues long, a quarter of a league wide, and a hundred feet thick. As already stated (ante, p.263), the effect of pressure is to enable water at great depths to retain a larger amount of mineral matter in solution. Hence, when the water ascends, it deposits its mineral contents, not only because it cools, but because the pressure is removed. There must in many cases be a copious deposit along the walls of the fissures up which the water flows on its way to the surface. Doubtless in this way many mineral veins have received their successive coatings of quartz, jasper, gypsum, calcite, and other minerals.
Brine— bringing to the surface a solution, more or less nearly saturated, of chloride of sodium. Springs of this kind appear where beds of solid rock-salt exist underneath. The water in its passage encounters the salt, dissolves it, and brings it to the surface. The brine springs of Cheshire in England, the Salzkammergut in Austria, Bex in Switzerland, &c., have long been well known. Some of the English brines contain about one per cent, of salts, of which the chloride of sodium may range from a half to three-fourths or more. Other brines, however, yield a far larger amount; one at Clemenshall, Wiirtemburg, gave upwards of 26 per cent, of salts, of which almost the whole was chloride of sodium. The other substances contained in solution in the water of brine springs are usually such as exist also in sea-water, such as sulphate and carbonate of lime, chlorides of magnesium and potassium, &c.
Medicinal— a vague term applied to mineral springs which have or are believed to have curative effects in different diseases. Medical men recognize various qualities, distinguished by the particular substance most conspicuous in each—as Alkaline Waters, containing lime or soda and carbonic acid, as those of Vichy or Saratoga; Bitter Waters, with sulphate of magnesia and soda— Sedlitz, Kissingen; Salt or Muriated Waters, with common salt as the leading mineral constituent—Wiesbaden, Cheltenham; Earthy Waters, lime, either a sulphate or carbonate, being the most marked ingredient—Bath, Lucca; Sulphurous Waters, with sulphur as sulphuretted hydrogen and in sulphides—Aix-la-Chapelle, Harrogate.
Oil.— Mineral oil is carried up by some ordinary springs, and floats in dark drops on the surface of the water. But in some parts of the avorld, as in a wide region in the Northern States of the American Union and in Canada, the oil ascends with little or no water, and forms the oil-springs which in recent years have become so remarkable and abundant a source of illuminating oils, paraffin, and other hydrocarbon compounds.
Results of Chemical Action of Underground Water.— Since every spring is busily engaged in bringing mineral substances from below ground to the surface, there must evidently be a vast amount of subterranean waste, and many tunnels, channels, and caverns must in consequence be formed. To take one illustration: the warm springs of Bath, with a mean temperature of 120° Fahr., are impregnated with sulphates of lime and soda, and chlorides of sodium and magnesium. Professor Ramsay has estimated their annual discharge of mineral matter to be equal to a square column 9 feet in diameter and 140 feet in height. It is in calcareous regions that the extent of the subterranean loss can be most strikingly seen. Sometimes a district of limestone is drilled with vertical cavities (swallσw-holes or sinhs) formed by the solution of the rock by the descent of carbonated rain-water. Surface-drainage is there intercepted, and passes at once under ground, where, in course of time, an elaborate system of channels may be dissolved out of the solid rock. Such has been the origin of the Peak caverns of Derbyshire, the intricate grottoes of Antiparos and Adelsberg, and the vast labyrinths of the Mammoth Cave of Kentucky. In the course of time the underground rivers open out new courses, and leave their old ones dry. By the falling in of the roofs of caverns near the surface, brooks and rivers are occasionally engulphed, which, after a long subterranean course, may issue to the surface again in a totally different surface area of drainage to that in which they took their rise, and sometimes, as in Florida, with volume enough to be navigable almost up to their outflow. In such circumstances lakes may be formed over the sites of the broken-in caverns; and valleys may thus be deepened, or perhaps even formed. Mud, sand, and gravel, with the remains of plants and animals, are swept below ground, and sometimes accumulate in deposits there. This has been the origin of ossiferous caverns, and of the loam and breccia so often found in them.
These wonderful results of the subterranean circulation of water appeal to the imagination, and are those usually most dwelt upon as evincing the potency of this kind of geological agency. And yet the thoughtful observer who reflects upon this subject will perhaps be led to perceive that even more important than these visible caverns and grottoes are the silent unobtrusive changes so constantly in progress in the solid heart of rocks. As far down as percolating water reaches there is not a particle of mineral matter safe from its attacks. And as we have seen, it is hardly possible to find any rock which does not bear throughout its minute grains and pores evidence that water has filtered through it, removing some substances and putting others in their place.
(2.) Resïdts of Mechanical Action.— In its passage along fissures and channels of the rocks, the underground water not merely dissolves materials chemically and removes them in solution, it likewise loosens some of the finer particles from the sides of these subterranean conduits and carries them along in mechanical suspension. We may occasionally observe, where a spring gushes forth at the surface, that grains of sand are brought up in the clear sparkling water. This removal of material sometimes produces remarkable surface changes when it takes place along the side of a steep slope or cliff, such as occur in river valleys, or by the sea-coast. Let us suppose a thin layer of some porous material, like loose sand or ill-compacted sandstone to lie between two more impervious rocks such as masses of clay or limestone, and that this porous stratum sloping down from higher ground comes out to the surface near the base of a line of abrupt cliff. The water which finds its way down into this layer will use it as its channel of escape, and travelling along its course will issue in springs or in a morer general oozing forth along its outcrop at the foot of the declivity. Under these circumstances the support of the overlying mass of rock is apt to be loosened. The water not only removes piece-meal the sandy layer on which that overlying mass rests, but as it were lubricates the rock underneath. Consequently at intervals portions of the upper rock may break off and slide down into the valley or plain below. Such dislocations are known as landslips. Many illustrative examples might be cited. Thus in the year 1839 a mass of chalk on the Dorsetshire coast slipped over a bed of clay into the sea, leaving a rent three-quarters of a mile long, 150 feet deep, and 240 feet wide. The shifted mass, bearing with it houses, roads, and fields, was cracked, broken, and tilted in various directions, and was thus prepared for further attack and removal by the waves. On many parts of the coasts of Britain there are landslips on a large scale which doubtless took place many centuries ago, or even in some cases beyond the times of human history. The undercliff of the Isle of Wight, the cliffs west of Brandon Head, county Kerry, the basalt escarpments of Antrim, and the edges of the great volcanic plateaus of Mull, Skye, and Raasay furnish illustrations of such prehistoric landslips. Of Continental examples, the well-known fall of the Rossberg, behind the Righi in Switzerland, is one of the most memorable. After a rainy summer in 1806 a large part of one side of the mountain, consisting of sloping beds of hard red sandstone and conglomerate, resting upon soft sandy layers, gave way. Thousands of tons of solid rock suddenly swept across the valley of Goldau, burying four villages, with about 500 of their inhabitants. In 1855 a mass of debris, 3500 feet long, 1000 feet wide, and 600 feet high, slid into the valley of the Tiber, which, dammed back by the obstruction, overflowed the village of San Stefano to a depth of 50 feet, until drained off by a tunnel.
§ 3. Brooks and Rivers.
These will be considered under four aspects:—(1) their sources of supply, (2) their discharge, (3) their flow, and (4) their geological action.
L Sources of Supply. —Rivers are the natural drains of a land surface. They carry out to sea the surplus water after evaporation, and not water only, but a vast and almost incredible amount of material annually worn off the land. Their contents are derived partly from rain (including mist and dew) and melted snow, partly from springs. In a vast river system like that of the Mississippi, the area of drainage is so extensive as to embrace many different climates and varieties of rainfall, so that on the whole the amount of discharge, being in a great measure independent of local variations in the weather, remains tolerably uniform. But in smaller rivers, such as those of Britain, whose basins lie in a region having the same general features of climate, the quantity of water is regulated by the local rainfall. A wet season swells the streams, a dry one diminishes them. Were rivers entirely dependent, however, upon direct supplies of rain, they would only flow in rainy seasons, and disappear in dry weather. This does not happen, because they derive a great deal of their water not directly from rain, but indirectly through the intermediate agency of springs. Hence they continue to flow even in very dry weather, because, though the superficial supplies have failed, the underground sources still continue available. In a long drought, however, the latter begin to fail, the surface springs ceasing first, and gradually drying up in their order of depth, until at last only deep-seated springs furnish a perhaps daily diminishing quantity of water. It is a matter of great economic as well as scientific interest to know how long any river would continue to yield a certain amount of water during a prolonged drought. So far as we can tell, no rule could possibly be laid down for a generally applicable calculation, everyarea having its own peculiarities of underground drainage. Mr Joseph Lucas gives some particulars which show what may happen in a chalk district. The river Wandle drains an area of 51 square miles of the chalk downs in the southeast of England. For eighteen months, from May 1858 to October 1859, as tested by gauging, there was very little absorption of rainfall over the drainage basin, and yet the minimum recorded flow of the Wandle was 10,000,000 gallons a-day, which, Mr Lucas says, represents not more than ’ 4090 inch of rain absorbed on the 51 square miles of chalk. The rock is so saturated that it can continue to supply a large yield of water for eighteen months after it has ceased to receive supplies from the surface, or at least has received only very much diminished supplies.
II. Discharge. —As the natural drains of the land, rivers carry the surplus moisture out to sea. What proportion of the total rainfall is thus discharged by them is a question of great geological and industrial interest. From the very moment that water takes visible form as mist, cloud, dew, rain, snow, or hail, it is subject to evaporation. When it reaches the ground, or flows off into brooks, rivers, lakes, or the sea, it undergoes continual diminution from this cause. Hence in regions where rivers receive no tributaries, they grow smaller in volume as they move onward, till they sometimes even disappear. Apart from temperature, the amount of evaporation is very largely regulated by the nature of the surface from which it takes place, one soil or rock differing from another, and all of them probably from a surface of water. There is no question in meteorology where full and detailed observations are more wanting than in the determination of the relation of evaporation to rainfall and river discharge. During severe storms of rain, the water discharged over the land of course to a very large extent finds its way at once into brooks and rivers, where it causes floods, and whence it reaches the sea. Mr David Stevenson remarks that, acccording to different observations, the amount carried off in floods varies from 1 to 100 cubic feet per minute per acre. But though floods cannot be deemed exceptional phenomena, forming as they do a part of the regular system of water circulation over the land, they do not represent the ordinary proportions between rainfall and river discharge in such a climate as that of Britain, where the rainfall is not crowded into one season, but is spread more or less equally throughout the year. According to Beardmore’s table, the Thames at Staines has a mean annual discharge of 32*40 cubic inches per minute per square mile, equal to a depth of 7‘31 inches of rainfall run off, or less than a third of the total rainfall. The most carefully collected data at present available are probably those given by Humphreys and Abbot for the basin of the Mississippi and its tributaries as shown in the subjoined table :— [table]
Perhaps in Great Britain not more than a fourth part of the total moisture deposited on the land from the atmosphere is carried out to sea by streams. But this is a point on which, until far more facts have been gathered, no definite statement can be accepted as at all trustworthy.
III. Flow. —Rivers, in obedience to the law of gravitation, always move from a higher to a lower level. Where the channel of a river becomes vertical, or nearly so, a waterfall is formed; a steep rocky declivity in the channel gives rise to rapids; a fiat plain allows the stream to linger with a scarcely perceptible current; while a lake renders the flow nearly or altogether imperceptible. Thus the rate of flow is regulated in the main by the angle of inclination and form of the channel, but partly also by the volume of water, an increase of volume in a narrow channel increasing the rate of motion even without an increase of slope.
The course of a great river may be divided into three parts :—1. The Mountain Track,— where, amidst the clouds and snows it takes its rise as a mere brook, and, fed by innumerable similar torrents, dashes rapidly down the steep sides of the mountains, leaping from crag to crag in endless cascades, growing every moment in volume, until it enters lower ground. 2. The Valley Track. —It now flows through the lower hills or undulations which traverse or flank a great mountain chain, and is found at one time in a wide fertile valley, then in a dark gorge, now falling headlong in a cataract, now expanding into a broad lake. This is the part of its career where it assumes the most varied aspects, receives the largest tributaries, and fulfils most characteristically the various conditions which are present to our minds in the idea of a river. 3. The Plain Track.— Having quitted the undulating region, it finally emerges upon broad plains, probably wholly, or in great part, made by itself. Here it winds sluggishly in wide curves, perhaps divides so as to enclose tracts of flat meadow or marsh, and finally, amid banks of mud and sand, passes out into the great ocean. In Europe the Rhine, Rhone, and Danube, in Asia the Ganges and Indus, in America the Mississippi and Amazon, in Africa the Nile, more or less fully illustrate this typical course of a great river.
If we draw a longitudinal section of the course of any such river from its source, or from the highest peaks around that source to its mouth at the sea, we find that the line forms a concave curve. Steep at first, where it slopes from the mountain crests down into the valleys, the curve grows less and less through the middle portion, until it finally can hardly be distinguished from a horizontal line. Though characteristic of great rivers, this feature is not confined to their courses, but belongs to the architecture of the continents.
It is evident that a river must flow, on the whole, fastest in the first portion of its course, and slowest in the last. The common method of comparing the fall or slope of rivers is to divide the difference of height between their source and the sea-level by their length, so as to give the declivity per mile. This mode, however, often fails to bring out the real resemblances and differences of rivers, even in regard to their angle of slope. Foι ’ example, two streams rising at a height of 1000 feet, and flowing 100 miles, would each have an average slope of 10 feet per mile; yet they might be wholly unlike each other, one making its descent almost entirely in the first or mountain part of its course, and lazily winding for most of its way through a vast low plain, the other toiling through the mountains, then keeping among hills and table-lands, so as to form on the whole a tolerably equable and rapid flow. The great rivers of the globe have probably a less average slope than 2 feet per mile. The Missouri has a descent of 28 inches per mile. The average slope of the channel of the Thames is 21 inches per mile; of the Shannon about 11 inches per mile, but between Killaloe a,nd Limerick about 6∣ feet per mile; of the Nile, below Cairo, 3 ’ 25 to 5 ’ 5 inches per mile; of the Doubs and Rhone, from Besançon to the Mediterranean, 24 ’ 18 inches per mile; of the Volga, from its source to the sea, a little more than 3 inches per mile. Higher angles of descent are those of torrents, as the Arve, with a slope of 1 in 616 at Chamounix, and the Durance, whose angle varies from 1 in 467 to 1 in 208. The slope of a navigable river ought not, if possible, to exceed 10 inches per mile, or 1 in 6336.
But not only does the rate of flow of a river vary at different parts of its course, it is not the same in every part of the cross section of the river taken at any given point. The sides and bottom, being retarded by friction against the channel, move less rapidly than the centre. The central piers of a bridge have thus a greater velocity of river current to bear than those at the banks. It follows that whatever tends to diminish the friction of the moving current will increase its rate of flow. The same body of water, other conditions being equal, will move faster through a narrow gorge with steep smooth walls than over a broad rough rocky bed. For the same reason, when two streams join, their united current, having in many cases a channel not much larger than that of one of the single streams, flows faster, because the water encounters now the friction of only one channel. The average rate of flow of rivers is much less than might be supposed, even in what are termed swift rivers. A moderate rate is about 1∣ mile in the hour; even that of a torrent does not exceed 18 or 20 miles in the hour. Mr D. Stevenson states that the velocity of such rivers as the Thames, the Tay, or the Clyde may be found to vary from about one mile per hour as a minimum to about three miles per hour as a maximum velocity.
It may be remarked, in concluding this part of the subject, that elevations and depressions of the land must have a powerful influence upon the slope of rivers. The upraising of the axis of a country must increase the slope, and consequently the rate of flow which, on the contrary, will be diminished by a depression of the axis or by an elevation of the maritime regions.
IV. Geological Action. —Like all the other forms of moving water, the streams which traverse a country have both a chemical and mechanical action. The latter receives most attention, as it undoubtedly is the more important; but the former ought not to be omitted in any survey of the general waste of the earth’s surface.
i. Chemical.— The water of rivers must possess the powers of a chemical solvent like rain and springs, though-its actual work in this respect can be less easily measured, seeing that river water is directly derived from rain and springs, and necessarily contains in solution mineral substances supplied to it by them and not by its own operation. Nevertheless, it is sometimes easy to prove that streams dissolve chemically the rocks of their channels. Thus in limestone districts the base of the cliffs of river ravines may be found eaten away into tunnels, arches, and overhanging projections, presenting in their smooth surfaces a great contrast to the angular jointed faces of the same rock where exposed to the influence only of the weather on the higher parts of the cliff.
The composition of the river waters of western Europe is well shown by numerous analyses. The substances held in solution include variable proportions of the carbonates of lime, magnesia, and soda; silica; peroxides of iron and manganese; alumina; sulphates of lime, magnesia, potash, and soda; chlorides of sodium, potassium, calcium, and magnesium; silicate of potash; nitrates; and organic matter. The minimum proportion of mineral matter among the analyses collected by Bischof was 2 ’ 61 in 100,000 parts of water in a mountain stream 3800 feet above the sea. On the other hand as much as 54 ’ 5 parts in the 100,000 were obtained in the waters of the Beuvronne, a tributary of the Loire above Tours. The average of the whole of these analyses is about 21 parts of mineral matter in 100,000 of water, whereof carbonate of lime usually forms the half, its mean quantity being 11 ’ 34. Bischof calculated that, assuming the mean quantity of carbonate of lime in the Rhine to be 9 ’ 46 in 100,000 of water, which is the proportion ascertained at Bonn, enough carbonate of lime is carried into the sea by this river for the annual formation of three hundred and thirty-two thousand millions of oyster shells of the usual size. The mineral next in abundance is sulphate of lime, which in some rivers constitutes nearly half of the dissolved mineral matter. Silica amounting to 4 - 88 parts in 100,000 of water has been found in the Rhine, near Strasburg. The largest amount of alumina was 0^71 in the Loire, near Orleans. The proportion of mineral matter in the Thames, near London, amounts to about 33 in 100,000 parts of water, 15 of which (nearly half of the whole) consist of car-bouate of lime.
It requires some reflection properly to appreciate the amount of solid mineral matter which is every year carried iû solution from the rocks of the land and diffused by rivers into the sea. According to recent calculations by Mr T. Mellard Reade, C.E., a total of .8,370,630 tons of solids in solution is every year removed by running water from the rocks of England and Wales, which is equivalent to a general lowering of the surface of the country from that cause alone at the rate of ’ OO77 of afoot in a century or one foot in 12,978 years. The same writer computes the annual discharge of solids in solution by the Rhine to be equal to 92 ’ 3 tons per square mile, that of the Rhone at Avignon 232 tons per square mile, and that of the Danube at 72 ’ 7 tons per square mile; and he supposes that on an average over the whole world there may be every year dissolved by rain about 100 tons rocky matter per English square mile of surface. ii. Hechanical.— The mechanical work of rivers is threefold:—(1) to transport mud, sand, gravel, or blocks of stone from higher to lower levels; (2) to use these loose materials in eroding their channels; and (3) to deposit these materials where possible, and thus to make new geological formations.
1. Transporting Power.—It is one of the distinctions of river water, as compared with that of springs, that, as a rule, it is less transparent, that is, it contains more or less mineral matter in suspension. The same stream differs much at successive intervals in the amount of material thus transported. In dry weather when the water is low it maybe tolerably clear; but a -sudden heavy shower or a season of wet weather will render it turbid. The mud thus so frequently noticeable in rivers is partly derived from the surface of the ground on either side, whence it is washed into the main streams by rain and brooks, but partly also by the abrasion of the water-channels through the operations of the streams themselves. In the mountain tributaries of a river we find the channels choked with large fragments of rock disengaged from the cliffs and crags on either side. Traced downwards the blocks are seen to become gradually smaller and more rounded. They are ground against each other and upon the rocky sides and bottom of the channel, getting more and more reduced as they descend, and at the same time abrading the rocks over or against which they are driven. Hence a great deal of debris is produced, and is swept along by the onward and downward movement of the brooks and rivers. The finer portions, such as mud and fine sand, are carried in suspension, and impart the characteristic turbidity to rivers; the coarser sand and gravel are driven along the river bottom.
The transporting capacity of a stream depends (a) on the volume and velocity of the current, and (6) on the size, shape, and specific gravity of the sediment, (a) According to the calculations of II0pkin3, the capacity of transport increases as the sixth power of the velocity of the current; thus the motive power of the current is increased 64 times by the doubling of the velocity, 729 times by trebling, and 4096 times by quadrupling it. It has been found by experiment that “ordinary sandy soil is moved by a current having a velocity of about half a mile an hour, and that a current of about one mile per hour will move fine gravel, while heavy gravel resists a current of upwards of two miles per hour.” Mr David Stevenson gives the subjoined table of the power of transport of different velocities of river currents. [table]
We must never lose sight, however, of the fact that it is not the surface velocity, nor even the mean velocity, of a river which can be taken as the measure of its power of transport, but the bottom velocity—that is, the rate at which the stream overcomes the friction of its channel, (δ) The average specific gravity of the stones in a river ranges between two and three times that of pure fresh water; hence these stones lose from a half to a third of their weight in air when borne along by the river. Huge blocks which could not be moved by the same amount of energy applied to them on dry ground are swept along with ease when they have found their way into a strong river current. The shape of the fragments greatly affects their portability, when they are too large and heavy to be carried in mechanical suspension. Rounded stones are of course most easily moved; flat and angular ones are moved with comparative difficulty.
Besides their ordinary powers of transport, rivers gain at times considerable additional force from several causes. Those liable to sudden and heavy falls of rain acquire by flooding an enormous increase of transporting and excavating power. More work may thus be done by a stream in a day than could be accomplished by it during years of its ordinary condition. Another source of increase to the action of rivers is provided when, from landslips, formed by earthquakes, by the undermining influence of springs, or otherwise, a stream is temporarily dammed back, and the barrier subsequently gives way. The bursting out of the arrested waters produces great destruction in the valley. Blocks as big as houses may be set in motion, and carried down for considerable distances. Again, the transporting power of rivers is greatly augmented in countries where they freeze in winter. As the ice gathers along the banks it encloses gravel, sand, and even large blocks of rock, which, when thaw comes, are lifted up by the ice and carried down the stream. Ground-ice likewise appears in cold latitudes on the bottoms of the rivers, whence, rising in cakes to the surface, it carries with it sand, mud, or stones lying on the bottom, which are then swept seaward. When rivers such as those of northern Russia and Siberia, flowing from south to north, have the ice thawed in their higher courses before it breaks up farther down, much disaster is sometimes caused by the piling up of the ice, and then by the bursting of the impeded river through the temporary ice-barrier. In another way ice sometimes vastly increases the destructive powers of small streams where avalanches or an advancing glacier cross a valley and pond back its drainage. The valley of the Dranse, in Switzerland, has several times suffered from this cause. In 1818 the glacier barrier extended across the valley for more than half a mile, with a breadth of 600 and a height of 400 feet. The waters above the ice-dam accumulated into a lake containing 800,000,000 cubic feet. By a tunnel driven through the ice, the water was drawn off without desolating the plains below.
That rivers differ vastly from each other in the amount of material they transport is made evident by the great diversities in their relative muddiness. It should be borne in mind that the actual amount of sediment borne downwards by a river is not necessarily determined by the carrying power of the current. The swiftest streams are not always the muddiest. The proportion of sediment is partly dependent upon the hardness or softness of the rocks of the channel, the number of tributaries, the nature and slope of the ground forming the drainage basin, the amount and distribution of the rainfall, the size of the glaciers (where such exist) at the sources of the river, &c. A rainfall spread with some uniformity throughout the year may not sensibly darken the rivers with mud, but the same amount of fall crowded into a few weeks or months may be the means of sweeping a vast amount of earth into the rivers, and sending them down in a greatly discoloured state to the sea. Thus the rivers of India during the rainy season become rolling currents of mud.
In his journeys through equatorial Africa Livingstone came upon rivers which appear usually to consist more of sand than of water. He describes the Zingesi as “a sand rivulet in flood, 60 or 70 yards wide, and waist-deep. Like all these sand-rivers it is for the most part dry; but, by digging down a few feet, water is to be found, which is percolating along the bed on a stratum of clay. In trying to ford it,” he remarks, “I felt thousands of particles of coarse sand striking my legs, which gave me the idea that the amount of matter removed by every freshet must be very great. . . . These sand rivers remove vast masses of disintegrated rock before it is fine enough to form soil. In most rivers where much wearing is going on, a person diving to the bottom may hear literally thousands of stones knocking against each other. This attrition, being carried on for hundreds of miles in different rivers, must have an effect greater than if all the pestles and mortars and mills of the world were grinding and wearing away the rocks.”
The amount of mineral matter transported by rivers can be estimated by examining their waters at different periods and places, and determining their solid contents. A complete analysis should take into account what is chemically dissolved, what is mechanically suspended, and what is driven or pushed along the bottom. We have already dealt with the chemically dissolved ingredients. In determinations of the mechanically mixed constituents of river water, it is most advantageous to obtain the proportion first by weight, and then from its average specific gravity to estimate its bulk as an ingredient in the water. The Ganges, according to Everest, contains during the four months of flood earthy matter in the proportion of τ⅛∙by weight or by volume,—the mean average for the year being τ i γj ∙ by weight or r5 1 s - 0 by volume. According to Mr Login, the waters of the Irrawaddy contain τ √ 075 - by weight of sediment during floods, and 5-⅛5∙ during a low state of the river. The most elaborate measurements and calculations yet made regarding this aspect of the operations of a river are those by Messrs Humphreys and Abbot on the Mississippi, who found, as the mean of many observations carried on continuously at different parts of the river for months together, that the average proportion of sediment contained in the water of the Mississippi is r5 1 o - by weight, or τ ⅛ yτ by volume. But besides the matter held in suspension, they observed that a large amount of coarse detritus is constantly being pushed along the bottom of the river. They estimated that this moving stratum carries every year into the Gulf of Mexico about 750 million cubic feet of sand, earth, and gravel. Their observations led them to conclude that the annual discharge of water by the Mississippi is 19,500,000 million cubic feet, and consequently that the weight of mud annually carried into the sea by this river must reach the sum of 813,500 million lb. Taking the total annual contributions of earthy matter, whether in suspension or moving along the bottom, they found them to equal a prism 268 feet in height, with a base of one square mile.
2. Excavating Power.—In transporting its freight of sediment a river performs a vast amount of abrasion. In the first place it rubs the loose stones against each other, breaks them into smaller pieces, rounds off their edges, reduces them to rounded pebbles and finally to sand or mud. In the next place by driving these loose materials over the rocks it wears down the sides and bottom of its channel which is thereby widened and deepened.
The familiar effect of running water upon fragments of rock, in reducing them to smoothed rounded pebbles, is expressed by the common phrase “water-worn.” Every stream which descends from high rocky ground may be compared to a grinding mill; large boulders and angular blocks of rocks, disengaged by frosts, springs, and general atmospheric waste, fall into the upper end, and only fine sand and silt are discharged into the sea. Μ. Daubrée has instituted some ingenious experiments for ascertaining the circumstances under which angular fragments are converted into rounded pebbles with the production of sand and mud. Using fragments of granite and quartz, he caused them to slide over each other in a hollow cylinder partially filled with water, and rotating on its axis with a mean velocity of 0 ’ 80 to 1 metre in a second. He found that after the first 25 kilometres (about 15⅛ English miles) the angular fragments of granite had lost τ ⅛- of their weight, while in the same distance fragments already well-rounded had not lost more than - r ⅜ σ - to j⅜ σ . The fragments rounded by this journey of 25 kilometres in a cylinder could not be distinguished either in form or in general aspect from the natural detritus of a river bed. A second product of these experiments was an extremely fine impalpable mud, which remained suspended in the water several days after the cessation of the movement. During the production of this fine sediment the water acted chemically upon the granite fragments, for after a day or two it was found, even though cold, to have dissolved a very sensible proportion of silicate of potash. After a journey of 160 kilometres, 3 kilogrammes (about 6⅛ tt> avoirdupois) had yielded 3 ’ 3 grammes (about 50 grains) of soluble salts consisting chiefly of silicate of potasE A third product was an extremely fine angular sand consisting almost wholly of quartz, with scarcely any felspar, almost the whole of the latter mineral having passed into the state of clay. The sand grains, as they are continually pushed onward over each other upon the bottom of a river, become rounded as the larger pebbles do. But, as M. Daubrée points out, a limit is placed to this attrition by the size and specific gravity of the grains. So long as they are carried in suspension they will not abrade each other, but remain angular; for he found that the milky tint of the Rhine at Strasburg in the months of July and August was due not to mud but to a fine angular sand (with grains about ∙ 2 ⅛ millimetre in diameter) which constitutes of the total weight of water. Yet this sand had travelled in a rapidly flowing tumultuous river from the Swiss mountains, and had been tossed over waterfalls and rapids in its journey. He ascertained also that sand with a mean diameter of grain of τ ⅛ mm. will float in feebly agitated water; so that all sand of finer grain must remain angular. The same observer has noticed that sand composed of grains with a mean diameter of ⅜ mm., and carried along by water moving at a rate of 1 metre per second, gets rounded, and loses about ∙ πr ⅞ 7πr of its weight in every kilometre travelled.
The effects of abrasion upon the loose materials on a river bed are only a portion of the erosive work performed by the stream. Where the bottom is covered with a layer of debris only the upper portion of which is pushed onward by the current, the solid rock of the river channel is there protected from waste. But this protection is only local, and is apt to be swept away from time to time by violent floods. In those parts of a river channel where the current is strong enough to keep gravel and boulders moving along, these loose materials rub down the rocky bottom over which they are driven. As the shape and declivity of the channel vary constantly from point to point, with at the same time frequent changes in the nature of its rocks, this erosive action is liable to continual modifications. While there is a general abrasion of the whole bottom over which loose detritus is rolled, the erosion goes on most briskly in the numerous hollows and grooves along which chiefly these loose materials travel. Wherever an eddy occurs in which gravel is kept in gyration, erosion is much increased. The stones in their movement excavate a hole in the channel, while, as they themselves are reduced to sand and mud, or are swept out by the force of the current, their places are taken by fresh stones brought down by the stream. Such pot-holes, as they are termed, vary in size from mere cup-like depressions to huge cauldrons or pools. As they often coalesce by the giving way of the intervening walls between two or more of them they materially increase the deepening of the river bed.
The shape of a river channel at any given point in its course depends mainly upon the nature and structure of the material out of which it has been eroded. One of the most characteristic features of streams, whether large or small, is the tendency to wind in serpentine curves when the angle of declivity is low and the general surface of the country tolerably level This peculiarity may be observed in every stream which traverses a flat alluvial plain. Some slight weakness in one of its banks enables the current to cut away a portion of the bank at that point. By degrees a concavity is formed, whence the water is deflected to the opposite side, there to break with increased force against the bank. Gradually a similar concavity is cut out on that side, and so, bending alternately from one side to the other, the stream is led to describe a most sinuous course across the plain. By this process, however, while the course is greatly lengthened, the velocity of the current proportionately diminishes, until it may, before quitting the plain, become a lazy, creeping stream, which in England is bordered perhaps with sedges and willows. Such meandering courses are most frequent in soft alluvial plains, but they may also be found in solid rock if the original form of the surface was tolerably flat. The windings of the gorges of the Moselle and Rhine through the table-land between Treves, Mainz, and the Siebengebirge form a notable illustration.
Abrupt changes in the geological structure or lithological character of the rocks of a river-channel may give rise to waterfalls. In many cases waterfalls have originated in lines of escarpment over which the water at first found its way, or in the same geological arrangement of hard and soft rocks by which the escarpments themselves have been produced. In the case of the falls of Niagara, for example, the stream may have fallen over the Queenstown eliff when the river first sought its way to the sea. But much more probably the escarpment and waterfall began to arise simultaneously and from the same geological structure. As the escarpment grew in height, it receded from its starting point. The river-ravine likewise crept backward, but at a more rapid rate, and the result has been that at present the cliff worn down by atmospheric causes stands at Queenstown, while the ravine extends 7 miles further inland, with a width of from 200 to 400 yards, and a depth of from 200 to 300 feet. In this as in other cases the waterfall has cut its way backward up the course of its stream, and will continue to do so as long as the structure of the gorge continues as it is now—a thick bed or beds of limestone resting horizontally upon soft shales. The softer strata at the base are undermined, and slice after slice is cut off from the cliff over which the cataract pours. It has been estimated that at their present rate of recession the Niagara Falls must have taken about 35,000 years to cut their way backward and excavate the gorge between their present position and Queenstown. In other cases waterfalls have been produced by the existence of a harder and more resisting band or barrier of rock crossing the course of the stream, as, for instance, where the rocks have been cut by an intrusive dyke or mass of basalt. In these and all other cases the removal of the harder mass destroys the waterfall, which, after passing into a series of rapids, is finally lost in the general abrasion of the river-channel. The most marvellous river gorges in the world are those of the Colorado region in North America. The rivers there flow in ravines thousands of feet deep and hundreds of miles long, through vast tablelands of nearly horizontal strata. The Grand Caîion (ravine) of the Colorado river is 300 miles long, and in some places more than 6000 feet in perpendicular depth. The country is hardly to be crossed except by birds, so profoundly has it been trenched by these numerous gorges. Yet the whole of this excavation has been effected by the erosive action of the streams themselves.
In the excavation of a ravine, whether by the recession of a waterfall or of a series of rapids, the action of the river is more rapid than that of the atmospheric agents. The sides of the ravine consequently retain their vertical character. But where, from the nature of the ground, the denuding action of rain, frost, and general weathering is more rapid than that of the river, a wider and opener valley is hollowed out, through which the river flows, and from which it carries away the materials discharged into it from the surrounding slopes by the rain and brooks.
3. Reproductive Power.—Every body of water which when in motion carries along sediment drops it when at rest. The moment a current has its rapidity checked it is deprived of some of its carrying power, and begins to lose hold upon its sediment, which tends more and more to sink and halt on the bottom the slower the motion of the water. In the course of every brook and river there are frequent checks to the current. If these are examined, they will usually be found to be each marked by a more or less conspicuous deposit of sediment. We may notice seven different situations in which stream deposits or alluvium may be accumulated.
(a.) At the foot of Mountain Slopes.— When a runnel or torrent descends a steep declivity it tears down the soil and rocks, cutting a deep gash out of the side of the mountain. On reaching the level ground at the base of the slope the water, abruptly checked in its velocity, at once drops its coarser sediment, which gathers in a fan-shaped pile or cone, with the apex pointing up the water course. Huge accumulations of boulders and shingle may thus be seen at the foot of such torrents,—the water flowing through them often in several channels which re-unite in the plain beyond.
(7>.) In River-beds.— This is characteristically shown in many of the rivers of Britain, by the accumulation of a bed of sand or shingle at the concave side of each sharp bend of the river course. While the main current is making a great sweep round the opposite bank, the water lingers along the inner side of the curve and drops there its freight of loose detritus, which, when laid bare in dry weather, forms the familiar sand-bank or shingle beach. Again, when a river, well supplied with sediment, leaves rough ground where its course has been rapid, and enters a region of level plain,it begins to drop its burden on its bed, which is thereby heightened, till sometimes, as in parts of the courses of the Fo, Adige, and Brenta across the plains of Lombardy, it is higher than the surrounding level region. This could not happen were it not that in floods a river deposits sediment along its banks, which are thus also heightened so as to retain the river in its ordinary state. But in such cases, if man lives along the margin of the river, he needs all his skill and labour to keep the banks secure. And even with his utmost efforts the river will now and then break through, sweeping down the barrier which it has itself made, as well as any additional embankments constructed by him, and carrying its flood far and wide over the plain. Left to itself, the river would incessantly shift its course, until in turn every part of the plain had been again and again traversed. It is indeed in this way that a great alluvial plain is gradually levelled and heightened.
(c.) On River-banks and Flood-plains.— This deposit is partly implied in the action described in the foregoing paragraph. It is laid down on the level tracts or floodplain over which a river spreads in flood, and consists usually of fine silt, mud, earth, or sand, though close to the river it may be partly made up of much coarser materials. When a flooded river overflows, the portions of water which spread out on the plains, by losing velocity and consequently power of transport, are compelled to let fall some or all of their mud and sand. If the plains happen to be covered with woods, bushes, scrub, or even tall grass, the vegetation acts the part of a sieve, and filters the muddy water, which may rejoin the main stream comparatively clear. Every flood increases the height of the plain, until, partly from this cause and partly, in the case of a rapid stream, from the erosion of its bed, the river can no longer overspread it. As the channel is more and more deepened, the river continues, as before, to be liable from inequalities in the material of its banks, sometimes of the most trifling kind, to be turned from side to side in wide curves and loops, and cuts into its old alluvium, making eventually a newer plain at a lower level. Continued erosion carries the channel to a still lower level, where the stream can attack the later alluvial deposit, and form a still lower and newer one, The river comes by this means to be fringed with a series of terraces, each of which represents a former flood-level of the stream. In Britain it is common to find three such terraces, but sometimes as many as six or seven or even more may occur. In North America the river terraces exist on so grand a scale that the geologists of that country have named one of the later periods of geological history, during which those deposits were formed, the Terrace Epoch. In the attempt to reconstruct the history of the old river-terraces of a country, we have to consider whether they have been entirely cut out of older alluvium (in which case, of course, the valleys must have been as deep as now before the formation of the terraces); whether they afford any indications of having been formed during a period of greater rainfall, when the rivers were larger than at present; whether they point to any upheaval of the interior of the country which would accelerate the erosive action of the streams, or to any depression of the interior or rise of the seaward tracts, which would diminish that action and increase the deposition of alluvium. Professor Dana has connected those of America with the elevation of the axis of that continent.
(<7.) In Lakes.— When a river enters a lake its current is at once checked, and its sediment begins to gather over the lake bottom. If the lake be long enough in proportion to the volume of the river, the whole of the detritus may be deposited, so that, at its outflow, the river becomes as clear as when its infant waters began their course from the springs, snows, and mists of the far mountains. Thus the Rhone enters the Lake of Geneva turbid and impetuous, but it escapes at Geneva as blue translucent water. Its sediment is laid down on the floor of the lake, and chiefly at the upper end. Hence, lakes act as filters or sieves to intercept the sediment which is travelling in the rivers from the high grounds to the sea. If we look down from a height even upon a small lake among mountains, we observe that at the mouth of each torrent or brook which enters it there lies a little tongue of flat land (a true delta), through which the streamlet winds in one or more branches before mingling its waters with those of the lake. Each of these tongues consists of alluvium, laid down in obedience to the same law which governs the formation of river alluvium elsewhere, and continually creeping further out from the land as the deposit of sediment advances. Two streams entering a lake from opposite sides may join their alluvia so as to divide the lake into two, like the once united lakes of Thun and Brienz at Interlaken. Or the lake may be finally filled up altogether, as has happened in innumerable cases in all mountainous countries; the hilly tracts of Britain, for example, furnish abundant illustrations of every stage in this process. Where a large river with abundant sediment enters an important lake (as the Rhone at the head of the Lake of Geneva), the accumulation of its alluvium or delta may quite rival that of a great river in the sea, as described in paragraph (∕.) below.
(e.) Bars and Lagoon-Barriers.— If we take a broad view of the degradation of the land, we must admit that the deposit of any sediment on the land is only temporary; the inevitable destination of all this material is the ocean. Most rivers which enter the sea have their mouths crossed by a bar of gravel, sand, or mud. The formation of this barrier results from the conflict between the river and the ocean. Although the muddy fresh water floats on the heavier salt water, its current is lessened, and it can no longer push along the mass of detritus on its bed. It has been ascertained, moreover, that, though fresh water can retain for a long while fine mud in suspension, this sediment is rapidly thrown down when the fresh is mixcd with saline water. Hence, apart from the necessary loss of transporting power by the checking of the river current at the mouth, the mere mingling of a river with the sea must of itself be a cause of the deposit of sediment. But a large body of fresh water may float for a long distance before it is thoroughly mingled with the heavier water of the ocean. Ultimately, however, the fine detritus dropped by a river, together with the coarser materials on the bottom, are arrested by the sea. Moreover, in many cases the sea itself piles up great part of the sand and gravel of the bar. Heavy river-floods push the bar farther to sea, or even temporarily destroy it; storms from the sea, on the other hand, drive the bar farther up the stream. Another remarkable illustration of the contest between the alluvium-carrying streams and the land-eroding ocean is shown by the vast lines of bar or bank which, both in the Old World and the New, stretch along the coast. The streams do not flow straight into the sea, but run sometimes for many miles parallel to the coast, accumulating behind the barriers into broad and long lagoons, but eventually breaking through the barriers of alluvium and entering the sea. The lagoons of the Italian coast and of the Baltic near Dantzic are familiar examples. A conspicuous series of these alluvial bars fronts the American mainland for many hundred miles round the Gulf of Mexico and the shores of Florida, Georgia, and North Carolina. A. space of several hundred miles on the east coast of India is similarly bordered. M. F,. de Beaumont, indeed, has estimated that about a third of the whole of the coast-lines of the continents is fringed with such alluvial bars.
( ∕ ’ .) Deltas in the Sea.— The tendency of sediment to accumulate in a tongue of flat land when a river loses itself in a lake is exhibited on a far vaster scale where the great rivers of the continents enter the sea. It was to one of these maritime accumulations, that of the Nile, that the Greeks gave the name Delta, from its resemblance to their letter Δ, with the apex pointing up the river, and the base fronting the sea. This shape being the common one in all such alluvial deposits at river mouths, the term delta has come to be always applied to them. A delta therefore consists of successive layers of detritus, brought down from the land and spread out in the sea at the mouth of a river until they reach the surface, and then, partly by growth of vegetation and partly by flooding of the river, forma plain, of which the inner and higher portion comes eventually to be above the reach of the floods. Large quantities of driftwood are often carried down, and bodies of animals are swept off to be buried in the delta, or even to be floated out to sea. Hence, in deposits formed at the mouths of rivers, we may always expect to find terrestrial organic remains.
When a river enters upon the delta-portion of its course it assumes a new character. Iα the previous parts of its journey it is always being augmented by tributaries; but now it begins to split up iuto branches, which wind to and fro through the flat alluvial land, often coalescing and thus enclosing insular spaces of all dimensions. The feeble current, no longer able to bear along all its weight of sediment, allows much of it to sink to the bottom and to gather over the tracts which are from time to time submerged. Hence many of the channels get choked up, while others are opened out in the plain, to be in turn abandoned, and thus the river restlessly shifts its channels. The seaward ends of at least the main channels grow outwards by the constant accumulation of detritus pushed into the sea, unless this growth chances to be checked by any marine current sweeping past the delta.
The European rivers furnish many excellent illustrations of delta-growth. Thus the Rhine, Meuse, Sambre, Scheldt, and other rivers have formed the wide maritime plain of Holland and the Netherlands. The Rhone has deposited an important delta in the Mediterranean Sea. The upper reaches of the Adriatic Sea are being rapidly shallowed and filled up by the Po, Adige, and other streams. Thus Ravenna, originally built in a lagoon like Venice, is now 4 miles from the sea. The port of Adria, so well known in ancient times as to have given its name to the Adriatic, is now 14 miles inland, while in other parts of that coast-line the breadth of land gained within the last 1800 years has been as much as 20 miles. On the opposite side, also, of the Italian peninsula, great additions have been made to the coast-line within the historical period. It is computed that the Tuscan rivers lay down as much as 12 million cubic yards of sediment every year within the marshes of the Maremma. The “yellow” Tiber, as it was aptly termed by the Romans, owes its colour to the abundance of the sediment which it carries to sea. It has long been adding to the coast-line at its mouth at the rate of from 12 to 13 feet per annum. Hence the ancient harbour of Ostia is now more than 3 miles inland. Its ruins are at present (1879) being excavated, but every flood of the river leaves a thick deposit of mud on the streets and on the floors of the uncovered houses. Whence it would seem that the Tiber has not only advanced its coast-line, but has raised its bed on the plains by the deposit of alluvium, so that it now overflows places which, 2000 years ago, could not have been so frequently under water. In the Black Sea a great delta is rapidly growing at the mouths of the Danube. At the Kilia outlets the water is shallowing so fast that the lines of soundings of 6 feet and 30 feet are advancing into the sea at the rate of between 300 and 400 feet per annum.
The typical delta of the Nile has a seaward border 180 miles in length, the distance from which to the apex of the plain where the river bifurcates is 90 miles. That of the Mississippi contains an area of 40,000 square miles. The united delta of the .Ganges and Brahmaputra covers a space of between 50,000 and 60,000 square miles, and has been bored through to a depth of 481 feet.
(7.) Sea-borne Sediment.— Although more properly to be noticed under the section on the sea, the final course of the materials worn by rains and rivers from the surface of the land may be referred to here. By far the larger part of these materials sinks to the bottom close to the land. It is only the fine mud carried in suspension in the water which is carried out to sea, the distance depending on the velocity of the stream, the specific gravity and shape of the particles of the mud, and the help or hindrance given by marine currents. The sea fronting the Amazon is discoloured for 300 miles by the mud of that river. The soundings taken by the “Challenger” brought up land-derived detritus from depths of 1500 fathoms,—several hundreds of miles distant from the nearest shores.
The amount of material carried by a river into the sea may be taken as the measure or gauge of the general lowering of the surface of the basin drained by the river. If we ascertain the annual quantity of mineral matter thus delivered into the sea, and know the superficial extentof ground from which it has been derived, the one sum divided by the other gives the extent by which the mean level of the country is reduced in one year. Both the fine mud mechanically suspended in the water and the salts dissolved in it should be taken into account, as well as the coarser detritus pushed along the bottom. It is the mechanically suspended mud which has generally been measured. According to the data of Messrs Humphreys and Abbot, already cited, the proportion of sediment in the Mississippi is r ⅛ π by weight, or 777 ⅛ 75 by volume. The annual discharge of sediment is 7,459,267,200 cubic feet, and the drainage basin 1,147,000 square miles. This is equal to a loss of of a foot of rock from the general surface of the drainage basin in 1 year, or 1 foot in 6000 years. Other rivers work faster than this rate. The Ganges has been estimated to remove 1 foot of rock from its drainage area in 2358 years, and the Po 1 foot in 729 years. Such computations are at the best only approximations to the truth, but they are useful in showing how great an amount of change must be effected even within comparatively short geological periods by the various agents which are disintegrating the surface of the land.
§ 4. Lakes.
Depressions filled with water on the surface of the land, and known as lakes, occur abundantly in the northern parts of both hemispheres, and more sparingly, but often of largo size, in warmer latitudes. They do not belong to the normal system of erosion in which running water is the prime agent, and to which the excavation of valleys and ravines must be attributed. On the contrary, they are exceptional to that system, and the constant tendency of running water is to fill them up. Their origin, therefore, must be sought among some of the other geological processes. See part vii.
Lakes are conveniently classed as fresh or salt. Those which possess an outlet contain in almost all cases fresh water; those which have none are usually salt.
1. Fresh-water Lakes.— These, in a vast number of cases, are simply depressions or expansions of the valleys in which they lie. They receive a river at the upper end, together probably with many minor tributaries from the sides, and let the accumulated waters overflow at the lower end. In all these cases, they act as filters for the river water, allowing its sediment to settle, and discharging it purified at the outflow, to continue its course of erosion and mudmaking down the valley. A river which flows through a succession of lakes cannot carry much sediment to the sea, unless it has a long course to run after it has passed the lowest lake, and receives one or more muddy tributaries. But the sediment which would have reached the sea and gone to form a delta or deposit on the sea-bed serves its purpose in contributing to fill up the lakes. Hence it is rare to find any lake with an inflowing and outflowing river, where proofs of the gradual encroachment of the land upon the water may not be gathered.
In other cases lakes do not lie in the natural drainage lines of a country, but are scattered apparently at random over the surface, fed by springs, rains, and streamlets from the slopes, or, if of large size, lying as great basins, receiving the collected waters of a wide region, and forming in this way the source out of which full-formed rivers emerge. From the little tarns of Wales and the lochans of Scotland a series may be traced, through innumerable grades of size and form, in Scandinavia, Finland, and Canada, till we reach such vast sheets of inland water as Lakes Huron, Erie, and Superior, and the great equatorial lakes of Africa. In lakes of this kind also the process of filling up may often be traced. Each tributary stream pushes its delta into the water. Where the lakes are shallow, and lie in temperate countries, vegetation comes in to aid in the conversion of the water, first into marsh, then into peat-bog, and finally into dry ground. In Scotland during the last few centuries great changes of this kind have been going un.
On large lakes the wind throws the water into waves which almost rival those of the ocean in size and destructive power. Beaches, sand-dunes, shore-cliffs, and the other familiar features of the meeting line between land and sea reappear along the margins of such great fresh-water seas as Lake Superior.
Three geological functions of lakes are (1) to arrest and equalize the drainage by regulating the outflow and preventing or lessening the destructive effects of floods; (2) to filter river water and permit of the undisturbed accumulation of new deposits, which in some modern cases may cover thousands of square miles of surface, and might attain a thickness of nearly 3000 feet (Lake Superior has an area of 32,000 square miles; Lago Maggiore is 2800 feet deep); (3) to furnish an abode for a lacustrine fauna and flora, to receive the remains of the plants and animals washed down from the surrounding country, and to entomb all these organisms in the growing deposits, so as to preserve a record of the terrestrial life of the period. The deposits in lakes consist of alternations of sand, silt, mud, and gravel, with occasional irregular seams of vegetable matter, and layers of calcareous marl formed from the accumulation of lacustrine shells, Entomostraca, &c. In a lake receiving much sediment there will be little or no marl formed, at least not during the time when the sediment is being deposited. In clear lakes, on the other hand, where there is very little sediment or where it only comes occasionally at wide intervals of flood, beds of white marl, formed entirely of organic remains, may gather on the bottom to a depth of many yards.
2. Salt Lakes may be divided into two classes—(a) those which owe their saltness to the evaporation and concentration of the fresh water poured into them by their feeders; and (6) those which were originally parts of the ocean. Salt lakes of the first kind are abundantly scattered over the inland areas of drainage in the heart of conti nents, as in the great Lake of Utah, and numerous other minor lakes in North America, and the abundant salt lakes of the great plateau of Central Asia. These sheets of water were doubtless fresh at first, but they have progressively increased in salinity, because though the water is evaporated as fast as it is received, there is no escape for the dissolved salts, which consequently remain in the increasingly concentrated liquid. Salt lakes of the second class are comparatively few in number. In their case portions of the sea have been isolated by movements of the earth’s crust, and these detached areas, exposed to evaporation, which is only partially compensated by inflowing rivers, have shrunk in level, and at the same time have sometimes grown much salter than the parent ocean. The Caspian Sea, 180,000 square miles in extent, and with a maximum depth of from 2000 to 3000 feet, is a magnificent example. The shells are chiefly the same as those still living in the Black Sea. Banks of them may be traced between the two seas, with salt lakes and marshes and other evidence to prove, not only that the Caspian was once joined to the main ocean, but that a great firth ran up between Europe and Asia, and possibly stretched completely across what are now the steppes and plains of the Tundras till it merged into the Arctic Sea. Even at present, by means of canals connecting the rivers Volga and Dwina, vessels can pass from the Caspian into the White Sea. But the surface of the Caspian is now more than 80 feet below that of the Black Sea. At present the amount of water supplied by rivers to the Caspian just balances that removed by evaporation, so that the level appears to be no longer sinking. But though, owing to the enormous volume of fresh water poured into it by these rivers, the Caspian is not as a wflιole so salt as the main ocean, and still less so than the Mediterranean, nevertheless the inevitable result of evaporation is there manifested. Along the shallow pools which border this sea a constant deposition of salt is taking place, forming sometimes a pan or layer of rose-coloured crystals on the bottom, or gradually getting dry, and covered with drift sand. This concentration of the water is still more marked in the great offshoot called the Karaboghaz, which is connected with the middle basin of the Caspian by a channel 150 yards wide and 5 feet deep. Through this narrow mouth there flows from the main sea a constant current, which Von Baer estimated to carry daily into the Karaboghaz 350,000 tons of salt. An appreciable increase of the saltness of that gulf has been noticed: seals, which once frequented it, have forsaken its barren shores. Layers of salt are gathering on the mud at the bottom, and the sounding-line, when scarcely out of the water, is covered with saline crystals. These facts furnish an illustration of the circumstances under which the rocksalt deposits in the New Red Sandstone and other geological formations were probably accumulated.
The following table shows the proportion of the saline materials in the waters of some salt lakes :— [table]
II. FRESH WATER IN THE SOLID STATE—ICE.
Fresh water under ordinary circumstances, when it reaches a temperature of 32° Fahr., passes into the solid state by crystallizing into ice. In this condition it performs a series of important geological operations before being again melted and relegated to the general mass of liquid terrestrial waters. Five conditions under which ice occurs on the land deserve notice, viz., frost, frozen rivers and lakes, hail, snow, and glaciers.
1. Frost.— Water in freezing expands. If it be confined in such a way that expansion is impossible, it remains liquid •even at temperatures far below the freezing point; but the instant that the pressure is removed this chilled water becomes solid ice. There is a constant effort on the part of the water to become solid, and very considerable pressure is needed to counterbalance its expansive power. The lower the temperature the greater this exerted pressure becomes. At a temperature of 30° Fahr, the pressure must amount to 146 atmospheres, or the weight of a column of ice a mile high, or 138 tons on the square foot. Consequently when the water freezes at a lower temperature its pressure on the walls of its enclosing cavity must exceed 138 tons on the square foot. Bomb-shells and cannon filled with water and hermetically sealed have been burst in strong frosts by the expansion of the freezing water within them. It is easy to see, therefore, that we have here a geological agent of great potency. It is true that in nature the enormous pressures which can be obtained artificially occur rarely or not at all, because the spaces into which water penetrates can hardly ever be so securely closed as to permit the water to be cooled down very considerably below 32° Fahr, before freezing. Still ice forming at even two or three degrees below the freezing point exerts an enormous disruptive force.
Soils and rocks are all porous, and usually contain a good deal of moisture. When frost congeals this interstitial water, the particles of the soil or rock are pushed asunder by the expanding ice; their cohesion is loosened or destroyed, so that when a thaw comes, they seem as if they have been ground down in a mortar. Water lodges also in the numerous joints and crevices of rocks. Freezing there, it exerts great pressure upon the walls between which it lies, pushing them asunder as if a wedge were driven between them.
When this ice melts, the separated masses do not return to their original position. Their centre of gravity in successive winters becomes more and more displaced, until the sundered masses fall apart. In mountainous districts, where the winters are severe, and in high latitudes, a great deal of waste is thus produced on exposed cliffs and loose blocks of rock. Some measure of its magnitude may be seen in the heaps of angular rubbish which in these regions are so frequently to be met with at the foot of crags and steep slopes. At Spitzbergen and on the coast of Greenland the amount of destruction caused by frost is enormous. The short and warm summer, melting the snow, fills the pores and joints of the rocks with water, which when it freezes splits off large blocks of rock from the hills, and sends them to the base of the declivities, where they are further broken up by the same cause.
2. Frozen Rivers and Lahes. — In countries where the winter temperature falls considerably below 32° Fahr., the lakes and rivers become solidly frozen over. The amount of geological change effected during the process is probably hardly appreciable. But when the ice breaks up in spring its power as a geological agent becomes apparent. In lakes, such as Lake Superior, the ice in forming encloses beachpebbles and boulders, and when thaw sets in, floats these off, so as either to drop them in deeper water or to strand them on some other part of the coast. Should a gale arise during the breaking up of the ice, vast piles of the latter, with mingled gravel and boulders, may be driven ashore and pushed up the beach. By this means blocks of stone, even of considerable size, are sometimes forced to a great height inland on some of the Canadian lakes, tearing up the soil on their way, and helping to form, a bank above the water level. It has been observed that during a severe frost ice occasionally forms on the bottoms of rivers where it encloses stones and large boulders. These are borne up to the surface in cakes of ground-ice to join the rest of the superficial ice-borne detritus. Great damage is frequently done to quays and bridges in Canada by masses of river-ice driven against them on the arrival of spring. Reference has already been made to the increased power of transport and erosion acquired by rivers liable to be frozen over, and especially when their ice is broken up in the higher parts of their courses, before it gives way in the lower.
3. Hail.— When rain or aqueous vapour is cooled down in the atmosphere to the freezing point of water, it is frozen, and falls to the earth as hail or snow. The formation of hail is not yet well understood. It is chiefly in summer and during thunderstorms that hail falls. When the pellets of ice are frozen together so as to reach the ground in lumps as large as a pigeon’s egg, or larger, great damage is often done to cattle, flying birds, and vegetation. Trees have their leaves and fruit torn off, and farm crops are beaten down.
4. Snow.— In those parts of the earth’s surface where, either from geographical position or from elevation into the upper cold regions of the atmosphere, the mean annual temperature is below the freezing point, the condensed moisture falls chiefly assnow, and remains in great measure unmelted throughout the year. A line can be traced below which the summer heat suffices to cause the disappearance of the snow, but above which the snow continues to cover the whole or great part of the surface. This line has received the name of the snow-line, or line of perpetual snow. It comes down to the sea within the polar circles. Between these limits it rises gradually in level till it reaches its highest elevation in tropical latitudes. In northern Scandinavia it is less than 3000 feet above the sea. None of the British mountains quite reach it. In the Alps it stands at 8500 feet, on the Andes at 18,000 feet, and on the northern slopes of the Himalayas at 19,000 feet.
Snowexhibits two different kinds of geological behaviour, one conservative, the other destructive. Lying stationary and unmelted it exercises a protective influence on the face of the land, shielding rocks, soils, and vegetation from the effects of frost. On low grounds this is doubtless its chief function. When snow falls in a partially melted state it is apt to accumulate on branches and leaves, until by its weight it breaks them off, or even bears down entire trees. Snow which falls thickly on steep mountain slopes is frequently during spring and summer detached in large sheets. These rush down the declivities as avalanches, and often create much destruction of trees, soil, crops, and houses in their course, Another indirect effect of snow is seen in the sudden rise of the rivers when warm weather rapidly melts the mountain snows. Many summer floods are caused in this way in Switzerland.
5. Glaciers.— (1.) Nature and Origin.—A glacier is a river of ice formed by the slow movement and compression of the snow which by gravitation creeps downward into a valley descending from a snow-field. The structure and physics of glaciers are described elsewhere (see Glaciers). From a geological point of view these ice-rivers may be regarded as the drainage of the snow-fall above the snow-line, as rivers are the drainage of the rainfall. In a mountainous region, such as the Alps, or a table-land like Scandinavia, where a considerable mass of ground lies above the snowline, three varieties of glaciers have been observed, (a.) Glaciers of the first order, where the ice-river comes down well below the snow and extends into the valley, even it may be far below the upper limits of cultivation, or in northern regions approaches or even reaches the sea. In the Alps such glaciers may be 20 or 30 miles long, by a mile or more wide, and 600 feet or more deep. (&.) Glaciers of the second order, which hardly creep beyond the high recesses wherein they are formed, and do not therefore reach as far as the nearest valley. Many beautiful examples of this type may be seen along the steep declivities which intervene between the snow-covered plateau of Arctic Norway and the sea. (c.) Re-cemented glaciers, consisting of fragments which fall from an ice-cliff crowning precipices of rock, and are re-frozen at the bottom into a solid mass, creeping downward as a glacier usually of the second order. Probably the best illustrations in Europe are furnished by the Nus Fjord, and other parts of the north of Norway. In some cases a cliff of blue ice appears at the top of the precipice,—the edge of the great “suee-fond,” or snow-field,—while several hundred feet below, in the corrie or cwm at the bottom, lies the re-cemented glacier (glacier remanie of the Swiss), white at its upper edge, but acquiring somewhat of the characteristic blue gleam of compact ice as it moves towards its lower margin.
But it is in high Arctic, and still more in Antarctic, latitudes that land-ice, formed from the drainage of a great snow-field, attains its greatest dimensions. The land in these regions is completely buried under an ice-cap, which ranges in thickness up to a depth (in the South Polar circle) of 10,000 feet (2 miles) or even more. Greenland lies under such a pall of snow that all its inequalities, save the mere steep mountain peaks, are concealed. The snow creeping down the slopes, and mounting over the minor hills, passes beneath by pressure into compact ice. From the main valleys great glaciers like vast tongues of ice, 2000 or 3000 feet thick, and sometimes 50 miles or more in breadth, push out to sea, where they break off in huge fragments, which float away as icebergs.
A glacier, like a river, is always in motion, though so slowly that it seems to be solid and stationary. The motion also, like that of a river, and for the same reason, is unequal in the different parts, the centre moving faster than the sides and bottom. This important fact was first ascertained through accurate measurement by J. D. Forbes, who found that in the Mer de Glace of Chamouni, the mean daily rate of motion in the summer and autumn was from 20 to 27 inches in the centre, and from 13 to 19∣ near the side. The consequence of this differential motion is seen in the arrangement of the lines of rubbish thrown down at the end of a glacier, which often present a horse-shoe shape, corresponding to that of the end of the ice by which they were discharged.
There are some features of geological importance also in the behaviour of the ice as it descends its valley. When it has to travel over a very uneven floor, some portions may get embayed, while overlying parts slide over them. A massive ice-sheet may thus have many local eddies in its lower portions, the ice there even travelling for various distances, according to the nature of the ground, obliquely to the general flow of the main mass. In descending by a steep slope from an upper to a lower and more level part of its course, a glacier becomes a mass of fissured ice in great confusion. It descends by a slowly creeping ice-fall, where a river would shoot over in a rushing waterfall. A little below the fall the fractured ice is pressed together again into a solid mass as before. The body of the glacier throughout its length is traversed by a set of fissures called crevasses, which, though at first as close-fitting as cracks in à sheet of glass, widen by degrees as the glacier moves on, till they form wide yawning chasms, reaching, it may be, to the bottom of the ice, and travelling down with the glacier, but apt to be effaced by the pressing of their walls together again as the glacier winds down its valley. The glacier continues to descend until it reaches that point where the supply of ice is just equalled by the liquefaction. There it ends, and its place down the rest of the valley is taken by the tumultuous river of muddy water which escapes from under the melting extremity of the ice. A prolonged augmentation of the snow-fall will send the foot of the glacier further down the valley; a diminution of the snowfall with a general rise of temperature will cause it to retreat farther up.
(2.) Work done by Glaciers.—Glaciers have two important geological tasks to perform,—(1) to carry the debris of the mountains down to lower levels; and (2) to erode their beds.
a. Transport.— This takes place chiefly on the surface of the glacier. Descending its valley, the glacier receives and bears along on its margin the earth, stones, and rubbish which, loosened by frost, or washed down by rain and rills, slip from the cliffs and slopes to the level of the ice. In this part of its work the glacier resembles a river which carries down branches and leaves from the woods on its banks. Most of the detritus rests on the surface of the ice. It includes huge masses of rock, sometimes as big as a large cottage, all which, though seemingly at rest, are slowly travelling down the valley with the ice, and liable at any moment to slip into the crevasses which may open below them. When they thus disappear they may descend to the bottom of the ice, and move with it along the rocky floor, which is no doubt the fate of the smaller stones and sand. But the large stones seem sometimes at least to be cast up again by the ice to the surface of the glacier at a lower part of its course. Whether, therefore, on the ice, in the ice, or under the ice, a vast quantity of detritus is continually travelling with the glacier down towards the plains. The rubbish lying on the surface is called moraine stuff. Naturally it accumulates on either side of the glacier, where it forms the so-called lateral moraines. When two glaciers unite, their two adjacent lateral moraines are brought together, and travel thereafter down the centre of the glacier as a medial moraine. A glacier, formed by the union of many tributaries in its upper parts, may have many medial lines of moraine, so many indeed as sometimes to be entirely covered with debris to the complete concealment of the ice. At such parts the glacier resembles a bare field or earthy plain rather than a solid mass of clear ice of which only the surface is dirty with rubbish. At the point where the glacier ends, the pile of loose materials is tumbled upon the valley in what is called the terminal moraine.
In such comparatively small and narrow ice-sheets as the present glaciers of Switzerland, the rock-bottom on which the ice moves is usually, as far as it can be examined, swept clean by the trickle or rush of water over it from the melting ice. But when the ice does not flow in a mere big drain (which, after all, the largest Alpine valley really is), but overspreads a wide area of uneven ground, there cannot fail to be a great accumulation of rubbish here and there underneath it. Wheu the broad central plain of Switzerland between the Alps and the Jura was filled with ice, the latter certainly pushed a vast deal of mud, sand, and stones over the floor of the valley. This material is known to Swiss geologists as the moraine profonde or Grundmoräne }
When from any cause a glacier diminishes in size, it may drop its blocks upon the sides of its valley, and leave them there sometimes in the most threatening positions. Such stranded stones are known as perched blocks. They abound in the Swiss valleys, extending even across the great plain of Switzerland, and appearing in numbers high upon the flanks of the Jura. Since the latter mountains consist chiefly of limestone, and the blocks are of various crystalline rocks belonging to the higher parts of the Alps, the proof of transport is irrefragable. The agent of removal is now recognized to have been an enormous extension of the glacier system of the Alps, whereby the whole country was buried under snow and ice. Similar evidence abounds in the valleys among the mountainous parts of Britain, as well as in other parts of Europe and America, no longer the abode of glaciers.
b. Erosion.— The manner in which glacier ice erodes its channel differs in many respects from that in use by any other geological agent, and forms therefore one of the distinguishing features of ice-action. This erosion is effected not by the mere contact and pressure of the ice upon the rocks, though undoubtedly fragments of rock must now and then be detached from this cause. It is by means of the fine sand, stones, and blocks of rock, which fall between the ice and the rocks on which it moves, that the grinding work of the glacier is done. These materials, held by the ice as it creeps along, are pressed against the rocky sides and bottom of the valley so firmly and persistently as to descend into each little hollow and mount over each ridge, yet all the while moving along steadily in one dominant direction with the general movement of the glaciers. As a result, the most compact resisting rocks are ground down, smoothed, polished, and striated. The striae vary from such fine lines as may be made by the smallest grains of quartz up to deep ruts and grooves. They sometimes cross each other, one set partially effacing an older one, and thus pointing to shiftings in the movement of the ice. On the retirement of the glacier, hummocky bosses of rock having smooth undulating forms like dolphins’ backs are conspicuous. These have received the name of roches moutonnées. The stones by which this scratching and polishing are effected suffer in exactly the same way. They are ground down and striated, and since they must move in the line of least resistance, or “end on,” their striae run in a general sense lengthwise. It will be seen, when we come to notice the traces of former glaciers, how important is the evidence given by these striated stones.
As rocks present great diversities of structure and hardness, and consequently vary much in the resistance they offer to denudation, they are necessarily worn down unequally. The softer, more easily eroded portions are scooped out by the grinding action of the ice, and basin-shaped or various irregular cavities are dug out below the level of the general surface. Similar effects may be produced by an augmented excavating power of a glacier, as where the ice is strangled in some narrow part of a valley, or where, from change in declivity, it is allowed to accumulate in greater mass as it moves more slowly onward. Such hollows, on the retirement of the ice, become receptacles for water, and form pools, tarns, or lakes, unless indeed they chance to have been already filled up with glacial rubbish. It is now some years since Professor A. C. Ramsay drew attention to this peculiar power of land-ice, and affirmed that the abundance of excavated rock-basins in northern Europe and America was due to the fact that these regions had been extensively eroded by sheets of land-ice. This glaciation was due not to independent glaciers but to the pressure and grinding power of vast masses of continental ice. In short the more northern parts of Europe and North America must have been in a condition like that of North Greenland at the present day. It is therefore among the ice-fields of Greenland rather than among the valley glaciers of isolated mountain-groups that we ought to look for analogies to the operations which produced the widespread general glaciation of the period of the rock-basins. A single valleyglacier retires towards its parent snow-field as the climate ameliorates, leaving its roches moutonnées, moraine-mounds, and rock-basins, yet at times discharging its water-drainage in such a way perhaps as to sweep down the moraine-mounds, fill up the basins, bury the ice-worn hummocks of rock, and strew the valley with gravel, earth, sand, and big blocks of rock. Hence the actual floor of the glacier is apt to be very much obscured. But in the case of a vast sheet of land-ice covering continuously a wide region, there can be but little superficial debris. When such a mass of ice retires it must leave behind it an ice-worn surface of country more or less strewn with the subsoil which accumulated under the ice and was pushed along by it. This infra-glacial detritus forms the Grundmoräne (moraine profonde) or bottom moraine. We know as yet very little regarding its formation in Greenland. Most of our knowledge regarding it is derived from a study of the till or boulder-clay of Britain, which is believed to represent the bottom moraine of an ancient ice-sheet. In countries where true boulderclay occurs, numerous rock-basins are commonly to be met with among the uncovered portions of the rocks.
The abundant fine sediment which gives the characteristic milky turbidity to all streams that escape from the melting ends of glaciers is an index of the amount of erosion unceasingly effected by the ice. From the end of the Aar glacier, for example, though by no means one of the largest in Switzerland, it has been estimated that there escape every day in the month of August 2 million cubic metres (440 million gallons) of water, containing 284,374 kilogrammes (280 tons) of sand.
B. Oceanic Waters.
The area, depth, temperature, density, and composition of the sea have been already treated of in part ii. Viewed as a dynamical agent in geology, the ocean may be studied under two aspects—(1) its movements, and (2) its geological work.
I. Movements of the Ocean.
These may be grouped as—(1) tides, (2) currents, and (3) waves.
1. Tides.— These are oscillations of the mass of the oceanic waters caused by the attraction of the sun and moon. We have at present to deal with them merely in so far as their geological bearings are concerned. In a wide deep ocean the tidal elevation probably produces no perceptible geological change. It passes at a great speed; in the Atlantic its rate is 500 geographical miles an hour. But as this is merely the passing of an oscillation whereby the particles of water are gently raised up and let down again, there can hardly be any appreciable effect upon the deep ocean bottom. When, however, the tidal wave enters a narrow and shallow sea, it has to accommodate, itself to a smaller channel, and encounters more and more the friction of the bottom, Hence, while its rate of motion is diminished, its height and force are increased. It is in shallow water and along the shores of the land that the tides acquire their main geological importance. They there show themselves in an alternate advance upon and retreat from the coast. Their upper limit has received the name of highwater mark, their lower that of low-water mark, the space between being termed the beach. If the coast is precipitous, a beach can only occur in the shelving bays and creeks, since elsewhere the tides will rise and fall against a face of rock, as they do on the piers and bulwarks of a port. On such rocky coasts the line of high water is sometimes admirably defined by the grey crust of barnacles adhering to the rocks. Where the beach is flat, and the rise and fall of the tide great, an area of several hundred square miles of sand or mud may be laid bare in one bay at low-water,
The height of the tide varies from zero up to 60 or 70 feet. It is greatest where, from the form of the land, the tidal wave is cooped up within a narrow inlet or estuary. Under such circumstances the advancing tide sometimes gathers itself into one or more large waves, and rushes furiously up between the converging shores. This is the origin of the “bore” of the Severn, which rises to a height of 9 feet, while the rise and fall of the tide there amounts to 40 feet. In like manner the tides which enter the Bay of Fundy, between Nova Scotia and New Brunswick, get more and more cooped up as they ascend that strait, till they reach a height of 70 feet.
While the tidal swelling is increased in height by the shallowness and convergence of the shores, it gains at the same time force and rapidity. No longer a mere oscillation or pulsation of the great ocean, the tide acquires a true movement of translation, and gives rise to currents which rush past headlands and through narrows in powerful currents and eddies. The rocky and intricate navigation of the west of Scotland and Scandinavia furnishes many admirable illustrations of the rapidity of these tidal currents. The famous whirlpool of Corryvreckan, the lurking eddies in the Kyles of Skye, the breakers at the Bore of Duncansbay, and the tumultuous tideway, grimly named by the northern fishermen the Merry Men of Mey, in the Pentland Firth, bear witness to the strength of these sea rivers. At the last-mentioned strait the current at its strongest runs at the rate of 10 miles an hour, which is fully three times the speed of most of our larger rivers.
2. Currents.— Recent researches in ocean temperature have disclosed the remarkable fact that beneath the surface layer of water affected by the temperature of the latitude there lies a vast mass of cold water, the bottom temperature of every ocean in free communication with the poles being little above and sometimes actually below the freezing point of fresh water. In the North Atlantic a temperature of 40° Fahr, is reached at an average depth of about 800 fathoms, all beneath that depth being progressively colder. In the equatorial parts of the same ocean the same temperature comes to within 300 fathoms of the surface. In the South Atlantic, off Cape of Good Hope, the mass of cold water (below 40°) comes likewise to about 300 fathoms from the surface. This distribution of temperature proves that there must be a transference of cold polar water towards the equator, for in the first place the temperature of the great mass of the ocean is much lower than that which is normal to each latitude, and in the second place it is lower than that of the superficial parts of the earth’s crust underneath. On the other hand, the movement of water from the poles to the equator requires a return movement of compensation from the equator to the poles, and this must take place in the superficial strata of the ocean. Apart therefore from those rapid river-like streams which traverse the ocean, and to which the name of currents is given, there must be a general drift of warm surface water towards the poles. This is doubtless most markedly the case in the North Atlantic, where besides the current of the Gulf-stream there is a prevalent set of the surface waters towards the northeast. As the distribution of life over the globe is everywhere so dependent upon temperature, it becomes of the highest interest to know that a truly arctic submarine climate exists everywhere in the deeper parts of the sea. With such uniformity of temperature we may anticipate that the abyssal fauna will be found to possess a corresponding sameness of character, and that arctic types may be met with even on the ocean-bed at the equator.
But besides this general drift or set, a leading part in oceanic circulation is taken by the more defined streams termed currents. The tidal wave only becomes one of translation as it passes into shallow water, and is thus of but local consequence. But a vast body of water, known as the Equatorial Current, moves in a general westerly direction round the globe. Owing to the way in which the continents cross its path, this current is subject to considerable deflexions. Thus that portion which crosses the Atlantic from the African side strikes against the mass of South America and divides, one portion turning towards the south and skirting the shores of Brazil, the other bending north-westward into the Gulf of Mexico, and issuing thence as the well-known Gulf-stream. This equatorial water is comparatively warm and light. At the same time the heavier and colder polar water moves towards the equator, sometimes in surface currents like those which skirt the eastern and western shores of Greenland, but more generally as a cold under-current which creeps over the floor of the ocean even as far as the equator.
Much discussion has arisen in recent years as to the cause of oceanic circulation. Two rival theories have been given. According to one of these the circulation entirely arises from that of the air. The trade-winds blowing from either side of the equator drive the water before them until the north-east and south-east currents unite in equatorial latitudes into one broad westerly-flowing current. Owing to the form of the land portions of this main current are deflected into temperate latitudes, and, as a consequence, portions of the polar water require to move towards the equator to restore the equilibrium. According to the otheι∙ view the currents arise from differences of temperature (and according to some, of salinity also); the warm and light equatorial water is believed to stand at a higher level than the colder and heavier polar water; the former, therefore, flows down as it were pole wards, while the latter moves as a bottom inflow towards the equator; the cold bottom water under the tropics is constantly ascending to the surface, whence, after being heated, it drifts away towards the pole, and on being cooled down there, descends and begins another journey to the equator. There can be no doubt that the winds are directly the cause of such currents as the Gulf-stream, .and therefore, indirectly, of return cold currents from the polar regions. It seems hardly less certain that, to some extent at least, differences of temperature, and therefore of density, must occasion movements in the mass of the oceanic waters. The discussion of this subject, however, belongs to another part of this work. The main facts for the geological reader to grasp are—that a system of circulation exists in the ocean; that warm currents move round the equatorial regions, and are turned now to the one side now to the other by the form of the continents along and round which they sweep; that cold currents set in from poles to equator; and that, apart from actual currents, there appears to be an extremely slow “creep” of the polar water under the warmer upper layers towards the equator.
3. Waves and Ground-Swell.— A gentle breeze curls into ripples the surface of water over which it blows. A strong gale or furious storm raises the surface into waves. The agitation of the water in a storm is prolonged to a great distance beyond the area of the original disturbance, and then takes the form of the long heaving undulations termed ground-swell. Waves which break upon the land are called breakers, and the same name is applied to the gr0und-3well as it bursts into foam and spray upon the rocks. The concussion of earthquakes sometimes gives rise to very disastrous earthquake-waves, as already explained.
The height and force of waves depend upon the breadth and depth of sea over which the wind has driven them, and the form and direction of the coast-line. The longer the “fetch,” and the deeper the water, the higher the waves. A coast directly facing the prevalent wind will have larger waves than a neighbouring shore which presents itself at au angle to this wind or bends round so as to form a lee-shore. The highest waves in the narrow British seas probably never exceed 15 or 20 feet, and usually fall short of that amount. The greatest height observed by Dr Scoresby among the Atlantic waves was 43 feet.
Ground-swell propagated across a broad and deep ocean produces by far the most imposing breakers. So long as the water remains deep and no wind blows, the only trace of the passing ground-swell on the open sea is the huge broad heaving of the surface. But when the water shallows, the superficial part of the swell travelling faster than the bottom begins to curl and crest as a huge billow or wall of water, which finally bursts with enormous force against the shore. Such billows, even when no wind is blowing, often cover the cliffs of the north of Scotland with sheets of water and foam up to heights of 100 or even nearly 200 feet. At Dunnet Head during north-westerly gales the windows of the lighthouse, at a height of upwards of 300 feet above high-water mark, are said to be sometimes broken by stones swept up the cliffs by the sheets of sea water which then deluge the building.
A single roller of the ground-swell 20 feet high falls, according to Mr Scott Russell, with a pressure of about a ton on every square foot. Mr Thomas Stevenson conducted some years ago a series of experiments on the force of the breakers on the Atlantic and North Sea coasts of Britain. The average force in summer was found in the Atlantic to be 611 lb per square foot, while in winter it was 2086 B>, or more than three times as great. But on several occasions, both in the Atlantic and North Sea, the winter breakers were found to exert a pressure of three tons per square foot, and at Dunbar as much as three tons and a half. Besides the waves produced by ordinary wind action, others of an extraordinary size and destructive power are occasionally caused by a violent cyclone-storm. The mere diminution of atmospheric pressure in a cyclone must tend to raise the level of the ocean within the cyclone limits. But the further furious spiral in-rushing of the air towards the centre of the low pressure area drives the sea onward, and gives rise to a wave or succession of waves having great destructive power. Thus, on 5th October 1864, during a great cyclone which passed over Calcutta, the sea rose in some places 24 feet, and swept everything before it with irresistible force, drowning upwards of 48,000 people.
4. Ice on the Sea.— In this place may be most conveniently noticed the origin and movements of the ice which in circumpolar latitudes covers the sea. This ice is derived from two sources—(1) the freezing of the sea itself, and (2) the seaward prolongation of land-ice.
1. Three chief types of sea-ice have been observed, (a.) In the Arctic sounds and bays the littoral waters freeze along the shores and form a cake of ice which, upborne by the tide and adhering to the land, is thickened by successive additions below, as well as by snow above, until it forms a shelf of ice 120 to 130 feet broad and 20 or 30 feet high. This shelf, known as the ice-foot, serves as a platform on which the abundant debris loosened by the severe frosts of an arctic winter gathers at the foot of the cliffs. It is more or less completely broken up in summer, but forms again with the early frosts of the ensuing autumn, (δ.) The surface of the open sea likewise freezes over into a continuous solid sheet which in summer breaks up into separate masses sometimes of large extent. This is what navigators term floe-ice, and the separate floating cakes are known asyfoes. Ships fixed among these floes have been drifted with the ice for hundreds of miles until at last liberated by its disruption. (c.) In the Baltic Sea, off the coast of Labrador and elsewhere, ice has been observed to form on the sea-bottom. It is known as ground-ice or anchor ice. In the Labrador fishing-grounds it forms even at considerable depths. Seals caught in the lines at those depths are brought up sometimes solidly frozen.
2. In the Arctic regions the vast glaciers which drain the snow-fields and descend to the sea extend for some distance from the land until large fragments break off and float away seawards. These detached masses are icebergs. Their shape and size greatly vary, but lofty peaked forms are common, and they sometimes rise from 200 to 300 feet above the level of the sea. As only about a ninth part of the ice appears above water, these larger bergs must sometimes be from 2000 to 3000 feet thick from base to top. They consequently require water of some depth to float them, but they are often seen aground. In the Antarctic regions, where one vast sheet of ice envelops the land and extends into the sea as a high rampart of ice, the detached icebergs often reach a great size, and are characterized by the frequency of a flat tabular form.
II. Geological Wokk of the Sea.
I. Influence on Climate. —Were there no agencies in nature for distributing temperature, there would be a regular and uniform diminution in the mean annual temperature from equator to poles, and the isothermal lines, or lines of equal heat, would coincide with lines of latitude. But no such general correspondence actually exists. If we look at a chart of the globe with the isothermal lines drawn across it, we shall find that their divergences from the parallels are striking, and most so where they approach and cross the ocean. Currents from warm regions raise the temperature of the tracts into which they flow: those from cold regions lower it. The ocean, in short, is the great distributor of temperature over the globe. As an illustration the two opposite sides of the Atlantic may be taken. The cold arctic current flowing southward along the north-east coast of America reduces the mean annual temperature of that region. On the other hand, the Gulf-stream brings to the shores of the north-west of Europe a temperature much above what they would otherwise enjoy. Dublin and the south-eastern headlands of Labrador, lie in the same parallel of latitude, yet differ as much as 18° in their mean annual temperature, that of Dublin being 50°, and that of Labrador 32° Fahr. Dr Croll has calculated that the Gulfstream conveys nearly half as much heat from the tropics as is received from the sun by the entire Arctic regions.
II. Erosion. —The chemical action of the sea upon the rocks of its bed and shores has not yet been properly studied. It is evident, however, that changes analogous to those effected by fresh water on the land must be in progress. Oxidation, and the formation of carbonates, no doubt continually take place. We may judge indeed of the nature and rapidity of some of these changes by watching the decay of stones and material employed in the construction of piers. At the Bell Rock lighthouse, twenty-five different kinds and combinations of iron were exposed to the action of the sea, and all yielded to corrosion. Mr Mallet—as the result of experiments with specimens sunk in the sea—concluded that from - r ¾ths ∙⅛ths of an inch in depth in iron castings 1 inch thick, and about 1 ⅜∙ths of an inch of wrought iron, will be destroyed in a century in clear salt water. Mr Stevenson, in referring to these experiments, remarks that he has in his possession specimens of iron which show even a more rapid rate of decay. In castings used at the Bell Rock the loss has been at the rate of an inch in a century. “One of the bars which was free from air holes had its specific gravity reduced to 5 - 63, and its transverse strength from 7409 to 4797 K>, and yet presented no external appearance of decay. Another apparently sound specimen was reduced in strength from 4068 lb to 2352 Bb, having lost nearly half its strength in fifty years.” Similar results were recently observed by Mr Grothe, resident engineer at the railway bridge across the Firth of Tay. A cast-iron cylinder which had been below water for only sixteen months was found to be so corroded that a penknife could be stuck through it in many places.
An examination of the shore will sometimes reveal a good deal of quiet chemical change on the outer crust of rocks exposed to the waves. Such rocks as basalt have their felspar decomposed, and show the presence of carbonates by effervescing briskly with acid. One of their minerals, augite, is occasionally replaced by pseudomorphs of carbonate of iron.
It is mainly by its mechanical action that the sea accomplishes its erosive work. This can only take place where the water is in motion, and, other things being equal, is greatest where the motion is strongest. Hence we cannot suppose that erosion to any appreciable extent can be effected in the abysses of the sea, where the only motion possible is that slow creeping of the polar water along the bottom already referred to. But where the currents are powerful enough to move grains of sand and gravel, very slow erosion may take place even at considerable depths. It is in the upper portions of the sea, however, where, owing to currents, tides, and waves, the water suffers most disturbance, that the main mechanical erosion goes on. The depth to which the influence of waves and ground-swell may extend seems to vary greatly according to the situation. The astronomer-royal states that groundswell may break in 100 fathoms water. It is common to find boulders and shingle disturbed at a depth of 10 fathoms, and even driven from that depth to the shore, and waves may be noticed to become muddy from the working up of the silt at the bottom when they have reached water of 7 or 8 fathoms in depth, Gentle movement of the bottom water is said to be sometimes indicated by ripple-marks on the fine sand of the sea-floor at a depth of 600 feet. A good test for the absence of serious abrasion is furnished by the presence of fine mud on the bottom. Wherever that is found, we may be tolerably sure that the bottom at that place lies beyond the reach of ordinary breaker action. From the upper limit at which the accumulation of mud is possible to high-water mark, and in exposed places up to 100 feet or more above high-water mark, lies the zone within which the sea does its work of abrasion. To this zone, even where the breakers are heaviest, a greater extreme vertical range can hardly be assigned than 300 feet, and in most cases it probably falls far short of that extent.
The mechanical work of erosion by the sea is done in four ways :—(1) the enormous force of the breakers suffices to tear off fragments of the solid rocks; (2) the alternate compression and expansion of the air in the crevices of rocks exposed to heavy breakers dislocates rocks even above the limits of wave-action; (3) the hydraulic pressure of those portions of large waves which enter fissures and cavities forces asunder masses of rock; (4) the waves make use of the loose fragments within their reach in battering down the cliffs exposed to their fury.
(1.) Abundant examples of the dislodgement of huge blocks of rock from their parent masses are furnished by the precipitous shores of Caithness, and of the Orkney and Shetland Islands. It sometimes happens that demonstration of the height to which the effective force of breakers may reach is furnished at lighthouses built on exposed parts of the coast. Thus, at Unst, a door was broken open at a height of 195 feet above the sea, and at the Bishop Rock lighthouse a bell was wrenched off at a level of 100 feet above high-water mark. Some of the most remark able instances of the power of breakers have been observed by Mr Thomas Stevenson among the islands of the Shetland group. On the Bound Skerry he found that blocks of rock up to 9 ∣ tons in weight had been washed together at a height of nearly 60 feet above the sea, that blocks weighing from 6 to 13∣ tons had been actually quarried out of their original bed, at a height of from 70 to 75 feet, and that a block of nearly 8 tons had been driven before the waves at the level of 20 feet above the sea, over very rough ground, to a distance of 73 feet (figs. 7 and 8). He likewise records the moving of a 50-ton block by the waves at Barrahead, in the Hebrides. At Plymouth also, blocks of several tons in weight have been known to be washed about the breakwater like pebbles.
(2.) But, besides their mechanical force, waves acquire a singular and most effective aid from the air. It is a fact familiar to engineers that, even from a vertical and apparently perfectly solid wall of well-built masonry exposed to heavy seas, stones will sometimes be started out of their places, and that when this happens a rapid enlargement of the cavity may be effected, as if the walls were breached by a severe bombardment. At the Eddystone lighthouse, during a storm in 1840, a door which had been securely fastened against the force of the surf from without, was actually driven outward by a pressure acting from within the tower, in spite of the strong bolts and hinges, which were broken. We may infer that, by the sudden sinking of a mass of water hurled against the building, a partial vacuum was formed, and that the air inside forced out the door in its efforts to restore the equilibrium. This explanation may partly account for the way in which the stones are started from their places in a solidly built sea-wall. But besides this cause we must also consider a perhaps still more effective one in the condensation of the air driven before the wave between the joints and crevices of the stones, and its subsequent instantaneous expansion when the wave drops. During gales when large waves are driven to shore, many tons of water are poured suddenly into each cleft and cavern within reach. These volumes of. water, as they rush in, compress the air into every joint and pore of the rock at the further end, and then quickly retiring, exert such a suction as from time to time to bring down part of the walls or roof. The sea may thus gradually form an inland passage for itself to the surface above, in a “blow-hole” or “puffing-hole,” through which spouts of foam and spray are in storms shot high into the air. On the more exposed portions of the west coast of Ireland numerous examples of such blow-holes occur. In Scotland, likewise, they may often be observed, as in the Bullers (boilers) of Buchan on the coast of Aberdeenshire, and the Geary Pot near Arbroath. Magnificent instances occur among the Orkney and Shetland Islands, some of the more shattered rocks of these northern coasts being, as it were, honeycombed by seatunnels, many of which open up into the middle of fields or moors.
(3.) The sea-water which, as part of an inrushing wave, fills the gullie3 and chinks of the shore-rocks exerts the same pressure upon the walls between which it is confined as the rest of the wave is doing upon the face of the cliff. Each cleft so circumstanced becomes a kind of hydraulic press, the potency of which is to be measured by the force with which the waves fall upon the rocks outside—a force which often amounts to three tons on the square foot. There can be little doubt that by this means considerable pieces of a cliff are from time to time dislodged.
(4.) But probably by far the largest amount of erosion accomplished by the sea is due not to its own direct mechanical impetus, but to the blows dealt by the boulders, gravel, or sand which it hurls against the shores. This action was aptly compared by Playfair to a kind of artillery. During a storm upon a shingly coast we may hear, at a distance of several miles, the grind of the stones against each other, as they are dragged back by the recoil of the waves which had launched them forward. In this tear and wear the loose stones are ground smaller, and acquire the smooth round form so characteristic of a surf-beaten beach. At the same time they bruise and wear down the solid cliffs against which they may be driven. Wherever the rock is much jointed, or from any cause presents less resistance to attack, it is excavated into gullies, creeks, and caves; its harder parts standing out as promontories are pierced; gradually a series of detached buttresses and sea· stacks appears as the cliff recedes, and these in turn are wasted until they become mere skerries and sunken surf-beaten reefs. Of this progress of destruction the more exposed parts of the British coast-line furnish many admirable examples. The west coast of Ireland, exposed to the full swell of the Atlantic, is in innumerable localities completely undermined by caverns, into which the sea enters from both sides. In many places the cliffs are as vertical as walls, this feature depending upon their joints, which enable slice after slice to be undermined and removed. The precipitous coasts of Skye, Sutherland, Caithness, Forfar, Kincardine, and Aberdeenshire abound in the most impressive lessons of the waste of a rocky sea-margin; while the same picturesque features are prolonged into the Orkney and Shetland Islands, the magnificent cliffs of Hoy towering as a vast wall some 1200 feet above the Atlantic breakers, which are tunnelling and fretting their base.
If such is the progress of waste where the materials consist of the most solid rocks, we may expect to meet with at least equally impressive proofs of decay where the coast-line can oppose only soft sand or clay to the march of the breakers. Again, the geological student in Britain can examine for himself many illustrations of this kind of destruction around the shores of these islands. Within the last few hundred years entire parishes with their towns and villages have been washed away, and the tide now ebbs and flows over districts which in old times were cultivated fields and cheerful hamlets. The coast of Yorkshire between Flamborough Head and the mouth of the Humber, and also that between the Wash and the mouth of the Thames, suffer at a specially rapid rate, for the cliffs in these parts consist in great measure of soft clay. In some places this loss iβ said to amount to 3 feet per annum.
While investigating the proofs of decay along the shore, the geologist endeavours to ascertain to what extent the action of the waves is assisted by that of rain, springs, frosts, and general atmospheric disintegration. He often finds that the progress of the waves depends not so much upon then own labours as upon those of the terrestrial agencies already described. A crumbling cliff, battered and wasted by the breakers, will yield to him abundant evidence of the manner in which the other agents of destruction prepare the way for its final demolition and removal by the sea; and he will learn that the very blocks of stone which give the waves so much of their efficacy are in great measure furnished to them by these co-operating agents. If the cutting back of a cliff were mainly the work of the sea, we ought to find the cliff overhanging, because the sea acts only at its base. But the fact that in the vast majority of cases sea-cliffs, instead of overhanging, slope backward, at a greater or less angle, from the sea, shows that the waste from subaerial action is really greater than that from the action of the breakers. What the sea chiefly does is to break down and wash away the rubbish that falls from the cliff's, and thus to leave an ever fresh surface for renewed denudation.
(5.) Among the erosive operations of the sea must be included what is performed by floating ice. Along the margin of arctic lands a good deal of work is done by the broken up floe-ice and ice-foot. These cakes of ice, driven ashore by storms, tear up the soft shallow-water or littoral deposits, rub and scratch the rocks, and push gravel and blocks of rock before them as they strand on the beach. Icebergs also, when they get aground in deep water, must greatly disturb the sediment accumulating there, and must grind down any submarine rock on which they grate as they are driven along.
The general result of the erosive action of the sea on the land is the production of a submarine plain. As the sea advances by cutting slice after slice away from the coast, successive lines of beach pass under low-water mark. The whole of the littoral belt, as far down as wave action has influence, is continually being ground down by the moving detritus. If no change of level between sea and land should take place, the sea might conceivably eat its way slowly far into the land, and produce a gently sloping yet almost horizontal selvage of plain covered permanently by the waves. In such a submarine plain the influence of geological structure, and notably of the relative powers of resistance of different rocks, would make itself conspicuous. The present promontories caused by the superior hardness of their component rocks would no doubt be represented by ridges on the subaqueous plateau, while the existing bays and creeks worn out of softer rocks would be marked by lines of valleys or hollows.
III. Transport. —The sea by means of its surface-drifts and currents carries sedimentary material to great distances and strews them over its floor. Near land, where the movements of the water are active, much coarse detritus is transported along shore or swept farther out to sea. A prevalent wind, by creating a current in a given direction along a coast-line, will cause the shingle to travel coastwise, the stones getting more and more rounded and reduced in size as they recede from their sources. The Chesil Bank, which runs as a natural breakwater 16 miles long connecting the Isle of Portland with the mainland of Dorsetshire, consists of rounded shingle which is constantly being driven westwards. On the Moray Firth the reefs of quartz-rock about Cullen furnish abundance of shingle, which moves westwards along the coast for more than 15 miles. The coarser sediment probably seldom goes much beyond the littoral zone. Fine gravel, however, is pushed along the bottom by currents even at 600 fathoms; for at that depth in the North Atlantic between the Faroe Islands and Scotland small pebbles of volcanic and other rocks are dredged up which have probably been carried by an arctic under-current from the north. At greater depths the force of currents at the bottom must be too feeble to push along any detritus. But much fine sediment is carried in suspension by the sea for long distances from land. Some rivers, as the Amazon, pour so much silt into the sea as to discolour its water for several hundred miles away from land. After wet weather the coast-waters round the shores of the British Islands are sometimes made turbid from the quantity of mud brought down from the land. Dr Carpenter found the bottom waters of the Mediterranean to be everywhere permeated by an extremely fine mud, derived no doubt from the rivers and shores of that sea, borne away out far from land, and settling slowly down upon the bottom. He remarks that the characteristic blueness of the Mediterranean may be explained, like that of the Lake of Geneva (as shown by Dr Tyndall), by the diffusion of those exceedingly minute sedimentary particles through the water.
But the most startling evidence of the wide extent to which transport takes place in the ocean is that supplied by the observations made during the voyage of the “Challenger.” From the abysses of the Pacific Ocean, at the furthest distances from land, the dredge brought up bushels of rounded pieces of pumice of all sizes up to blocks a foot in diameter. These fragments were all evidently water-worn, and almost certainly were derived from the land. Some small pieces indeed were taken on the surface in the tow-net. Round volcanic islands, and off the coasts of volcanic tracts of the mainland, the sea is sometimes covered with floating pieces of water-worn pumice swept out by flooded rivers. These fragments drift away for hundreds or even thousands of miles until, becoming water-logged, they sink to the bottom. Their universal distribution was one of the most noticeable features in the dredgings of the “Challenger.” The clay which is found on the bottom of the ocean at the greatest distances from any shore may be partly due to its transport in that condition from land, but more probably to the decomposition of the drifted pumice.
Another not unimportant process of marine transport is that performed by floating ice. Among the arctic glaciers moraine stuff is of rare occurrence; but occasional blocks of rock and heaps of earth and stones fall from the cliffs which rise above the general waste of snow. Hence on the icebergs that float off from these glaciere, rock debris sometimes may be observed, It is transported southward for hundreds of miles until, by the shifting or melting of the bergs, it is dropped into deep water. The floor of certain portions of the North Atlantic in the pathway of the bergs must be plentifully strewn with this kind of detritus. By means of the ice-foot also, an enormous quantity of earth and stones is every year borne away from the shore as the ice breaks up, and strewn over the floor of the sounds, bays, and channels.
IV. Reproduction. —The sea being the receptacle for the material worn away from the land must receive and store up in its depths all that vast amount of detritus by the removal of which the level and contours of the land are in the course of time so greatly changed. The deposits which take place within the area covered by the sea may be divided into two groups-—the inorganic and organic. It is the former with which we have at present to deal; the latter will be discussed with the other geological functions of plants and animals. The inorganic deposits of the seafloor are partly (a) land-derived or terrigenous, partly (δ) abyssal.
(a.) Land-derived or Terrigenous.— These may be conveniently grouped according to their relative places on the sea-bed.
(1.) Shore Deposits.—The most conspicuous and familiar are the layers of gravel and sand which accumulate between tide-marks. As a rule, the coarse materials are thrown up about the upper limit of the beach. They seem to remain stationary there; but if watched and examined from time to time, they will be found to be continually shifted by high tides and storms, so that the bank or bar of shingle retains its place though its component pebbles are being constantly moved. Below the limit of coarse shingle upon fhe beach lies the zone of fine gravel, and then that of sand. These zones are far from being constant; yet when they all occur on the same beach, they tend to range themselves according to their relative coarseness, the rougher detritus lying at the upper, and the finer towards the lower edge of the shore. The nature of the littoral accumulations on any given part of a coast-line must depend either upon the character of the shore-rocks which at that locality are broken up by the waves, or upon the set of the shore-currents and the kind of detritus they bear with them. Coasts exposed to heavy surf, especially where of a rocky character, are apt to present beaches of coarse shingle between their projecting promontories. Sheltered bays, on the other hand, where wave action is comparatively feeble, afford a gathering ground for fine sediment such as sand and mud. Estuaries and inlets into which rivers enter frequently show wide muddy flats at low water. The mud brought down by the fresh water is allowed to sink to the bottom when the motion of the current is checked as it enters the sea.
(2.) Infra-Littoral and Deeper-Water Deposits.—These extend from below low-water mark to a depth of sometimes as much as 2000 fathoms, and reach a distance from land varying up to 200 miles or even more. Near land, and in comparatively shallow water, they consist of banks or sheets of sand more rarely mixed with gravel. The bottom of the North Sea, for example, which between Britain and the continent of Europe lies at a depth never reaching 100 fathoms, is irregularly marked by long ridges of sand enclosing here and there hollows where mud has been deposited. In the English Channel large banks of gravel extend through the Straits of Dover as far as the entrance to the North Sea. These features seem to indicate the line of the chief mud-bearing streams from the land, and the general disposition of currents and eddies in the sea which covers that region, the gravel ridges marking the tracks of the more rapidly moving currents, while the muddy hollows point to the eddie≡ where the fine sediment is permitted to settle on the bottom. It is possible, however, that the inequalities on the floor of the North Sea, and their peculiarities of sediment, may not be due wholly to modern accumulations, but partly to the contour of the ground before it was submerged and the land connexion between Britain and Europe was destroyed
During the course of the voyage of the “Challenger,” the approach to land could always be foretold from the character of the bottom, even at distances of 150 and 200 miles from land. The deposits were found to consist of blue and green muds derived from the degradation of older crystalline rocks. At depths of 100 to 700 fathoms they are often coloured green by glauconite. At greater depths they consist of blue or dark slate-coloured mud with a thin upper layer of red or brown. Throughout these land-derived sediments particles of mica, quartz, and other minerals are distributed, the materials becoming coarser towards land. Pieces of wood, portions of fruits, and leaves of trees occur in them, and further indicate the reality of the transport of material from the land. Shells of pteropods, larval gasteropods, and lamellibranchs are tolerably abundant in these muds, with many infra-littoral species of Foraminifera, and diatoms. Below 1500 or 1700 fathoms pteropod shells seldom appear, while at 3000 fathoms hardly a foraminifer or any calcareous organism remains (Murray, Proc. Roy. Soc. Edin., 1876, p. 519). Round volcanic islands the bottom is found to be covered with grey mud and sand derived from the degradation of volcanic rocks. These deposits can be traced to great distances, as at Hawaii for 200 miles or more. Pieces of pumice, scoriae, <fcc.∙, occur in them, mingled with marine organisms, and more particularly with abundant grains, incrustations, and nodules of an earthy peroxide of manganese. Near coral-reefs the sea-floor is coated with a white calcareous mud derived from the abrasion of the corah The east coast of South America supplies a peculiar red mud which is spread over the Atlantic slope down to depths of more than 2000 fathoms.
(6.) Abyssal.— Passing over at present the organic deposits which form so characteristic a feature on the floor of the deeper and more open parts of the ocean, we come to certain red and grey clays found at depths of more than 2000 fathoms down to the bottoms of the deepest abysses, These consist of exceedingly fine clay, coloured sometimes red by iron-oxide, sometimes of a chocolate tint from manganese oxide, with grains of quartz, mica, pumice, scoriae, peroxide of manganese, and other mineral substances, together with Foraminifera, and in some regions a large proportion of siliceous Radiolaria. Mr Murray has shown the high probability that these clays result from the decomposition of pumice and fine volcanic dust transported from volcanic islands into mid-ocean. The extreme slowness of their deposit is strikingly brought out in the tracts farthest removed from land. From these localities great numbers of sharks’ teeth, with ear-bones and other bones of whales, were dredged up in the “Challenger” expedition,—some of them quite fresh, others partially crusted with peroxide of manganese, and some completely and thickly surrounded by that substance. We cannot suppose that sharks and whales so abound in the sea as to cover the floor of the ocean with a continuous stratum of their remains. No doubt each haul of the dredge which brought up so many bones represented the droppings of many generations. The successive stages of manganese incrustation point to a long, slow, undisturbed period, when so little sediment accumulated that the bones dropped at the beginning remained at the end still uncovered, or only so slightly covered as to be easily scraped up by the dredge. In these deposits, moreover, Mr Murray has found numerous minute spherular particles of metallic iron which there is every reason to believe are of cosmic origin—portions of the dust of meteorites which in the course of ages have fallen upon the seabottom. Such particles no doubt fall all over the ocean; but it is only on those parts of the bottom which, by their distance from any land, receive accessions of deposit with extreme slowness, and where therefore the present surface may contain the dust of a long succession of years, that it has been possible to detect them.
The abundant deposit of peroxide of manganese over the floor of the deep sea is one of the most singular features of recent discovery. It occurs as an earthy incrustation round bits of pumice, bones, and other objects. The nodules possess a concentric arrangement of lines not unlike those of urinary calculi. That they are formed on the spot, and not drifted from a distance, was made abundantly clear from their containing abyssal organisms, and enclosing more or less of the surrounding bottom, whatever its nature might happen to be. Mr Murray refers their origin to the decomposition of the manganese-bearing minerals in the universally diffused volcanic detritus. Quite recently Mr J. Y. Buchanan has dredged similar manganese concretions from some of the deeper parts of Loch Fyne. In connexion with the chemical reactions indicated by these nodules as taking place on the sea-bottom, reference may be made to a still more remarkable but yet unpublished discovery made by Mr Murray in the course of his examinations of the materials brought up from the same abyssal deposits. He has detected abundant minute concretions or bundles of crystals which on analysis are found to resemble olivine in composition. These silicates (there may be several of them) have certainly been formed directly on the sea-bottom, for they are found gathered round abyssal organisms. It is difficult to overestimate the importance of this fact in reference to the chemistry of marine deposits.
From a comparison of the results of the dredgings made in recent years in all parts of the oceans, it is impossible to resist the conclusion that there is nothing in the character of the deep-sea deposits which finds a parallel among the marine geological formations visible to us on the land. It is only among the comparatively shallow water accumulations of the existing sea that we encounter analogies to the older formations. And thus we reach by another and a new approach the conclusion which on very different grounds has been arrived at, viz., that the present continental ridges have existed from the remotest times, and that the marine strata which constitute so large a portion of their mass have been accumulated not as deep water formations, but in comparatively shallow water along their flanks.
Section III.—Life.
Among the agents by which geological changes are carried on upon the surface of the globe living organisms must be enumerated. Both plants and animals co-operate with the inorganic agents in promoting the degradation of the land; and in some cases, on the other haud, they protect rocks from decay. Again by the accumulation of their remains they form extensive formations both upon the land and in the sea. Their operations may hence be described as alike destructive, conservative, and reproductive. Under this heading also we may notice the influence of man as a geological agent.
I. Destructive Action. —Plants aid in the general progress of disintegration in various ways. 1. By keeping the surfaces of rocks moist, and thus promoting both the mechanical and chemical dissolution of the rocks. This action is especially shown by liverworts, mosses, and other plants which only thrive in copious moisture. 2. By producing through their decay carbonic and other acids, which, with decaying organic matter taken up by passing moisture, become potent in effecting the chemical decomposition of rocks, and in promoting the disintegration of soils. 3. By inserting their roots or branches between joints of rock, which are thereby loosened, so that large slices may be eventually wedged off. On the sides of wooded hills and cliffs this process may often be seen; even among old ruins an occasional sapling ash or elm may be found to have cast its roots round a portion of the masonry and to be slowly detaching it from the rest of the wall. 4. By attracting rain, as thick woods, forests, and mosses do, and thus accelerating the general scouring of a country by running water. The indiscriminate destruction of the woods in the Levant has been assigned with much plausibility as the main cause of the present desiccation of that region. 5. By promoting the decay of diseased and dead plants and animals, as when fungi overspread a damp rotting tree or the carcase of a dead animal.
Of the destructive influences of animal life numerous illustrations might be given. 1. The composition and arrangement of soil are affected. Worms are continually engaged in bringing up the lower portions of the soil to the surface, and thus increase its fertility and its capability of being washed away by rain, Burrowing animals, by throwing up the soil and subsoil, expose these to be dried and blown away by the wind. At the same time their subterranean passages serve to drain off the superficial water and to injure the stability of the surface of the ground above them. In Britain the mole aud rabbit are familiar examples. In North America the prairie dog has undermined extensive tracts of pasture land in the west. In Cape Colony wide areas of open country seem to be in a constant state of eruption from the burrowing operations of multitudes of Dathyergi and Chrysochloris— small mole-like animals which bring up the soil and bury the grassy vegetation under it. 2. The flow of streams is sometimes interfered with, or even diverted, by the operations of animals. Thus the beaver, by constructing dams, checks the current of water-courses, intercepts floating materials, and sometimes even diverts the water into new channels. This action is typically displayed in Canada and other parts of North America. The embankments of the Mississippi are sometimes weakened to such an extent by the burrowings of the cray-fish as to give way and allow the river to inundate the surrounding country. Similar results have happened in Europe from the subterranean operations of rats. 3. Some Mollusca (P holas, Saxicava, Teredo, &c.) bore into stone or wood, and by the number of contiguous perforations greatly weaken the material. Pieces of drift-wood are soon riddled with long holes by the teredo; while wooden piers, and the bottom of wooden ships, are often rapidly perforated. The saxicavous shells, by piercing rocks and leaving open cavities for rain and sea water to fill, promote the decay of the stone. 4. Many animals exercise a ruinously destructive influence upon vegetation. Of the many insect plagues of that kind it will be enough to enumerate the locust, phylloxera, and Colorado beetle. The pasture in some parts of the south of Scotland has in recent years been much damaged by mice, which have increased in numbers owing to the indiscriminate shooting and trapping of owls, hawks, and other predaceous creatures. Grasshoppers cause the destruction of vegetation in some parts of Wyoming and other western territories of the United States. The way in which animals destroy each other, often on a great scale, may likewise be included among the geological operations now under description.
IT. Conservative Action. —This is admirably shown by many kinds of vegetation. 1. The formation of a stratum of turf protects the soil and rocks from being rapidly disintegrated and washed away by atmospheric action. Hence the surface of a district so protected is denuded with extreme slowness except along the lines of its water-courses. 2. Many plants, even without forming a layer of turf, serve by their roots or branches tα protect the loose sand or soil on which they grow from being removed by wind. The common sand-carex and other arenaceous plants bind the loose sand-dunes of our coasts, and give them a permanence which would at once be destroyed were the sand laid bare again to storms. In North America the sandy tracts of the western territories are in many places protected by plants known as sage-brush and grease-wood. The growth of shrubs and brushwood along the course of a stream not only keeps the alluvial banks from being so easily undermined and removed as would otherwise be the case, but serves to arrest the sediment in floods, filtering the water, and thereby adding to the height of the flood plain. On some parts of the west coast of France extensive ranges of sand-hills have been gradually planted with pine woods which, while preventing the destructive inland march of the sand, also yield a large revenue in timber, and have so improved the climate as to make these districts a resort for pulmonary invalids. In tropical countries the mangrove grows along the margin of the sea, and not only protects the land, but adds to its breadth, by forming and increasing an alluvial belt along the coast. 3. Some marine plants likewise afford protection to shore rocks. This is done by the calcareous nullipores, which form upon them a hard incrustation; likewise by the tangles and smaller fuci which grow abundantly on the littoral zone and break the force of the waves, or diminish the effects of ground swell. 4. Forests and brushwood protect the soil, especially on slopes, from being washed away by rain. This is shown by the disastrous results of the thoughtless destruction of such woods. According to Reclus (La Terre, p. 410), in the three centuries from 1471 to 1776, the “vigueries,” or provostry-districts of the French Alps, lost a third, a half, and even three-fourths of their cultivated ground, and the population has diminished in somewhat similar proportions. From 1836 to 1866 the departments of Hautes and Basses Alpes lost 25,000 inhabitants, or nearly one-tenth of their population—a diminution which has with plausibility been assigned to the reckless removal of the pine forests, whereby the steep mountain sides have been washed bare of their soil. The desiccation of the countries bordering the eastern Mediterranean has been ascribed to a similar cause. 5. In mountain districts pine forests exercise also an important conservative function in preventing the formation or arresting the progress of avalanches. In Switzerland some of the forests which cross the lines of frequent snow-falls are carefully preserved.
Animals do not exert any important conservative action upon the earth’s surface, save in so far as they form new deposits, as will be immediately referred to. In the prairie regions of Wyoming and other tracts of North America, some interesting minor effects are referable to the herds of roving animals which migrate over these territories. Professor Comstock describes the trails made by the bison, the elk, and the big-horn or mountaimsheep as firmly-trodden tracks on which vegetation will not grow for many years. All over the region traversed by the bison numerous circular patches of grass are to be seen which have been formed on the hollows where this animal has wallowed. Originally they are shallow depressions formed in great numbers where a herd of bisons has rested for a time. On the advent of the rains they become pools of water; thereafter grasses spring up luxuriantly, and so bind the soil together that these grassy patches, or “bison-wallows,” may actually become slightly raised above the general level if the surrounding ground becomes parched and degraded by the winds (Reconnaissance of H. W. Wyoming, 1875, p. 175).
III. Reproductive Action. —Both plants and animals contributematerialstowardsnewgeologicalformations. Their remains are enclosed in deposits of sand and mud and there preserved. But they form of themselves not unimportant accumulations. Of plant formations the following illustrative examples may be given. (1.) Peat-Mosses.— These are accumulations of marshy vegetation which occur in temperate and arctic latitudes, sometimes to a depth of 40 feet or more. In Europe they have been largely formed by plants of the genus Sphagnum, which, growing as a spongy fibrous mass over wet ground, die in their lower parts and send out new fibres above. It is this lower decaying stratum which forms the peat. Every stage of the process may be seen in a large moss, from the green living plants at the top, through fibrous brown turf full of the scarcely decayed rootlets of the Sphagnum, down to the compact browu or almost black peat at the bottom. Many peatmosses were at one time lakes which have been gradually filled up by the accumulation of marsh-plants. Peat possesses a great antiseptic power; the bodies of animals which have been entombed in it are sometimes preserved for many centuries. (2.) Mangrove Swamps.— On the low moist shores and river mouths of tropical countries, the mangrove tree plays an important geological part. It grows in such situations in a dense jungle, sometimes 20 miles broad, which fringes the coast as a green selvage, aud runs up if it does not quite occupy creeks and inlets. The mangrove flourishes in sea-water even down to low-water mark, forming there a dense thicket which, as the trees drop their radicles and take root, grows outward into the sea. It is singular to find terrestrial birds nestling in the branches above and crabs and barnacles living among the roots below. By this network of subaqueous radicles and roots the water is filtered of its sediment, which, retained among the vegetation, helps to turn the spongy jungle into a firm soil. On the coast of Florida the mangrove swamps stretch for long distances as a belt from 5 to 20 miles broad, which winds round the creeks and inlets. At Bermuda the mangroves co-operate with grasses and other plants to choke up the creeks and brackish lakes. In these waters calcareous algae abound, and as their remains are thrown up amidst the sand aud vegetation they form a remarkably calcareous soil. (3.) Diatom Mud or Earth.— As the minute siliceous plants called diatoms occur both in *resh and salt water, the deposit formed from their congregated remains is found both on the sites of lakes and on the sea-floor. “Infusorial” earth and “tripoli powder” consist mainly of the frustules and fragmentary debris of diatoms which have accumulated on the bottoms of lacustrine areas. Towards the Antarctic circle the “Challenger” met with Diatomacece in abundance, both in the surface waters of the ocean and on the bottom. They form at depths of from 1260 to 1975 fathoms a pale straw-coloured deposit, which when dried is white and very light.
Animal formations are chiefly composed of the remains of the lower grades of the animal kingdom, especially of Mollusca, Actinozoα, and Foraminifera. (1.) In some cases they are calcareous. Lime, chiefly in the form of carbonate, is the mineral substance of which the solid parts of animals are mainly built up. Hence the great majority of the accumulations formed of animal remains are calcareous. In fresh water they are represented by the marl of lakes—a white, chalky deposit consisting of the mouldering remains of Mollusca, Entomostraca, and partly of fresh-water algae. On the sea-bottom in shallow water they consist of beds of shells, such as the oyster-banks of English seas. The fringing, barrier, and atoll coral-reefs of warm seas are conspicuous examples of wide and thick masses of rock formed from the accumulated growth of animal organisms. The great reef of Australia, for example, is 1250 miles long, from 10 to 90 miles broad, and more than 1800 feet thick. The coral rock, though formed by the continuous growth of the polyps, gradually loses any distinct organic structure, and acquires an internal crystalline character owing to the infiltration of water through its mass, whereby carbonate of lime is carried down and deposited in the pores and crevices as in a growing stalactite. Great quantities of calcareous mud are produced by the breakers which beat upon the outer edge of the reefs. This mud is partly washed up upon the reefs and aids in their consolidation, but in great measure it is swept away by the ocean currents and distributed over many thousands of square miles of the sea-floor. In deep water over the bed of the Atlantic and other oceans a remarkable calcareous ooze occurs which is formed of the remains of Foraminifera, and chiefly of species of the genus Globigervna. It is next in abundance to the red and grey clays of the deep sea. It is a pale-grey marl, sometimes red from peroxide of iron, or brown from peroxide of manganese; and it usually contains more or less clay, even with occasional fragments of pumice. (2.) Siliceous deposits formed from animal exuviae are illustrated by another of the deep-sea formations brought to light by the“Challenger” researches. In certain regions of the western and middle Pacific Ocean, the bottom was found to be covered with an ooze consisting almost entirely of Radiolaria. These minute organisms occur, indeed, more or less abundantly in almost all deep oceanic deposits. From the deepest sounding yet taken (4575 fathoms, or more than 5 miles) a radiolarian ooze was obtained. The spicules of sponges likewise furnish materials towards these siliceous accumulations. (3.) Phosphatic deposits, in the great majority of cases, betoken some of the vertebrate animals, seeing that phosphate of lime enters largely into the composition of their bones and occurs in their excrement. The most typical modern accumulations of this nature are the guano beds of rainless islands off the western coasts of South America and Southern Africa. In these regions immense flocks of sea-fowl have in the course of centuries covered the ground with an accumulation of their droppings to a depth of sometimes 30 to 80 feet, or even more. This deposit, consisting chiefly of organic matter and ammoniacal salts, with about 20 per cent, cf phosphate of lime, has acquired a high value as a manure, and is being rapidly cleared off. It could only have been preserved in a rainless or almost rainless climate. On the west of Europe isolated stacks and rocky islands in the sea are often seen to be white from the droppings of clouds of sea birds; but it is merely a thin crust, which is not allowed to grow thicker in a climate where rains are frequent and heavy.
IV. Man as a Geological Agent. —No survey of the geological workings of plant and animal life upon the surface of the globe can be complete which does not take account of the influence of man—an influence of enormous and increasing consequence in physical geography, for man has introduced, as it were, an element of antagonism to nature. Not content with gathering the fruits and capturing the animals which she has offered for his sustenance, he has, with advancing civilization, engaged in a contest to subdue the earth and possess it. His warfare indeed has often been a blind one, successful for the moment, but leading to sure and sad disaster. He has, for instance, stripped off the woodland from many a region of hill and mountain, gaining his immediate object in the possession of their stores of timber, but thereby laying bare the slopes to parching droughts or fierce rains. Countries once rich in beauty, and plenteous in all that was needful for his support, are now burnt and barren, or washed bare of their soil. It is only in comparatively recent years that he has learnt the truth of the aphorism— “Homo Naturae minister et interpres.”
But now, when that truth is coming more and more to be recognized and acted on, man’s influence is none the less marked. His object still is to subdue the earth, and he attains it, not by setting nature and her laws at defiance, but by enlisting her in his service. Within the compass of this article it is impossible to give more than merely a reference to this vast subject. The action of man may be witnessed on climate, on the flow of water, on the character of the terrestrial surface, and on the distribution of life.
1. On Climate.— Human interference affects meteorological conditions—(1) by removing forests and laying bare to the sun and winds areas which were previously kept cool and damp under trees, or which, lying on the lee side, were protected from tempests; as already stated, it is supposed that the wholesale destruction of the woodlands formerly existing in countries bordering the Mediterranean has been in part the cause of the present desiccation of these districts; (2) by drainage, the effect of this operation being to remove rapidly the discharged rainfall, to lessen the evaporation, and thereby to diminish the rainfall and somewhat increase the general temperature of a country; (3) by the other processes of agriculture, such as the transformation of moor and bog into cultivated land, and the clothing of bare hillsides with green crops or plantations of coniferous and hardwood trees.
2. On the Flora of Water.— 1. By increasing or diminishing the rainfall man directly affects the course of the waters over the land. 2. By his drainage operations he makes the rain to run off more rapidly than before, and thereby increases the floods in the rivers. 3. By wells, bares, mines, or other subterranean works he interferes with the underground waters and consequently with the dis charge of springs. 4. By embanking rivers he confines them to narrow channels, sometimes increasing their scour, and enabling them to carry their sediment further seaward, sometimes causing them to deposit it over the plains and raise their level.
3. On the Surface of the Land.— Man’s operations alter the aspect of a country in many ways :—(1) by changing forest into bare mountain, or clothing bare mountains with forest; (2) by promoting the growth or causing the removal of peat-mosses; (3) by heedlessly uncovering sand-dunes, and thereby setting in motion a process of destruction which may convert hundreds of acres of fertile land into waste sand, or by prudently planting the dunes with sandloving vegetation or pines, and thus arresting their landward progress; (4) by so guiding the course of rivers as to make them aid him in reclaiming waste land, and bringing it under cultivation; (5) by piers and bulwarks, whereby the ravages of the sea are stayed, or by the thoughtless removal from the beach of stones which the waves had themselves thrown up, and which would have served for a time to protect the land; (6) by forming new deposits either designedly or incidentally. The roads, bridges, canals, railways, tunnels, villages, and towns with which man has covered the surface of the land will in many cases form a permanent record of his presence. Under his hand the whole surface of civilized countries is very slowly covered by a stratum, either formed wholly by him, or due in great measure to his operations, and containing many relics of his presence. The soil of old cities has been increased to a depth of many feet by the rubbish of his buildings; the level of the streets of modern Rome stands high above that of the pavements of the Caesars, and that again above the roadways of the early republic. Over cultivated fields his potsherds are turned up in abundance by the plough. The loam has risen within the walls of his graveyards, as generation after generation has mouldered there into dust.
4. On the Distribution of Life.— It is under this head, perhaps, that the most subtle of human influences come. Some of man’s doings in this domain are indeed plain enough, such as the extirpation of wild animals, the diminution or destruction of some forms of vegetation, the introduction of plants and animals useful to himself, and especially the enormous predominance given by him to the cereals and to the spread of sheep and cattle. But no such extensive disturbance of the normal conditions of the distribution of life can take place without carrying with it many secondary effects, and setting in motion a wide cycle of changeand of reactionin the animaland vegetable kingdoms. For example, the incessant warfare waged by man against birds and beasts of prey in districts given up to the chase leads sometimes to unforseen results. The weak game is allowed to live, which would otherwise be killed off and give more room for the healthy remainder. Other animals which feed perhaps on the same materials as the game are by the same cause permitted to live unchecked, and thereby to act as a further hindrance to the spread of the protected species. But the indirect results of man’s interference with the régime of plants and animals still require much prolonged observation.
From this brief and imperfect outline the reader may perceive that man takes an important place as a geological agent, and that in future ages the traces of his interference will introduce a new element of difficulty into the study of geological phenomena.
PART IV.—STRUCTURAL GEOLOGY,
OR THE ARCHITECTURE OF THE EARTH’S CRUST.
Having considered the nature of the materials constituting the crust of the earth, and the operation of the different agencies by which these materials are produced, arranged, and modified, we may now proceed to examine the structure of the crust itself with the view of marking how its component parts have been put together. Since by far the largest portion of the crust consists of sedimentary or aqueous rocks, it will be of advantage to treat of them first, noting, in the first place, their original characters as resulting from the circumstances under which they were formed, and afterwards, the modifications subsequently effected upon them. Many of these superinduced structures, which are not peculiar to sedimentary, but occur more or less markedly in all rocks, may be conveniently described together. The distinctive characters of the igneous or eruptive rocks, as portions of the architecture of the crust will then be described; and lastly, those of the crystalline schists and other associated rocks to which the name of metamorphic is usually applied.
I. Stratification and its Accompaniments.
The term “stratified,” so often applied as a general designation to the aqueous or sedimentary rocks, expresses their leading structural feature. They are arranged in layers or strata, an arrangement characteristic of them alike in hand-specimens and in the cliffs of mountains. Not that every morsel of aqueous rock exhibits evidence of stratification. But it is this characteristic which is least frequently absent. The general aspects of stratification will be best followed in an explanation of the terms by which they are expressed.
Laminae are the thinnest paper-like layers of deposit in a stratified rock. Such fine layers only occur where the material is fine-grained, as in mud or shale, or where fine scales of some mineral have been plentifully deposited, as in micaceous sandstone. In some laminated rocks the laminae cohere so firmly that they can hardly be split open, and the rock will break more readily across them than in their direction. More usually, however, the planes of lamination serve as convenient divisional planes by means of which the rock can be split open. The frequency with which laminae can be separated from each other, indicating, as it does, a failure of coherence between the layers of deposit, may probably be taken as a proof that these layers were originally laid down at intervals of sufficient duration to admit of a considerable amount of consolidation of one layer before the deposition of the next. It is quite possible that in many, if not in most cases, these intervals were of longer duration than those required for the successive deposit of the laminae. In estimating therefore the length of time represented by say one foot of such finely laminated rock, we might reasonably regard the actual time occupied in deposition as only a small fraction of the whole interval.
The existence of laminae points to tranquil conditions of slow intermittent deposit. The sediment has been borne at intervals and fallen over the same area of undisturbed water. Regularity of thickness and persistence of lithological character among the laminae may be taken to indicate periodic currents, of approximately equal force, from the same quarter. In some cases successive tides in a sheltered estuary may have been the agent of deposition. In others the sediment was doubtless brought by recurring river-floods. A great thickness of laminated rock, like the massive shales of Palaeozoic formations, points to a prolonged period of quiescence, and probably, in most cases, to slow, tranquil subsidence of the sea-floor. On the other hand, the alternation of thin bands of laminated rock with others coarser in texture and non-laminated suggests considerable oscillation of currents from different quarters bearing different qualities and amounts of sediment.
Strata or Beds are layers of rock varying from an inch or less up to many feet in thickness. A stratum may be made up of many laminae, if the nature of the sediment and mode of deposit have favoured the production of this structure. This has very commonly been the case where the sediment has been exceedingly fine-grained. Where the materials are of coarser grain, the strata, as a rule, are not laminated, but form the thinnest parallel divisions of the mass of rock. Strata, like laminae, may either cohere firmly, or, as more usually happens, be separable with more or less ease from each other. In the former case we may suppose the upper to have followed the lower bed without the lapse of an interval long enough to allow of the consolidation of the latter. The common merging of a stratum into that which 0verlie3 it must no doubt be regarded as evidence of more or less gradual change in the conditions of deposit. Where the overlying bed shows no cohesion with that below it, the interval was probably of some duration. A stratum may be one of a series of similar beds in the same mass of rock. Thus a thick sandstone consists of many individual strata, varying it may be very considerably in their respective thicknesses. Or a stratum may be complete and distinct in itself, as where one of limestone or ironstone runs through the heart of a series of shales. As a general rule we may conclude that wherever among sedimentary accumulations stratification is exceedingly well-marked the rocks were formed rather slowly, and that where it is weak or absent the conditions of deposit were more rapid, without the intervals and changes necessary for the production of the distinctly stratified structure.
False-bedding, Current-bedding.— Some strata, particularly sandstones, are marked by an irregular lamination, wherein the laminae, though for short distances parallel to each other, are oblique to the general stratification of the mass, at constantly varying angles and in different directions. The accompanying section (fig. 9) illustrates this structure, which is known by the name of false-bedding or current-bedding. The finer lines in this drawing represent the laminae of deposit, the stronger lines mark successive surfaces on which these laminae were laid down. Such a structure points to frequent changes in the direction of the currents by which the sediment was carried along and deposited. Sand pushed over the bottom of a sheet of water by varying currents tends to accumulate irregularly in bands and ridges, which often advance with a steep slope in front. The upper and lower surfaces of the bank or bed of sand may remain parallel with each other as well as with the underlying bottom, yet the successi ve laminae composing it may lie at an angle of 30° or even more. We may illustrate this structure by the familiar formation of a railway embankment. The top of the embankment on which the permanent way is to be laid is kept level, but the advancing end of the earth-work shows a steep slope over which the workmen are constantly discharging waggon-loads of rubbish. Hence the embankment, if cut open longitudinally, would present a “false-bedded” structure, for it would be found to consist of many irregular layers inclined at a high angle in the direction in which the formation of the mound had advanced. In the accompanying figure (fig. 10) the water moving in the direction of the arrow may drop sand at b, which will correspond in lamination with the general stratification of the locality; but when the current reaches the steep front of one of the advancing sand sheets, it will allow the sand to roll down the slope, and may continue to bring fresh supplies of sediment until the slope is gradually effaced. Now and then, however, instead of laying down sediment, a current of greater strength than usual may appear and sweep away portions of the sediment already deposited.
Irregularities of Bedding due to Inequalities of Deposition or of Erosion.— A sharp ridge of sand or gravel may be laid down under water by current-action of some strength. Should the motion of the water diminish, finer sediment may be brought to the place and be deposited around and above the ridge. In such a case the stratification of the later accumulation will end off abruptly against the flanks of the older ridge, which will appear to rise up through the overlying bed. In fig. 11, for example, the lower bed seems to have been locally heaped up into the shape of a mound or ridge before the coal was accumulated over it. Appearances of this kind are not uncommon in some coalfields, where they are known to the miners as “rolls,” “swells,” or “horses’ backs.” A structure exactly the reverse of the preceding occurs where a stratum has been scooped, out before the deposition of the layers which cover it. This has often been observed in mining for coal. Channels have been cut out of a coal-seam, or rather out of the bed of vegetation which ultimately became coal, and these channels, ramifying and winding sometimes like those of streamlets on flat ground, have been filled up with sandy or muddy sediment. In fig. 12 a section is given of a remarkable series of such erosions, where beds of clay and sandstone have been extensively denuded in the intervals between the deposit of the successive beds. In these and similar cases it is evident that the erosion took place contemporaneously with the accumulation of the deposits as α whole. We cannot tell, of course, how long an interval elapsed between the formation of a given stratum and that of the next stratum which lies upon its eroded surface, nor how much depth of rock may have been removed in the erosion. When, however, as in the instances with which we are dealing, the structure occurs among conformable strata, evidently united as one lithologically continuous series of deposits, we may reasonably infer that the missing portions are of small moment and that the erosion was merely due to the irregular and more violent action of the very currents by which the sediment of the successive strata was supplied.
The case is very different when the eroded strata are inclined at a different angle to those above them, and are strongly marked off by lithological distinctions. In some of the coal-mines in central Scotland, for instance, deep channels have been met with entirely filled with sand, gravel, or clay belonging to the general superficial drift of the country. These channels have evidently been watercourses worn out of the coal-measure strata at a comparatively recent geological period, and subsequently buried under the glacial accumulations. There is a complete discordance between them and the Palaeozoic strata below, pointing to the existence of a vast interval of time.
Ripple-mark.— The surface of many beds of sandstone is marked with lines of wavy ridge and hollow, such as may be seen on any shore from which the tide has retired. This kind of surface is known as “ripple-mark.” It may be formed on dry blown sand by the action merely of the wind, and it is of everyday occurrence under shallow water, not merely on sea-shores, but on the floors of lakes and of river-pools. The water, gently agitated by the wind in a given direction, throws the surface of the underlying sediment into ripples which tend to run at right angles to the course of movement. But as the wind veers from point to point, producing corresponding changes in the direction of the water-currents, the ripples on the bottom are not strictly parallel, but often coalesce, intersect, and undulate in their course. Their general direction, however, suffices to indicate the quarter whence the chief movement of the water has come. No satisfactory inference can be drawn from the existence of a rippled surface as to the depth of water in which the sediment was accumulated. As a rule it is in water of only a few feet or yards in depth that ripple-mark is formed. But it may be produced at any depth to which the agitation caused by wind on the upper waters may extend.
On an ordinary beach each tide usually effaces the ripplemarks made by its predecessor, and leaves a new series to be obliterated by the next tide. But where the markings are formed in water which is always receiving fresh accumulations of sediment, a rippled surface may be gently overspread by the descent of a layer of sediment upon it and may thus be preserved. Another series of ripples may then be made in the overlying layers, which in turn may be buried and preserved under a renewed deposit of sand. In this way a considerable thickness of such ripple-marked strata may be accumulated, as has frequently taken place among geological formations of all ages.
An examination of any sandy beach from which the sea has recently retired brings before us many modifications of the perfect ripple-mark. The ridges may be seen to grow more and more notched and irregular, until at last the beach seems to be dotted over with little, flat, dome-shaped mounds, or as if the ridges of the ripple-mark had been furrowed across. These modifications are doubtless due to the partial effacement of the ridges by subsequent action of the water agitated by wind from a different quarter. Such indications of shallow-water conditions may often be observed among old arenaceous deposits, as in the Cambrian and Silurian rocks. In like manner we may frequently detect, among these ancient formations, small isolated or connected linear ridges directed from some common quarter, like the current-marks frequently to be found behind projecting fragments of shell, stones, or bits of sea-weed on a beach from which the tide has just retired.
Sun-cracks, Pain-pittings, <kc.— Proofs may not infrequently be found that during deposition aqueous strata have been laid bare to air and sun. The nature and validity of this evidence will be best ascertained by observations made at the margin of the sea, or of any inland sheet of water, which from time to time leaves tracts of mud or fine sand exposed to sun and rain. The way in which the muddy bottom of a dried-up pool cracks into polygonal cakes when exposed to the sun may be illustrated abundantly among geological formations of all ages. These desiccation-cracks, or sun-cracks, could not have been produced so long as the sediment lay under water. Their existence therefore among any strata proves that the surface of rock on which they lie was exposed to the air and dried before the next layer of water-borne sediment was deposited upon it.
With these markings are not infrequently associated prints of rain-drops. The familiar effects of a heavy shower upon a surface of moist sand or mud may be witnessed among rocks even as old as parts of the Cambrian system. In some cases the rain-prints are found to be ridged up on one and the same side in such a manner as to indicate that the rain-drops as they fell were driven along in a slanting direction. The prominent side of the markings therefore indicates the side towards which the wind blew.
Numerous proofs of shallow shore-water, and likewise of exposure to the air, are supplied by markings left by animals. Castings and trails of worms, tracks of mollusks and crustaceans, fin-marks of fishes, footprints of birds, reptiles, and mammals, may all be preserved and give their evidence regarding the physical conditions under which sedimentary formations were accumulated. It may frequently be noticed that such impressions are associated with ripple-marks, rainprints, or sun-cracks; so that more than one kind of evidence may be gleaned from a locality to show that it was sometimes laid bare of water.
Gas-spurts.— The surfaces of some strata, usually of a dark colour and containing much organic matter, may be observed to be raised into little heaps of various indefinite shapes, not, like the heaps associated with worm burrows, connected with pipes descending into the rock, nor composed of different material from the surrounding sandstone or shale. These may be conjectured to be due to the intermittent escape of gas from the decomposing organic matter in the original sand or mud, as we may sometimes witness in operation among the mud flats of rivers and estuaries. On a small scale these protrusions of the upper surface of a deposit may be compared with the well-known mud-lumps at the mouths of the Mississippi, where the muddy bottom rises into mounds sometimes to a height of several yards above the water, from the top of which great quantities of carburetted hydrogen gas make their escape, together with water and mud.
Concretions.— Many sedimentary rocks are marked by the occurrence of concretions in them, either distinct in aspect and composition from the general mass of the rock, or forming really part of that mass, though separated from the rest by their being agglutinated into concretionary forms. Such concretions, where they differ in petrographical characters from the surrounding matter, are almost invariably of original or contemporaneous formation, that is, were formed at the same time as the strata among which they lie. Where, however, they appear to be merely compacted portions of the stratum, they may be regarded aa generally due to some subsequent change effected upon the rock.
Contemporaneous concretions most commonly consist of carbonate of iron, carbonate of lime, or silica. Many clayironstone beds assume a nodular form, and this mineral occurs abundantly in the shape of separate nodules in shales and clay-rocks. The nodules have frequently formed round some organic body such as a fragment of plant, a shell, bone, or coprolite. That the carbonate of iron was slowly precipitated during the formation of the bed of shale in which its nodules lie may often be satisfactorily proved by the lines of deposit passing continuously through the nodules. In many cases the internal first-formed parts of a nodule have contracted more than the outer and more compact crust; and have cracked into open polygonal spaces which are commonly filled with calcite. Similar concretions of carbonate of lime occur in some clays and in connexion with limestones. Concretions of silica occur in limestone of many geological ages (see ante, p. 239). The flints of the English chalk are a familiar example, but similar siliceous concretions occur even in Lower Silurian limestones. The silica in these cases has not infrequently been deposited round organic bodies such as sponges, sea-urchins, and mollusca, which are completely enveloped in it and have even themselves been silicified. Iron-bisulphide (pyrite or marcasite) often assumes the form of concretions, more particularly among clay-rocks, and these, though presenting many eccentricities of shape, round like pistol-shot or cannon-balls, kidney-shaped, botryoidal, &c., agree in usually possessing an internal fibrous radiated structure. Phosphate of lime is found as concretions in formations where the coprolites and bones of reptiles and other animals have been collected together.
Concretions produced subsequently to the formation of the rock may be observed in some sandstones, which, when exposed to the weather, decompose into large round balls. Some shales exhibit this structure in a still more striking manner, inasmuch as the concretions consist of the general mass of the laminated shale, and the lines of stratification pass through them and mark them out distinctly as superinduced upon the rock. Some magnesian limestones are so concretionary as to resemble masses of conglomerate; yet the concretions, among all their fantastic shapes and with their acquired crystalline texture, may often be found to retain traces of the original stratification of the rock. Beds of rock-salt may likewise be observed to be marked with traces of a concretionary arrangement.
Order of Superposition—the Foundation of Geological Chronology.— As sedimentary strata are laid down upon one another in a more or less nearly horizontal position, the underlying beds must be older than those which cover them. This simple and obvious truth is termed the law of superposition. It furnishes the means of determining the chronology of rocks, and though other methods of ascertaining this point are employed, they must all be based originally upon the observed order of superposition. The only case where the apparent superposition may be deceptive is where the strata have been inverted. In the Alps, for example, the rocks composing huge mountain masses have been so completely overturned that the highest beds appear as if regularly covered by others which ought properly to underlie them. But these are exceptional occurrences, where the true order can usually be made out from other sources of evidence.
Alternations of Strata.— Though great variations occur in the nature of the strata composing a mass of sedimentary rocks, it may often be observed that certain repetitions occur. Sandstones, for example, are found to be interleaved with shale above, and then to pass into shale; the latter may in turn become sandy at the top and be finally covered by sandstone, or may assume a calcareous character and pass up into limestone. Such alternations bring before us the conditions under which the sedimentation took place. A sandstone group indicates water of comparatively little depth, moved by changing currents, bringing the sand now from one side now from another. The passage of such a group into one of shale points to a diminution in the motion and transporting power of the water, perhap3 to a sinking of the tract, whereby only fine mud was then intermittently brought into it. The advent of a limestone above the shale serves to show that the water cleared, owing to a deflexion of the sediment-carrying currents, or to continued and perhaps more rapid subsidence, and that Foraminiferα, corals, crinoids, Mollusca, or other lime-secreting organisms, established themselves upon the spot. Shale overlying the limestone would tell of fresh inroads of mud, which destroyed the animal life that had been flourishing on the bottom; while a return of sandstone beds would mark how, in the course of time, the original conditions of troubled currents and shifting sandbanks returned. Such alternating groups of sandy, calcareous, and argillaceous strata are well illustrated among the Jurassic formations of England.
Associations of Strata.— Certain kinds of strata very commonly occur together, because the conditions under which they were formed were apt to arise in succession. One of the most familiar examples is the association of coal and fire-clay. A seam of coal is almost invariably found to lie on a bed of fire-clay, or on some argillaceous stratum. The reason of this union becomes at once apparent when we learn that the fire-clay formed the soil on which the plants grew that went to form the coal. Where the clay was laid down under suitable circumstances vegetation sprang up upon it. Again conglomerate and sandstone occur together rather than conglomerate and shale, because the agitation of the water which could form and deposit coarse detritus, like that composing conglomerate, was too great to admit of the accumulation of fine silt. For a similar reason we may look for shale or clay rather than sandstone as an accompaniment of limestone.
Relative Persistence of Strata.— Observation of what takes place on any lake bottom, estuary, or sea-margin teaches that some kinds of sediment are much more widely spread than others, and prepares us to find that the same has been the case in past time, and therefore that some kinds of sedimentary rocks possess far greater persistence than others. As a general rule it may be said that the coarser the grain the more local the extent of a rock. Conglomerates are thus by much the mast variable and inconstant of all sedimentary formations. They suddenly sink down from a thickness of several hundred feet to a few yards, or die out altogether, to reappear perhaps further on, in the same wedge-like or lenticular fashion. Sandstones are less liable to such extremes of inconstancy, but they too are apt to thin away and to swell out again. Shales are much more persistent, the same zone being often traceable for many miles. Limestones sometimes occur in thick local masses, as among the Silurian formations of Wales and Scotland, but they often also display remarkable continuity. Three thin limestone bands, each of them only 2 or 3 feet in thickness, and separated by a considerable thickness of intervening sandstones and shales, can be traced through the coal-fields of central Scotland over an area of at least 1000 square miles. Coal-seams also possess great persistence. The same seams, varying slightly in thickness and quality, may often be traced thoughout the whole of an extensive coal-field.
What is thus true of individual strata may be affirmed also of groups of such strata. A thick mass of sandstone will be found as a rule to be more continuous than one of con glomerate, but less so than one of shale. A series of limestone-beds will usuallybe found to stretch further than either of them. But even to the most extensive stratum or group of strata there must be a limit. It must end off and give place to others, either suddenly, as a bank of shingle is succeeded by the sheet of sand heaped against its base, or very gradually, by insensibly passing into other strata on all sides.
Great variations in the character of stratified rocks may frequently be observed in passing from one part of a country to another along the outcrop of the same rocks. Thus at one end we may meet with a thick series of sandstones and shales which, traced in a certain direction, may be found passing into limestones. A group of strata may consist of massive conglomerates at one locality, and maygraduate into fine fissile flagstones in another. A thick mass of clay maybe found to alternate more and more with shelly sands as it is traced outward, until it loses its argillaceous nature altogether. No difficulty need be felt in admitting the strict contemporaneity of these diverse layers of sediment. At the present time we see how coarse shingle may be formed along the beach at the same time that the finest mud is being laid down on the same seabottom further from land. Could we raise up that bottom, we should doubtless find as gradual a passage from the littoral to the deeper water deposits as we do among the geological formations of the earth’s crust. The existing differences of character between the deposits of the shore and of the opener sea would no doubt continue to be maintained, with slight geographical displacements, even if the whole area were undergoing subsidence, giving rise to a thick group of littoral beds in one tract and of deeper-water accumulations at another. In like manner among the formations of former geological periods the same conditions of deposit appear sometimes to have continued for a considerable period. Hence the thick Mountain or Carboniferous Limestone of Derbyshire is gradually replaced northwards by the thicksandstoneshales, ironstones, and coal-seams of Scotland.
Overlap.— When strata have been laid down in a subsiding region wherein the area of deposit gradually increased, the sediment must have spread over a progressively augmenting surface. By this means the later portions of a sedimentary seri.es will extend beyond the limits of the older parts, and will repose directly upon the shelving bottom, with none of those older strata underneath them. This relation is called Overlap (see fig. 59). The higher or newer members are said to overlap the older. This structure may often be detected among formations of all geological ages. It brings before us the shore line of ancient land-surfaces, and shows how, as these sank under water, the gravels, sands, and silts gradually advanced and covered them.
Relative Lapse of Time represented by Strata and by the Intervals between them.— Of the absolute length of time represented by any strata or groups of strata we can form no satisfactory estimates. Certain general conclusions may indeed be drawn, and comparisons may be made between different series of rocks. Sandstones full of false-bedding were probably accumulated more rapidly than finely-laminated shales or clays. It is not uncommon in certain Carboniferous formations to find huge coniferous trunks imbedded in an inclined position in sandstone. These trees seem to have been carried along and to have sunk, their heavier or root-end touching the bottom, and their upper end pointing upward in the direction of the current, exactly as in the case of the snags of the Mississippi The continuous deposit of sand at last rose above the level of the trunks and buried them. It is clear then that the rate of deposit must have been sufficiently rapid to have allowed a mass of 20 or 30 feet of sand to accumulate before the decay of the wood; though modern instances are known where, under certain circumstances, submerged trees may last for some centuries. Continuous layers of the same kind of deposit suggest a persistence of geological conditions; numerous alternations of different kinds of sedimentary matter point to vicissitudes or alternations of conditions. As a rule, we should infer that the time represented by a given thickness of similar strata was less than that shown by the same thickness of dissimilar strata, because the changes needed to bring new varieties of sediment into the area of deposit would usually require the lapse of some time for their completion. But this conclusion might often be erroneous. It would be best supported when, from the very nature of the rocks, wide variations in the character of the waterbottom could be established. Thus a group of shales followed by a fossiliferous limestone would almost always mark the lapse of a much longer period than an equal depth of sandy strata. Limestones made up of organic remains which lived and died upon the spot, and whose remains are crowded together generation above generation, must have demanded many years for their formation.
Eut in all speculations of this kind we must bear in mind that the length of time represented by a given depth of strata is not to be estimated merely from their thickness or lithological characters. It has already been pointed out that the interval between the deposit of two successive laminae of shale may have been as long as, or even longer than, that required for the formation of one of the laminae. In like manner, the interval needed for the transition from one stratum or kind of strata to another may often have been more than equal to the time required for the formation of the strata on either side. But the relative chronological importance of the bars or lines in the geological record can seldom be satisfactorily discussed merely on lithological grounds. This must mainly be decided on the evidence of organic remains, as will be shown in part v. By this kind of evidence it can be made nearly certain that the intervals represented by strata were in many cases much shorter than those not so represented,—in other words, that the time during which no deposit of sediment went on was longer than that wherein deposit did take place.
Groups of Strata.— Passing from individual strata to large masses of stratified rock, the geologist finds it needful for convenience of reference to subdivide these into groups. He avails himself of two bases of classification—(1) lithological characters, and (2) organic remains.
1. The subdivision of stratified rocks into groups according to their mineral aspect is an obvious and easily applied classification. Moreover, it often serves to connect together rocks formed continuously in certain circumstances which differed from those under which the strata above and below were laid down,—so that it expresses natural and original subdivisions of strata. In the middle of the English Carboniferous system of rocks, for example, a zone of sandy and pebbly beds occurs, known as the Millstone Grit. No abrupt and sharp line can be drawn between these strata and those above and below them. They shade upward and downward into the beds between which they lie. Yet they form a conspicuous belt, traceable for many miles by the scenery to which it gives rise. The red rocks of central England, with their red sandstones, marls, rock-salt, and gypsum, form likewise a well-marked group or rather series of groups. It is obvious, however, that characters of this kind, though sometimes wonderfully persistent over wide tracts of country, must be at best but local. The physical conditions of deposit must always have been limited in extent. A group of strata showing great thickness in one region will be found to die away as it is traced into another. Or its place is gradually taken by another group which, even if geologically contemporaneous, possesses totally different lithological characters. Just as at the present time a group of sandy deposits gradually gives place along the sea-floor to others of mud, and these to others of shells or of gravel, so in former geological periods contemporaneous deposits were not always lithologically similar. Hence mere resemblance in mineral aspect usually cannot be regarded as satisfactory evidence of contemporaneity except within comparatively contracted areas. The Carboniferous Limestone of Ireland is a thick calcareous group of rocks, full of corals, crinoids, and other organisms, which bear witness to the formation of these rocks in the open sea. But if these limestones, with their characteristic marine fossils, are traced into the north of England and Scotland, they are found to pass into sandstones and shales, with numerous coal-seams, and only a few thin beds of limestone. The soft clay beneath the city of London is represented in the Alps by hard schists and contorted limestones. We conclude therefore that lithological agreement when pushed too far is apt to mislead us, partly because contemporaneous strata often vary greatly in their lithological character, and partly because the same lithological characters may appear again and again in different ages. By trusting too implicitly to this kind of evidence, we may be led to class together rocks belonging to very different geological periods, and on the other hand to separate groups which really, in spite of their seeming distinction, were formed contemporaneously.
2. It is by the remains of plants and animals imbedded among the stratified rocks that the most satisfactory subdivisions of the geological record can be made, as will be more fully stated in parts v. and vi. A chronological succession of organic forms can be made out among the rocks of the earth’s crust. A certain common facies or type of fossils is found to characterize particular groups of rock, and to hold true even though the lithological constitution of the strata should greatly vary. Moreover, though comparatively few species are universally diffused, they possess remarkable persistence over wide areas, and even when they are replaced by others, the same general facies of fossils remains. Hence the stratified formations of two countries geographically distant, and having little or no lithological resemblance to each other, may be compared and paralleled zone by zone, simply by means of their enclosed organic remains.
II. Joints.
All rocks are traversed more or less distinctly by vertical or highly inclined divisional planes termed Joints. Soft rocks indeed, such as loose sand and uncompacted clay, do not show these lines; but wherever a mass of clay has been subjected to some’pressure and consolidation, it will usually be found to have acquired them. It is by means of the intersection of joints that rocks can be removed in blocks; the art of quarrying consists in taking advantage of these natural planes of division. Joints differ in character according to the nature of the material which they traverse; those in sedimentary rocks are usually distinct from those in crystalline masses.
1. In Sedimentary Rocks.— Joints vary in sharpness of definition, in the regularity of their perpendicular and horizontal course, in their lateral persistence, in number, and in the directions of intersection. As a rule, they are most sharply defined in proportion to the fineness of grain of the rock. In limestones and close-grained shale3, for example, they often occur so clean-cut as to be invisible until revealed by fracture or by the slow disintegrating effects of the weather. The rock splits up along these concealed lines of division whether the agent of demolition be the hammer or frost. In coarse-textured rocks, on the other hand, joints are apt to show themselves as irregular rents along which the rock has been shattered, so that they present an uneven sinuous course, branching off in different directions. In many rocks they descend vertically in straight lines at not very unequal distances, so that the spaces between them are thus marked off into so many walllike masses. But this symmetry often gives place to a more or less tortuous course with lateral joints in various random directions, more especially where the different strata vary considerably in lithological characters. A single joint may be traced sometimes for many yards, or even for several miles, more particularly when the rock is finegrained, as in limestone. But where the texture is coarse and unequal, the joints, though abundant, run into each other in such a way that no one in particular can be identified for so great a distance. The number of joints in a mass of stratified rock varies within wide limits. Among strata which have undergone little disturbance the joints may be separated from each other by intervals of several yards. But in other cases where the terrestrial movement appears to have been considerable, the rocks are so jointed as to have acquired therefrom a fissile character that has nearly or wholly obliterated their tendency to split along the lines of bedding.
An important feature in the joints of stratified rocks is the direction in which they intersect each other. As the result of observation we learn that they possess two dominant trends, one coincident in a general way with the direction in which the strata are inclined to the horizon, and the other running transversely at a right angle or nearly so. The former set is known as dip-joints, because they run with the dip or inclination of the rocks, the latter is termed strike-joints, inasmuch as they conform to the general strike or mean outcrop. It is owing to the existence of this double series of joints that ordinary quarrying operations can be carried on. Large quadrangular blocks can be wedged off, which would be shattered if exposed to the risk of blasting. A quarry is usually worked to the dip of a rock, hence the strike-joints form clean-cut faces in front of the workmen as they advance. These are known as “backs, ’ and the dip-joints which traverse themas “cutters.” The way in which this double set of joints occurs in a quarry may be seen in fig. 15, where the parallel lines which traverse the shaded and unshaded faces mark the successive strata. The broad white spaces running along the length of the quarry behind the seated figure are strike-joints or “backs,” traversed by some highly inclined lines which mark the position of dip-joints or “cutters.” The shaded ends looking towards the spectator are “cutters” from which the rock has been quarried away on one side.
In some conglomerates the joints may be seen traversing the enclosed pebbles as well as the surrounding matrix. Large blocks of hard quartz are cut through by them as sharply as if they had been sliced in a lapidary’s machine, and the same joints can be traced continuously through many yards of the rock. Such facts show that the agency to which the jointing of rocks was due must have operated with considerable force. Further indication of movement is often supplied by the rubbed and striated surfaces of joints. These surfaces, termed slickensides, have evidently been ground against each other. They are often coated with haematite, calcite, chlorite, or other mineral, which has taken a cast of the striae and then seems itself to be striated.
Joints form natural lines for the passage downward and upward of subterranean water. They likewise furnish an effective lodgment for surface water which, frozen by a lowering of temperature, expands into ice, and wedges off blocks of rock in the manner already described. As they serve, in conjunction with bedding, to divide stratified rocks into large quadrangular blocks, their effect on cliffs and other exposed masses of rock is seen in the apparently splintered, dislocated aspect so familiar in mountain scenery.
Occasionally a prismatic or columnar form of joints may be observed among stratified rocks. When this occurs among unaltered strata it is usually among those which have been chemically formed, as in gypsum, where, as observed by Mr Jukes in the Paris Basin, some beds are divided from top to bottom by vertical hexagonal prisms. A columnar structure has often been superinduced upon stratified rocks by contact with intrusive igneous masses. Sandstones, shale, and coal may be observed in this condition. The columns diverge perpendicularly to the surface of the injected and altering substance, so that when the later is vertical the columns are horizontal, or when it undulates the columns follow its curvatures. Beautiful examples of this character occur among the coal-seams of Ayrshire.
2. In Crystalline (Igneous) Rocks.— While in stratified rocks the divisional planes consist of lines of bedding and of joint, cutting each other usually at a high if not a right angle, in massive igneous rocks they include joints only; and as these do not as a rule present the same parallelism as lines of bedding, unstratified rocks, even though as full of joints, have not the same regularity of arrangement as in the stratified formations. Granite, for example, is traversed by two sets of chief or “master-joints,” cutting each other somewhat obliquely. Their effect is to divide the rock into long quadrangular, rhomboidal, or even polygonal columns.
But a third set may usually be noticed cutting across tho columns, though less continuous and dominant than the others. When these transverse joints are few in number or occasionally absent, columns many feet in length can be quarried out entire. Such monoliths have been from early times employed in the construction of obelisks and pillars.
In rocks of finer grain than granite, such as many diorites and dolerites, the numerous perpendicular joints give the rock a prismatic character. The prisms however are unequal in dimensions, as well as in the number and proportions of their sides, a frequent diameter being 2 or 3 feet, though they may sometimes be observed three times thicker, and extending up the face of a cliff for 300 or 400 feet. It is by means of joints that precipitous faces of rock are produced and retained, for, as in the case of those in stratified masses, they serve as openings into which frost drives every year its wedges of ice, whereby huge slices are stripped off. They likewise give rise to the formation of those fantastic pinnacles and fretted buttresses so generally to be observed among igneous rocks in which they occur.
But undoubtedly the most striking series of joints to be found among igneous rocks is in the regularly columnar, or as it is often called, basaltic structure. This structure has been already (ante, p. 249) described in connexion with modern volcanic rocks. It may be met with in rocks of all ages. It is as well displayed among the felsites of the Lower Old Red Sandstone, and the basalts of the Carboniferous Limestone in central Scotland, as among the Tertiary lavas of Auvergne or the Vivarais.
3. In Foliated Pocks.— The schists likewise possess their joints, which approximate in character to those among the massive igneous rocks, but they are on the whole less distinct and continuous, while their effect in dividing the rocks into oblong masses is considerably modified by the transverse lines of foliation. These lines play somewhat the same part as those of stratification do among the stratified rocks, though with less definiteness and precision.
III. Inclination of Rocks.
The most casual observation is sufficient to satisfy us that the rocks now visible at the earth’s surface are seldom in their original position. We meet with sandstones and conglomerates composed of water-worn particles, yet forming the angular scarps of lofty mountains; shales and clays full of the remains of fresh-water shells and land-plants, yet covered by limestones made up of marine organisms, and these limestones rising into great ranges of hills, or undulating into fertile valleys, and passing under the streets of busy towns. Such facts, now familiar to every reader, and even to many observers who know little or nothing of systematic geology, point unmistakably to the conclusion that the rocks have in many cases been formed under water, sometimes in lakes, more frequently in the sea, and that they have been elevated into land.
But further examination discloses other and not less convincing evidence of movement. Judging from what takes place at the present time on the bottoms of lakes and of the sea, we confidently infer that when the strata now constituting so much of the solid framework of the land were formed, they were laid down either horizontally or at least at low angles. When, therefore, we find them inclined at all angles, and even standing on end, we conclude that they have been disturbed. Over wide spaces they have been upraised bodily with little alteration of their original horizontality; but in most places some departure from that original position has been effected.
The inclination thus given to rocks is termed their dip. Its amount is expressed in degrees measured from the plane of the horizon. Thus a set of rocks half-way between the horizontal and vertical position would be said to dip at an angle of 45°, while if vertical they would be marked with the angle of 90°. The edges of strata, where they come up to the surface, are termed their outcrop or basset. When they crop out, that is, rise to the surface, along a perfectly level piece of ground, the outcrop runs at a right angle to the dip. But any inequalities of the surface, such as valleys, ravines, hills, and ridges will cause the outcrop to describe a circuitous course, even though the dip should remain perfectly steady all the while. If a line of precipitous gorge should run directly with the dip, the outcrop will there be coincident with the dip. The occurrence of a gently shelving valley in that position will cause the outcrop to descend on one side and to mount in a corresponding way on the other, so as to form a V-shaped indentation in its course. A ridge, on the other hand, will produce a deflexion in the opposite direction. Hence a series of parallel ridges and valleys running in the same direction as the dip of the strata underneath would cause the outcrop to describe a widely serpentinous course. Again, should the rocks be vertical, the outcrop will necessarilycorrespond with the dip, and continue to do so irrespective altogether of any irregularities of the ground. The lower therefore the angle of inclination the greater is the effect of surface inequalities upon the line of outcrop; the higher the angle the less is that influence, till when the beds stand on end it ceases.
A line drawn at a right angle to the dip is called the strike of the rocks. From what has just been said this line must Coincide with outcrop when the surface of the ground is quite level, and also when the beds are vertical. At all other times they are not strictly coincident, but the outcrop wanders to and fro across the strike according to the changes in the angle of inclination and in the form of the ground. The strike may be a straight line or may curve rapidly in every direction, according to the behaviour of the dip. If, for instance, a set of beds dips for half a mile continuously to the north, the strike will run for that distance as a straight east and west line. If the dip gradually changes to north-west and west, and then by southwest to south, it is obvious that the strike must curve round by north-east, north, and north-west till it once more becomes parallel with its former course. Both of the parallel lines of strike run in an east-and-west direction, but in the one the dip is to the south, and in the other to the north.
The strike may be conceived as always a level line on the plane of the horizon, so that no matter how much the ground may undulate, or the outcrop may vary, or the dip may change, the strike will remain level. Hence in mining operations it is commonly spoken of as the level course or level-hearing. A level or underground road-way, driven through a coal-seam at right angles to the dip, will undulate in its course if the dip changes in direction, but it may be made perfectly level and kept so throughout a whole coal-field so long as it is not interfered with by any dislocations or other disturbances of the regularity of the rocks.
The accompanying figures (figs. 18 and 19) will serve to show some of these terms as expressed on maps and sections. Fig. 18 represents a geological map in which a series of strata dips in a south-south-easterly direction (S. 28° E.). The angle of inclination increases from 35° at the northern to 50° at the southern end of the beach. On the flat shore (AA) outcrop and strike coincide, but along the inner margin, where the ground ascends in a line or cliff (BB) to the inland country (ς CC), the outcrop is seen to be deflected a little so as to cross the plateau along a slightly more northerly line than on the beach. A section drawn at a right angle to the strike along the line DD would show the structure represented in fig. 19. Such a section, expressing graphically the result of careful measure ment in the field, would give not only the order of succession of beds at the surface, but their actual depth at any point beneath it. Thus a bore or shaft sunk at the point marked d on the map would have to pass through rather more than 425 feet of rock before reaching the stratum l>. The total thickness of rock measured at right angles to the dip in fig. 19 is somewhat more than 850 feet. These various strata, if restored to their original position, would lie one over the other to that depth. If they were on end they would occupy exactly that breadth of ground. But the inclined position of strata makes them cover more horizontal space; in the present instance it increases that space to 1200 feet.
A convenient rule was given many years ago by the late Mr Charles Maclaren of Edinburgh for estimating the thickness of strata inclined at angles of Jess than 45°. The real thickness of a mass of inclined strata is -⅛th of its apparent thickness for every 5° of dip. Thus if a set of beds dip steadily in one direction at 5° for a horizontal space of 1200 feet measured across the strike, their actual thickness will be ⅛th or 100 feet. If the dip be 15°. the true thickness will be ∙⅛ths or 400 feet, and so on.
IV. Curvatures of Rocks.
A little reflexion will show that though, so far as regards the trifling portions of the rocks visible at the surface, we might regard the inclined surfaces of the strata as parts of straight lines, they must nevertheless be parts of large curves. Take, for example, the section given in fig. 19. At the north end of that section we observe the beds to plunge one after another into the earth at an angle of 35°. By degrees the inclination increases until it reaches 50°. As there is no dislocation or abrupt change of angle, but a gradual transition, it is evident that the beds at the north end cannot proceed indefinitely downward at the same angle which they have at the surface, but must bend round to accommodate themselves to the higher inclination which sets in southwards. By prolonging the lines of the beds for some way beneath the sea-level, we can show graphically the nature of the curve. In every instance therefore where, in walking over the surface, we traverse a series of strata which gradually, and without dislocations, increase or diminish in inclination, we cross part of a great curvature in the strata of the earth’s crust.
Such foldings, however, can often be distinctly seen, either on some cliff or coast-line, or in the traverse of a piece of hilly or mountainous ground. The observer cannot long continue his researches in the field without discovering that the rocks of the earth’s crust have been almost everywhere thrown into curves, usually so broad and gentle as to escape observation except when specially looked for. The outcrop of beds at the surface is commonly the truncation of these curves. The strata must once have risen above the present surface, and in many cases may be found descending to the surface again with a contrary dip, the intervening portion of the undulation having been worn away.
If then the inclination of rocks is so closely connected with their curvature, a corresponding relation must hold between their strike and curvature. In fact, the prevalent strike of a region is determined by the direction of the axes of the great folds into which the rocks have been thrown. If the curves are gentle and inconstant there will be a corresponding variation in the strike. But should the rocks be strongly plicated, there will necessarily be the most thorough coincidence between the strike and the direction of the plication.
The curvature occasionally shows itself among horizontal or gently inclined strata in the form of an abrupt inclination, and then an immediate resumption of the previous flat or sloping character. The strata are thus bent up and continue on the other side of the tilt at a higher level. ►Such bends are called monoclines or moηoclinal folds, because they present only one fold, or one half of a fold, instead of the two which we see in an arch or trough. The most notable instance of this structure in Britain is that of the Isle of Wight, of which a section is given in fig. 20. The Cretaceous rocks on the south side of the island rapidly rise in inclination till they become nearly vertical.
The Lower Tertiary strata follow with a similar steep dip, but rapidly flatten down towards the north coast. Some remarkable cases of the same structure have been brought to light by Mr J. W. Powell in his survey of the Colorado region.
It much more frequently happens that the strata have been bent into arches and troughs, so that they can be seen dipping under the surface on one side of the axis of a fold, and rising up again on the other side. Where they dip away from the axis of movement the structure is termed an anticline or anticlinal fold where they dip towards the axis, it is a syncline or synclinal fold. The diagram in fig. 21 may be taken to represent a series of strata (1-17) thrown into an anticline (AA') and syncline (BB'). A section drawn across these folds in the line CD would show the structure given in fig. 22. Here we see that, at the part of the anticlinal axis (A) where the section crosses, bed No. 4 forms the crown of the arch, Nos. 1, 2, and 3 being concealed beneath it. On the east side of the axis the strata follow each other in regular succession as far as No. 13, which, instead of passing here under the next in order, turns up with a contrary dip and forms the centre of a trough or syncline (B). From underneath No. 13 on the east side, the same beds rise to the surface which passed beneath it on the west side. The particular bed marked EF has been entirely removed by denudation from the top of the anticline, and is buried deep beneath the centre of the syncline.
Such foldings of strata must always die out unless they are abruptly terminated by dislocations. In the cases given in fig. 21, both the arch and trough are represented as diminishing, the former towards the north, the latter towards the south. The observer in passing northwards along the axis of that anticline finds himself getting into progressively higher strata, as the fold sinks down. On the other hand, in advancing southwards along the synclinal axis, he loses stratum after stratum and gets into lower portions of the series. When a fold diminishes in this way it is said to “nose out.” In fig. 21 there is obviously a general inclination of the beds towards the north, besides the outward dip from the anticline and the inward dip from the syncline. Hence the anticline noses out to the north and the syncline to the south.
Tt occasionally happens that the maximum movement either of upheaval or subsidence has taken place not along a line of axis but at some one point. Hence arise, on the one hand, dome-shaped elevations of strata where the dip is outward from a centre (quaquaversal), round which the beds are disposed in successive parallel layers or rings, and, on the other hand, circular basin-shaped depressions, towards the centre of which there is a general inclination of the rocks.
So great has been the compression to which rocks have been subjected during the process of curvature that the folds may often be found inverted. This has taken place abundantly in regions of great plication. The Silurian uplands of the south of Scotland, for instance, have the arches and troughs tilted in one direction for miles together, so that in one half of each of them the strata lie bottom upwards. It is in large mountain-chains, however, that inversion can be seen on the grandest scale. The Alps furnish numerous striking illustrations. On the north side of that chain the older Tertiary rocks have been so completely turned over for many miles that the lowest beds now form the tops of the hills, while the highest lie deep below them. Individual mountains, such as the Glärnisch, present stupendous examples of inversion, great groups of strata being folded over and over above each other as we might fold carpets.
Where curvature has been carried so far, we may nearly always discover localities at which it has been so intensified that the strata have been corrugated and crumpled till it becomes almost impossible to follow out any particular bed through the disturbance. On a small scale instances of such extreme contortion may now and then be found at landslips, where fissile shales have been pressed forward by advancing heavy masses of more solid rock. But it is of course among the more, plicated parts of mountain-chains that the structure receives it best illustrations. Few travellers who have passed the upper end of the Lake of Lucerne can have failed to notice the remarkable cliff's of contorted rocks near Fluelen. But innumerable examples of equal or even superior grandeur may be observed among the more precipitous valleys of the Swiss Alps. No more impressive testimony could be given to the potency of the force by which mountains were upheaved.
V. Dislocations of Rocks.
The movements which the crust of the earth has undergone have not only folded and corrugated the rocks, but have fractured them in all directions. These dislocations may be either simple fissures, that is, rents without any vertical displacement of the mass on either side, or faults, that is, rents where one side has been pushed up or has sunk down. It is not always possible in a shattered rock to discriminate between joints and true fissures. The joints indeed have sometimes served as lines along which fissuring has taken place. It is common to meet with traces of friction along the walls of fissures even when no proof of actual vertical displacement can be gleaned. The rock is more or less shattered on either side, and the contiguous faces present numerous slickensided surfaces. Mineral deposits may also commonly be observed encrusting the cheeks of a fissure, or filling up, together with broken fragments of rock, the space between the two walls.
In a large proportion of cases, however, there has been displacement as well as fracture, and the rents have become faults as well as fissures. Faults on a small scale are sometimes sharply-defined lines, as if the rocks had been sliced through and fitted together again after being shifted (fig. 25). In such cases, however, the harder portions of the dislocated rocks will usually be found slickensided. More frequently some disturbance has occurred on one or both sides of the fault. Sometimes in a series of strata the beds on the side which has been pushed up are bept down against the fault, while those on the opposite side are bent up (fig. 26). Most commonly the rocks on both sides are considerably broken, jumbled, and crumpled, so that the line of fracture is marked by a belt or wall-like mass of fragmentary rock. Where a dislocation has occurred through materials of very unequal hardness, such as solid limestone bands and soft shales, or where its course has been undulating, the relative shifting of the two sides has occasionally brought opposite prominences together so as to leave wider interspaces, as in fig. 27. The actual breadth of a fault may vary from a mere chink into which the point of a knife could hardly be inserted up to a band of broken rock many yards wide. But in these latter cases we may usually suspect that so great a breadth of fractured materials has been produced not by a single fault but by a series of closely adjoining and parallel faults.
Faults are sometimes vertical, but are generally inclined. The largest faults, that is, those which have the greatest vertical displacement, slope at high angles, while those of only a few feet or yards may be inclined as low as 18° or 20°. The inclination of a fault from the vertical is called its hade. In fig. 28, for example, the fault between A and C being vertical has no hade, but that between C and B hades at an angle of 70° from the vertical to the right hand. The amount -of displacement is represented as the same in both instances, so that the level of the bed a is raised between the two faults at C above the uniform horizon which it retains beyond them.
That faults are vertical displacements of parts of the earth’s crust is most clearly shown when they traverse stratified rocks, for the regular lines of bedding and the originally flat position of these rocks afford a measure of the disturbance. Accordingly we may consider here the effects of faults as they traverse (1) horizontal, (2) inclined, or (3) undulating strata.
1. In the above section (fig. 28) two faults are supposed to traverse a set of horizontal strata, and to displace them in opposite directions. Hence the portion between them appears as if it had been pushed up, or as if the part on either side had slipped down. The amount of vertical displacement is measured from the end of any given stratum, say α, on one side of the fault, to its corresponding end on the other side. Suppose, for example, that the black band in fig. 29 represents a known stratum such as a seam of coal, which, having been explored in underground operations, is known to be cut by a fault at a depth of a hundred yards below the surface at A, and to lie 200 yards deep on the other side of the fault below B. The amount of displacement is the vertical distance between the two severed ends a and δ. This is termed the throw of a fault. From these two sections (figs. 28 and 29) we see that the horizontal distance to which the two ends of a faulted stratum may be separated does not depend upon the amount of throw but upon the angle of the hade. In the left-hand fault in fig. 28 there is no hade, for the fault is vertical; consequently there is no lateral displacement. In fig. 29, however, where the fault hades considerably, there is a lateral shift of the bed, the end a being 150 yards to the left of b. In this example the lateral shift is half as much again as the vertical. It is obvious that a fault of this kind must seriously affect the value of a coal-field; for while the coal-seam might be worked up to a on the one side and to b on the other, there would be a space of 150 yards of barren ground between these two points where the seam never could be found. The lower the angle of hade the greater the breadth of such barren ground. Hence the more nearly vertical the lines of fault, the better for the coal-fields.
In the vast majority of cases faults hade in the direction of downthrow, in other words, they slope away from the side which has risen. Consequently the mere inspection of a fault in any natural or artificial section suffices in most cases to show which side has been elevated. In mining operations the knowledge of this rule is invaluable, for it decides whether a coal seam, dislocated by a fault, is to be sought for by going up or down. In fig. 29, for example, a miner working from the right and meeting with the fault at b, would know from its hading towards him that he must ascend to find the coal. On the other hand were he to work from the left and catch the fault at a, he would see that it would be necessary to descend. According to this rule a normal fault never brings one part of a bed below another part, so as to be capable of being pierced twice by the same vertical shaft. Exceptional cases, however, where the hade is reversed, do occasionally appear. In fig. 30 a series of strata, 1 to 11, are represented as folded in an inverted anticline, and broken through by a fault along the axis, the portion on the right side having been pushed up.
The effect of the movement has been to make the ends of the beds on that 3ide overlie higher beds on the other side. A shaft would thus pierce the same stratum twice. Instances of reversed faults are chiefly met with in much disturbed districts, such as mountain chains, where the rocks have been affected by great undulations and corrugations. But instances on a small scale, like that in fig. 31, may now and then be encountered even in lowland districts, where no great disturbance has taken place.
2. Faults traversing inclined strata usually group themselves into two series, one running in the same general direction as the dip of the strata, the other approximating to the trend of the strike. They are accordingly classified as dip faults and strikefaults. They are not always to be sharply marked off from each other, for the dip-faults will often be observed to deviate considerably from the normal direction of dip, and the strike-faults from the prevalent strike, so that in such cases they pass into each other.
A dip-fault produces at the surface the effect of a lateral shift of the strata. This effect increases in proportion as the angle of dip lessens. It ceases altogether when the beds are vertical. Fig. 32 may be take⅛ as a plan of a dip-fault traversing a series of strata which dip northwards at 25°. The beds on the east side look as if they had been pushed horizontally southwards. That this apparent horizontal displacement is due really to a vertical movement, and to the subsequent planing down of the surface by denuding agents, will be clear if we consider what must be the effect of the vertical ascent or descent of the inclined beds on one side of a dislocation. Take the bed a in fig. 32, and suppose it to be still unbroken by the fault. It will then run in a straight east and west line. When the fault takes place, the part on the west side is pushed up, or, what comes to the same, that on the east side is let down. A horizontal plane cutting the dislocated stratum will show the portion on the west side lying to the north of that on the east side of the fracture. The effect of denudation has usually been practically to produce such a plane, and thus to exhibit an apparently lateral shift. This surface displacement has been termed the heave of a fault. Its dependence upon the angle of dip of the strata may be seen by a comparison of figs. 33 and 34. In the former figure the bed α, once prolonged above the present surface (marked by the horizontal line), is represented as having dropped from db to ec, the angle of inclination being 25°. The heave amounts to the horizontal distance between b and e. But if the angle should rise to 60°, as in fig. 34, though the amount of throw or vertical displacement remains the same, we see that the heave or horizontal shift diminishes to about a quarter of what it is in fig. 33. This diminu tion would continue with every increase of inclination in the strata till among vertical beds there would be no heave at all.
Strike-faults, where they exactly coincide with the strike, may sometimes remove the outcrop of some strata by never allowing them to reach the surface. Fig. 35 shows a plan of one of these faults (FF), having a downthrow to the north. In crossing the ground from north to south we pass successively over the edges of all the beds, except Nos. 3
and 4, which are cut out by the fault as shown in fig. 36, which is a section drawn across the ground at a right angle to the strike. It seldom happens, however, that such strict coincidence between faults and strike continues for more than a short distance. The dip is apt to vary a little even among comparatively undisturbed strata, and every such variation causes the strike to undulate and thus to be cut more or less obliquely by the line of dislocation, which may nevertheless run quite straight. Moreover, any increase or diminution in the throw of a strike fault will of course have the effect of bringing the dislocated ends of the beds against the line of dislocation. In fig. 37, for in stance, which represents in plan another strike fault, we see that the amount of throw is diminishing towards the left so as to allow lower beds to successively appear, until, at the extreme left side of the ground, the fault merely brings one part of the same bed (No. 5) against another part.
3. Their effects become more complicated where faults traverse undulating and contorted strata. Sometimes we can distinctly trace an undulation as the result of a fault. In the flat limestone beds shown in fig. 38, for example, there can be no doubt that the gentle depression from d to c would not have taken place but for the existence of the fault ab. But in all countries where the rocks have been thrown iuto folds and corrugations these structures are traversed by faults. It then often happens that the same fault appears to be alternately a downthrow on opposite sides. Let us suppose a series of gently rolling strata to be cut by a transverse fault as in the diagram in fig. 39.
At each of the two ridges on the near side of the fault the effect is an upthrow, while in the intervening valley it is a downthrow. On the opposite side of the fault each of these effects is reversed. It rarely happens, however, that a fault makes any such visible crack at the surface. The rocks have all been worn down so much that it is usually only by careful examination of their dip that the existence of faults can be determined.
The influence of faults upon curvatures may be illustrated by a plan and sections of a dislocated anticline and syncline, which will also show clearly how the apparently lateral displacement of outcrop produced by dip-faults is due to vertical movement. Fig. 40 represents a plan of strata thrown into an anticlinal fold AA and a synclinal fold SS, and traversed by a fault FF, which is an upthrow to the left hand. We have seen that a dip-fault always shifts the outcrop to the dip on the upthrow side, and this will be observed to be the case here. Beginning at the upper side of the diagram, which may be called north, we notice that the bed aa, dipping towards the lower side or south at 60°, is truncated by the fault at u, and that the portion on the upthrow side is shifted forwards or southward. Crossing the syncline we meet with the same bed, and as the upthrow of the fault still continues on the same side we must go some way southwards on the downthrow side before we meet with its continuation. On the southern slope of the anticline the same bed once more appears, and again is shifted forwards as before. A section along the left or upcast side (uιι) of the fault would give the structure represented in fig. 41; while one along the downcast side
(√<7) would be as in fig. 42. These two sections clearly prove that the shifting of the outcrops at the surface can be simply explained by a mere vertical movement. They also show that faults which cross anticlinal and synclinal foldδ narrow the anticlines but widen the synclines on the downthrow side, while they widen the anticlines and narrow the synclines on the upthrow side.
Dislocation may take place either by a single fault or as the combined effects of two or more. Where there is only one fault, as in fig. 43, one of its sides may be pushed up or let down, or there may be a simultaneous opposite movement on either side. In such cases, there must be a gradual dying out of the dislocation towards either end; and there will usually be one or more points where the displacement has reached a maximum. Sometimes, as shown in fig. 44, a fault with a considerable maximum throw (35 feet, yards, or fathoms, in the drawing) splits into minor faults at the terminations. Examples of this kind occur not infrequently in coal-work ings. In other cases the offshoots take place along the line of the main fissure (fig. 45). Exceedingly complicated examples occur in some coal-fields, where the connected faults become so numerous that no one of them deserves to be called the main or leading dislocation.
The subsidence or elevation of a large mass or block of rock has more usually taken place by a combination of faults. If we suppose two fissures to meet at a point, as at b in fig. 46, and to die out respectively at a and c, the intervening triangular mass ed may be moved upwards or downwards, or it may remain stationary while the surround ing ground is displaced. The maximum displacement in such an instance would be sought for towards b ; in the direction e there would be no displacement at all.
It often happens that, by a succession of parallel and adjoining faults, a series of strata is so dislocated that a given stratum which may be near the surface on one side is carried down by a series of steps to some distance below. Excellent examples of these stepfaults (fig. 47) are to be seen in the coal-fields on both sides of the upper part of the estuary of the Forth. Instead, however, of having the same downthrow, parallel faults frequently show a movement in opposite directions. If the mass of rock between them has subsided relatively to the surrounding ground, they are troughfaults (fig. 48). They enclose wedge-shaped masses, of which the apices, formed by the junction of two faults, point downwards. In the accompanying section (fig. 49) of a portion of the thick coal of South Staffordshire, drawn to scale by Mr Johnson of Dudley {Records of Geol. Survey, vol. i. part 2, p. 313), the commencement of a trough-fault is shown in the centre of the figure.
The late Mr Jukes carefully described this interesting section, and showed that the coal must once have been more arched than now, and that on the cessation of the elevatory process the fractured pieces adjusted themselves to their new position by means of dislocations. The mass of higher beds (A) driven as a wedge into the coal, has hindered the bed from regaining its horizontality, and at the same time has caused the adjacent parts of the coal (BB) to be so crushed by the enormous pressure as to have been reduced to “a paste of coal dust and very small coal” {Memoir on South Staffordshire Coal-field, 2d ed., p. 194).
It will be observed that the hade of the faults is towards the downthrow side, and that the wedged-shaped masses with broad bottoms have risen, while those with narrow bottoms and broad tops have sunk.
It has been already {ante, p. 261) pointed out that faults are. traceable to the effects of elevation. The general hade or inclination of faults towards the side of downthrow was satisfactorily explained by the late Mr Jukes in the last edition of the present work,
“Suppose,” he says, “that in diagram fig. 50 we have a portion of the earth’s crust, of which AB is the surface, and CD a plane acted on by some widespread force of expansion tending to bulge ιιpwards the part ABCD. If then a fracture takes place along the line EF, it is obvious that the expanding force will, on the side of AC, have the widest base CF to act upon, while it will have a proportionately less mass to move in the part AECF, which grows gradually smaller towards the surface, than on the other side of the fault, where, with the smaller base FD, the mass FDBE continually grows larger towards the surface. The mass G will consequently be much more easily raised into the position AeC∕, than the mass H into the position D∕'B< the elevation of which could hardly take place without leaving a great open gap along the line of fault between FE and f'e', and, moreover, without leaving the projecting piece d overhanging without any support.
“This is yet more clearly perceptible if we suppose two such fissures, as in fig. 51, inclining towards each other, since, if we sup pose the included piece I to be elevated into the position indicated by the dotted lines, it becomes utterly unsupported unless we suppose huge dykes or ejections of igneous rock to issue out along each fault. But this would remove the case from the class of fractures we are at present considering.”
Trough-faults offer at first some difficulty. In fig. 48, for example, it is evident that in both the wedge-shaped masses (A and B) there has been subsidence. The bed X is cut by four faults. In the space B two of these faults hade towards each other, and as they have the same amount of throw the level of the bed remains unchanged on either side. In the other trough, however, the fault a has a throw twice as much as that of l> which it completely cuts off. The two faults d and /neutralize each other, and are connected with a vertical fissure without any throw. The fault a however descends with its persistent hade and dislocates the bed Z and the other strata below. Mr J ukes proposed the following satisfactory explanation of this kind of structure.
“Suppose the beds AA, BB, &c., (fig. 52) to have been formerly in a state of tension, arising from the bulging tendency of an internal force, and one fissure, FE, to have been formed below, which on its course to the surface splits into two, ED and EC. If the elevatory force were then continued, the wedge-like piece of rock W between these two fissures, being unsupported, as the rocks on each side separated, would settle down into the gap as in fig. 53. If the elevatory action were greater near the fissure than farther from it, the single fissure below would have a tendency to gape upwards, and swallow down the wedge, so that eventually this might settle down, and become fixed at a point much below its previous relative position. Considerable friction and destraction of the rocks, so as to cut off the corner gh (fig. 53) on either side, would probably take place along the sides of tne fissures, and thus widen the gap, and allow the wedge-shaped piece W to settle down still further.
“When the forces of elevation were withdrawn, the rocks would doubtless have a tendency to settle down again, but these newly-
included wedge-shaped, and other masses, would no longer fit into the old spaces, so that great compression and great lateral pressure might then take place.”
In fig. 49 an excellent illustration is afforded of how an arched mass of strata ha3 been faulted, and how trough-faults have been formed.
VI.—CLEAVAfrE.
There is yet another system of divisional planes, termed cleavage, by which rocks are sometimes traversed. When this structure is well developed it divides a rock into parallel laminae, which run at a high angle quite independently of stratification or any other divisional planes. It is most perfect in proportion to the fineness of grain of the material in which it occurs. Hence fine argillaceous rocks show it admirably. An ordinary roofing slate may be taken as an illustration of a cleaved rock; its opposite surfaces are cleavage-planes, while the opposite faces of a slab of shale would be stratification-planes. Though most perfectly exhibited by clay-slate, cleavage occure in other rocks, even in old lavas and tuffs, limestones, and sandstones or greywackes; but as the texture increases in coarseness the cleavage lines become more undecided and further apart. The structure may be observed to vary in distinctness in the same face of rock, being well-defined among bands of slate, but becoming faint or even disappearing in intercalated beds of sandstone or grit.
The direction of cleavage usually remains persistent over considerable regions, and, as was shown by Sedgwick, corre-spends on the whole with the strike of the rocks, β It is, however, independent of bedding. Among curved rocks the cleavage planes may be seen traversing the contortions without sensible deflexion from their normal direction, parallelism, and high angle. Mr Jukes pointed out that over the whole of the south of Ireland the trend of the cleavage seldom departs 10° from the normal direction E. 25° N., no matter what may be the differences in character and age of the rocks which it crosses. Some of the more obvious characters of cleavage are shown in fig. 54, which represents a block of cleaved variegated slate about 18 inches in height. The left side of the block which is in shadow is formed by a smooth cleavage plane, and the whole block might be split into laminae parallel to that plane as shown by the cleavage lines in front. The lines of stratification are marked by the white and dark contorted bands, the axes of which evidently correspond nearly with the direction of the cleavage. These bands are commonly marked in nature by zones of different colour, and sometimes of texture. In the present instance the white bands are more sandy than the rest of the mass, and the cleavage-planes only partially enter them. This specimen is further interesting as it bear3 witness by its puckered bedding to the great lateral pressure in virtue of which, as we have already seen (ante, p. 261), the cleavage structure has been produced.
VII. Igneous Rocks as Part of the Structure of the Earth’s Crust.
In this section we shall consider the part taken by igneous rocks in the architecture of the earth’s crust. Their lithological differences having already been described in part ii., it is their larger features in the field that now require attention,—features which in some cases can be well illustrated by reference to the action of modern volcanoes, and in other cases bring before us parts of the economy of volcanoes which can never be reached in any recent cone. A study of the igneous rocks of former ages thus serves to augment our knowledge of volcanic action.
At the outset an obvious distinction must be drawn between those igneous masses which reached the surface and consolidated there, like modern lava streams or showers of ashes, and those which we must believe never found their way to the surface but consolidated at a greater or less depth beneath it. There must be the same division to be drawn in the case of every active volcano of the present day. But we can examine only the materials which reach the surface, and we can but speculate as to the nature and arrangement of what still lies underneath. In the revolutions to which the crust of the earth has been subjected, however, the subterranean continuations of volcanic sheets have ofteu been laid bare, and not only so, but sections have been opened into the very heart of masses which, though molten and eruptive, seem never to have been directly connected with actual volcanic outbursts. All those subterranean intruded masses, which are now revealed at the surface only after the removal of the depth of rock which once covered them, may be grouped together into one division under the names plutonic, intrusive, or subsequent. On the other hand, all those which came up to the surface as ordinary volcanic rocks, whether molten or fragmental, and were consequently contemporaneously interstratified with the formations which happened to be in progress on the surface at the time, may be classed in a second group under the names volcanic, interbedded, or contemporaneous.
It is obvious of course that these are only relative terms. Every truly volcanic mass which, by being poured out as a lava-stream at the surface, came to be regularly interstratified with contemporaneous accumulations, must have been directly connected below with molten matter which did not reach the surface. One part of the total mass therefore would be included in the second group, while another portion, if ever exposed by geological revolutions, would be classed with the first group. Seldom, however, can the same masses which flowed out at the surface be traced directly to their original underground prolongations. It is evident that an intrusive rock, though necessarily subsequent in age to the rocks through which it has been thrust, need not be long subsequent. Its relative date can only be certainly affirmed with reference to the rocks through which it has broken. It may be older than other rocks through which it has not been intruded but which lie almost immediately above it. The probable geological date of its eruption must be decided by the evidence to be obtained from the grouping of the rocks all around. Its intrusive character can only certainly determine the limit of its antiquity. We know that it must be younger than the rocks it has invaded; how much younger must be otherwise determined. On the other hand, an interbedded or contemporaneous igneous rock has its date precisely fixed by the geological horizon on which it lies. A lava-bed or tuff intercalated among strata containing Sphenoptems afhnis, Lepidodendron veltheimianum, Leperditia, and other associated fossils, would unequivocally prove the existence of volcanic action at the surface during the Lower Carboniferous period, and at that particular part of the period represented by the horizon occupied by the volcanic bed. An interbedded and an intrusive mass found on the same platform of strata would not necessarily be coeval. On the contrary, the latter, if clearly intruded along the horizon of the former, would necessarily be posterior in date. It will be understood then that the two groups have their respective limits determined solely by their relations to the rocks among which they may happen to lie.
The value of this classification for geological purposes is great. It enables the geologist to place and consider by themselves the granites, quartz-porphyries, and other crystalline masses which, though lying sometimes perhaps at the roots of ancient volcanoes, and therefore intimately connected with volcanic action, yet owe their special characters to their having consolidated under pressure at some depth within the earth’s crust; while he arranges in another series the lavas and tuffs which, thrown out to the surface, bear the closest resemblance to the ejected materials from modern volcanoes. He is thus presented with the records of hypogene igneous action in the one group, and with those of superficial volcanic action in the other. He is furnished with a method of chronologically arranging the volcanic phenomena of past ages, and is thereby enabled to collect materials for a history of volcanic action over the globe.
In adopting this classification for unravelling the geological structure of a region where igneous rocks abound, the geologist will encounter instances where it may be difficult or impossible to decide in which group a particular mass of rock must be placed. He will bear in mind, however, that after all, such schemes of classification are proposed only for convenience in systematic work, and that there are no corresponding hard and fast lines in nature. He will recog nize that all crystalline or glassy igneous rocks, whether the portion visible be interbedded or intrusive, must be intrusive at a greater or less depth from the surface. Every contemporaneous sheet has proceeded from some internal pipe or mass, so that though interbedded and contemporaneous with the strata at the top, it is intrusive in relation to the strata below. But we cannot always assert that an intrusive mass must have been connected with an outflowing interbedded sheet above.
Section I.—Plutonic, Intrusive, or Subsequent Igneous Pods. Under this section we have to consider the part played by igneous rocks which, either possessing a crystalline (sometimes glassy or felsitic) structure have been injected in a fluid or at least viscous condition into other rocks, or having been blown into fragments have consolidated iu volcanic pipes. After some practice in the field the geologist learns to recognize these rocks, and to distinguish them from the similar masses which must be placed in the contemporaneous series. As a rule their crystalline texture is coarser than in that series; only in a few rare cases does a cellular or amygdaloidal character appear, and the fragmental accompaniments so characteristic of the contemporaneous sheet are only found in the actual vents of eruption. Granite, syenite, felsite, diorite, basalt, and agglomerate occur in this form.
The general law which has governed the intrusion of igneous rock within the earth’s crust may be thus stated: every fluid mass impelled upwards by pressure from below, or by the expansion of its own imprisoned vapour, has sought egress along the line of least resistance. What that line was to be has depended in each case upon the structure of the terrestrial crust and the energy of eruption. In many instances it has been determined by an already existent dislocation; in others by the planes of stratification, or by the surface of junction of two unconformable formations, or by irregular cracks and rents, or by other more complex lines of weakness. Sometimes the intruded mass has actually fused and obliterated some of the rock which it has invaded, incorporating this portion into its own substance. The shape of the channel of escape has necessarily determined the form of the intrusive rock, as the mould regulates the form assumed by a mass of cast-iron. This offers a very convenient means of classifying the intrusive rocks. According to the shape of the mould in which they have solidified, they may be arranged as—(1) amorphous masses, (2) sheets, (3) veins and dykes, and (4) necks.
I. Amorphous Masses. —These consist chiefly of crystalline coarse-textured rocks. Granite and syenite are the most conspicuous, but there are to be included also various quartz-porphyries, felsites, diorites, &c. Where rocks occur in this form which also are found in sheets and dykes as well as contemporaneous beds, it is commonly observed that they are more coarsely crystalline in the form of amorphous masses than in any other. Doleritic rocks afford many examples of this characteristic.
Granitic Bosses.— It was once a firmly-held tenet that granite is the oldest of rocks, the foundation on which all other rocks have been laid down. This idea no doubt originated in the fact that granite is found rising from beneath gneiss, schist, and other crystalline masses which in their turn underlie very old stratified formations. The intrusive character of granite, shown by its numerous ramifying veins, proved it to be later than at least those rocks which it had invaded. Nevertheless the composition and structure of gneiss and mica-schist were believed to be be3t explained by supposing these rocks to have been derived from the waste of granite, and thus, though the existing intrusive granite had to be recognized as posterior in date, it was regarded as only a subsequent protrusion of the vast underlying granitic crust. In this way the idea of the primeval or fundamental nature of granite held its ground.
From what has already (ante, p. 258) been said regarding the fusion and consolidation of rocks, and the evidence supplied on this subject by granite itself, it will readily be understood that the first or original crust could hardly have been one of granite. That rock, so far as can be made out by careful microscopic examination, appears, to have always consolidated under considerable pressure, and in the presence of superheated water and even of liquid carbonic acid—-conditions which probably never obtained at the earth’s immediate surface. The original crust may have been of a glassy character like some of the vitreous lavas; but whatever it was, no trace of it has ever been or is ever likely to be found.
The presence of granite at the existing surface must in all cases be due to the removal by denudation of the masses of rock under which it originally consolidated. The fact that, wherever extensive denudation of an ancient series of crystalline rocks has taken place, a subjacent granite nucleus is apt to appear does not prove that rock to be of a primeval origin. It shows, however, that the lower portions of crystalline rocks very generally assume a granitic type, and it suggests that if at any part of the earth we could bore deep enough into the cru3t we should probably come to a granitic layer. That this layer, even if general round the globe, is not always of the highest geological antiquity is abundantly clear from the fact that in many cases it can be proved to be of later date than fossiliferous formations the geological position of which is known; that is, the granitic layer has invaded these formations, rising up through them, and probably melting down portions of them in its progress. This is true not only of ancient Palaeozoic but of other stratified rocks of various much more recent ages. So that we must conclude that granite does not belong exclusively to the earliest nor to any one geological period, but rather that it has been formed at various epochs, and may even be forming now, wherever the conditions required for its production have existed. As a matter of fact granite occurs much more frequently in association with older and therefore lower than with newer and higher rocks. But a little reflexion shows us that this must be the case. Granite having a deep-seated origin must rise through the lower and more ancient masses before it can reach the overlying more recent formations. But many protrusions of granite would doubtless never ascend beyond the lower rocks. Subsequent denudation would be needed to reveal these protrusions, and this very process would remove the later formations and at the same time any portions of the granite which might have reached them.
Granite frequently occurs in the central parts of mountain chains; sometimes it forms there a kind of core round which the various gneisses, schists, and other crystalline rocks are arranged with more or less irregularity. More frequently it appears in large eruptive bosses which traverse indifferently the rocks on the line of which they rise. Sometimes it even overlies the schistose and other rocks, as in the Piz de Graves in the upper Engadine, where a wall-like mass of granite, with syenite, diorite, and altered rocks, may be seen resting upon schists. In the Alps and other mountain ranges it is found likewise in large bed-like masses which run in the same general direction as the rocks with which they are associated.
Many of the most characteristic features of granitic bosses can be admirably studied where the rock has risen through contorted sedimentary formations, which form undulating or hilly ground rather than mountains. The granite of the south-east and east of Ireland, the south of Scotland, and the south-west of England may be taken as illustrative examples.
In the south-east of Ireland a mass of granite 70 miles in length and from 7 to 17 in width stretches from northeast to south west, nearly along the strike of the Lower Silurian rocks. These strata, however, have not been upraised by it in such a way as to expose their lowest beds dipping away from the granite. On the contrary, they seem to have been contorted prior to the appearance of that rock; at least they often dip towards it, or lie horizontally or undulate upon it, apparently without any reference to movements which it could have produced. As Mr Jukes has shown, the Silurian strata are underlaid by a vast mass of Cambrian rocks, all of which must have been invaded by the granite before it could have reached its present horizon. He infers that the granite must have slowly and irregularly eaten its way upward through the Silurian rocks, absorbing much of them into its own mass as it rose. For a mile or more the stratified beds next the granite have been altered into mica-schist, and are pierced by numerous veins from the invading rock. Within the margin of the granitic mass belts or rounded irregular patches of schist are enclosed; but in the central tracts where the granite is widest, and where therefore we may suppose the deepest parts of the mass have been laid bare, no such included patches of altered rock occur. From the manner in which the schistose belt is disposed round the granite, it is evident that the upper surface of the latter rock where it extends beneath the schists must be very uneven. Doubtless it rises in some places much nearer to the present surface of the ground than at others, and sends out veins and strings which do not appear above ground. If, as Mr Jukes supposes, a thousand feet of the schists could be restored at some parts of the granite belt, no doubt the belt would there be entirely buried; or if, on the other hand, the same thickness of rock could be stripped off some parts of the band of schist, the solid granite underneath would be laid bare. The extent of granite surface exposed must thus be largely determined by the amount of denudation, and by the angle at which the upper surface of the granite is inclined beneath the schists. Where the inclination is high, prolonged denudation will evidently do comparatively little in widening the belt. But where the slope is gentle, and especially where the surface undulates, the removal for some distance of a comparatively slight thickness of rock may uncover a large breadth of underlying granite.
Recent observations by Professor Hull and Mr Traill of the Geological Survey have shown that in the Mourne Mountains a mass of granite has in some parts risen up through highly inclined Silurian rocks, which consequently seem to be standing almost upright upon an underlying boss of granite. The strata are sharply truncated by the crystalline mass, and are indurated but not otherwise altered. The intrusive nature of the granite is well shown by the way in which numerous dykes of dark melaphyre are cut off when they reach that rock.
In the Lower Silurian tract of the south of Scotland several large intrusive bosses of granite occur. The strata do not dip away from them on all sides, but with trifling exceptions maintain their normal N.E. and S.W. strike up to the granite on one side, and resume it again on the other. The granite indeed occupies the place of so much Silurian greywacke and shale. There is usually a metamorphosed belt of variable width in which, as they approach the granite, the stratified rocks assume a schistose or gneissoid character. Numerous small, dark, often angular patches or fragments of mica-schist may be observed along the marginal parts of the granite. Similar features are presented by the granite bosses of Devon and Cornwall which have risen through Devonian strata.
The manner in which some bosses of granite penetrate the rocks among which they occur strongly reminds one of the structure of volcanic necks or pipes. The granite is found as a circular or elliptical mass which seems to descend vertically through the surrounding rocks without seriously altering or disturbing them, as if a tube-shaped opening had been blown out of the crust of the earth up which the granite had risen. Several of the granite masses of the south of Scotland exhibit this character very strik ingly. That granite and granitoid rocks have actually been associated with volcanic action is shown by the way in which they occur in connexion with the Tertiary volcanic rocks of Skye, Mull, and other islands in the Inner Hebrides. As Mr Jukes suggested many years ago, granite or granitoid masses may lie at the roots of volcanoes, and may be the source whence the more silicated lavas, such as trachyte and liparite, proceed.
That some granite, however, is of metamorphic origin, that is to say, has been produced by the gradual softening and recrystallization of other rocks at some depth within the crust of the earth, seems to be now satisfactorily established. Such granite may be looked upon as the extreme of metamorphism, the various schists and gneisses being less advanced stages of the process. Provided the chemical composition of the altered rock be the same as that of granite, it is not necessary that the granite resulting from its alteration should be supposed to differ in any noteworthy particular from ordinary intrusive or igneous granite. The members of the Geological Survey of Ireland have indeed distinguished two granites in Galway, one of which they regard as metamorphic, the other as igneous. The former is characterized by the occurrence of two felspars (orthoclase and oligoclase); the latter contains only one (orthoclase). More recently, however, in the east of the country thêy have separated two groups of granites, of which the intrusive masses are composed of dark-coloured quartz, orthoclase, albite, and black mica (Mourne Mountains), while the metamorphic variety is formed of grey felspar, quartz, and black mica. The mineralogical composition of granite formed by the metamorphism of other and specially sedimentary rocks must necessarily vary with that of the masses out of which it has arisen. In some cases there is a regular gradation from true granite outward into the schistose and gneissose masses. But this passage need not always occur, for if the granite was subject to unequal pressure (which it assuredly would in most cases be) it would in its soft, pasty condition undoubtedly be squeezed into any rents made in the surrounding rocks, and would thu3 imitate exactly a truly igneous mass, which in actual fact it would then be. When a mass of granite rises through unaltered or only locally altered strata, it may fairly be assumed to be igneous and intrusive. When, on the other hand, it is intimately associated with extensive masses of schist and gneiss, many of which can only be distinguished from it by their foliated structure, its metamorphic origin may at least be strongly suspected. Fundamentally, indeed, igneous and metamorphic granite seem to be due only to different modifications of the same subterranean processes. A mass of originally sedimentary rocks may be depressed to a depth of several thousand feet within the earth’s crust, subjected there to vast pressure and considerable heat in presence of interstitial water or steam, and may thus be metamorphosed into crystalline schists. A portion of this mass, undergoing extreme alteration, may so completely lose all trace of its original fissile structure as to become amorphous crystalline granite, some of which may even be thrust as veins into the less highly changed parts above and around. One stage further would bring before us a connexion opened between the surface and such a deep-seated granitic mass, and the consequent ascent and outburst of acid lavas and their fragmental accompaniments.
Amorphous Masses of Diorite, Ac.— On a smaller scale usually than granite, other crystalline rocks assume the condition of amorphous bosses. Syenite, diorite, quartzporphyry, and members of the basalt family have often been erupted in irregular masses, partly along fissures, partly along the bedding, but often involving and apparently melting up portions of the rocks through which they have made their way. Such bosses have frequently tortuous boundary-lines, since they send out veins into or cut capriciously across the surrounding rocks. In Wales, as shown by the maps and sections of the Geological Survey, the Lower Silurian formations are pierced by huge bosses of different crystalline rocks, mostly included under the old term “greenstone,” which, after running for some way with the strike of the strata, turn round and break acr033 it, or branch and traverse a considerble thickness of stratified rock. In central Scotland numerous masses of dolerite and quartziferous diabase have been intruded among the Lower Carboniferous formations. One horizon on which they are particularly abundant lies about the base of the Carboniferous Limestone series. Along that horizon they rise to the surface for many miles, sometimes ascending or descending in geological position, and breaking here and there abruptly across the strata. There can be little doubt that they have actually melted down some parts of the stratified rocks, particularly the limestone. Considerable petrographical differences occur among them which may perhaps be in some measure due to the incorporation of such extraneous material into their mass. Gaps occur where these intrusive rocks do not rise to the surface, but as they resume their position again not far off, it may be presumed that they are really connected under these blank intervals.
The amount and nature of the alteration produced on contiguous rocks by the invasion of an intrusive boss vary necessarily with the character and bulk of the igneous mass, as well as with the susceptibility of the surrounding rock to metamorphism. Induration is generally traceable; shales are hardened into porcelain, jasper, Lydian-stone, or some other flinty argillaceous rock. Sandstones are converted into a kind of lustrous quartz-rock. Limestones are made to assume a granular or crystalline texture, passing into marble or sometimes into dolomite. Under favourable conditions crystals of garnet, analcime, pyrite, and other minerals are developed in the surrounding altered rock.
There can be little doubt that, though the portions of these rocks now visible consolidated under a greater or less depth of overlying rock, they must in many cases have been directly connected with superficial volcanic action. Some of them may have been underground ramifications of the ascending molten rock which poured forth at the surface in streams of lava. Others may mark the position of intruded masses which were arrested in their ascent in the unsuccessful attempt to open a new volcanic vent.
II. Sheets. —These are masses of crystalline rock which have been intruded as sheets between other rocks, and now appear as more or less regularly defined beds. In almost all cases it will be found that these intrusions have taken place between the planes of stratification. The ascending mass of molten matter, after breaking across the rocks, or rather after ascending through fissures either previously formed or opened at the time of the outburst, has at last found its path of least resistance to lie along the bedding planes of the strata. Accordingly it has thrust itself between the beds, raising up the overlying mass and solidifying as a nearly or exactly parallel cake or bed.
It is evident that one of these intercalated intrusive sheets of igneous rock must present such points of resemblance to a truly contemporaneous bed of lava as to make it occasionally a somewhat difficult matter to determine its true character, more especially when, owing to extensive denudation, only a small portion of the rock can now be seen. The following characters mark intrusive sheets, though they must not be supposed to be all present in every case. (1.) They do not rigidly conform to the bedding, but sometimes break across it and run along on another platform. (2.) They catch up and involve portions of the surrounding strata. (3.) They are commonly most close-grained at their upper and under surfaces, and most coarsely crystalline in the central portions. (4.) They are very rarely cellular or amygdaloidal. (5.) The rocks both above and below them are usually hardened and otherwise more or less altered.
Many of the older volcanic recks occur in this form, as felstone, quartz-porphyry, diorite, melaphyre, diabase, dolerite, basalt, and others. The remarks above made regarding the connexion of intrusive bosses with volcanic action may be repeated with even greater definiteness here. Intrusive sheets abound in old volcanic districts intimately associated with dykes and surface outflows, and thus bringing before our eyes traces of the underground mechanism of the volcanoes.
The same kinds of alteration may be observed along the line of junction of intrusive sheets with the adjacent rocks as in the case of amorphous masses; but as the boundary lines are often very sharply defined they present the process of alteration in a more generally accessible and interesting form. Sandstone, for example, besides being indurated and acquiring the distinct lustre of quartzite, may occasionally be seen to possess a distinctly prismatic structure—the prisms or columns diverging at right angles to the line of junction with the igneous rock. Even microscopic black microlites, like those which occur in basalt-rocks, have been detected in altered sandstone, in the minute fissures of which they may be supposed to have been sublimed from the molten injected mass. Argillaceous rocks are commonly converted into hard flinty textures to which the names of flinty-slate, Lydian-stone, jasper, and porcellanite have been applied. Coal-seams when invaded by intruded sheets of igneous matter assume different aspects according to the thickness and nature of the invading sheet, the depth of the coal-seam, and probably to other less easily recognizable causes. In some cases the coal has been fused and has acquired a blistered or vesicular texture, the gas cavities being either empty or filled with mineral matter such as calcite. In other cases it has nearly disappeared, the remaining portion being a black soot or ash. In others it has become hard and brittle, and has been converted into a kind of anthracite or “blind-coal,” owing to the loss of its more volatile portions. In the Ayrshire coal-fields the coal seams have sometimes become beautifully columnar owing to the intrusion of a sheet of basalt along them. The hexagonal and pentagonal columns diverge like rows of stout pencils from the surfaces of the basalt. In one coalfield of that county a seam of coal lias been converted into graphite. The accompanying section (fig. 55) by the late
Mr Jukes represents one of the numerous sheets of “white rock” intruded into the South Staffordshire coal-field. The horizontal distance shown in thi3 section is more than 100 yards. The coal (&) resting on sandstone (c) is traversed by irregular strings and sheets (a) of what the miners term “white-rock,” which proceed from the large basalt masses of the district. The coal has there become dull and anthracitic, and Î3 not worth being extracted.
When a coal-field is much invaded by igneous rocks the seams of coal are usually found to have suffered more than the other strata, not merely because they are specially liable to alteration from the proximity of heated surfaces, but because they have presented lines of more easy escape for the igneous matter pressed from below. The molten rock has very generally insinuated itself along the coal-seams, sometimes taking the lower, sometimes the upper surface, and not infrequently forcing its way along the centre.
In the destruction or alteration of coal and bituminous shales a process of subterranean distillation must often have been set in progress. The gases evolved would find their way to the surface through joints and pores of the overlying rock. The liquid products, on the other hand, would be apt to collect in fissures and cavities. In central Scotland, where the coal-fields have been so abundantly pierced by igneous masses, petroleum and asphaltum are of frequent occurrence in many districts, sometimes in chinks and veins of sandstones and other sedimentary strata, sometimes in the cavities of the igneous rocks themselves.
It is a remarkable fact that, striking as is the change produced by the intrusion of basalt into coals and bitumin ous shales, it is hardly more conspicuous than the alteration effected on the invading masses themselves. A compact crystalline black heavy basalt or dolerite, when it sends sheets and veins into a coal or highly carbonaceous shale, becomes yellow or white, earthy, and friable, loses weight, ceases to have any apparent crystalline texture, and in short passes into what any observer would at first unhesitatingly pronounce to be a mere clay. It is only when the distinctly intrusive character of this substance is recognized in the veins and fingers which it sends out, and in its own irregular course in the coal, that its true nature is made evident (see fig. 55). Microscopical examination shows that this “white-rock” or “white-trap” is merely an altered form of basalt, wherein the felspar crystals, though much decayed, can yet be traced, the augite, olivine, and magnetite being more or less completely changed into a mere pulverulent earthy substance. A specimen of this altered rock was analysed by Mr Henry with the following results :— [table]
It is evident that most of the alkalies and much of the silica have been removed, and that most of the iron exists as carbonate of the protoxide.
In connexion with the alteration produced by igneous sheets upon their contiguous stratified rocks, reference may here be made to the lithological differences traceable within the igneous masses. The close grain already referred to as characteristic of the upper and under portions of an intrusive sheet evidently depends upon more rapid cooling towards the surface of contact with the adjacent cold rocks. When thin slices of these marginal parts are placed under the microscope, they sometimes show abundant black microlites which disappear as the rock is traced away from the margin. They may be regarded as incipient stages in the crystallization of the magnetite, arrested in their development by the rapid consolidation of the outer parts of the rock. In the central portions they have had an opportunity of coalescing into octahedra or groups of definite isometric crystals. A series of sections of a rock, from the outer edge where the arrested crystallites occur to the centre where definitely-built crystals appear, brings in this way before us a history of the stages in the consolidation of the mass.
But considerable differences in composition may also be detected in different portions of the same intrusive sheet. A rock which at one place gives under the microscope a coarsely crystalline texture with the petrographical elements of dolerite will at a short distance show abundant orthoclase and free quartz—minerals which do not belong to normal dolerite. These differences, like those above referred to as noticeable among amorphous bosses, seem too local and sporadic to be satisfactorily referred to original differences in the composition of various parts of the molten mass, or to segregation by gravitation or otherwise. They suggest rather that the great intrusive sheets, in their passage through the rocks underneath, have here and there involved and melted down portions of these rocks, and have thus acquired locally an abnormal composition.
III. Veins and Dykes. —Veins of igneous rock may occur indifferently in igneous, aqueous, or metamorphic rocks. They may range in diameter from mere thread-like filaments up to huge bands many feet or yards broad. In regard to their origin they may be grouped into two series —(1) veins of segregation, and (2) veins of intrusion.
Veins of Segregation.— These include most of what were formerly and not very happily termed “contemporaneous veins.” They are peculiar to crystalline rocks. They abound in many granites, likewise in some gneisses and schists. They may not infrequently be observed in sheets of diorite, dolerite, and diabase. They run as straight, curved, or branching ribands, seldom exceeding a foot in thickness. Most frequently they are finer in texture than the rock which they traverse, though now and then the reverse is the case, more especially in granite. Close examination of them shows that they are not sharply defined by a definite junction line with the enclosing rock, but that on the contrary they are welded into that rock in such a way that they cannot easily be broken along the plane of union. This welding is found to be due to the mutual protrusion of the component crystals of the vein and of the surrounding rock—a structure sometimes admirably revealed under the microscope. Veins of this kind are evidently to be referred to the earliest condition of the rocks iu which they occur. They point to some process, still unexplained, whereby into rents formed in the deeply buried, and at least partially consolidated or possibly colloid, mass there was a transfusion or exosmosis of some of the crystallizing minerals.
Veins of Intrusion.— These are portions of once-melted or at least pasty matter which have been injected into rents of previously solidified rocks. When traceable suffi ciently far, they may be seen to swell out and merge into their large parent mass, while in the opposite direction they may become attenuated into mere threads. Sometimes they run for many yards in tolerably straight lines, and when this takes place along the stratification they look like beds. At these parts, they are of course really intrusive sheets. But they may frequently be found to start suddenly upward or downward, and to break across the bedding in a very irregular manner. In fig. 56 t represents an intrusive igneous rock running through horizontal strata s. If we saw merely the horizontal portion below or above, the really igneous and intrusive nature of the rock t might escape us, but the intermediate connecting vein makes its character at once apparent.
No rock exhibits so admirably as granite the varieties assumed by veins. In many cases the veins which traverse the granite itself must be regarded as segregation or infiltration veins, as already described. But where they proceed from the granite, and traverse surrounding rocks, they are probably in most cases intrusive, though where granite and highly granitic gneiss are in contact we may conceive that some of the veins traversing both rocks may be segregation veins. Most large masses of granite send veins into the surrounding rocks. Frequently the veins so abound as to form a complicated net-work. They vary in breadth from several feet or even yards down to fine filaments at the ends of the smaller branches. They frequently cross each other, not only outside of the granite mass, bu t even within it. They vary much in texture and in composition. Sometimes they are coarsely crystalline; but probably most of the veins of this kind are due rather to segregation than intrusion Most frequently granite veins traversing granite are finer-grained than the main mass. Veins which are clearly intrusive are not only finer in grain than the parent granite, but sometimes present considerable differences in mineralogical composition. The mica, for example, may be reduced to exceedingly minute and not very abundant flakes, and may almost disappear. The quartz also occasionally assumes a subordinate place, and the rock of the veins passes into eurite, elvanite, or one of the varieties of porphyry.
The rocks surrounding a granite mass and traversed by granite veins are almost always more or less metamorphosed in a belt varying in breadth from a few yards up to a mile or more. It is in this zone that the granite veins typically occur. The altered rocks have assumed the characters of gneiss, mica-schist, or other metamorphic product, but resume their usual condition as we trace them away from the granite. Curious angular portions of them may often be observed within the granite veins and in the main granite miss. In Cornwall the granite and surrounding slates are traversed by veins of quartz-porphyry termed elvans, which are most numerous near the granite. They vary in width from a few inches or feet to 50 fathoms, their central portions being commonly more crystalline than the sides. In the great granite region of Leicester Mr Jukes traced some of the elvans for several miles running in parallel bands, each only a few feet thick, with intervals of 200 or 300 yards between them. Many of the other intrusive rocks likewise present the phenomena of veins; diorite, diabase, mslaphyre, and basalt furnish numerous illustrations.
Dylces are wall-like masses of igneous rock, filling vertical or highly-inclined fissures. They differ therefore from veins in the greater parallelism of their sides, their verticality, and their greater regularity of breadth and persistence of direction. They present as great a variety of thicknees as that which is shown by veins. Sometimes they occur as mere plates of rock not more than an inch or two in thickness; at other times they attain a breadth of ten or twelve fathoms. The smaller or thinner dykes can seldom be traced more than a few yards; but the larger examples may be followed sometimes for miles. Thus in the south of Scotland a remarkable series of Basaltdykes can be traced across all the geological formations of that region and even across powerful faults. They run parallel to each other in a general north-west and south-east slirection for distances of 20 and 30 mile3. A remarkable dyke crosses the north of England from the coast of Yorkshire for fully GO miles inland.
The name dyke is applied to these masses of igneous rock on account of their resemblance to walls (Scotice, dykes). Their sides are often as parallel and perpendicular as those of a piece of masonry. Moreover, the resemblance to human workmanship is sometimes brought out still more by the numerous joints which, intersecting each other along the face of a dyke, remind us of well-fitted masonry. Where the surrounding rock has decayed, the dykes may be seen projecting above the ground exactly like walls; indeed in many parts of the west of Scotland they are made use of for enclosures. The material of the dykes has in other cases decayed, and deep ditch-like hollows are left to mark their sites. The coast-lines of many of the Inner Hebrides and of the Clyde Islands furnish numerous admirable examples of both kinds of scenery.
While veins have been injected into irregular branching cracks, dykes have been formed by the welling upwards of liquid rock in vertical or steeply inclined fissures. Sometimes the line of escape has been along a fault. In Scotland, however, which may be regarded as a typical region for this kind of geological structure, the vast majority of dykes rise along fissures which have no throw, and are therefore not faults. On the contrary the dykes may be traced across some of the largest faults in the midland counties.
While the term dyke might be applied to some of the wall-like intrusions of porphyry, elvanite, and even of granite, it is more typically illustrated among the augitic igneous rocks, such as basalt, diabase, &c., though also among diorites, porphyries, pitchstones, &c. The central parts of a dyke are usually most crystalline. Towards the margin the grain becomes finer, and even sometimes passes into a vitreous condition. Many of the basalt dykes in different parts of Scotland are coated along the sides with a film or crust of black vitreous tachylite. Lines of amygdaloidal kernels may not infrequently be traced along the centre of a dyke.
When the rock on one side is freshly stripped off, the dyke is usually observed to present a system of polygonal jointing. The joints start from each face or cheek, and either go right across to the opposite side, or branch and lose themselves about the centre. They thus divide the dyke into irregular prisms which, when the dyke is vertical, lie of course in a horizontal position, whence they depart in proportion as the dyke is inclined. Occasionally the prisms are as well-formed as in any columnar bed of basalt. A less prominent set of joints runs parallel with the two cheeks of a dyke.
There is usually some alteration of the stratified rocks in contact with a dyke; but the nature and amount of the change vary within a wide range. The most sensitive material to this influence is undoubtedly coal. A seam of coal 6 or 8 feet in thickness may be observed to grow dull and brittle at a distance of 50 yards from a large dyke, becoming what is termed “blind coal,” as it then burns without flame. Still nearer to the intrusive mass the coal passes into a kind of pyritous cinder scarcely half the original thickness of the seam. At the actual contact with the dyke it becomes by degrees a kind of caked soot not more perhaps than a few inches thick. Sandstones are hardened into a kind of lustrous quartzite, and sometimes made columnar, shales into flinty slate or porcellanite; limestones are occasionally rendered crystalline and even dolomitic. Occasionally a segregation of new minerals has taken place in the rocks adjoining a dyke. But cases are by no means infrequent where dykes have produced little or no appreciable change upon the contiguous rocks.
IV. Necks. —Under this term are included the filled-up pipes or funnels of former volcanic vents. Every series of volcanic sheets poured out at the surface must have been connected with one or more orifices which, on the cessation of the eruptions, would remain more or less completely filled with lava or with fragmentary matter. But unless subsequent denudation should remove the overlying cone audits surrounding piles of lava and tuff, these vents must remain buried under the materials which came out of them. So extensive, however, has been the waste of the surface in many old volcanic regions that the sites of the vent≡ have been laid bare. In the study of these we have before us some of the more deep-seated phenomena of volcanic action never to be seen in any modern volcano.
A neck is of a circular or elliptical, but occasionally of a more irregular branching form. It varies in diameter from a few yards up to a mile, or even more. It descends into the earth perpendicularly to the stratification of the formation to which it belongs. Thus, if a neck was formed and filled up during the accumulation of a certain group of strata, it would rise on the whole vertically through these strata, and its ejected lava or tuff would spread out conformably among them. Should the rocks be subsequently tilted the neck would of course be thrown out of the vertical. As a rule, however, the vertical descent of the necks into the earth’s crust has been comparatively little interfered with.
The materials filling up ancient volcanic orifices are sometimes crystalline, sometimes fragmental. The neck may be occupied by some form of lava, as felstone, quartzporphyry, diabase, porphyrite, basalt; or by the fragmentary materials which fell back into the throat of the volcano and finally solidified there; or by both kinds of rock combined. Among the Palaeozoic volcanic districts of Britain the necks not infrequently are filled with some siliceous crystalline rock, such as a quartz-porphyry or felstone, even where the surrounding lavas are basic. Necks of agglomerate and fine tuff abound among the Carboniferous and Permian volcanic regions of Scotland.
The fragmentary materials consist mainly of different lava-form rocks imbedded in a gravelly peperino-Wz matrix of more finely comminuted debris of the same rocks; but they also contain, sometimes in abundance, fragments of the strata through which the necks have been drilled. Pieces of fine stratified tuff not infrequently appear in the agglomerates. This fact, coupled with the not uncommon occurrence of a tumultuous fractured and highly-inclined bedding of the materials in the necks appears to show that the pipes were partly filled up by the subsidence of the tuff consolidated in beds within the crater and at the upper part of the funnel. Veins of basalt abound in many of the necks of Carboniferous age in central Scotland.
The strata round a neck are usually somewhat hardened. The sandstones have acquired sometimes a vitreous lustre; argillaceous beds have been indurated into porcellanite; coal-seams have been burnt and rendered unworkable. These changes may be due partly to the heat of the ascending column of molten rock or ejected fragments, partly to the ascent of heated vapours, even for a long time subsequently to the volcanic explosions. Proofs of a metamorphism probably due to the latter cause may sometimes be seen within the area of a neck. It is where the altered materials are of a fragmentary character that the nature and amount of this change can be best estimated. What was originally volcanic dust has been converted into a crystalline and even porphyritic mass, through which, however, the likewise intensely altered blocks interspersed through the agglomerate are still recognizable. Such blocks as, from the nature of their substance, must have offered most resistance to change,—pieces of sandstone or quartz, for example,—stand out prominently in the altered mass, though even they have undergone more or less modification, the sandstone being converted into vitreous quartz-rock.
Section II.—Volcanic, Interbedded, or Contemporaneous I y neons Rocks.
The rocks comprised in this section have all been ejected to the surface like the lava-streams and showers of ashes of modern volcanoes. It is evident that on the whole they must agree in lithological characters with those rocks, described in the previous section, which have been extravasated by volcanic efforts though not quite reaching the surface. Yet they have some well-marked general characters, of which the most important may be thus stated. (1.) They occur a≡ beds or sheets which conform to the bedding of the strata among which they are intercalated. (2.) They do not break into or involve portions of the overlying beds. (3.) The upper and under por tions of the lava sheets present commonly a scoriaceous or vesicular character, which may even be found extending throughout the whole of a sheet. (4.) Beds of tuff are frequently interstratified with the crystalline sheets.
I. Crystalline. —While the underground course of a protruded mass of molten igneous rock has widely varied according to the shape of the channel through which it proceeded, and in which, as in a mould, it solidified, the behaviour of the rock, once poured out at the surface, has been much more uniform. As in modern lava, the erupted rock has rolled along, varying in thickness and other minor characters, but retaining the broad general aspect of a bed or sheet. A comparison of such a bed with one of the intrusive sheets already described shows that in several important respects they differ from each other. An intrusive sheet is closest in grain near its upper and under surfaces. A contemporaneous bed or true lava-flow, on the contrary, is there usually most open and scoriaceous. In the one case we rarely see vesicles or amygdules, in the other they often abound. However rough the upper surface of an interbedded sheet may be, it never sends out veins into nor encloses portions of the superincumbent rocks, which, however, sometimes contain portions of it, and wrap round its hummocky irregularities, as shown in fig. 57. Occasionally it may be observed to be full of rents which have been filled up with sandstone or other sedimentary material. In these cases we see that the lava cracked in solidifying, and that sand was washed into the fissures where it consolidated. The amygdaloidal cavities throughout an interbedded sheet, but more especially at the top, may often be noticed with an elongated form, and even pulled out into tube-like hollows in one general direction, which w as obviously the line of movement of the yet viscous mass.
Some kinds of rock when occurring in interbedded sheets are apt to assume a system of columnar jointing. Basalt in particular is distinguished by the frequency and perfection of its columns. The Giant’s Causeway and the cliffs of Staffa, of Ardtun in Mull, and of Loch Staffin in Skye are well-known examples. The columns are set perpendicularly to the two cooling surfaces, that is, to the top and bottom of the bed. Any inclination from the horizontal in the disposition of the bed will cause a corresponding departure from the vertical on the part of the columns. Sometimes the columns are arched or curved, as in the Clam-shell Cave, Staffa.
A single interbedded sheet of crystalline rock seldom oecurs by itself without any other volcanic accompaniment. It is usually associated at least with bands of tuff showing that the emission of lava was not unattended with fragmentary ejections. In the majority of cases it will be found to form part of a series of interbedded sheets with intercalated tuffs. Vast piles of such consecutive flows, reaching a total depth of several thousand feet, remain to witness the energy of former volcanic vents.
II. Fragmental. —The rocks embraced under this term include all the fragmentary volcanic ejections which enter into the composition of the earth’s crust, from the coarsest agglomerate to the finest tuff. They differ in lithological character, according to the nature of the lavas with which they are associated and from which they have been derived. Thus in a region of trachyte-lavas, we have trachyte-tuffs, trachyte-breccias; in one of basalts, we find basalt-breccias, basalt-agglomerates, basalt-tuffs; in one of obsidians, we meet with pumiceous tuffs and breccias. The fragmentary matter has been ejected from volcanic vents, and has fallen partly back into the funnels of discharge, partly over the surrounding area. It is therefore apt to be more or less mingled with ordinary sedimentary detritus. We find it indeed passing insensibly into sandstone, shale, limestone, and other strata.
Great differences occur in the texture of fragmental rocks even in the same volcanic districts. They are often coarse and tumultuous at or near the vents, and fine-grained at a distance. Alternations of gravelly peperino-Xiko, tuff with a very fine-grained “ash” may frequently be observed. Large blocks of lava-form rock, as well as of the strata through which the volcanic explosions have taken place, occur in the tuffs of most old volcanic districts.
It has been already pointed out that agglomerate and tuff are not infrequently to be met with occupying the sites of the vents of eruption. Their most common disposition, however, is in beds either alone or associated with interbedded lavas. Masses of fine or gravelly tuff several hundreds of feet in thickness, without the intervention of any lava-bed, may be observed in the volcanic districts of the Old Red Sandstone and Carboniferous systems in Scotland. On the other hand, in these same areas thin seams of tuff may be seen interlaminated with ordinary sandstone, shale, or limestone. In the one case we have evidence of long-continued and powerful volcanic action, during which fragmentary materials were showered out and spread over the water-basins to the exclusion of ordinary sediment. In the other we have proof of feeble intermittent volcanic explosions, whereby light showers of dust were discharged, which settled down quietly amidst the sand, mud, or limestone accumulating around at the time. Under these latter circumstances tuffs often became f03silifer0us; they enclose the remains of such plants and animals as might be lying on the lake-bottom or sea-floor over which the showers of volcanic dust fell, and thus they form a connecting link between aqueous and igneous rocks.
VIII.— Metamorphic Rocks as Parts of the Architecture of the Earth’s Crust.
In part ii. (ante, p. 235) some account has been given of the composition of certain foliated rocks frequently met with in the central portions of mountain chains and elsewhere, lying beneath geological formations of high antiquity. In part iii., in the discussion of the hypogene causes of change within the earth’s crust, reference was again made to these rocks, and they were alluded to as examples of the effects of subterranean processes altering the original character of large mineral masses (ante, p. 263). They were there cited as metamorphic rocks, but their characters as integral parts of the earth’s crust were reserved for discussion in the present part of this article.
At the outset some caution must be employed as to the exact meaning in which the terms “metamorphism” and “metamorphosed” are employed. In a certain sense it may be said that all or nearly all rocks have been metamorphosed, since it is exceptional to find any, at least among such as are not in a geological sense of modern date, which do not show, when closely examined, proofs of having been altered by the action of percolating water or other daily acting metamorphic agent. Even a solid crystalline mass which, when viewed on a fresh fracture with a good lens, seems to consist of unchanged crystalline particles will usually betray under the microscope unmistakable evidence of alteration. And this alteration may go on until the whole internal organization of the rock has been readjusted, though the external form may still remain such as hardly to indicate the change, or to suggest that any new name should be given to the recomposed rock. Among many igneous rocks, particularly the more basic kinds, as basalts, diorites, olivine rocks, &c., metamorphism of this kind may be studied in all its stages.
But it is not to alteration of this nature, effected at the surface by meteoric water, that the term metamorphism is properly applied. That word is reserved for the process of subterranean change above treated of (ante, p. 258), whereby a more or less complete transformation has been effected throughout vast mineral masses which, while undergoing crystalline rearrangement, have usually suffered simultaneously enormous compression. Gneiss, mica-schist, and the other schistose or foliated rocks are typical examples of the results of this metamorphic process.
Three antagonistic opinions are at present entertained regarding the origin of these rocks. Some geologists regard the crystalline schists as plutonic rocks representing the early cooled crust of the earth, and suppose that a similar schistose structure has occasionally been superinduced by plutonic action on later sedimentary formations. Again, by some recent writers the Wernerian notion of chemical precipitation has been revived, and the idea of metamorphism has been discarded. These authors suppose that the schistose rocks, in common with many pyroxenic and hornblendic rocks (diabases, diorites, &c.), as well as masses in which serpentine, talc, chlorite, and epidote are prevailing minerals, have been deposited “for the most part as chemically-formed sediments or precipitates, and that the subsequent changes have been simply molecular, or at most confined in certain cases to reactions between the mingled elements of the sediments, with the elimination of water and carbonic acid.” To support this view, it is necessary to suppose that the rocks in question were formed during a period of the earth’s history when the ocean had a considerably different relative proportion of mineral substances dissolved in its waters, and consequently that they must be assigned to a very early geological period, anterior indeed to what are usually termed the Palaeozoic ages. And it becomes further needful to discredit the belief that any gneiss or schist can by possibility belong to one of the later stages of the geological record. The more thorough-going advocates of the pristine or “eozoic” date and original chemical deposition of the so-called “metamorphic” rocks do not hesitate to take this step, and endeavour, by ingeni ous explanations, to show that the majority of geologists have mistaken the geological structure of the districts where these rocks have been supposed to be metamorphosed equivalents of what elsewhere are Palaeozoic, Secondary, or Tertiary strata. Prevalent opinion supports the third or metamorphic theory, according to which the schistose rocks are held to be crystalline transformations of ordinary sedimentary strata. The problem whether or not certain masses of gneiss or schist represent equivalent unaltered sedimentary rocks of other districts is mainly one of structural geology. It must be decided by the geologist rather than the chemist. It has been answered in the affirmative by the great majority of stratigraphical geologists all over the world. We may not be entitled to assert that every mass of gneiss or schist is a metamorphosed sedimentary rock. Possibly some foliated rocks of extreme antiquity may have originated directly from chemical precipitation. But when it can be shown that ordinary stratified rocks have been converted into schist and gneiss, it seems permissible to hold that all such rocks have had a similar origin, at least until good reasons can be adduced against that view.
It is evident that if the so-called metamorphic rocks can anywhere be seen to graduate into unaltered strata, it is there that they ought to be specially studied, and that light may be expected to be cast on their origin and history. It is customary to speak of metamorphism as being either local or regional, that is, being confined to one limited portion of a mass of rock, or as extending throughout the whole of the rock and over wide districts. It is doubtless from the first of these developments that we may hope to learn most regarding metamorphic changes.
Local (Contact) Metamorphism.— The simplest and most obvious examples of this kind of alteration occur where a mass of igneous rock has invaded sedimentary strata, which have in consequence been affected in lithological character along the margin of contact with the intrusive rock. Allusion has already been made to changes of this kind effected by intrusive sheets and dykes, and likewise traceable round the edges of ancient volcanic vents. Sandstones are converted into a lustrous crystalline quartz-rock; shales and argillaceous strata are indurated into porcellanite, jasper, flinty-slate, or some analogous product. Limestones are made crystalline; coals are charred, turned into anthracite, and sometimes even into graphite. These metamorphisms extend to very variable distances from the intrusive rock. Sometimes they are scarcely perceptible at all, and disappear within a space of a few inches. In other cases they extend for some feet, and in the case of some coal-seams even to 50 yards or more.
It is around bosses of granite however that the most marked examples of local metamorphism can be observed. On a previous page some account has been given of the way in which the Silurian rocks of Ireland and the south of Scotland are pierced by large masses of granite, and how, as they'approach the granite even at a horizontal distance of a mile or more, they begin to assume a micaceous foliated texture which becomes more and more marked until, along the margin of the granite, they pass into true mica-schist and gneiss. The identity of origin between these schistose masses and the greywackes and shales beyond the metamorphic zone does not admit of any question. The unaltered rocks can be followed step by step into and through the stages of alteration, until they are found to have acquired the genuine gneissic or schistose structure. Moreover, it may be observed that all the bands of sedimentary rock do not give riee to the same kinds of metamorphic products. Some kinds of strata are more prone to alteration than others, and give rise to more perfect schists. In the south of Scotland, for example, certain greywackes and grits formed of a granular mixture of quartz-sand, felspar, clay, and various decomposition products have been metamorphosed into perfect mica-schist, while some bands of black anthracitic and graptolitic shale have merely been intensely indurated and shattered. Many districts in Scotland, in Ireland, in the lake country of England, and in Wales might be cited as furnishing examples of this gradual conversion of ordinary sedimentary rocks into truë schists.
Regional Metamorphism.— If then it can be proved that over limited areas thoroughly foliated rocks have been produced by the transformation of ordinary sedimentary strata, a presumption is established in favour of a similar mode of origin for foliated rocks elsewhere and over wider regions. There occur many vast spaces of the earth’s surface occupied by foliated rocks. In Finland, Scandinavia, and the Scottish Highlands tracts many thousands of square miles in extent consist of gneiss, mica-schist, hornblende-rock, and other members of the same great family of rocks. A large portion of British North America lies upon similar mineral masses. Rocks of this type commonly rise also along the core of great mountain ranges, as in the Alps and in the Rocky Mountains. If these rocks which cover such vast areas were originally ordinary sedimentary rocks, they must have been metamorphosed, not by mere local protrusions of igneous matter, but by some general process.
Gradations of Metamorphism. —We have seen above that the nature and extent of the alteration experienced by rocks have been regulated, not merely by the vigour of the metamorphic process, but by the composition and structure of the rocks themselves. A siliceous sandstone, for instance, containing little or no aluminous or other admixture, seems to be capable of retaining much of its original character, while surrounding or intercalated less purely quartzose beds have been completely changed. It is converted into quartz-rock, but still shows the rounded quartz grains of the original sand. In proportion as the sand has been mixed with clay it has produced a rock more susceptible of change. The argillaceous (or magnesian) cement has been attacked, and in the process of change the quartz-sand has been affected. Mica in one or other of its various forms, aluminous or magnesian, has very generally appeared, and in proportion to its development has the foliated structure been made apparent. Hence we may obtain every gradation from a quartz-rock or grit into a true mica-schist or gneiss.
Production of Foliation.— The term “foliation” means the separation of a rock into approximately parallel or lenticular crystalline layers or folia of different mineral composition. It implies a segregation and crystallization of mineral matter along distinct planes. Those who maintain the original chemical precipitation of the most ancient gneisses and schists believe that the folia mark the stratification of the successive layers of deposit. Those, on the other hand, who hold that all the schistose rocks about the origin of which any satisfactory conclusion can be reached were originally mechanical sediments of ordinary kinds, regard the folia as coincident generally with the stratification of these sediments.
The folia of a gneiss or schist are crystalline aggregates which along their planes of mutual contact are as it were welded or felted into each other by the interlacing of their several component crystals. They are destitute, as a rule, of the parallelism, flatness, and persistence so characteristic of stratification. On the contrary they are apt to swell out into thick concretionary aggregations and to die out rapidly; they exhibit a wavy, crumpled, or puckered arrangement traceable in vast folds on the side of a mountain, and yet descending even to such minute corrugation as can only be detected by the microscope.
Foliation occurring in altered sedimentary rocks must be due to a crystallization and rearrangement of the chemical constituents of these rocks along certain lines. On a former page (ante, p. 263) it was pointed out that this kind of metamorphism would as a rule proceed along the lines of stratification. Doubtless in the vast majority of cases the planes of foliation are coincident with the general direction of this stratification. The metamorphic rocks of the Scottish Highlands furnish admirable proofs of this fact. Bands of quartz-rock and schist alternate with each other and with zones of limestone, precisely as beds of sandstone, shale, and limestone do in unaltered formations. Thin seams of pebbly grit with well-rounded water-worn pebbles may be observed running parallel with the folia of a schist, as a seam of fine grit or conglomerate may be seen to do in a series of shales. Nay, even the false-bedding so characteristic of mechanical sediment may be observed among these metamorphic rocks. The metamorphism is not uniform in these regions. Here and there it becomes intensified, and the rocks assume a thoroughly crystalline and much disturbed aspect. But as they recede from these areas they are found to lose much of their foliated character, and indeed to present such slight traces of metamorphism that they can be at once recognized as greywackes, grits, and shales. Moreover, they have been thrown into anticlinal and synclinal folds, and in these and other larger features of geological structure they differ in no essential respect from ordinary unaltered strata. Lastly, fossiliferous limestones containing Silurian shells have been found at their base, so that they have all been formed long after the seas over the area of Britain had been tenanted by living organisms.
Professor Sedgwick and Mr Darwin pointed out many years ago that a crystalline rearrangement of mineral matter has in some cases taken place along the planes of cleavage. We have already observed that the water which has been the great agent of metamorphism must always have followed the dominant divisional planes of a rock. If these plane3 were those of cleavage, the foliation would doubtless be produced along them, irrespective of the original laminae of deposit. So long as the rock remained tolerably homogeneous in chemical composition, there seems no reason why foliation along the cleavage should differ in any material respect from that along stratification. But it may be doubted whether a cleavage foliation could run without sensible and even very serious interruptions over wide areas. For, in the first place, in most large masses of sedimentary matter we encounter alternations of different kinds of sediment, which could not but produce distinct kinds of rock under the influence of metamorphic change. In the second place, cleavage depends for its perfection and continuity on the fineness of grain of the rock through which it runs. While exceedingly perfect in a mass of argillaceous strata, it becomes feebler or even dies out in a coarse sandy or gritty rock. Hence, where foliation coincides with cleavage over large tracts, there will almost certainly be bands, more or less distinct, coincident with the original stratification, and running oblique to the general foliation, like bedding and cleavage, save where these two kinds of structure may happen to coalesce.
In a region of intense metamorphism the foliation of the schists may be observed to become here and there somewhat indefinite, until it disappears altogether, and the rocks assume a thoroughly granitic character. Between gneiss and granite there is no difference in mineralogical composition; in the one rock the minerals are arranged in folia, in the other they have no definite arrangement. Gneiss might be called a foliated granite; granite might be termed a non-foliated gneiss. The two rocks may be observed to graduate into each other. In Aberdeenshire, for example, the common grey mica-schist and gneiss may be seen to pass insensibly into the ordinary grey granite. In such cases it has been naturally concluded that granite is the ultimate stage of metamorphism. Judged merely from their composition and microscopic structure, an intrusive granite connected with igneous protrusions and a metamorphic granite representing the thorough transformation of stratified rock cannot be distinguished from each other.
There is thus nothing improbable in the idea that the same mineral particles may have gone through many successive cycles of change. We may suppose them to have been originally part of a grauite mass, and to have been subsequently exposed at the surface by enormous denudation. Worn away from their parent granite they would be washed down with other particles, and spread out under water as parts of sandy or muddy deposits. Buried under a gradual accumulation of sedimentary material thousands of feet in thickness they might be depressed deep beneath the surface, and be thus brought within the influence of metamorphism. Gradually recomposed, crystallized, and converted into schistose rock, they might be eventually reduced to the condition of granite, and protruded into some of the overlying less metamorphosed masses in the form of granilc veins. Or we may conceive, as already (ante, p. 309) suggested, that a communication was opened between the granite thus produced and the surface, and that the original mineral particles, whose vicissitudes we have been tracing, were erupted to the surface as part of a stream of lava.
Possible Metamorphism of Igneous Rocks.— In most large tracts of foliated rocks there occur masses less distinctly foliated or quite granitoid in texture, formed mainly of hornblende or of that mineral in combination with others. Zones or bosses of hornblende-rock and hornblende-schist frequently appear among gneiss and mica-schist. Varieties of quartz-porphyry occur in a similar way. Bands of fine unctuous chloritic schists may also often be traced. It is not easy to understand how such rocks, at least those containing a large percentage of magnesia, could be produced by the metamorphism of ordinary sediment. The difficulty may perhaps be removed if we regard them as having originally been igneous rocks, either erupted at the surface or intrusively injected among the surrounding rocks previous to metamorphism. Such mineral masses as varieties of syenite and diorite, rich in hornblende or ether magnesian silicates, might have been the original condition of many of the rocks here referred to. The fine magnesian schists might be regarded as having been at first tuffs associated with the lava-form masses.
Structure of Metamorphic Rocks in the Field.— As the series of metamorphic rocks ranges from scarcely altered sedimentary strata on the one hand to crystalline amorphous granitic masses on the other, they must obviously possess a great range of structure as parts of the architecture of the solid land. In particular they must under different circumstances present the features now of aqueous and now of igneous rocks. The most typical form of metamorphism being foliation, we may consider the structure of foliated rocks as the most characteristic. From what has been said above, it is evident that the planes of foliation give the rocks a general resemblance to stratified sedimentary masses. But these planes are seldom so definite and persistent as those of stratification. They do not impart to the rocks the same tendency to split up into well-marked parallel beds. On the contrary they are often so felted or welded together, especially in the coarse and most crystalline gneisses, that they hardly serve as divisional planes at all, but leave the firm tough rock to split up along other lines.
With care and patience lines of anticlinal and synclinal fold may often be traced among foliated as well as among unaltered rocks. But the unravelling of these and other features of structure is much more difficult than among ordinary stratified formations. This arises partly from the frequent absence of conspicuous and persistent bands which could be used as horizons in working out geological structure, partly from the abundant crumpling which most foliated rocks have undergone, whereby the continuity of the individual bands is much disturbed or entirely destroyed.
The joints among foliated rocks to which the regular and parallel folia impart a marked fissility resemble those among sedimentary strata. Where, however, the foliation is of a more massive kind, as in the coarser varieties of gneiss, the system of jointing approximates to that of granite or one of the more crystalline igneous rocks.
IX. Mineral Veins.
The fissures which so abundantly traverse the crust of the earth have in many instances served as places for the deposit of mineral matter quite distinct from that of the rocks through which they run. As metallic ores frequently occur among the minerals so deposited, and have been extensively worked, a large amount of information has been obtained by mining operations regarding these fissures, or, as they are termed, mineral veins. A general though not invariable relation exists between the nature of the minerals in the fissures and that of the contiguous rocks. When the latter are calcareous, calcite usually forms a conspicuous feature in the veins; among siliceous rocks quartz is abundant. These and the other minerals are for the most part well crystallized or at least largely crystalline in the veins, even when the adjoining rocks are granular or amorphous. They are termed veinstones. Since the joints, faults, and fissures which have been filled with new mineral substances are commonly highly inclined or vertical, mineral veins generally run as steep wall-like bands across the rocks in which they occur. Their minerals are arranged in strips, which on the whole run parallel with the walls of the vein (fig. 58).
Mineral veins become metalliferous, when among their vein-stones there occur detached crystals or particles, or branching strings and threads, or concretionary masses or parallel bands of native metal, or of the sulphides, oxides, or other ores of metals. The association of these various substances within the two walls or cheeks of a vein is often in a double set of parallel bands, those in one-half of the vein being repeated in the other. The middle of the vein, for example, may consist of galena flanked and partially mixed up with zinc-blendø and pyrites. On either side of this central rib there may be a layer of some veinstone, perhaps fluor spar, then a duplicate band of another veinstone, such as barytes or quartz, and so on, to the outer edge of the vein (fig. 58). In other cases, as in auriferous veins of quartz, the vein consists wholly or almost wholly of one vein-stone through which the ore is disseminated in minute grains and strings, so that the vein-stone must be extracted and crushed to obtain the metal by washing.
Mineral veins (lodes, rake-veins) vary in thickness from only an inch or less up to many fathoms. Extraordinary variations may be traced even in the course of the same vein, a breadth of several feet or yards rapidly diminishing until the two walls nearly or quite meet, to the exclusion. of the minerals of the vein. Similar diversities may be observed in the horizontal extent of veins, some being traceable for miles, others disappearing in a few yards.
They sometimes occupy fissures without any throw, but most frequently seem to occur along linβ3 of fault. In some cases indeed it can be shown that dislocation has taken place after some portion of the vein had been formed, but before the completion of the process. They usually send out branches, and in some mining districts do this to such an extent that it becomes hardly possible to identify the main vein among its numerous offshoots.
The direction of the veins varies in different districts. Two series may often be traced,—a principal series running in one general direction, and a minor set crossing the first at right angles or obliquely. Great differences in the richness of a metallic lode may be observed as it is worked vertically and horizontally, some of these depending in a way not easily explained upon the nature of the surround ing rock. Among the Cornish lodes, for example, some contain copper only where they traverse the Devonian slates, and lose it where they enter the granite, where tin takes its place. In the lead tracts of the north of England the metal diminishes where the veins lie in shale, and augments where they run through limestone.
In some rocks, more especially in limestones, large subterranean cavities have been filled with vein-stones and ores. The iron mines of the English lake district, for example, lie in the Carboniferous Limestone, where tunnels and caverns anciently dissolved out of the rock by percolating or running water have been subsequently filled up with haematite. In the lead districts of the north of England also, similar cavities have received a plentiful deposit of vein-stones and galena with its accompanying ores.
Various theories have been proposed to account for the infilling of mineral veins. Of these the most noteworthy are—(1) the theory of lateral segregation,—which teaches that the substances in the veins have been derived from the adjacent rocks by a process of solution and redeposit; and (2) the theory of infilling from below,·—according to which the minerals and ores were introduced from below dissolved in water or steam, or by sublimation, or by igneous fusion and injection.
The fact that the nature and amount of the minerals, and especially of the ores, in a vein vary with the nature of the surrounding rocks seems to show that these rocks have had a certain influence on the precipitation of mineral matter in the fissures passing through them. But that this mineral matter came chiefly from below appears almost certain. The phenomena of the ascent of hot water in volcanic districts afford a close analogy to what has occurred in mineral veins. It is known that at the present time various minerals, including silica, both crystalline and calcedonic, and various metallic sulphides, are being deposited in fissures up which hot water rises. At the same time it is conceivable that to some extent there may be a decomposition of the rocks on either side of a fissure, and that a portion of the mineral matter abstracted may be laid down in another form along the walls of the fissure, or, on the other hand, that the rocks on either side of the fissure may be permeated for some distance by the ascending waters, and that some of the mineral substances carried up in solution may be deposited in the pores and cavities of these r f κks as well as in the fissure itself.
X. Unconformability.
Where one series of rocks, whether of aqueous or igneous origin, has been laid down continuously and without disturbance upon another series, they are said to be conformable. Thus in fig. 59 the sheets of rock numbered 1, 2, 3, and 4 have succeeded each other in regular order, and exhibit a perfect conformability. They overlap each other, however, No. 2 extending beyond the edge of No. 1, No. 3 beyond that of No. 2, and so on. As already explained (p. 295), this structure points to a gradual subsidence and enlargement of the area of deposit. But all these conformable beds repose against the older platform A, with which they have no direct connexion. That platform may consist of horizontal or inclined strata, or contorted schist, or crystalline igneous rocks. In any case there is a complete break between it and the overlying rocks, which rest successively on different parts of the older mass. This relation is termed an unconforιnability. The upper conformable beds in fig. 59 are said to lie unconformably upon A.
It is evident that this structure may occur in ordinary stratified, or in igneous, or in metamorphic rocks, or between any two of these great series. It is most familiarly displayed among stratified masses, and can there be most satisfactorily studied. The lines of bedding furnish a ready means of detecting differences of inclination and discordance of superposition. But even among igneous protrusions and in ancient metamorphic masses, distinct evidence of un conformability is not always difficult to trace.
Though conformable rocks may usually be presumed to have followed each other continuously without any great disturbance of geographical conditions, we cannot always be safe in such an inference. But an unconformability leaves no room to doubt that it marks a decided break in the continuity of deposit. Hence no kind of geological structure is of higher importance in the interpretation of the history of the stratified formations of a country. In rare cases an unconformability may occur between two horizontal groups of strata. In fig. 60, for instance, a set of beds C is shown to lie conformably for some distance upon an older series d. Were nothing more to be seen than what appears towards the right hand, we might justifiably conclude the whole of the rocks to be conformable. By passing to the left, however, we should find evidence of the older group having been upraised and unequally denuded before the deposition of the newer. The denudation would show that the conformability was accidental, that the older rocks had really been upraised and worn down before the formation of the newer. In such a case the upheaval must have been so equable as not to disturb the horizontality of the lower rocks.
As a rule, however, it seldom happens that movements of this kiud have taken place over an extensive area so equably as not to produce a want of conformability somewhere between the older and newer rocks. Most frequently the older formations have been disturbed, tilted at various angles, or even placed on end. They have likewise been irregularly and enormously worn down. Hence, instead of lying parallel, the younger beds run transgressively across the upturned denuded ends of the older. The greater the disturbance of the older rocks the more marked is the unconformability. In fig. 61, for instance, the series of beds A is uncomformably covered by the series C. At both sides of the arch the uncomformability is strongly marked, but at the centre d the two series seem to be conformable. An un conformability forms one of the great breaks in the geological record. In the foregoing figure, by way of illustration, we see at once that a notable hiatus in deposition, and therefore in geological chronology, must exist between series A and C. The older rocks had been deposited, folded, upheaved, and worn down before the accumulation of the newer series upon their denuded edges. These changes must have demanded a considerable lapse of time. Yet, looking merely at the structure in itself, we have evidently no means of fixing, even relatively, the length of interval marked by an unconformability. The mere violence of contrast between a set of vertical beds below and a horizontal group above it is no reliable criterion of the relative lapse of time between their deposition, for an older portion of a given formation might be tilted on end and be overlaid unconformably by a later part of the same formation. A set of flat rocks of high geological antiquity might, on the other hand, be covered by a formation of comparatively recent date, yet in spite of the want of discordance between the two, they might have been separated by a large portion of the total sum of geological time. It is by the evidence of organic remains that the relative importance of unconformabilities must be measured, as will be explained in part v.
Paramount though the effect of an unconformability may be in the geological structure of a country, it must nevertheless be in almost all cases local. The disturbance by which it was produced can have effected but a comparatively circumscribed region, beyond the limits of which the continuity of sedimentation may have been undisturbed. We may therefore always expect to be able to fill up the gaps in one district from the more complete geological formations of another. In fig. 61 we see that something is wanting between A and C. But in the structure of another country or a different part of the same country we might discover the complete succession, as in fig. 62, where the whole of the rocks succeed each other conformably, and where the gap between A and C marked by the unconformability in fig. 61 is bridged over by the intermediate group of strata B.
PART V.—PALAeONTOLOGICAL GEOLOGY.
Palaeontology is the science which treats of the structure, affinities, classification, and distribution in time of the forms of plant and animal life embedded in the rocks of the earth’s crust. In one sense it may be regarded as a branch of zoology and of botany, its claim in this view to rank as a separate science resting almost solely on the fact that of the forms with which it deals but a small proportion belongs to the living world. In another aspect it may be looked upon as a branch of geology, seeing that its assistance is absolutely indispensable in many of the most familial’ and fundamental problems of the latter science. It is under this last aspect that we have to regard it here. We shall consider merely those leading features of palaeontological inquiry without some knowledge of which progress in modern geology would be impossible.
Fossils.— Palaeontological geology, then, deals with the fossils or organic remains preserved in the rocks, and endeavours to gather from them information as to the history of the globe and its inhabitants. The term “fossil,” meaning literally anything “dug up,” was formerly applied indiscriminately to any mineral substance taken out of the earth’s crust, whether organized or not. Ordinary minerals and rocks were thus included as fossils. For many years, however, the meaning of the word has been restricted, so as to include only the remains or traces of plants and animals preserved in any natural formation whether hard rock or superficial deposit. The idea of antiquity or relative date is not necessarily involved in this conception of the term. Thus the bones of a sheep buried under gravel and silt by a modern flood, and the obscure crystalline traces of a coral in ancient masses of limestone, are equally fossils.
Nor has the term fossil any limitation as to organic grade. It includes not merely the remains of organisms, but also whatever was directly connected with or produced by these organisms. Thus the resin which was exuded from trees of long-perished forests is as much a fossil as any portion of the stem, leaves, flowers, or fruit, and in some respects is even more valuable to the geologist than more determinable remains of its parent trees, because it has often preserved in admirable perfection the insects which flitted about in the woodlands. The burrows and trails of a worm preserved in sandstone and shale claim recognition as fossils, and indeed are commonly the only indications to be met with of the existence of annelide life among old geological formations. The droppings of fishes and reptiles, called coprolites, are excellent fossils, and tell their tale as to the presence of vertebrate life in ancient waters. The little agglutinated cases of the caddis-worm remain as fossils in formations from which perchance most other traces of life may have passed away. Nay, the very handiwork of man, when preserved in any natural manner, is entitled to rank among fossils; as where his flint-implements have been dropped into the prehistoric gravels of river-valleys, or where his canoes have been buried in the silt of lake-bottoms.
The term fossil, moreover, suffers no restriction as to the condition or state of preservation of any organism. In some rare instances the very flesh, skin, and hair of a mammal havejoeen preserved for thousands of years, as in the case of the mammoths entombed within the frozen mud cliffs of Siberia. In most cases all or most of the original animal matter has disappeared, and the organism has been more or less completely mineralized or petrified. It often happens that the whole organism has decayed, and a mere cast in amorphous mineral matter, as sand, clay, ironstone, silica, or limestone remains; yet all these variations must be comprised in the comprehensive term fossil.
Conditions for the Preservation of Organic Remains.— At the outset the question naturally suggests itself how the remains of plants and animals come to have been preserved in rocks at all. If we observe what takes place at the present day, and argue that it may fairly be taken as an indication of what has been the ordinary condition of things in the geological past, we see that there must have been so many chances against the conservation of either animal or plant remains that their occurrence among stratified formations should be regarded as exceptional, and as the result of various fortunate accidents.
I. Consider, in the first place, what chances exist for the preservation of remains of the present fauna and flora of a country. The surface of the land may be densely clothed with forest, and abundantly peopled with animal life. But the trees die and moulder into soil. The animals, too, disappear, generation after generation, and leave no perceptible traces of their existence. If we were not aware from authentic records that central and northern Europe was covered with vaβt forests at the beginning of our era, how could we know this fact! What has become of the herds of wild oxen, the bears, wolves, and other denizens of primeval Europe? How could we prove from tbe examination of the surface soil of any country that those creatures had once abounded there? We might search in vain for any such superficial traces, and would learn by so doing that the law of nature is everywhere “dust to dust.”
The conditions for the preservation of any relics of the plant and animal life of a terrestrial surface must therefore be always exceptional. They are supplied only where the organic remains can be protected from the air and superficial decay. Hence they may be observed in
1. Lakes.— Over the floor of a lake deposits of silt, peat, marl, <fcc., are formed. Into these the stems, branches, leaves, flowers, fruits, or seeds of plants from the neighbouring land may be carried, together with the bodies of land animals, insects, and birds. An occasional storm may blow the lighter debris of the woodlands into the water. Such portions of the wreck as did not float, and were not washed ashore again, might sink to the bottom. Of these the larger part would in most cases probably rot away, so that, in the end, only a very small fraction of the whole vegetable matter cast over the lake by the wind would be covered up and preserved at the bottom. In like manner the animal remains swept by winds or by river floods into the-lake would run so many risks of dissolution that only a proportion of them, and probably merely a small proportion, would be preserved. When we consider these chances against the conservation of the vegetable and animal life of the land, we must admit that, at the best, lake-bottoms can contain but a meagre and imperfect representation of the abundant life of the adjacent hills and plains.
Butlakeshavea distinct flora and faunaof theirown. Their aquatic plants may be entombed in the gathering deposits of the bottom. Their mollusks, of characteristic types, sometimes form, by the accumulation of their remains, sheets of soft calcareous marl, in which many of the undecayed shells are preserved. Their fishes, likewise distinctly lacustrine, no doubt must often be entombed in the silt or marl.
2. Peat-mosses.— Wild animals venturing on the more treacherous watery part3 of a peat-bog are sometimes engulphed or “laired.” The antiseptic qualities of the peat preserve such remains from decay. Hence from European peat-mosses numerous remains of deer and oxen have been exhumed. Evidently the larger beasts of the forest ought chiefly to be looked for in these localities.
3. Deltas at River Mouths.— From what has been said in previous pages (ante, pp. 276-8) regarding the geological operations of rivers, it is obvious that to some extent both the flora and the fauna of the land may be buried among the sand and silt of deltas. When we consider, however, that though occasional or frequent river-floods sweep down trees, herbage, and the bodies of land animals, the remains so transported run every risk of decaying or being otherwise destroyed while still afloat, and that even if they reach the bottom they will tend to dissolution there unless speedily covered up and protected by fresh sediment, we must perceive that delta formations can scarcely be expected to give us more than a meagre outline of the varied terrestrial flora and fauna,
4. Caverns.— These are eminently adapted for the preservation of the higher forms of terrestrial life. Most of our knowledge of the prehistoric mammalian fauna of Europe is derived from what has been disinterred from bone-caves. As these recesses lie for the most part in limestone or in calcareous rock, their floors are commonly coated with stalagmite from the drip of the roof; and as this deposit is of great closeness and durability it has effectually preserved whatever it has covered or enveloped. The caves have in many instances served predatory beasts, like the hyaena, cave-lion, and cave-bear, as dens in which they slept, and into which some of them dragged their prey. Tn other cases they have been merely holes into which different animals crawled to die, or into which they fell or were swept by inundations. Under whatever circumstances the animals left their remains in these subterranean retreats, the result has been that the bones have been covered up and preserved. Still, we must admit that after all but a mere fraction even of the mammals of the time would enter the caves, and therefore that the evidence of the cavern-deposits, profoundly interesting and valuable as it 13, presents us with merely a glimpse of one aspect of the life of the land.
II. In the next place, if we turn to the sea, we find certainly many more favourable conditions for the preservation of organic forms, but also many circumstances which operate against it. While the level of the land remains stationary, there can be but little effective entombment of marine organisms in littoral deposits; for only a limited accumulation of sediment will be formed until subsidence of the sea-floor takes place. In the trifling beds of sand or gravel thrown up on a stationary shore, only the harder and more durable forms of life, such as gasteropods and lamellibranchs, which can withstand the triturating effects of the beach waves, are likely to remain uneffaced.
Below tide-marks, along the margin of the land where sediment is gradually deposited, the conditions are favourable for the preservation of marine organisms. Sheets of sand and mud are there laid down. In those sediments the harder parts of many forms of life may be entombed and protected from decay. But only a small proportion of the total marine fauna may be expected to occur in such deposits. At the best, merely littoral and shallow-water forms will occur, and even under the most favourable conditions they will represent but a fraction of the whole assemblage of life in these juxta-terrestrial parts of the ocean. As we recede from the land the rate of deposition of sediment on the sea-floor must become feebler, until in the remote central abysses it reaches a hardly appreciable minimum. Except, therefore, where organic deposits, such as ooze, are forming in these more pelagic regions, the conditions must be on the whole unfavourable for the preservation of any adequate representation of the deep-sea fauna. Hard enduring objects, such as teeth and bones, may slowly accumulate and be protected by a coating of peroxide of manganese, or of some of the silicates above (p. 288) referred to as now forming here and there over the deep-sea-bottom. But such a deposit, if raised into land, would εupply but a meagre picture of the life of the sea.
We must conclude therefore that of the whole sea-floor the area best adapted for preserving a varied suite of marine organic exuviae is that belt which, running along the margin of the land, is ever receiving fresh layers of sediment transported by rivers and currents from the adjacent shores. The most favourable conditions for the accumulation of a thick mass of marine fossiliferous strata will arise when the area of deposit is undergoing a gradual subsidence. If the rate of depression and that of deposit were equal, or nearly so, the movement might proceed for a vast period without producing any great apparent change in marine geography, and even without seriously affecting the distribution of life over the sea-floor within the area of subsidence. Hundreds or thousands of feet of sedimentary strata might in this way be heaped up round the continents, containing a fragmentary series of organic remains belonging to those forms of shallow-water life which had hard parts capable of preservation.
There can be little doubt that such has in fact been ths history of the main mass of stratified formations in the earth’s crust. These piles cf marine strata have unquestionably been laid down in comparatively shallow water within the area of deposit of terrestrial sediment. Their great depth seems only explicable by prolonged and repeated movements of subsidence, interrupted, however, as we know, by other movements of a contrary kind. These geographical changes affected at once the deposition of inorganic materials and the succession of organic forms. One series of strata is sometimes abruptly succeeded by another of a very different character, and we generally find a corresponding contrast between their respective organic contents.
It follows from these conclusions that representatives of the abyssal deposits of the central oceans are not likely to be met with among the geological formations of past times. Thanks to the great work done by the “Challenger” expedition, we now know what are the leading characters of these abyssal deposits of the present day. They have absolutely no analogy among the formations of the earth’s crust. They differ, indeed, so entirely from any formation which geologists considered to be of deep-water origin as to indicate that, from early geological times, the present great areas of land and sea have remained on the whole where they are, and that the land consists mainly of strata formed at successive epochs of terrestrial debris laid down in the surrounding shallow sea.
Relative Value of Organic Remains as Fossils.— As the conditions for the preservation of organic remains exist more favourably under the sea than on land, marine organisms must be far more abundantly conserved than those of the land. This is true to-day, and has been true in all past geological time. Hence for the purposes of the geologist the fossil remains of marine forms of life far surpass all others in value. Among them there will necessarily be a gradation of importance regulated chiefly by their relative abundance. Now, of all the marine tribes which live within the juxta-terrestrial belt of sedimentation, unquestionably the Moll·usca stand in the place of pre-eminence as regards their aptitude for becoming fossils. In the first place they almost all possess a hard durable shell, capable of resisting considerable abrasion, and readily passing into a mineralized condition. In the next place they are extremely abundant both as to individuals and genera. They occur on the shore within tide mark, and range thence down into the abysses. Moreover, they appear to have possessed those qualifications from early geological times. In the marine Mollusca, therefore, we have a common ground of comparison between the stratified formations of different periods. They have been styled the alphabet of palaeontological inquiry. It will be seen, as we proceed, how much in the interpretation of geological history depends upon the testimony of sea-shells.
Looking at the organisms of the land, we perceive that, as a rule, the abundant terrestrial flora has a comparatively small chance of being well represented in a fossil state, that indeed, a3 a rule, only that portion of it of which the leaves, twigs, flowers, and fruits are blown into lakeβ is likely to be partially preserved. Terrestrial plants, therefore, occur in comparative rarity among stratified rocks, and furnish in consequence only limited means of comparison between the formations of different ages and countries. Of land animals the vast majority perish and leave no permanent trace of their existence. Predatory and other forms whose remains may be looked for in caverns or peat-mosses, must occur more numerously in the fossil state than birds, and are correspondingly more valuable to the geologist for the comparison of different strata.
Relative Age of Fossils.— Although absolute date3 cannot be fixed in geological chronology, it is not difficult to determine the relative age of different strata, and consequently of their enclosed organic remains. For this purpose the fundamental law is based on what is termed the “order of superposition.” This law may be thus defined :—in a series of stratified formations the older must underlie the younger. It is not needful that we should actually see the one lying below the other. If a continuous conformable succession of strata dips steadily in one direction we know that the beds at the one end must underlie those at the other, because we can trace the whole succession of beds between them. Rare instances occur where strata have been so folded by great terrestrial disturbance that the younger are made to underlie the older. But this inversion can usually be made quite clear from other evidence. The true order of superposition is decisive of the relative ages of stratified rocks.
If therefore formations lie regularly above each other, B upon A, C upon B, D upon C, and so on, it is evident that the organic remains found in A must have lived and died before those in B were entombed; the latter must have been covered up before those in C, and these again before those in D. The chronological sequence of fossils must be determined first of all by the order of superposition of their enclosing strata. There is nothing in the fossils themselvee, apart from experience, to fix their date. Unless, for example, we knew from observation or testimony that Rhynchonella pleurodon is a shell of the Carboniferous Limestone, and Rhynchonella tetrahedra is a shell of the Lias, we could not, from mere inspection of the fossils themselves, pronounce as to their real geological position. It is quite true that by practice a palaeontologist has his eye so trained that he can make shrewd approximations to the actual horizon of fossils which he may never have seen before; but he can only do this by availing himself of a wide experience based upon the ascertained order of appearance of fossils as determined by the law of superposition. For geological purposes therefore, and indeed for all purposes of comparison between the faunas and floras of different periods, it is absolutely essential first of all to have the order of superposition of strata rigorously determined. Unless this is done the most fatal mistakes may be made in palaeontological chronology. But when it has onoe been done in one typical district, the order thus established may be held as proved for a wide region where, from paucity of sections, or from geological disturbance, the true succession of formations cannot be satisfactorily determined.
Uses of Fossils in Geology.— There are two main purposes to which fossils may be put in geological research :—(1) to throw light upon former conditions of physical geography, such as the presence of land, rivers, lakes, and seas, in places where they do not now exist, changes of climate, and the former distribution of plants and animals; and (2) to furnish a guide in geological chronology whereby rocks may be classified according to relative date, and the facts of geological history may be arranged and interpreted as a connected record of the earth’s progress.
1. A few examples will suffice to show the manifold assistance which fossils furnish to the geologist in the elucidation of ancient geography.
(a.) Former land-surfaces are revealed by the presence of tree-stumps in their positions of growth, with their roots branching freely in the underlying stratum, which, representing the ancient soil, often contains leaves, fruits, and other sylvan remains, together with traces of the bones of land animals, remains of insects, land-shells, Ac. Ancient woodland surfaces of this kind are found between tide-marks, and even below low-water line, round different parts of the British coast. They unequivocally prove a subsidence of the land. Of more ancient date are the “dirt-beds” of Portland, which, by their layers of soil and tree-stumps, show that woodlands of cycads sprang up over an upraised sea-bottom and were buried beneath the silt of a river or lake. Still further back in geological history come the numerous coal-growths of the Carboniferous period, pointing to wide jungles of terrestrial or aquatic plants, like the modern mangrove swamps, which were submerged and covered with sand and silt.
(b.) The former existence of lakes can be satisfactorily proved from beds of marl or lacustrine limestone full of fresh-water shells, or from fine silt with leaves, fruits, and insect remains. Such deposits are abundantly forming at the present day, and they occur at various horizons among the geological formations of past times. The well-known nagelflue of Switzerland—a mass of conglomerate attaining a thickness of fully 6000 feet—can be shown from its fossil contents to be essentially a lacustrine formation.
(c.) Old sea-bottoms are vividly brought before us by beds of marine shells and other organisms. Layers of waterworn gravel and sand, with rolled shells of littoral and infra-littoral species, unmistakably mark the position of a former shore line. Deeper water is indicated by finer muddy sediment, with relics of the fauna which prevails beneath the reach of waves and ground-swell. Limestones full of corals, or made up of crinoids, point to the slow continuous growth and decay of generation after generation of organisms in clear sea-water.
(<7∙) Variations in the nature of the water or of the seabottom may sometimes be shown by changes in the size or shape of the organic remains. If, for example, the fossils in the central and lower parts of a limestone are large and well-formed, but in the upper layers become dwarfed and distorted, we may reasonably infer that the conditions for their continued existence at that locality must have been gradually impaired. The final complete cessation of these favourable conditions is shown by the replacement of the limestone by shale, indicative of the water having become muddy, and by the disappearance of the fossils, which had shown their sensitiveness to the change.
(i ’.) That the sea-floor represented by a fossiliferous stratum was not far from land is sufficiently proved by mere lithological characters, as has been already explained; but the conclusion may be further strengthened by the occurrence of leaves, stems, and other fragments of terrestrial vegetation which, if found in some numbers among marine organisms, would make it improbable that they had been drifted far from land.
(∕.) The existence of different conditions of climate in former geological periods is satisfactorily demonstrated from the testimony of fossils. Thus an assemblage of the remains of palms, gourds, and melons, with bones of crocodiles, turtles, and sea-snakes, proves a sub-tropical climate to have prevailed over the south of England in the time of the older Tertiary formations. On the other hand, the presence of an intensely cold or arctic climate far south in Europe during post-Tertiary time can be shown from different kinds of evidence, such as the existence of the remains of arctic animals even as far as the south of England and of France.
This is a use of fossils, however, where great caution must be used. We cannot affirm that, because a certain species of a genus lives now in a warm part of the globe, every species of that genus must always have lived in similar circumstances. The well-known example of the mammoth and woolly rhinoceros having lived in the cold north, while their modern representatives inhabit some of the warmest regions of the globe, may be usefully remembered as a warning against any such conclusions. When, however, we find that not one fossil merely, but the whole assemblage of fossils in a formation has its modern analogue in a certain general condition of climate, we may at least tentatively infer that the same kind of climate prevailed where that assemblage of fossils lived. Such an inference would become more and more unsafe in proportion to the antiquity of the fossils and their divergence from existing forms.
2. When the order of superposition has been determined in a great series of stratified formations, it is found that the fossils at the bottom are not quite the same as those at the top of the series. As we trace the beds upward we discover that species after species of the lowest platforms disappears, until perhaps not one of them is found. With the cessation of these older species others make their entrance. These in turn are found to die out and to be replaced by newer forms. After patient examination of the rocks, it is ascertained that every well-marked formation is characterized by its own species or genera, or by a general assemblage or facies of organic forms. This can only, of course, be determined by actual practical experience over an area of some size. When the typical fossils of a formation are known, they serve to identify that formation in its progress across a country. Thus, as we trace the formation into tracts where it would be impossible to determine the true order of superposition, owing to the want of sections, or to the disturbed condition of the rocks, we can employ the fossils as a means of identification, and speak with confidence as to the succession of the rocks. We may even demonstrate that in some mountainous ground the beds have been turned completely upside down, if we can show that the fossils in what are now the uppermost strata ought properly to lie underneath those in the beds below them.
Observations made over a large part of the surface of the globe have enabled geologists to divide the stratified part of the earth’s crust into systems, formations, and groups or series. These subdivisions are frequently marked off from each other by lithological characters. But mere lithological differences would afford at the best but a limited and local ground of separation. Two masses of sandstone, for example, having exactly the same general external and internal characters, might belong to very different geological periods. On the other hand, a series of limestones in one locality might be the exact chronological equivalent of a set of sandstones and conglomerates at another, and of a series of shales and clays at a third.
It is by their characteristic fossils that the divisions of the stratified rocks can be most satisfactorily made. Each formation being distinguished by its own assemblage of organic remains, it can be followed and recognized even amid the crumplings and dislocations of a disturbed region. The same general succession of organic types can be observed over a large part of the world, though, of course, with important modifications in different countries. This similarity of succession has been termed homotaxis— a term which expresses the fact that the order in which the leading types of organized existence have appeared upon the earth has been similar even in widely separated regions.
It is evident that in this way a method of comparison is furnished whereby the stratified formations of different parts of the earth’s crust can be brought into relation with each other. We find, for example, that a certain series of strata is characterized in Britain by certain genera and species of corals, brachiopods, lamellibranchs, gasteropods, and cephalopods. A group of rocks in Bohemia, differing more or less from these in lithological aspect, contains on the whole the same genera, and many even of the same species. In Scandinavia a set of beds may be seen unlike, perhaps, in external characters to the British type, but yielding many of the same fossils. In Canada and many parts of the northern United States, other rocks enclose many of the same, and of closely allied genera and species. All these groups of strata are classed together as homotaxial, that is, as having been deposited during the same relative period in the general progress of life in each region.
It was at one time believed, and the belief is still far from extinct, that groups of strata characterized by this community or resemblance of organic remains were chronologically contemporaneous. But such an inference rests upon most insecure grounds. We may not be able to disprove the assertion that the strata were strictly coeval, but we have only to reflect on the present conditions of zoological and botanical distribution, and of modern sedimentation, to be assured that the assertion of contemporaneity is a mere assumption. Consider for a moment what would happen were the present surface of any portion of central or southern Europe to be submerged beneath the sea, covered by marine deposits, and then re-elevated into land. The river-terraces and lacustrine marls formed before the time of Julius Caesar could not be distinguished by any fossil tests from those laid down in the days of Victoria, unless, indeed, traces of human implements were obtainable whereby the progress of civilization during 2000 years might be indicated. So far as regards the shells, bones, and plants preserved in the various formations, it would be absolutely impossible to discriminate their relative dates; they would be classed as “geologically contemporaneous,” that is, as having been formed during the same period in the history of life in the European area; yet there might be a difference of 2000 years or more between many of them. Strict contemporaneity cannot be asserted of any strata merely on the ground of similarity or identity in fossils.
But the phrase “geologically contemporaneous” is too vague to have any chronological value except in a relative sense. To speak of two formations as in any sense contemporaneous which may have been separated by thousands of years seems rather a misuse of language, though the phraseology has now gained such a footing in geological literature as probably to be inexpugnable. If we turn again for suggestions to the existing distribution of life on the earth we learn that similarity or identity of species and genera holds good on the whole only for limited areas, and consequently, if applied to wide geographical regions, ought to be an argument for diversity rather than for similar ty of age. If we suppose the British seas to be raised int > dry land, so that the organic relics preserved in their sandι and silts could be exhumed and examined, a general common facies or type wordd be found, though some species would be more abundant in or entirely confined to the north, while others would show a greater development in the opposite quarter. Still there would be such a similarity throughout the whole that no naturalist would hesitate to regard the organisms as those of one biological province, and belonging to the same great geological period. The region is so small, and its conditions of life so uniform and uninterrupted, that no marked distinction is possible between the forms of life in its different parts.
Widening the area of observation, we perceive that as we recede from any given point the forms of life gradually change. Vegetation alters its aspect from climate to climate, and with it come corresponding transformations in the character of insects, birds, and wild animals. A lake bottom would preserve one suite of organisms in England, but a very different group at the foot of the Himalaya Mountains, yet the deposits at the two places might be absolutely coeval, even as to months and days. Hence it becomes apparent that while strict contemporaneity cannot be predicated of deposits containing the same organic remains, it may actually be true of deposits in which they are quite distinct.
If, then, at the present time, community of organic forms obtains only in districts, regions, or provinces, it may have been more or less limited also in past time. Similarity or identity of fossils among formations geographically far apart, instead of proving contemporaneity, ought rather to be looked upon as indicative of great discrepancies in the relative epochs of deposit. For in any theory of the origin of species, the spread of any one species, still more of any group of species to a vast distance from the original centre of dispersion, must in most cases have been inconceivably slow. It must have occupied so prolonged a time as to allow of almost indefinite changes in physical geography. A species may have disappeared from its primeval birthplace while it continued to flourish in one or more directions in its outward circle of advance. The date of the first appearance and final extinction of that species would thus differ widely according to the locality at which we might examine its remains.
The grand march of life, in its progress from lower to higher forms, has unquestionably been broadly alike in all quarters of the globe. But nothing seems more certain than that its rate of advance has not everywhere been the same. It has moved unequally over the same region. A certain stage of progress may have been reached in one quarter of the globe thousands of years before it was reached in another; though the same general succession of organic forms might be found in each region.
The geological formations form the records of these ages of organic development. In every country where they are fully displayed, and where they have been properly examined, they can be separated out from each other according to their organic contents. Their relative age within a limited geographical area can be demonstrated by the mere law of superposition. When, however, the formations of distant countries are compared, all that we can safely affirm regarding them is that those containing the same or a representative assemblage of organic remains belong to the same epoch in the history of biological progress in each area. They are homotaxial ; but we cannot assert that they are contemporaneous, rmless we are prepared to include within that term a vague period of perhaps thousands of years.
Doctrine of Colonies.— M. Barrande, the distinguished author of the Système Silurien de lα Boheme, drew attention more than a quarter of a century ago to certain remarkable intercalations of fossih in the series of Silurian strata of Bohemia. He showed that, while these strata presented a normal succession of organic remains, there were nevertheless exceptional bands, which, containing the fossils of a higher zone, were yet included on different horizons among inferior portions of the series. He termed these precursory bands “colonies,” and defined the phenomena as consisting in the partial co-existence of two general faunas, which, considered as a whole, were nevertheless successive. He supposed that during the later stages of his second Silurian fauna in Bohemia the first phases of the third fauna had already appeared, and attained some degree of development in some neighbouring but yet unknown region. At intervals, corresponding doubtless to geographical changes, such as movements of subsidence or elevation, volcanic eruptions, &c., communication was opened between that outer region and the basin of Bohemia. During these intervals a greater or less number of immigrants succeeded in making their way into the Bohemian area, but as the conditions for their prolonged continuance there were not yet favourable, they soon died out, and the normal fauna of the region resumed its occupancy. The deposits formed during these partial interruptions, notably graptolitic schists, accompanied by igneous sheets, contain, besides the invading species, remains of some of the indigenous forms. Eventually, however, on the final extinction of the second fauna, and, we may suppose, on the ultimate demolition of the physical barriers hitherto only occasionally and temporarily broken, the third fauna, which had already sent successive colonies into the Bohemian area, now swarmed into it, and peopled it till the close of the Silurian period.
This original and ingenious doctrine has met with much opposition on the part of geologists and palaeontologists. Of the facts cited by M. Barrande there has been no question, but other explanations have been suggested for them, It has been said, for example, that the so-called colonies are merely bands of the Upper Silurian rocks or third fauna, which by great plications have been so folded with the older rocks as to seem regularly interstratified with them. But the author of the Système Silurien very justly contends that of such foldings there is no evidence, but that, on the contrary, the sequence of the strata appears normal and undisturbed. Again it has been urged that the difference of organic contents in these so-called colonies is due merely to a difference in the conditions of water and sea bottom, particular species appearing with the conditions favourable to their spread, and disappearing when these ceased. But this contention is really included in M. Barrande’s theory. The species which disappear and reappear in later stages must have existed in the meanwhile outside of the area of deposit, which is precisely what he has sought to establish. Much of the opposition which his views have encountered has probably arisen from the feeling that if they are admitted they must weaken the value of palaeontological evidence in defining geological horizons. A palaeontologist, who has been accustomed to deal with certain fossils as unfailing indications of particular portions of the geological series, is naturally unwilling to see his generalizations upset by an attempt to show that the fossils may occur on a far earlier horizon.
If, however, we view this question from the broad natural history platform from which it was regarded by M. Barrande, it is impossible not to admit that such phenomena as he has sought to establish in Bohemia must have constantly occurred in all geological periods and in all parts of the world. No one now believes in the sudden extinction and creation of entire faιfnas. Every great fauna in the earth’s history must have gradually grown out of some preexisting one, and must have insensibly graduated into that which succeeded. The· occurrence of two very distinct faunas in two closely consecutive series of strata does not prove that the one abruptly died out and the other suddenly appeared in its place. It only shows, as Darwin has so well enforced, the imperfection of the geological record. In the interval between the formation of two such contrasted groups of rocks the fauna of the lower strata must have continued to exist elsewhere, and gradually to change into the newer facies which appeared when sedimentation recommenced with the upper strata. Distinct zoological provinces have no doubt been separated by narrow barriers in former geological periods, as they still are to-day. There seems, therefore, every probability that such migrations as M. Barrande has supposed in the case of the Silurian fauna of Bohemia have again and again taken place. Two notable examples will be given in later pages, one in the Lower and one in the Upper Old Red Sandstone of Scotland.
Gaps in the Geological Record.— The history of life has been very imperfectly preserved in the stratified parts of the earth’s crust. Apart from the fact that, even under the moot favourable conditions, only a small proportion of the total flora and fauna of any period could be preserved in the fossil state, enormous gaps occur where no record has been preserved at all. It is as if whole chapters aud books were missing from an historical work. Some of these lacunae are sufficiently obvious. Thus, in some cases, powerful dislocations have thrown considerable portions of the rocks out of sight. Sometimes extensive metamorphism has so affected them that their original characters, including their organic contents, have been destroyed. Oftenest of all, denudation has come into play, and vast masses of fos3iliferous rock have been entirely worn away. That this cause has operated frequently is shown by the abundant unconformabilities in the structure of the earth’s crust.
While the mere fact that one series of rocks lies unconformably on another proves the lapse of a considerable interval between their respective dates, the relative length of this interval may sometimes be demonstrated by means of fossil evidence and by this alone. Let us suppose, for example, that a certain group of formations has been disturbed, upraised, denuded, and covered unconformably by a second group. In lithological characters the two may closely resemble each other, and there may be nothing to show that the gap represented by their unconformability is not of a trifling character. In many cases, indeed, it would be quite impossible to pronounce any well-grounded judgment as to the amount of interval, even measured by the vague relative standards of geological chronology. But if each group contains a well-preserved suite of organic remains, it may not only be possible, but easy, to say exactly how much of the geological record has been left out between the two sets of formations. By comparing the fossils with those obtained from regions where the geological record is more complete, it may be ascertained perhaps that the lower rocks belong to a certain platform or stage in geological history which for our present purpose we may call D, and that the upper rocks can in like manner be paralleled with stage H. It would be then apparent that at this locality the chronicles of three great geological periods E, F, and G were wanting, which are elsewhere found to be intercalated between D and H. The lapse of time represented by this unconformability would thus be equivalent to that required for the accumulation of the three missing formations in those regions where sedimentation went on undisturbed.
But fossil evidence may be made to prove the existence of gaps which are not otherwise apparent. As has been already remarked, changes in organic forms must, on the whole, have been extremely slow in the geological past. The whole species of a sea floor could not pass entirely away, and be replaced by other*forms, without the lapse of long periods of time. If then among the conformable stratified formations of former ages we encounter sudden and abrupt changes in the facies of the fossils, we may be certain that these must mark omissions in the record, which we may hope to fill in from a more perfect series elsewhere. The complete contrasts between unconformable strata are sufficiently explicable. It is not so easy to give a satisfactory account of those which occur where the beds are strictly conformable, and where no evidence can be observed of any considerable change of physical conditions at the time of deposit. A group of strata having the same general lithological characters throughout may be marked by a great discrepance between the fossils above and below a certain line. A few species may pass from the one into the other, or perhaps every species may be different. In cases of this kind, when proved to be not merely local but persistent over wide areas, we must admit, notwithstanding the apparently undisturbed and continuous character of the original deposition of the strata, that the abrupt transition from the one facies of fossils to the other must represent a loug interval of time which has not been recorded by the deposit of strata. Professor Ramsay, who called attention to these gaps, termed them “breaks in the succession of orgauic remains.” He showed that they occur abundantly among the Palaeozoic and Secondary rocks of England. It is obvious, of course, that these breaks, even though traceable over wide regions, were not general over the whole globe. There have never been any universal interruptions in the continuity of the chain of being, so far as geological evidence can show. But the physical changes which caused the breaks may have been general over a zoological district or minor region. They no doubt often caused the complete extinction of genera and species which had a small geographical range.
From all these facts it is clear that the geological record, as it now exists, is at the best but an imperfect chronicle of geological history. In no country is it complete. The lacunae of one region must be supplied from another. Yet in proportion to the geographical distance between the localities where the gaps occur and those whence the missing intervals are supplied, the element of uncertainty in our reading of the record is increased. The most desirable method of research is to exhaust the evidence for each area or province, and to compare the general order of its succession as a whole with that which can be established for other provinces. It is, therefore, only after long and patient observation and comparison that the geological history of different quarters of the globe can be correlated.
Subdivisions of the Geological Record by means of Fossils. — As fossil evidence furnishes a much more satisfactory and widely applicable means of subdividing the stratified rocks of the earth’s crust than mere lithological characters, it is made the basis of the geological classification of these rocks. Thus we may find a particular stratum marked by the occurrence in it of various fossils, one or more of which may be distinctive, either from occurring in no other bed above and below, or from special abundance in that stratum. These species might therefore be used as a guide to the occurrence of the bed in question, which might be called by the name of the most abundant species. In this way a geological horizon or zone would be marked off, and geologists would thereafter recognize its exact position in the series of formations. But before such a generalization can be safely made, we must be sure that the species in question really never does appear on any other platform. This evidently demands wide experience over an extended field of observation. The assertion that a particular species occurs only on one horizon manifestly rests on negative evidence as much as on positive. The palaeontologist who makes it cannot mean more than that he knows the fossil to lie on that horizon, and that, so far as his own experience and that of others goes, it has never been met with anywhere else. But a single example of the occurrence of the fossil on a different zone would greatly damage the value of his generalization, and a few such cases would demolish it altogether. Hence all such statements ought at first to be made tentatively. To establish a geological horizon on limited fossil evidence, and then to assume the identity of all strata containing the same fossils, is to reason in a circle and to introduce utter confusion into our interpretation of the geological record. The first and fundamental point is to determine accurately the order of superposition of the strata. Until this is done detailed palaeontological classification may prove to be worthless. But when once the succession of the rocks has been fixed palaeontological evidence may become paramount.
From what has been above advanced it must be evident that, even if the several groups in a formation or system of rocks in any district or country have been minutely subdivided by means of their characteristic fossils, and if, after the lapse of many years, no discovery has occurred to alter the established order of succession of these fossils, nevertheless the subdivisions can only be held good for the region in which they have been made. They must not be supposed to be strictly applicable everywhere. Advancing into another district or country where the petrographical characters of the same formation or system indicate that the original conditions of deposit must have been very different, we ought to be prepared to find a greater or less departure from the first observed or what might be regarded as the normal order of organic succession. There can be no doubt that the appearance of new organic forms in any locality has been in large measure connected with such physical changes as are indicated by diversities of sedimentary materials and arrangement. The Upper Silurian formations, for example, as studied by Murchison in Shropshire and the adjacent counties, present a clear sequence of strata well defined by characteristic fossils. But within a distance of 60 miles it becomes impossible to establish these subdivisions by fossil evidence. If we examine corresponding strata in Scotland, we find that they contain some fossils which never rise above the Lower Silurian formations in Wales and the west of England. Again, in Bohemia and in Russia we meet with still greater departures from the order of appearance in the original Silurian area, some of the most characteristic Upper Silurian organisms being there found far down beneath strata replete with records of Lower Silurian life. Nevertheless the general succession of life from Lower to Upper Silurian types remains distinctly traceable. Such facts warn us against the danger of being led astray by an artificial precision of palaeontological detail. Even where the palaeontological sequence is best established, it rests probably in most cases not merely upon the actual chronological succession of organic forms, but also, far more than is usually imagined, upon original accidental differences of local physical conditions. As these conditions have constantly varied from region to region, it must hardly ever happen that the same minute palaeontological subdivisions, so important and instructive in themselves, can be identified and paralleled, except over comparatively limited geographical areas.
It cannot be too frequently stated, nor too prominently kept in view, that, although gaps occur in the succession of organic remains as recorded in the rocks, there have been no such blank intervals in the progress of plant and animal life upon the globe. The march of life has been unbroken, onward and upward. Geological history, therefore, if its records in the stratified formations were perfect, ought to show a blending and gradation of epoch with epoch, so that no sharp divisions of its events could be made. But the progress has been constantly interrupted; now by upheaval, now by volcanic outbursts, now by depression. These interruptions serve as natural divisions in the chronicle, and enable the geologist to arrange his history into periods. As the order of succession among stratified rocks was first made out in Europe, and as many of the gaps in that succession were found to be widespread over the European area, the divisions which experience established for that portion of the globe came to be regarded as typical, and the names adopted for them were applied to the rocks of other and far distant regions. This application has brought out the fact that some of the most marked breaks in the European series do not exist elsewhere, and, on the other hand, that some portions of that series are much more complete than in other regions. Hence, while the general similarity of succession may remain, different subdivisions and nomenclature are required as we pass from continent to continent.
A bed, or limited number of beds, characterized by one or more distinctive fossils, is termed a zone or horizon, and, as already mentioned, is often known by the name of a typical fossil, as the different zones in the Lias are by their special species of ammonite. A series of such zones, united by the occurrence among them of a number of the same species or genera, is called a group. A series of groups similarly related constitute a formation, and a number of formations may be united into a system. The terminology employed in this classification will be discussed in the following part.
PART VI.—STRATIGRAPHICAL GEOLOGY.
This branch of the science arranges the rocks of the earth’s crust in the order of their appearance, and interprets the sequence of events of which they form the records. Its province is to cull from all the other departments of geology the facts which may be needed to show what has been the progress of our planet, and of each continent and country, from the earliest times of which the rocks have preserved any memorial. Thus from mineralogy and petrography it obtains information regarding the origin and subsequent mutations of minerals and rocks. From dynamical geology it learns by what agencies the materials of the earth’s crust have been formed, altered, broken, upheaved, and melted. From structural geology it understands how these materials were put together so as to build up the complicated crust of the earth. From palaeontological geology it receives in well-determined fossil remains a clue by which to discriminate the different stratified formations, and to trace the grand onward march of organized existence upon this planet·. Stratigraphical geology thus gathers up the sum of all that is made known by the other departments of the science, and makes it subservient to the interpretation of the geological history of the earth.
The leading principles of stratigraphy may be summed up as follows :—
1. In every stratigraphical research the fundamental requisite is to establish the order of superposition of the strata. Until this is accomplished it is impossible to arrange the dates and make out the sequence of geological history.
2. The stratified portion of the earth’s crust, or geological record, as it has been termed, may be subdivided into natural groups or formations of strata, each marked throughout by some common genera or species, or by a general resemblance in the type or character of its organic remains.
3. Many living species of plants and animals can be traced downward through the more recent geological formations; but they grow fewer in number as they are followed into more ancient deposits. With their disappearance we encounter other species and genera which are no longer living. These in turn may be traced backward into earlier formations, till they too cease, and their places are taken by yet older forms. It is thus shown that the stratified rocks contain the records of a gradual progression of organic forms. A species which has once died out does not seem ever to have reappeared. But as has been already pointed out in reference to Barrande’s doctrine of colonies, a species may within a limited area appear in a formation older than that of which it is characteristic, having temporarily migrated into the district from some neighbouring region where it had already established itself.
4. When the order of succession of organic remains among the stratified rocks has been determined, they become an invaluable guide in the investigation of the relative age of rocks and the structure of the land. Each zone and formation, being characterized by its own species or genera, may be recognized by their means, and the true succession of strata may thus be confidently established even in a country which has been shattered by dislocation, or where the rocks have been folded and inverted.
5. The relative chronological value of the divisions of the geological record is not to be measured by mere depth of strata. While it may be reasonably assumed that a great thickness of stratified rock must mark the passage of a long period of time, it cannot safely be affirmed that a much less thickness elsewhere represents a correspondingly diminished period. This may sometimes be made evident by an unconformability between two sets of rocks, as has already been explained. The total depth of both groups together may be, say 1000 feet. Elsewhere we may find a single unbroken formation reaching a depth of 10,000 feet; but it would be utterly erroneous to conclude that the latter represents ten times the length of.time shown by the two former. So far from this being the case, it might not be difficult to show that the minor thickness of rock really denoted by far the longer geological interval. If, for instance, it could be proved that the upper part of both the sections lay on one and the same geological platform, but that the lower unconformable series in the one locality belonged to a far lower and older system of rocks than the base of the thick conformable series in the other, then it would be clear that the gap marked by the unconformability really indicated a longer period than the massive succession of deposits.
6. Fossil evidence furnishes the chief means of comparing the relative value of formations and groups of rock. A break in the succession of organic remains marks an interval of time often unrepresented by strata at the place where the break is found. The relative importance of these breaks, and therefore, probably, the comparative intervals of time which they mark, may be estimated by the difference of the facies of the fossils on each side. If, for example, in one case we find every species to be dissimilar above and below a certain horizon, while in another locality only half of the species on each side are peculiar, we naturally infer, if the total number of species seems large enough to warrant the inference, that the interval marked by the former break was very much longer than that marked by the second. But we may go further and compare by means of fossil evidence tlιfe relation between breaks in the succession of organic remains and the depth of strata between them.
Three formations of fossiliferous strata, A, C, and H, may occur conformably above each other. By a comparison of the fossil contents of all parts of A, it may be ascertained that, while some species are peculiar to its lower, others to its higher portions, yet the majority extend throughout the formation. If now it is found that of the total number of species in the upper portion of A only one-third passes up into C, it may be inferred with some probability that the time represented by the break between A and C was really longer than that required for the accumulation of the whole of the formation A. It might even be possible to discover elsewhere a thick intermediate formation B filling up the gap between A and C. In like manner were it to be discovered that, while the whole of the formation C is characterized by a common suite of fossils, not one of the species and only one half of the genera pass up into H, the inference could hardly be resisted that the gap between the two formations marks the passage cf a far longer interval than was needed for the deposition of the whole of C. And thu3 we reach the remarkable conclusion that, thick though the stratified formations of a country may be, in some cases they may not represent so long a total period of time as do the gaps in their succession,—in other words, that non-deposition was more frequent and prolonged than deposition, or that the intervals of time which have been recorded by strata have not been so long as those which have not been so recorded.
In all speculations of this nature, however, it is necessary to reason from as wide a basis of observation as possible, seeing that so much of the evidence is negative. Especially needful is it to bear in mind that the cessation of one or more species at a certain line among the rocks of a particular district may mean nothing more than that, owing to some change in the conditions of life or of deposition, these species were compelled to migrate or became locally extinct at the time marked by that line. They may have continued to flourish abundantly in neighbouring districts for a long period afterward. Many examples of this obvious truth might be cited. Thus in a great succession of mingled marine, brackish-water, and terrestrial strata, like that of the Carboniferous Limestone series of Scotland, corals, crinoins, and brachiopods abound in the limestones and accompanying shales, but disappear as the sandstones, ironstones, clays, coals, and bituminous shales supervene. An observer meeting for the first time with an instance of the disappearance, and remembering what he had read about “breaks in succession,” might be tempted to speculate about the extinction of these organisms, and their replacement by other and later forms of life, such as the ferns, lycopods, ganoid fishes, and other fossils so abundant in the overlying strata. But further research would show him that high above the plant-bearing sandstones and coals other limestones and shales might be observed, once more charged with the same marine fossils as before, and still farther overlying groups of sandstones, coals, and carbonaceous beds followed by yet higher marine limestones. He would thus learn that the same organisms, after being locally exterminated, returned again and again to the same area. After such a lesson he would probably pause before too confidently asserting that the highest bed in which we can detect certain fossils marked really their final appearance in the history of life. A break in the succession may thus be extremely local, one set of organisms having been driven to a different part of the same region, while another set occupied their place until the first was enabled to return.
7. The geological record is at the best but an imperfect chronicle of th⅜,geological history of the earth. It abounds in gaps, some of which have been caused by the destruction of strata owing to metamorphism, denudation, or otherwise, some by original non-deposition, as above explained. Nevertheless from this record alone can the progress of the earth be traced. It contains the registers of the births and deaths of tribes of plants and animals which have from time to time lived on the earth. But a small proportion of the total number of species which have appeared in past time have been thus chronicled, yet by collecting the broken fragments of the record an outline at least of the history of life upon the earth can be deciphered.
The nomenclature adopted for the subdivisions of the geological record bears witness to the rapid growth of geology. It is a patch-work in which no system nor language has been adhered to, but where the influences by which the progress of the science has been moulded may be distinctly traced. Some of the earliest names are lithological, and remind us of the fact that mineralogy and petrography preceded geology in the order of birth—Chalk, Oolite, Greensand, Millstone Grit. Others are topographical, and often recall the labours of the early geologists of England—London Clay, Oxford Clay, Purbeck, Portland, Kimeπdge beds. Others are taken from local English provincial names, and remind us of the debt we owe to William Smith, by whom so many of them were first used —Lias, Gault, Crag, Combrash. Others of later date recognize an order of superposition as already established among formations—Old Red Sandstone, New Red Sandstone. By common consent it is admitted that names taken from the region where a formation or group of rocks is typically developed, are best adapted for general use. Cambrian, Silurian, Devonian, Permian, Jurassic, are of this class, and have been adopted all over the globe.
But whatever be the name chosen to designate a particular group of strata, it soon comes to be used as a chronological or homotaxial term, apart altogether from the stratigraphical character of the strata to which it is applied. Thus we speak of the Chalk or Cretaceous system, and embrace under that term formations which may contain no chalk; and we may describe as Silurian a series of strata utterly unlike in lithological characters to the formations in the typical Silurian country. In using these terms we unconsciously allow the idea of relative date to arise prominently before us. Hence such a word as chalk or cretaceous does not suggest so much to us the group of strata so called, as the interval of geological history which these strata represent. We speak of the Cretaceous, Jurassic, and Cambrian periods, and of the Cretaceous fauna, the Jurassic flora, the Cambrian trilobites, as if these adjectives denoted simply epochs of geological time.
The geological record is classified into five main divisions: •—(1) the Archaean, Azoic (lifeless), or Eozoic (dawn of life) Periods; (2) the Primary or Palaeozoic (ancient life) Periods; (3) the Secondary or Mesozoic (middle life) Periods; (4) the Tertiary or Cainozoic (recent life); and (5) the Quaternary or Post-Tertiary Periods. These divisions are further ranged into systems, each system into formations, each formation into groups, and each group or series into single zones or horizons. The subjoined generalized table exhibits the order in which the chief subdivisions appear.
Order of Succession of the Stratified Formation of the Earth's Crust. [table]
I. ARCIIAeAN.
Underneath the oldest unaltered stratified and fossiliferousr formations in Europe there occur masses of gneiss and other crystalline schistose rocks belonging perhaps to widely different geological periods, but, from want of satisfactory means of discrimination, necessarily united provisionally in one common series. That they are separated by a vast interval of time from the rocks which lie upon them is shown by the strong unconformability with which they are related to every formation of younger date than themselves. Everywhere thoroughly crystalline, they are disposed in rude, crumpled, often vertical beds, out of the ruins of which the overlying formations have been partly built.
Britain. —In no part of the European area are these ancient rocks better seen than in the north-west of Scotland. Their position there, previously indicated by MacCulloch and Hay Cunningham, was first definitely established by Murchison, who showed that they possess a dominant strike to N.N.W., and are unconformably overlaid by all the other rocks of the Scottish Highlands. They consist of a tough massive gneiss usually hornblendic, with bands of hornblende-rock, hornblende-schist, quartz-felsite, granite, and other crystalline rocks. In two or three places they enclose bands of limestone, but neither in these nor in any other parts of their mass has the least trace of any organic structure been detected. It is impossible at present to offer any conjecture as to their probable thickness. It must be many thousand feet; but its approximate amount, if ever ascertainable, will only be made out after the region where they occur has been mapped in detail. These gneisses and schists possess a massiveness and rudeness of bedding which strongly distinguishes them from all the other and younger metamorphic rocks of Britain. They form nearly the whole of the Outer Hebrides, and occupy a variable belt of the western parts of the counties of Sutherland and Ross. Murchison proposed to term them the Fundamental or Lewisian Gneiss from the isle of Lewis—the chief of the Hebrides. Afterwards he called them Laurentian, regarding them as the equivalent of some part of the great Laurentian system of Canada.
In recent years Mr Hicks and others have endeavoured to show that in Wales there exist here and there protrusions of an old crystalline group of rocks from beneath the Cambrian system, and they have described these “pre-Cambrian” masses as overlaid unconformably by younger formations, as in the north-west of Scotland. Professor Ramsay, however, who with his colleagues in the Geological Survey mapped the Welsh areas in detail, contends that the supposed older gneiss is merely a metamorphosed portion of the Cambrian rocks.
Continental Europe. —On the continent of Europe numerous areas of ancient gneiss rise from under the oldest fossiliferous formations. In Scandinavia the structure of part of the country resen1ble3 that of the north-west of Scotland: the fundamental-gneiss (ffryneiss), covering a large area, is overlaid unconformably by red sandstones which underlie the most ancient strata containing organic remains. The gneiss and its accompanying rocks range through Finland into the north-west of Russia, reappearing in the north-east of that vast empire in Petchora Land down to the White Sea, and rising in the nucleus of the chain of the Ural Mountains, and still further south in Podolia. In Central Europe they appear as islands in the midst of more recent formations. In the midst of the Carpathian Mountains they protrude at a number of points, but westwards in the Alpine chain they rise in a more continuous belt in the central portion of these crests, and show numerous mineralogical varieties, including protogine, mica-schist, and many other schists, as well as limestone and serpentine. But perhaps their most intelligible sections are those which they present in Bavaria and Bohemia between the valley of the Danube and the headwaters of the Elbe. They are there divided into two well-marked groups—(a) red gneis≡, covered by (δ) grey gneiss. According to Gümbel the former (called by him the Bojan gneiss) may be traced as a distinct formation associated with granite, but with very few other kinds of crystalline or schistose rocks, while the latter (termed the Hercynian gneiss) consists of gneiss with abundant interstratification of many other schistose rocks, graphitic limestone, and serpentine. The Hercynian gneiss is overlaid by mica-schist, above which comes a vast mass of argillaceous schists and shales. Gümbel some years ago found in the marbles associated with the younger gneiss what he considered to be an organism of the same genus as the Eozoon of Canada, to which reference will immediately be made. He named it Eo20on Bavaricum. More recently a similar substance was obtained in the Archaean series of Bohemia, and named by Fritsch Eozoon Bohemicum. America. —In. North America Archaean rocks cover a large part of the continent from the Arctic Circle southwards to the great lakes. They appear likewise, as in Europe, along the central parts of prominent mountain chains, as in the Rocky Mountain range and that of the Appalachians. They have been carefully studied in Canada, where the late Sir W. E. Logan, Director of the Geological Survey of the Dominion, estimated their depth at about 30,000 feet, but neither their top nor their base can there be found. He named them the Lauren tian system from their abundant development along the shores of the St Lawrence. They have been divided into two series—(1) a lower formation more than 20,000 feet thick, consisting chiefly of granitic, orthoclase gneiss, with bands of quartzrock, schists, iron-ore, and limestone; and (2) an upper formation fully 10,000 feet thick, composed also, for the most part, of gneis3, but marked by the occurrence of bands of Labrador felspar, as well as schist, iron-ore, and limestone. The upper division has been stated to lie unconf ormably on the lower. Mr Selwyn, however, has recently pointed out that this is almost certainly not the case, but that the limestone-bearing series rests conformably upon a massive granitoid gneiss, to which he would restrict the term Laurentian, classing the limestones in the next or Huronian system (Hat. Hist. Soc. Montreal, Feb. 1879).
In one of the Laurentian limestones of Canada, specimens have been found of a remarkable mixture of calcite and serpentine. These minerals are arranged in alternate layers, the calcite forming the main framework of the substance with the serpentine (sometimes loganite, pyroxene, &c.) disposed in thin, wavy, inconstant layers, as if filling up flattened cavities in the calcareous mass. So different from any ordinary mineral segregation with which he was acquainted did this arrangement appear to Logan, that he was led to regard the substance as probably of organic origin. This opinion was adopted, and the structure of the supposed fossil was worked out in elaborate detail by Dr Dawson of Montreal, who pronounced the organism to be the remains of a massive foraminifer which he called Eozoon, and which he believed must have grown in large thick sheets over the sea-bottom. This opinion was confirmed by Dr W. B. Carpenter, who from a large suite of additional and better preserved specimens, described a system of internal canals having the characters of those in true foraminiferal structures. (See Foraminifera.) Other observers, notably Profesors King and Rowney of Galway and Möbius of Kiel, have opposed the organic nature of Eozoon, and have endeavoured to show that the supposed canals and passages are merely infiltration veinings of serpentine in the calcite. In some cases, however, the “canalsystem” is not filled with serpentine but with dolomite, which seems to show that the cavities must have existed before either dolomite or serpentine were introduced into the substance. Dr Carpenter contends that the disposition of these passages in his decalcified specimens is very regular, and quite unlike any mineral infiltration with which he is acquainted.
The opinion of the organic nature of Eozoon has been supposed to receive support from the large quautity of graphite found throughout the Archaean rocks of Canada and the northern parts of the United States. This mineral occurs partly in veins, but chiefly disseminated in scales and laminae in the limestones and as independent layers. Dr Dawson estimates the aggregate amount of it in one band of limestone in the Ottawa district as not less than from 20 to 30 feet, and he thinks it is hardly an exaggeration to say that there is as much carbon in the Laurentian as in equivalent areas of the Carboniferous system. He compares some of the pure bands of graphite to beds of coal, and maintains that no other source for their origin can be imagined than the deoxidation of carbonic acid by living plants. In the largest of three beds of graphite at St John he has found what he considers may be fibrous structure indicative of the existence of land-plants.
Still further evidence in favour of organized existence during Archaean time in the North American area has been adduced from the remarkably thick and abundant masses of iron ore associated with the Laurentian rocks of Canada and the United States. Dr Sterry Hunt has called attention to these ores as proving the precipitation of iron by decomposing vegetation during the Laurentian period on a more gigantic scale than at any subsequent geological epoch. Some of the beds of magnetic iron ranged up to 200 feet in thickness. Large masses also of haematite and titaniferous iron, as well as of iron sulphides, occur in the Canadian Archaean series. These great bands of iron ore run southward, and fprm an important feature in the economic geology of the Northern States of the Union.
Above the Laurentian rocks in the region of Lake Huron lies a vast mass of slates, conglomerates, limestones, and quartz-rocks, attaining a depth of from 10,000 to 20,000 feet. They are termed Huronian. No fossils have yet been found in them; but they must be much younger than the Laurentian rocks, on which they rest unconformably, and from which they have been in part at least derived.
II. PALAeOZOIC.
Under the general term of Primary or Palaeozoic are now-included all the older sedimentary formations containing organic remains, up to the top of what is termed the Permian system. These rocks consist mainly of sandy and muddy sediment with occasional intercalated zones of limestone. They everywhere bear witness to comparatively shallow water and the proximity of land. Their frequent alternations of sandstone, shale, conglomerate, and other detrital materials, their abundant, rippled, and sun-cracked surfaces marked often with burrows and trails of worms, as well as the prevalent character of their organic remains, show that they must have been deposited in areas of slow subsidence, bordering continental or insularmasses of land. As regards the organisms of which they have preserved the casts, the Palaeozoic rocks, as far as the present evidence goes, may be grouped into two divisions—an older and a newer :—the former distinguished more especially by the abundance of its graptolitic, trilobitic, and brachiopodous fauna, and by the absence of vertebrate remains; the latter by the number and variety of its fishes and amphibians, the disappearance and extinction of graptolites and trilobites, and the abundance of its cryptogamie terrestrial flora.
Cambrian.
This name was applied by Sedgwick to the rocks of North Wales (Cambria), where he first investigated them. Their base is there nowhere seen, so that, though they attain a great depth, some part of their total mass must be concealed from view. They pass up continuously into the base of the Silurian system. Considerable diversity of opinion has existed, and still continues, as to the line where the upper limit of the Cambrian system should be drawn. Murchison contended that this line should be placed below the strata where a trilobitic and brachiopodous fauna begins, and that these strata cannot be separated from the overlying Silurian system. He therefore included in the Cambrian only the barren grits and slates of the Longmynd, Harlech, and Llanberis. Sedgwick, on the other hand, insisted on carrying the line up to the base of the Upper Silurian rocks. He thus left these formations as alone constituting the Silurian system, and massed all the Lower Silurian in his Cambrian system. Murchison worked out the stratigraphical order of succession from above, and chiefly by help of organic remains. He advanced from where the superposition of the rocks is clear and undoubted, and for the first time in the history of geology ascertained that the “transition-rocks” of the older geologists could be arranged into zones by means of characteristic fossils as satisfactorily as the Secondary formations had been classified in a similar manner by William Smith. Year by year, as he found his Silurian types of life descend farther and farther into lower deposits, he pushed backward the limits of his Silurian system. In this he was supported by the general consent of geologists and palaeontologists all over the world. Sedgwick, on the other hand, attacked the problem rather from the point of stratigraphy and geological structure. Though he had collected fossils from many of the rocks of which he had made out the true order of succession in North Wales, he allowed them to lie for years unexamined. Meanwhile Murchison had studied the prolongations of some of the same rocks into South Wales, and had obtained from them the abundant suite of organic remains which characterized his Lower Silurian formations. Similar fossils were found abundantly on the continent of Europe, and in America. Naturally the classification proposed by Murchison was adopted all over the world. As he included in his Silurian system the oldest rocks containing a distinctive fauna of trilobites and brachiopods, the earliest fossiliferous rocks were everywhere classed as Silurian, and the name Cambrian was discarded by geologists of other countries as indicative of a more ancient series of deposits not characterized by peculiar organic remains, and therefore not capable of being elsewhere satisfactorily recognized. Barrande, investigating the most ancient fossiliferous rocks of Bohemia, distinguished by the name of the “Primordial Zone” a group of strata underlying the Lower Silurian rocks, and containing a peculiar and characteristic suite of trilobites. He classed it, however, with the Silurian system, and Murchison adopted the term, grouping under it th? lowest dark slates which in Wales and the border English counties contained some of the same early forms of life.
Investigations during the last twelve years, however, chiefly by the late Mr Salter and Mr Hicks, have brought to light a much more abundant fauna from the so-called primordial rocks of Wales than they were supposed to possess. These fossils were found to be in large measure distinct from those in the undoubted Lower Silurian rocks. Thus the question of the proper base of the Silurian system was re-opened, and the claims of the Cambrian system to a great upward extension were more forcibly urged than ever. But these claims could now be urged on palaeontological evidence such as had never before been produced. Accordingly there has arisen a general desire among the geologists of Britain to revise the nomenclature of the older rocks. Though as yet a common accord of opinion has not been reached, there seems a strong probability that ultimately the boundary line between the Cambrian and Silurian systems will be drawn above the primordial zone along the base of the great Arenig group or Lower Llandeilo rocks of Murchison. All his Silurian strata of older date than these rocks vzill be classed as Cambrian.
According to this classification, the Cambrian system, as developed in North Wales and the border English counties, consists of purple, reddish-grey, and green slates, grits, sandstones, and conglomerates. Its true base is nowhere seen, yet even the visible mass of strata has been estimated to reach the enormous thickness of 25,000 feet. By far the larger part of this vast depth of rock is unfossiliferous. Indeed it is only in some bands of the upper 6000 feet, or thereabouts, that fossils occur plentifully. By fossil evidence the Cambrian system may be divided into Lower and Upper, and each of these sections may be further subdivided into two groups, as in the following table :— [table]
1. Harlech and Longmynd Group.— This includes purple, red, and grey flags, sandstones, and slates, with conglomerates. These strata attain a great thickness, estimated at 4000 feet in South Wales, hut more than 8000 in North Wales. They were formerly supposed to be nearly barren of organic remains; but in recent years, chiefly through the researches of Mr Hicks at St Davids, they have yielded a tolerably abundant fauna, consisting of 30 species. Among these are 16 species of trilobite (Paradoxides, Plutonia, Microdiscus, Paloeo- pyge, Agnostus, Conocoryphe), four annelides (Arenicolites), a sponge (Protospongia), five brachiopods (Discina, Lingulella), two pteropods (Theca), &c. Many of the surfaces of the strata in some parts of this group are marked with ripples, sun-cracks, and rainpittings as well as with trails of worms—indicative of shallowwater and shore-conditions of deposit. 14 of the 30 species, according to Mr Etheridge, F.R.S., pass up into the Menevian group, and 7 continue into the Lingula flags.
2. Menevian Group.— This subdivision has been proposed for a series of sandstones and shales, with dark-blue slates and flags, dark-grey flags and grey grits, which are seen near St David’s (Menevia), where they attain a depth of about 600 feet. They pass down conformably into the Harlech group with which, as just stated, they are connected by 14 species in common. The Menevian beds have yielded upwards of 50 species of fossils, of which 24 are confined to the Menevian, while 18 pass up into the lower Lingula flags. Among these the trilobites are specially prominent, Some of them attained a great size, Paradoxides Davidis being nearly two feet long. But with these were mingled others of diminutive size. It is noteworthy also, as Mr Hicks has pointed out, that while the trilobites had attained their maximum size at this early period, they are represented among the older Cambrian rocks by genera indicative of almost every stage of development, “from the little Agnostus with two rings in the thorax, and Microdiscus with four, to Erinnys with twenty-four,” while blind genera occur together with those having the largest eyes. Upwards of 30 l species of trilobites have been obtained from the Menevian beds, the genera Agnostus (7. species), Conocoryphe (7 species), and Paradoxides being specially characteristic. F our species of sponges (Protospongia) and some annelide-tracks likewise occur. The mollusca are represented by 6 species of brachiopoda of the genera Discina, Lingulella, and Obolella∙, 5 pteropods (Theca) have been met with. The earliest entomostraca (Eniomis) and the first cystidean (Proto-cystites) yet discovered occur in the Menevian fauna.
3. Lingula Flags.— These strata, consisting of bluish and black slates and flags, with bands of grey flags and sandstones, attain in some parts of Wales a thickness of more than 5000 feet. They received their name from the discovery by Mr E. Davis (1846) of vast numbers of a Lingula (Lingulella Davisii) in some of their layers. They rest conformably upon, and pass down into, the Menevian beds below them, and likewise graduate into the Tremadoc group above. They are distinguished by a characteristie suite (78 species) of organic remains. The trilobites include the genera Agnostus, Anapolcnus, Conocoryphe, Dikcloccphahιs, Ennnys, Olenus, and Paradoxides. The earliest phyllopods {Hymcnocuris) and heteropods {Bellerophon} occur in these beds. The brachiopods include species of Lingulcllo {L. Davisii), Discina, Obolclla, and Orthis. The pteropods are represented by three species of Theca. Several annelides {Cruziana) and polyzoa {Fenestella) likewise occur.
According to a careful census by Mr Etheridge, the Lingula flags may be grouped into three zones, each characterized by a peculiar assemblage of organic remains. The lower division contains 37 species, of which 9 are peculiar to it. The middle zone has yielded 5 species, 2 of which {Conocoryphe bucephala and Lingulella Davisii) pass down into the lower division, 1 {Kutorgia cingulata) into the upper, and 2 {Lingula squamosa and Bellerophon Cambrensis) are peculiar. The upper zone has yielded 40 species. Of these 9 pass up into the Tremadoc beds, while 2 {Lingulella lepis and L. Davisii) continue on into the Arenig group.
4. Tronadoc Slates.— This name was given by Sedgwick to a group of dark grey slates, about 1000 feet thick, found near Tremadoc in Carnarvonshire, and traceable thence to Dolgelly. Their importance as a geological formation was not recognized until the discovery of a remarkably abundant and varied fauna in them. They contain the earliest crinoids, star-fishes, lamellibranchs, and cephalopods yet found. The trilobites embrace 14 genera, among which, besides some, as Agnostus, Conocoryphe, and Olenus, found in the Lingula flags, we meet for the first time with Angelina, Asaρhus, Cheirurus, Neseuretus, Niobe, Ogygia, Psilo∙ cephaliLs, ke. The same genera, and in some cases species, of brachiopods appear which occur in the Lingula flags, Orthis Carausii and Lingulella Davisii being common forms. Mr Hicks has described 12 species of lamellibranchs from the Tremadoc beds of Ramsay Island and St Davids, belonging to the genera Cteno∙ donta, Palaearca, Glyptarca, Davidia, and Modiolopsis. The cephalopods are represented by Orthoceτas sericeum and Cyrtoccras proecox ; the pteropods by Theca Davidii, T. operculata, and Gonu-laria Homfrayi ; the echinoderms by a beautiful star-fish {Palas-terina ramsegensis) and by a crinoid {Dendrocrimιs Cambrensis).
Careful analysis of the fossils yielded by the Tremadoc beds suggests a division of this formation into two zones. According to a census by Mr Etheridge, the Lower Tremadoc rocks have yielded in all 56 species, of which 9 pass down into the Lingula flags and 10 ascend into the Upper Tremadoc zone, 31 being peculiar. The Upper Tremadoc beds contain, as at present ascertained, 33 species, of which 9 are peculiar, and 13 or possibly 15 pass up into the Arenig group. It is at the top of the Upper Tremadoc strata that the line between the Cambrian and Silurian systems is here drawn. According to Professor Ramsay, there is evidence of a physical break at the top of the Tremadoc beds of Wales, so that on a large scale the next succeeding or Arenig strata repose unconformably upon everything older than themselves. Mr Etheridge also shows that the palaeontological break is nearly complete, only about 7 per cent, of the fossils of the one series passing over into the other. Out of 184 known Arenig species, not more than 13 are common to the Tremadoc beds underneath. Besides these important facts the character of the Arenig fauna strongly distinguishes it from that of the formations below, and further supports the line of division here adopted between the Cambrian and Silurian systems.
In the north-west of Scotland a mass of reddish-brown and chocolate-coloured sandstone and conglomerate (at least 8000 feet thick in the Loch Torridon district) lies unconformably upon the fundamental gneiss in nearly horizontal or gently inclined beds. It rises into picturesque groups of mountains which stand out as striking monuments of denudation, seeing that the truncated ends of their component flat strata can be traced even from a distance forming parallel bars along the slopes and precipices. The denudation must have been considerable even in early Silurian times, for the sandstones are unconformably overlaid by quartz-rocks and limestones containing Lower Silurian fossils. No trace of organic remains of any kind has been found in the red sandstones themselves. They were at one time regarded as Old Red Sandstone, though Macculloch, and afterwards Hay Cunningham, pointed out that they underlie parts of the schistose racks of the northern Highlands. The discovery by Air C. W. Peach of Lower Silurian shells in the overlying limestones showed that the massive red sandstones of western Ross and Sutherland could not be paralleled with those of the eastern tracts of those counties, but must be of older date than part of the Llandeilo rocks of the Lower Silurian period. Sir R. Murchison classed them as Cambrian—an identification which has much support in the lithological resemblance between these rocks of the north-west Highlands and muoh of the Lower Cambrian system of Wales.
In the south-east of Ireland masses of purplish, red, and green shales, slates, grits, quartz-rocks, and schists occupy a considerable area and attain a depth of 14,000 feet without revealiug their base, while their top is covered by unconformable formations (Lower Silurian and Lower Carboniferous). They have yielded Oldhamia, described originally as a sertularian zoophyte, but now regarded by many palaeontologists as an alga; also numerous burrows and trails of annelides (Histioderma Hibernicum, Arenicolites didymus, A. sparsus, Haughtonia poecila). No Upper Cambrian forms have been met with in these Irish rocks, which are therefore placed with the Lower Cambrian, the unconformability at their top being regarded as equivalent to the interval required for the deposition of the intervening formations up to the time of the Llandeilo rocks, as in the north-west of Scotland. Some portions of the Irish Cambrian series have been intensely metamorphosed. Thus on the Howth coast they appear as schists and quartz-rocks; in Wexford they pass into gneiss and granite. In West Galway Air Kinahan has described a vast mass of schists, quartz-rocks, and limestones (8000 feet and upwards) passing up into schistose, hornblendic, and unaltered rocks containing Llandeilo fossils, and he agrees with Griffith and King in regarding these as probably Cambrian. He suggests that they are Upper Cambrian, which would imply that Upper Cambrian rocks pass conformably into the Llandeilo formation without thp occurrence of the thick Arenig rocks of Wales. In a difficult country, however, broken by faults and greatly metamorphosed, an unconformability might easily escape detection.
Continental Europe. —According to the classification adopted by AI. Barrande, the older Palaeozoic rocks of Europe suggest an early division of the area of this continent into two regions or provinces,—a northern province, embracing the British Islands, and extending through North Germany into Scandinavia, on the one hand, and into Russia on the other, and a central-European province, including Bohemia, France, Spain, Portugal, and Sardinia.
Bohemia.— The classic researches of AL Barrande have given to the oldest fossiliferous rocks of Bohemia an extra ordinary interest. He has made known the existence there of a remarkable suite of organic remains representative of those which characterize the Cambrian rocks of Britain. At the base of the geological formations of that region lie the Archaean gneisses already described. These are overlaid by vast masses of schists, conglomerates, quartzites, slates, and igneous rocks, which have been more or less metamorphosed, and are singularly barren of organic remains, though some of them have yielded traces of annelides. They pass up into certain grey and green fissile shales, in which the earliest well-marked fossils occur. The organic contents of this zone (Étage C) form what Al. Barrande terms his primordial fauna, which contains 40 or more species, of which 27 are trilobites, belonging to the characteristic Cambrian genera— Paradoxides ( 12), Agnostus (5), Conocoryphe (4), Ellipsoceρhalus (2), Hydrocephalus (2), Amoηellus (1), Sao (1). Not a single species of any one of these genera, save Agnostus (of which 4 species appear in the second fauna), has been found by M. Barrande higher than his primordial zone. Among other organisms in this primordial fauna, the brachiopods are represented by 2 species (Orthis and Orbicula), the pteropods by 5 (Theca), and the echinoderms by 5 cystideans.
Scandinavia.— In Norway the vast masses of Archaean gneiss (Teilemark) are overlaid by schists, red sandstones, and conglomerates. These are termed the Sparagmite formation, and have hitherto proved barren of fossils. They are covered, however, by beds containing Dictyonema Norvegicum, which may represent the primordial zone of Barrande. In Sweden the sparagmite formation has been more productive of organic remains. It is there represented by a sandy zone not more than 50 or 60 feet thick·—a poor equivalent for the great mass of strata in the Cambrian system of Wales. It was originally termed the Begio Fucoidarum by Angelin, from the fucoids alone found in it. In more recent years, however, its list of organic remains has been considerably increased; 12 species of plants, chiefly fucoids, but including some (Eophyton) of higher grade, 9 species of annelides, 4 brachiopods, a pteropod, a bryozoan, a coral, a crinoid, and a sponge have been obtained. Above the strata containing these organisms comes a zone which has yielded 77 species of primordial trilobites, including the genera Agnostus (19 species), Cono-cοτyphe (13), Olenus (21), Paradoxides (9).
North America. —Rocks corresponding in position and in the general character of their organic contents with the Cambrian formations of Britain have been recognized in different parts of the United States and Canada. They appear in Newfoundland, whence, ranging by Nova Scotia and New Brunswick, they enter Canada, the northern parts of New York, Vermont, and eastern Massachusetts. They rise again along the Appalachian ridge, in Wisconsin, Minnesota, Missouri, Arkansas, Texas, and Georgia. Westwards from the great valley of the Mississippi, where they have been found in many places, they reappear from under the Secondary and younger Palaeozoic rocks of the Rocky Mountains. They have been divided by American geologists into two formations—(-1) Acadian, a mass (2000 feet) of grey and dark shales and some sandstones; and (2) Potsdam (or Georgian), which attains in Newfoundland a depth of 5600 feet, but thins away westward and southward till in the valley of the St Lawrence, where it was studied by Logan and his associates of the Geological Survey of Canada, it is only from 300 to 600 feet thick.
Among the organic remains of the North American Cambrian rocks fucoid casts appear in many of the sandstones, but no traces of higher vegetation. The Acadian formation has yielded primordial trilobites of the genera Paradoxides, Conocoryplιe, Agnostus, and some others; brachiopods of the genera Lingulella, Discina, Obolella, and Orthis ’ , and several kinds of annelide-tracks. The Potsdam rocks contain a few sponges, the earliest forms of graptolite, some brachiopods, including, besides the genera in the Acadian beds, Obolus, Camarélla, and Orthisina ; some pteropods (Hyalites or Theca) ; two specie3 of Orthoceras; annelide tracks; trilobites of the genera Conocoryphe, Agnostus, Dikelocephalus, Olenellus, Ptychaspis, Charioce-phalus, Aglaspis, and Illoenurus.
M. Barrande has called attention to the remarkable uniformity of character in the organic remains of his primordial zone over the continents of Europe and America. He published in 1871 the subjoined table, to show how close is the parallelism between the proportions in which the different classes of the animal kingdom are represented. [table]
Silurian.
The important system of rocks next to be described was first investigated by the late Sir R. Murchison in Wales and the bordering counties of England. He found it to be characteristically developed over the tract once inhabited by the Silures, an ancient British tribe, and he thence chose the name of Silurian as a convenient designation. It there passes down conformably into the Tremadoc slates at the top of the Cambrian series, and is covered conformably by the base of the Old Red Sandstone.
Great Britain. —In the typical area where Murchison’s discoveries were first made he found the Silurian rocks divisible into two great and well-marked series, which he termed Lower and Upper. This classification has been found to hold good over a large part of the world. The subjoined table shows the present arrangement and nomenclature of the various subdivisions of the Silurian system. [table]
A. Lower Silurian.
1. Arenig or Stiper Stone Group.— These rocks consist of dark slates, shales, flags, and bands of sandstone, They are abundantly developed in the Arenig mountain, where, as originally described by Sedgwick, they contain masses of associated porphyry. Throughout that district they have been deposited at a time when streams of lava and showers of volcanic ashes were thrown out in great quantity from submarine vents. They contain an abundant suite of organic remains (184 species), of which only 13 species are common to the Tremadoc beds below. Trilobites occur of the genera Aeglina, Agnostus, Ampyx, Barrandea, Calymene, Cheirurus, Illceηοpsis, Illoenus, Ogygia, Phacops, and Trimιcleus. Three species of pteropods (C anularia, Theca), 18 species of brachiopods (Lingula, Lingulella, Obolella, Piscina, Siphonotreta, Orthis), 8 lamellibranchs, 3 gasteropods, and 5 cephalopods have been found; but the most abundant organisms are the graptolites, of which the Arenig rocks of St David’s, in Pembrokeshire, have yielded 48 species, which belong to 20 genera, including Didymograptus, Tetragraptus, Diρlograptus, Dendrograptus, and Callograptus A Altogether 78 species of hydrozoa have been obtained from the British Arenig rocks, but none from any older strata. This sudden and great development of these organisms gives a distinctive aspect to the Arenig rocks. It continues in the overlying Llandeilo group, so that the graptolites form in Britain a convenient character by which to mark off the Cambrian from the Lower Silurian fauna.
2. Llandeilo Flag Group.— Dark argillaceous flagstones, sandstones, and shales, some parts often calcareous. These beds were first described by Murchison as occurring at Llandeilo, in Carmarthenshire. They reappear on the coast of Pembrokeshire, and at Builth, in Radnorshire. Up to the present time they have yielded 227 species of fossils. Of these 13 are common to the Arenig below, 82 to the Caradoc or Bala above, while 145 are peculiar. The hydrozoa are still the most abundant forms, 94 species being here met with, no fewer than 81 of these being confined to Llandeilo rocks, and only 9 passing down into the Arenig group. Of crustacea 44 species have been obtained. These include the characteristic trilobites — Ampyx nudus, Asaphus tyrannus, Barrandea Cordai, Calymene duplicata, C. Cambreηsis, Cheirurus Sedgwickii, Ogygia Buchii, Trinτιcleus concentricus, T. Lloydii. The brachiopods number 37 species, including the genera Orthis, Lept<xna, Strophomenα, Lingula, Siphonotreta. The lamellibranchs are represented by 6 species, the gasteropods by 10 (Murchisonia, Cyclonema, Loxonema), the heteropods by 7 (Bellerophon), the pteropods by 2 (Conulama, Theca), the cephalopods by 8 (Orthoceras, Cyrtoceras).
A remarkable feature in the history of the Llandeilo rocks in Britain was the outbreak of volcanic action abundantly in North Wales and in Cumberland. Vast piles of lava and ashes were thrown out, which even to this day remain in mass sufficient to form groups of important hills, as Cader Idris, Aran Mowddwy, the Arenigs, and the Moelwyns in Wales, and Helvellyn and Scaw Fell in Westmoreland and Cumberland.
3. Caradoc or Bala Group.— Under this name are placed the thick yellowish and grey sandstones of Caer Caradoc in Shropshire, and the grey and dark slates, grits, and sandstones round Bala in Merionethshire. In the Shropshire area some of the rocks arc so shelly as to become strongly calcareous. In the Bala district the strata contain two limestones separated by a sandy and slaty group of rocks 1400 feet thick. The lower or Bala limestone (25 feet thick) has been traced as a variable band over a large area in North Wales. It is usually identified with the Coniston limestone of the Westmoreland region. The upper or Hirnaut limestone (10 feet) is more local. Bands of volcanic tuff and large beds of various felsitic lavas occur among the Bala beds, and prove the contemporaneous ejection of volcanic products. These attain a thickness of several thousand feet in the Snowdon region.
A large suite of fossils has been obtained from this formation :—the sponges represented by Sphoerospongia and other genera; the graptolites by Diplograp)tu8 pristis, Graptolithus priodoη, and G. Sedgwickii, &c.; the corals by species of Heliolites, Favosites, Monticulipora, Halysites, Petraia; the echinoderms by encrinites of the genera Cyathocrinus and Glyptocmnus, by cystideans of the genera Echinosph<xrites and Sphaeronites, and by star-fishes of the genera Paloeaster and Stenaster; the annelides by Serpulites, Τentaculites, and numerous burrows and tracks; the trilobites by many species of the genera Phacops, Cheirurus, Cybele, Lichas, Acidaspis, Calymene, Remopleurides, Asaphus, lllοenus, Ampyx, and Trinucleus; the polyzoa by Fenestella, Glauconome, and Ρtilodictυa : the brachiopods by Atrypa, Rhynchonella, Leptarna, Orthis (many species), Strophomena, Discina, and Lingula·, the lamellibranchs by Modiolopsis, Mytilus, Paloearca, Pteriηea, Orthonota, and Ctenodonta ; the gasteropods by Murchisonia, Pleura-tomaria, Raphistama, Cyclonema, Euornphalus, Maclurea, Holopea ; the pteropods by Coηularia, Theca, and Ecculi-omj)halus∙, the heteropods by various species of Bellerophon', and the cephalopods by many species of Orthoceras, with forms of Cyrtoceras and Lituites.
4. Lower Llandovery Group.— In North Wales the Bala beds about 5 miles S.E. of Bala Lake begin to be covered with grey grits, which gradually expand southwards until they attain a thickness of 1000 feet in South Wales. These overlying rocks are well displayed near the town of Llandovery, where they contain some conglomerate bands, and where Mr Aveline detected an unconformability between them and the Bala group below them, so that the subterranean movements had already begun, which in Wales marked the close of the Lower Silurian period. Elsewhere they seem to graduate downwards conformably into that group. They cover a considerable breadth of country in Cardigan and Carmarthenshire, owing to the numerous undulations into which they have been thrown. Their chief interest lies in the transition which they present between the fauna of the Lower and Upper Silurian formations. They have yielded in all about 128 species of fossils, whereof 11 are peculiar, 93 are common to the Bala group below, and 83 pass up into Upper Llandovery rocks above. Some of the peculiar fossils are Nidulites favus, Meristella crassa, Μ. angustifrons, and Murchisonia angulata. Among the forms which come up from the Bala group and disappear here are the corals Heliolites interstinctus, Petraia subduplicata, and Favosites aspera; the trilobites Lichas laxatus and Illoeηus Bowmanni; the brachiopods Orthis Aclonioe and O. insularis ; the gasteropods Murchisonia gyrogoηia and Cyclonema crebiistria; and the cephalopod Orthoceras tenuicinctum. But many of the Lower Silurian forms continue on into the Upper Llandovery beds. From the abundance of the peculiar brachiopods termed Pentamerus in the Lower, but still more in the Upper Llandovery rocks, these strata were formerly grouped together under the name of “Pentamerus beds.” Though the same species are found in both divisions, Pentamerus oblongus is chiefly characteristic of the upper group and comparatively infrequent in the lower, while Stricklandinia (Pentamerus) lens abounds in the lower but appears more sparingly in the upper.
The Lower Silurian rocks, typically developed in Wales, extend over nearly the whole of Britain, though largely buried under more recent formations. They rise into the hilly tracts of Westmorelaud and Cumberland, where they consist of the following subdivisions in descending order :— [table]
Apart from the massive intercalation of volcanic rocks these strata present considerable lithological and palaeontological differences from the typical subdivisions in Wales. The Skiddaw slates are black or dark-grey argillaceous, and in some beds sandy rocks, often much cleavèd though seldom yielding workable slates, sometimes soft and black like Carboniferous shale. As a rule they are singularly unfossiliferous, but in some of their less cleaved and altered portions they have yielded about 40 species of graptolites (chiefly of the genera Didymοgrαptus, Diplοgraptus, Dichograptus, Τclragraptus, Ρhyllograptus, and Climacograptus) Lingula brevis, traces of annelides, a few trilobites (Aeglina, Agnostus, Asaphus, &c. ), some phyllopods (Caryocaris), and remains of plants (But hoir ∙cphis, &c.). In many places the slates have been metamorphosed, passing into chiastolite-slate, mica-schist, andalusite-schist, &c., with protrusions of granite, syenite, and other crystalline rocks. Towards the close of the long period represented by the Skiddaw slates, volcanic action manifested itself, first by intermittent shθλvers of ashes and streams of lava which were interstratified with the ordinary marine sediment, and then by a more powerful and continuous series of explosions, whereby a 'huge volcanic mountain or group of cones was piled up above the sea-level. The length of time occupied by this volcanic episode in Cumbrian geology may be inferred from the fact that all the Llandeilo and nearly all the Bala beds are absent here. The volcanic island slowly sank into a sea where Bala organisms flourished. Among these λve find such familiar Bala species as Favosites fibrosa, Heliolites interstinctus, Cybele verrucosa, Leptaena sericea, Orthis Actonioe, O. biforata, Ο. caligramma, O. elegantula, O. porcata, and Strophomena rhomboidalis. These organisms and their associates gathered on the submerged flanks of the sinking volcano into a bed of limestone— the Coniston limestone—which can still be traced for many miles through the Westmoreland hills, as the Bala limestone which it represents can be followed through the volcanic tracts of North Wales. The Coniston limestone is covered by certain flags and grits which from their organic remains are referred to the Upper Silurian series.
In the South of Scotland, according to the detailed researches of the Geological Survey, the Lower Silurian formations are represented by the subjoined groups of strata in descending order :— [table]
As a whole these strata are singularly barren of organic remains. Most of the fossils which the Llandeilo groups contain lie in the bands of dark anthracitic shale which have been traced across nearly the whole breadth of the country. These shales are crowded with graptolites of recognizable Llandeilo forms, Glimacograptus teretiusculus, Diplograptus ρristis, and Graptolithus sagittarius being particularly abundant. Crustacea are exceedingly rare, but two phyllopods, Discinocaris Browniana and Peltocharis ap>tychoides, occur; while from Dumfriesshire two obscure trilobites are referred doubtfully to Encrinurus and Phacops. The vast thickness of sandy, gritty, and shaly unfossiliferous strata is the distinguishing feature of the Lower Silurian series in the south of Scotland. The Caradoc or Bala group lies unconformably upon the upper parts of the Llandeilo rocks. It contains in the eastern districts some calcareous conglomerates which here and there swell out into local masses of limestone. In the south-west of Ayrshire the limestones attain considerable dimensions. In these calcareous bands numerous Caradoc species have been found, among them Gheirwrus gelasinosus, Encrinurus punctatus, with species of Illaenus and Asaphus, Orthis calligramma, O. confinis, Leptaena sericea, Maclurca, and such corals as Heliolites, Favosites, Omphyma, and Strephodes. In the south-west of Ayrshire certain shales and sandstones full of Caradoc fossils are overlaid with sandstones, shales, and conglomerates containing Pentamerus oblongus, Atrypa hemispherica, Meristella angustifrons, Lichas laxatus, Petraia elongata, Nidulites favus, and. numerous other fossils which indicate the horizon of the Llandovery rocks.
The Highlands of Scotland consist mainly of crystalline rocks— gneiss, mica-schist, chlorite-schist, clay-slate, quartz-rock, schistose flagstone, and many others, often much invaded by granite and other intrusive masses. It was at one time supposed that these rocks all belonged to the so-called primary or primitive series, older than any of the fossiliferous systems. But the discovery by Mr C. W. Peach, already referred to, that recognizable fossils occur in the limestone of Durness in Sutherlandshire, led Murchison to infer that the whole of the overlying gneissose and schistose masses are really metamorphosed Lower Silurian rocks—a generalization which has been completely confirmed by subsequent investigation. At the base of this great series of rocks masses of white quartz-rock are found lying with a marked unconformability upon the red sandstones described in a previous page. These quartzose beds are merely hardened and somewhat metamorphosed sandstones; they still show their original false-bedding, and the casts of sea-weeds and worm-burrows. They contain a band of limestone which in Assynt swells out to a thickness of 1000 feet or more, and can be traced almost continuously from the Kyles of Skye to the north coast of Sutherlandshire. Over these strata, in perfect conformable sequence, and with a complete lithological gradation, come quartzose flagstones dipping like the rocks below at gentle angles to the southeast. They become more schistose and crumpled as they are traced upwards, until, after a thickness of several thousand feet has been passed over, they begin to undulate in steep folds and pass into the ordinary schistose rocks which coveι∙ so much of the Highlands. The gradation from the comparatively unaltered lower quartz-rocks and limestones on the west to the intensely crumpled crystalline upper schists and flagstones on the east can be followed step by step in numerous fine natural sections from the north_of Sutherland to the Kyles of Skye. The proof is thus complete that a vast mass of schists and other crystalline rocks overlies fossiliferous limestones in the Scottish Highlands. It therefore becomes of the utmost importance to determine the geological horizon of the fossils in the limestones. This was done by the late Mr Salter, who declared his conviction that they were unequivocally Lower Silurian, and bore a most remarkable resemblance to a group of fossils from the Lower Silurian rocks of North America. Five of the species he regarded as identical with known American forms f)rthoceras arcuoliratum, Hall; Orthis striatula, Emmons; Ophileta compacta, Salt.; Murchisonia gracilis, Hall; Μ. bellicincta, Hall), 4 as representative, 3 doubtful, and 1 new genus, found also in Canada. “That this truly North American assemblage,” he remarks, “should be found in the extreme north of Scotland on the same parallel as the Canadian,—that species of Maclurea and Raphistoma, resembling those of the St Lawrence basin, and Orthocerata, bearing large siphuncles like those of North America, Scandinavia, and Kussia, should occur in Scotland and yet be scarcely known further south, is at least suggestive of a geographical distribution—perhaps even of climatal conditions—not very unlike that of more modern times.” From this palaeontological decision it follows that the overlying schistose series-of the Scottish Highlands is a mass of metamorphosed Silurian strata. Examined in detail they show very unequal and sporadic metamorphism. Some portions are scarcely more changed than the ordinary greywackes and shales of unaltered districts. False-bedding, pebble-beds, and other common features of sedimentation occur abundantly throughout the whole vast series of schists. Here and there the metamorphism has become extreme, the rocks passing into coarsely crystalline schists full of garnets, with bands of hornblende-rock, actinolite-schist, and other metamorphic products, and passing even into granitic gneiss and true granite. No more convincing proof could be obtained that vast masses of schist do not necessarily belong to an azoic period of the earth’s history, but may have been produced by the alteration of previously existing sediments.
It is not necessary to believe that the sediments so altered were in all cases mere ordinary marine sand and mud. The white quartz-rocks were no doubt at one time pure white siliceous sand, the rounded grains of which can still be readily detected in them. The quartzose flagstones were stratified sand with thin partings of clay or mud. The clay slates were evidently thick accumulations of mud. But the rocks containing a marked percentage of magnesia, such as chlorite-slate, actinolite-schist, hornblende-rock, &c., may have resulted from the alteration of volcanic sediments and submarine lavas. The evidence from Cumberland and Wales proves how voluminous and long continued were the volcanic eruptions of the Lower Silurian period in Britain. The abundant diffusion of volcanic detritus over the present sea-bottom is now well known. The “Challenger” researches have also shown us that, besides the glauconite previously known to be deposited from sea-water in the chambers of foraminifera and other dead organisms on the oceanbottom, true magnesian silicates are now in the process of elimination from sea-water in some of the abysses of the ocean. It is quite possible therefore that some of the rocks of the metamorphic series rich in magnesian silicates may have arisen from the alteration of volcanic tuffs or submarine lavas, and that others may owe their distinctive composition to original chemical precipitation, as ably contended by Sterry Hunt, though their present crystalline structure must be regarded as a part of the general metamorphism by which the whole of the Lower Silurian rocks of the Highlands have been affected.
In the south-east of Ireland, grey, greenish, and purple grits, and grey and dark shales, lie uncomformably upon the Cambrian rocks, and contain a few fossils of Llandeilo age. They present interstratified beds of tuff and felsitic lavas indicating contemporaneous volcanic action. In the north-east of the island a broad belt of Lower Silurian rocks runs from the coast of Down into the heart of Roscommon and Longford. This belt is evidently a prolongation of that in the southern uplands of Scotland. It is marked by the occurrence of similar dark anthracitic shales crowded with graptolites. The richest fossiliferous localities among the Irish Lower Silurian rocks are found at the Chair of Kildare, Portrane near Dublin, Γomeroy in Tyrone, and Lisbellan in Fermanagh, where small protrusions of the older rocks rise as oases among the surrounding later formations. Portlock brought the northern and western localities to light, and Murchison pointed out that, while a number of the trilobites (Trinuclcus, Phacops, Calymene, and lllccnus), as well as the simple plaited Grthidoe, Leptaenae, and Stropho mena, some spiral shells, and many Grthocerata, are specifically identical with those from the typical Caradoc and Bala beds of Shropshire and Wales, yet they are associated with peculiar forms, first discovered in Ireland, and very rare elsewhere in the British Islands. Among these distinctive fossils he cites the trilobites, Remopleurides, Harpes, Amphion, and Bronteus, with the smooth forms of Asaphus (Isotelus), which, though abundant in Ireland and America, seldom occur in Wales or England, and never on the Continent.
In the north and west of Ireland a large area of surface is occupied by crystalline rocks—gneiss, schists, quartz-rocks, limestone, granite, &c.—which are manifestly a continuation of those of the Highlands of Scotland. They run south-westward parallel with the belt of unaltered Lower Silurian rocks from which, in some places, as in county Tyrone, they are only a few miles distant. The district of Pomeroy, so rich in Silurian fossils, promises to afford the greatest light on the interesting but difficult problem of the metamorphism of the Lower Silurian rocks of the Scottish Highlands and the north-west of Ireland. It will be seen from the evidence furnished by the sections in West Mayo (p. 337) that the metamorphism must have taken place prior to the deposition of the Upper Silurian formations of the west of Ireland,
B. Upper Silurian.
The formations which in the British Islands are classed as Upper Silurian occur in two very distinct types. So great indeed is the contrast between these types that it is only by a comparison of organic remains that the whole can be grouped together as the deposits of one great geological period. In the original region described by Murchison, and from which his type of the system was taken, the strata are comparatively flat, soft, unaltered, consisting mainly of somewhat incoherent sandy mud with occasional bauds of limestone. But as these rocks are followed into North Wale3, they are found to swell out into a vast series of grits and shales so like portions of the hard altered Lower Silurian rocks that, save for the evidence of fossils, they would naturally be grouped as part of that more ancient series. In Westmoreland and Cumberland, and still further north in the border counties of Scotland, also in the south-west of Ireland, it is the North Welsh type which prevails, so that in Britain the general lithological characters and minute palaeontological subdivisions ascertained in the typical Silurian district are almost confined to that limited region, while over the rest of the British area for thousands of square miles the hard sandy and shaly type of North Wales is prevalent.
Taking first the Silurian tract of the south-west of England, and the east and south of Wales, we find a decided unconformability separating the Lower from the Upper Silurian formations. In some places the latter are found passing across the edges of the former, group after group, till they come to lie directly upon the Cambrian rocks. Indeed, in one district between the Longmynd and Wenlock edge, the base of the Upper Silurian rocks is found within a few miles to pass from the Caradoc group across to the Lower Cambrian rocks. It is evident, therefore, that in the Welsh region very great disturbance and extensive denudation preceded the commencement of the deposition of the Upper Silurian rocks. As Professor Ramsay has pointed out, the area of Wales, previously covered by a wide though shallow sea, was ridged up into a series of islands, round the margin of which the conglomerates at the base of the Upper Silurian series began to be laid down. This took place during a time of submergence, for these conglomeratic and sandy strata are found creeping up the slopes and even capping some of the heights, as at Bogmine, where they reach a height of 1150 feet above the sea. The subsidence probably continued during the whole of the interval occupied by the deposition of the Upper Silurian strata, which thus were piled to a depth of from 3000 to 5000 feet over the disturbed and denuded platform of Lower Silurian rocks.
Arranged in tabular form, the subdivisions of the Upper Silurian rocks of Wales and the adjoining counties of England are in descending order as follows :— [table]
1. Upper Llandovery Group.— (a.) Hay Hill Sandstones. —-The position of these rocks as the true base of the Upper Silurian formations was first shown in 1853 by Sedgwick, who named them the May Hill Sandstones from the locality in Gloucestershire where they are so well displayed. Appearing on the coast of Pembrokeshire at Marloes Bay, they range across South Wales until they are overlapped by the Old Red Sandstone. They emerge again in Carmarthenshire, and trend north-eastward as a narrow strip at the base of the Upper Silurian series, from a few feet to 1000 feet or more in thickness, as far as the Longmynd, where as a marked conglomerate wrapping round that ancient Cambrian ridge they disappear. In the course of this long tract they pass successively and unconformably ovei Lower Llandovery, Caradoc, Llandeilo, and Cambrian rocks. They consist of yellow and brown ferruginous sandstones, often full of shells, which are apt to weather out and leave casts. Their lower parts are commonly conglomeratic, the pebbles being largely derived from older parts of the Silurian formations. Here and there, where the organic remains become extraordinarily abundant, the strata pass into a kind of sandy limestone, known as the “Pentamerus limestone,” from the numbers of this brachiopod contained in it. The species of fossils found in the May Hill Sandstones number about 230.
Among these are some traces of fucoids; sponges (Cliοnα, Ischaditcs) ; the widely diffused Graptοlithus priodoη ; a number of corals (jPctraia, Heliolites, Faνositcs, Halysites, Syringopora, &c.); a few crinoids; some annelides, particularly the Tcntaculites anglicus, which is abundant; a number of trilobites, of which Phacops Stokesii, P. Weavcri, Encrinunιs punctatus, and Calymene Blumenbachii are common; numerous brachiopods, as A trypa hemispherica, A. reticularis, Pentamerus oblongus, Stricklandinia lirata (f>. lens also occurs), Leptoena transversalis, Orthis calligramma, O. clegantula, O. reversa, Strophomcna compressa, S. pecten, and Lingula parallela·, lamellibranchs of the mytiloid genera Orthonota, Mytihιs, and Modiolopsis, with forms of Pterinea, Ctcnodonta, and Lyro-desma·, gasteropods, particularly the genera Murchisonia, Plcuro-tomaria, Cyclonema, Holopclla∙, and cephalopods, chiefly Orthoccrata, with some forms of Actinoceras and Phragmoccras, and the old species Lituites comu-arietis.
(δ.) Tarannon Shale.— Above the Upper Llandovery beds comes a very persistent zone of fine, smooth, light grey or blue slates, which has been traced down the whole length of Wales from the mouth of the Conway into Carmarthenshire. These rocks, termed the “paste-rock” by Sedgwick, have an extreme thickness of 1000 to 1500 feet. Barren in organic remains, their chief interest lies in the fact that the persistence of so thick a band of rock between what were supposed to be continuous and conformable formations should have been unrecognized until it was proved by the detailed mapping of the Geological Survey.
2. Wenlock Croup.—(aj) Woolhope Limestone.— In the original typical Upper Silurian tract of Shropshire and the adjacent counties, the Upper Llandovery rocks are overlaid by a local group of grey shales containing nodular limestone which here and there swells out into beds having an aggregate thickness of 30 or 40 feet. These strata are well displayed in the picturesque valley of Woolhope in Herefordshire, which lies upon a worn quaquaversal dome of Upper Silurian strata rising in the midst of the surrounding Old Red Sandstone. They are seen likewise to the northwest at Presteign, Nash Scar, and Old Radnor in Radnorshire, and to the east and south in the Malvern Hills (where they include a great thickness of shale below the limestone), and May Hill in Gloucestershire. These strata have yielded many characteristically Upper Silurian fossils, among which may be mentioned Bumastus Barriensîs, Homalonotus del-phiηoceplιalus, Phacops cazulatus, Λtrypa reticularis, Orthis calligramrna, Strophomena imbrex, Rhynchonella borealis, R. Wilsoni, Euomphalus sculptus, Orthoceras annulatum.
It is a characteristic of the older Palaeozoic limestones to dccuγ in a very lenticular form, swelling in some places to a great thickness and rapidly dying out, to reappear again perhaps some miles away with increased proportions. This local character is well exhibited by the Woolhope limestone. Where it dies out, the shales underneath and intercalated with it join on continuously to the overlying Wenlock shale, and no line for the Woolhope group can then be satisfactorily drawn. The same discontinuity is strikingly traceable in the Wenlock limestone to be immediately referred to.
(δ.) Wenlock Shale.— This is a group of grey and black fine shales, traceable from the banks of the Severn near Coalbrook Dale across Radnorshire to near Carmarthen—a distance of about 90 miles. The same strata reappear in the protrusions of Upper Silurian rock which rise out of the Old Red Sandstone plains of Gloucestershire, Herefordshire, and Monmouthshire. In the Malvern Hills they were estimated by Professor Phillips to reach a thickness of 640 feet, but towards the north they thicken out to 1000 or even 1400 feet. On the whole the fossils are identical with those of the overlying limestone. The corals, however, so abundant in that rock are here comparatively rare. The brachiopods (of the genera Lep>tarna, Orthis, Strophomena, Atrypa, and Rhynchonella) are generally of small size— Orthis biloba, O. hybrida, and the large flat O. rzιstica, being characteristic. Of the higher mollusca thin-shelled forms of Orthoceras are specially abundant. Among the trilobites, Encriηurus punctatus, E. variolaris, Calymene Blumenbachii, C. tuberculosa, Phacops caudatus, and P. longicaudatus are common. The G raptolithus priodon, so frequent among the Bala beds of the Lower Silurian series, also occurs in the Wenlock shale. Graptolithus Flemiηgii is here a characteristic species.
(c.) Wenlock Limestone is a thick-bedded, sometimes flaggy, usually more or less concretionary limestone, grey or pale pink, often highly crystalline, occurring in some places as a single massive bed, in others as two or more strata separated by grey shales, the whole forming a thickness of rock ranging from 100 to 300 feet. As its name denotes, this stratum is typically developed along Wenlock Edge in Shropshire, where it runs as a prominent ridge for fully 20 miles, also between Aymestry and Ludlow. It likewise appears at the detached areas of Upper Silurian strata above referred to, being specially well seen near Dudley (whence it is often spoken of as the Dudley limestone), Woolhope, Malvern, May Hill, and Usk in Monmouthshire.
A distinguishing characteristic of the Wenlock limestone is the abundance and variety of its corals, of which 53 species have been described. The rock seems indeed to have been formed in part by massive sheets and bunches of coral. Among characteristic species are Ηαlysites catenzιlaria, Heliolites interstinctus, H. tubulatus, Alveolites Labcchei, Favositcs aspera, F. fibrosa, F. Gothlandicu, Comités juniperinus, Syringopora fascicularis, and Oznphyma turbinatum. The crinoids are also specially abundant, and are often beautifully preserved: Periechocrimus moniliformis is one of the most frequent species; others are Crotalo-crinzιs rugosus, Gyathocrinus goniodactylus, and Marsupiocrinus coelatus ; with several cystideans, as Pseudo erinites guadrifasciatzιs. The crustaceans include numerous trilobites, among which we miss some of the persistent Lower Silurian genera, such as Asaphus, Ogygia, and Trinucleus, none of which ascend into the Wenlock group. The most abundant trilobite is the long-lived Calymene Blumenbachii, which ranges from the Llandeilo flags up to near the top of the Upper Silurian formations. It occurs abundantly at Dudley, where it received the name of the ‘‘ Dudley Locust. ’ ’ Other common forms are Encrinzιruβ punctatus, E. variolaris, Phacops caudatus, P. Downingioe, P. Stokesii, Bumastzzs Barriensis, Homalonotus dclphinocephalus, and Cheirurus bimucronatus. The brachiopods continue to be abundant; among typical species may be noted Atrypa reticularis, Mcristella tumida, Spirifcτ elevatus, S. plicatcllzιs, Rhynchonella borealis (very common), R. cuneata, R. Wilsoni, Orthis elegantula, O. rustica, Strophomena rhoinboidalis, and Pcntamerzcs galeatus. The lamellibranchs are not well represented; but several species of Ptcrinea are abundant, with Grazn-rnysia cingulaia, and some species of Modiolopsis and Gtenodozιta. The gasteropods are most characteristically marked by 8 or 9 species of Buomphalus, 3 or more of Murchisonia, with species of Pleurotomaria, Acroculia, and Cyclonema. The cephalopods are confined to few genera, Lituites, Actizιoceras, Cyrtoceras, Orthoceras, and Phragmoceras ; of these the orthoceratites are by far the most abundant both in species and individuals. Orthoceras azιnulaiuzn is the most common form. The pteropods appear in the beautiful and very abundant Conzιlaria Sowcrbyi, and the heteropods in the common and characteristic Bellerophon Wenlockensis.
3. Ludlνw Group.— This series of strata consists essentially of shales, with occasionally a calcareous band in the middle. It graduates downward into the Wenlock group, so that when the Wenlock limestone disappears the Wenlock and Ludlow shales form one continuous argillaceous formation. It is in this united form that the two groups stretch to the south-west through Brecon and Carmarthen. The Ludlow rocks are typically seen between Ludlow and Aymestry. They appear likewise at the detached Silurian areas from Dudley to the mouth of the Severn. They were grouped by Murchison into three zones.
(a.) Lower Lzιdlovo Rock.— This is a group of soft darkgrey to pale greenish-brown or olive sandy shales, often with calcareous concretions. Much of the rock, however, presents so little fissile structure as to get the name of mudstone, weathering out into concretions which fall to angular fragments as the rock crumbles down. It becomes more sandy and flaggy towards the top. From the softness of the shales this zone of rock has been extensively denuded, and the Wenlock limestone rises up boldly from under it.
An abundant suite of fossils has been yielded by these shales. No fewer than 18 species of star-fishes, belonging to 6 genera, have been described fPrοtaster, like the brittle-stars of the British seas, Paloeocoma, Palasterizιa'). A few graptolites occur, particularly the persistent Graptolithzιs priodozι (common), G. colonus, and G. FlczningU. A few of the Wenlock corals survive in the Lower Ludlow rock, but the conditions of deposit were evidently unfavourable for their growth. The trilobites are less numerous than in older beds; they include the venerable Calymene Blumenbachii, Phacops cazulatzιs, and its still longer-tailed variety P. longi-cazzdatzzs ; also Acidaspis Brightii, Homalonotus delphizwcephalzzs, and Cyphaspis megalops. But other forms of crustacean life occur in some number. As the trilobites begin to wane numerous phyllopods appear, the genus Ceratiocaris being represented by 10 or more species. Large eurypterids now make their entrance upon geological history — Eurypterzιs, Pterygotus, and Hemiaspis. Though brachiopods are not scarce, hardly any seem to be peculiar to the Lower Ludlow rock, the Lingula lata, which Murchison suggested might be peculiar, having been obtained from what is supposed to be representative of this group of strata in Westmoreland. Rhynchonella Wilsoni, Spirifer exporrectzιs, Strophomena euglypha, Atrypa reticularis, and Chonetes minima are not infre-queut. Among the more frequently recurring species of lamelli-braucħs the following may be named— Cardiola interrupta, C. striata, Orthonota rigida, 0. scmisulcata, and a number of species of Pterinea. The orthoceratites are numerous, as Grthoceras Ludense, 0. subundulatum, also species of Phragmoceras and Lituitcs. The numbers of these straight and curved cephalopods form one of the distinguishing features of the zone. At one locality, near Leint-wardine in Shropshire, which has been prolific in Lower Ludlow fossils, particularly in star-fishes and eurypterid crustaceans, a fragment of the fish Pteraspis was discovered in 1859. This is the earliest trace of vertebrate life yet detected. It is interesting to note that the Pteraspis does not stand low in the scale of organization, but has affinities with our modern sturgeon.
(6.) Aymestry Limestone is a dark grey somewhat earthy coucretionary limestone in beds from 1 to 5 feet thick. Where at its thickest it forms a conspicuous feature, rising above the soft and denuded Lower Ludlow shales and, owing to the easily removable nature of some fuller’s earth on which it lies, it has here and there been dislocated by large landslips. It is still more inconstant than the Wen lock limestone. Though well developed at Aymestry it soon dies away into bands of calcareous nodules, which finally disappear, and the lower and upper divisions of the Ludlow group then come together. The most characteristic fossil is the Pentamerus Kηightii∙ i other common forms are Rhynchonella Wilsoni, Lingula Lewisii, Strophomena euglypha, Bellerophon dilatatus, Pterinea Sowerbyi, with many of the same shells, corals, and trilobites found in the Wenlock limestone. Indeed, as Murchison has pointed out, except in the less number of species and the occurrence of some of the shells more characteristic of the Upper Ludlow zone, there is not much palaeontological distinction between the two limestones.
(c.) Upper Ludlow Rock.— In the original Silurian district described by Murchison, the Aymestry limestone is covered by a calcareous shelly band full of Rhynchonella navicula, sometimes 30 or 40 feet thick. This layer is succeeded by grey sandy shale or mudstone, often weathering into concretions, as in the Lower Ludlow zone, and assuming externally the same rusty-brown or greyish olive-green hue. Its harder beds are quarried for building stone; but the general character of the deposit, like that of the argillaceous portions of the Upper Silurian formations as a whole in the typical district of Siluria, is soft, incoherent, and crumbling, easily decomposing once more into the original mud, and presenting in this respect a contrast to the hard fissile and often slaty shales of the Lower Silurian series. Many of the sandstone beds are crowded with ripple-marks, rill-marks, and annelid-trails, indicative of the shallow littoral waters in which they were deposited. One of the uppermost sandstones is termed the “Fucoid Bed,” from the number of its cylindrical sea-weed-like stems. It likewise contains numerous inverted pyramidal bodies, which are believed to be casts of the cavities made in the muddy sand by the rotatory movement of crinoids rooted and halfburied in the micaceous mud. At the top of the Upper Ludlow rock near the town of Ludlow, a brown layer occurs from a quarter of an inch to 3 or 4 inches in thickness, full of fragments of fish, Pterygotus, and shells. This layer, termed the “Ludlow Bone-bed,” is the oldest from which any considerable number of vertebrate remains has been obtained. In spite of its insignificant thickness it has been detected at numerous localities from Ludlow as far as Pyrton passage, at the mouth of the Severn—a distance of 45 miles from north to south, and from Kington to Ledbury and Malvern—a distance of nearly 30 miles from west to east; so that it probably covers an area (now largely buried under Old Red Sandstone) not less than 1000 square miles in extent, yet it appears never to exceed and usually to fall short of a thickness of 1 foot. Fish remains, however, are not confined to this horizon. They have been detected in strata above the original bone-bed at Ludlow, together with some minute globular bodies believed to be the sporangia of a lycopod. These, with some other plant remains from the same district, are the earliest traces of land vegetation yet found. The higher parts of the Ludlow rock consist of fine yellow sandstone and harder grits known as the Downton sandstone. Originally the whole of these flaggy upper parts of the Ludlow group were called “Tilestones” by Murchison, and being often red in colour were included by him as the base of the Old Red Sandstone, into which they gradually and conformably ascend. Undoubtedly they show the gradual change of physical conditions which took place at the close of the Silurian period in the west of England, and brought in the deposits of the Old Red Sandstone. But as their organic contents are still unequivocally those of the Ludlow group, they are now classed as the uppermost zone of the Silurian system.
A considerable suite of organic remains lias been obtained from the Upper Ludlow rock, which on the whole are the same as those in the zones underneath. Vegetable remains, some of which seem to be fucoids, but most of which are probably terrestrial and lycopodiaeeous, abound in the Downton sandstone and passage-beds into the Old Red Sandstone. Corals, as might be supposed from the muddy character of the deposit, seldom occur, though Murchison mentions that the encrusting form Alveolites fibrosus may not infrequently be found enveloping shells, Cycloηemα corallii and Mτιrchisonia corallii being, as their names imply, its favourite habitats. Some annelides (Serρulites longisρinus, Cornulites serρularius, Tcntaculites tenuis, and. Trachyderma coriacea) are not uncommon. The Crustacea are represented chiefly by small ostracods {Beyrichia Kloedeni, Leρerditia marginata, Entomis tuberosa), and by species of Ccratic-caris, Didyocaris, Eurypterus, Hemiaspis, Pterygotus, and Stylonurus; the trilobites having still further waned, though Homalonotus Knightii, Encrimιrus punctatus, Phacops Downingice, and a few others still occur, and even the persistent Calymene Blumen· bachii may occasionally be found. Of the brachiopods the most abundant forms are Rhynchonella mιcula, Chonetcs striatella, Piscina rugata, and Lingula cornea. The most characteristic lamellibranchs are Orthonota amygdalina, Goniophora cymbceformis, Pterinea lineata, P. retroflexa ; some of the commonest gasteropods are Murchisonia corallii, Platyschisma helicites, and Holopclla obsoleta. The orthoceratites are specifically identical with those of the Lower Ludlow rock, and are sometimes of large size, Grthoceras bullatum being specially abundant. The fish remains consist of bones, teeth, shagreen-like scales, plates, and fin-spines. They include some plagiostomous (placoid) forms (7'helodus, shagreenscales, Sphagodιcs, skin, Onclιus, spines) and some ostracosteans (Cephalaspis, Auchenaspis, and Pteraspis).
Iu the typical Silurian region of Shropshire and tlιe adjacent counties, nothing can be more decided than the lithological evidence for the gradual disappearance of the Silurian sea, with its crowds of graptolites, trilobites, and brachiopods, and for the gradual introduction of those geographical conditions which brought about the deposit of the Old Red Sandstone. The fine grey and olive-coloured muds, with their occasional zones of limestone, are succeeded by bright red clays, sandstones, cornstones, and conglomerates. The evidence from fossils is equally explicit. Up to the top of the Ludlow rocks the abundant Silurian fauna continues in hardly diminished numbers. But as soon as the red strata begin the organic remains rapidly die out, until at last only the fish and the large eurypterid crustaceans continue to occur.
Turning now from the interesting and extremely important though limited area in which the original type of the Upper Silurian rocks is developed, we observe that whether we pass northwards or south-westwards the soft mudstones and thick limestones give way to hard slates, grits, and flagstones, among which it is scarcely possible sometimes even to discriminate what represents the Wenlock from what may be the equivalent of the Ludlow group. It is in Denbighshire and the adjacent counties that this change becomes most marked. The Tarannon shale above described passes into that region of North Wales, where it forms the base of the Upper Silurian formations. It is covered by a series of grits or sandstones which in some places are at least 3000 feet thick. These are covered by and pass laterally into hard shales, which are believed to represent parts of the true Wenlock group, perhaps even some portion of the Ludlow rocks. It is evident, however, that in spite of the wide extent over which these Silurian rocks of North Wales are spread, and the great thickness which they attain, they do not present an adequate stratigraphical equivalent for the complete succession in the original Silurian district. Instead of passing up conformably into the base of the Old Red Sandstone, as at Ludlow, they are covered by that formation unconformably. In fact they have been upturned, crumpled, faulted, and cleaved before the deposition of those portions of the Old Red Sandstone which lie upon them. These great physical changes took place in Denbighshire when, so far as the evidence goes, there was entire quiescence in the Shropshire district; yet the distance between the two areas was not more than about 60 miles. These subterranean movements were doubtless the precursors of those more widely extended upheavals which converted the floor of the Silurian sea into a series of isolated basins, in which the Old Red Sandstone was laid down.
In Westmoreland and Cumberland a vast mass of hard slates, grits, and flags was identified by Sedgwick as of Upper Silurian age. These form the varied ranges of hills in the southern part of the lake district from near Shap to Duddon mouth. The following are the local subdivisions with the conjectural equivalents in Siluria. [table]
In the northern part of the Lake district a great anticlinal fold takes place. The Skiddaw slates arch over and are succeeded by the base of the volcanic series above described. But before more than a small portion of that series has appeared the whole Silurian area is overlapped unconformably by the Carboniferous Limestone series. It is necessary to cross the broad plains of Cumberland and the south of Dumfriesshire before Silurian rocks are again met with. In this intervening tract a synclinal fold must lie, for along the southern base of the uplands of the south of Scotland a belt of Upper Silurian rocks, dipping on the whole to the south-east, can be traced from the heart of the Cheviot Hills to the headlands of Wigtownshire. These rocks must reach a thickness of several thousand feet, but their top is nowhere seen. They repose on some of the older parts of the Llandeilo series, with so close a coincidence of dip and strike that no decided unconformability has yet been traced between them. They consist essentially of shales, with a considerable proportion of greywacke bands towards the base. At different horizons they contain lenticular bands of a calcareous pebbly grit. But their most characteristic feature, and one which at once distinguishes them locally from the adjoining Lower Silurian rocks, is the occurrence of a nearly black, highly fissile shale, composed of layers in most cases as thin as ordinary writing paper and usually crowded with graptolites. These peculiar bands occur throughout the whole series of rocks from bottom to top. They are sometimes so thin that 20 or 30 seams or ribs, each finely fissile, may be seen intercalated within the space of an inch of the ordinary shale or greywacke. Occasionally they form zones 80 to 100 feet thick, consisting entirely of finely leaved graptolitic shales. As a whole these Upper Silurian strata resemble lithologically the corresponding series in Westmoreland, though here and there they assume the character of mudstones not unlike those of Shropshire. The abundant fossils in them are simple graptolites (Graptolithus Sedgwickii, G. Becki, G. Flemingii, G. colonus, G. Griestonensis, Retiolites G eiηitziaηus, <kc.). Orthoceratite3 come next in paint of numbers (Orthoceras annulatum, O. tenuicinctum, &c.). In some of the shales crustacean fragments are numerous. They include large pieces of the carapace of Dictyocams, with remains of Pterygotus and Ceratiocaris. The pebbly grits contain Petraia and crinoid stems. In the south of Kirkcudbright certain limestones and conglomerates intercalated among these shales have yielded a more varied fauna, having on the whole a decidedly Wenlock character. It includes Faυosites, Catenipora, Beyrichia tuberculata, Phacops caudatus, Meristella, Leptaεna sericea, Atrypa reticularis, Strophomena imbrex, Murchisonia, Orthoceras tenuicinctum, &c.
It is impossible in the south of Scotland to separate the Upper Silurian rocks into Wenlock and Ludlow groups. On the whole these rocks seem to be representative mainly of the older half of the Upper Silurian formations. They are covered unconformably by Lower Old Red Sandstone and later formations. In the counties of Edinburgh and Lanark, however, the base of the Lower Old Red Sandstone is found to graduate downward into a thick series of brown,, olive, and grey shales, sandstones, and grits, containing undoubted Ludlow fossils. It is deserving of remark also that the peculiar lithological type so characteristic of the strata in the original Silurian area reappears in the centre of Scotland, many of the concretionary brown shales and olive-coloured mudstones being undistinguishable from those in the typical sections at Ludlow. Some of these beds are crowded with fossils. Among the most characteristic are Legtoena transver sedis, Orthonota amygdalina, Platyschisma helicites, Beyrichia Kloedeηi, Orthoceras Maclareni, with many crustaceans of the genera Ceratiocaris, Eurypterus, Pterygotus, Slimoηia, and Stylonurus. In the Pentland Hills these strata are estimated to attain a thickness of 3500 to 4000 feet, but their base is nowhere reached; in Lanarkshire they are at least as thick. Their lower portions may represent some of the higher parts of the Wenlock group.
Ireland furnishes some interesting evidence regarding the geographical changes in the west of Europe between thp close of the Lower Silurian and the beginning of the Upper Silurian period. It has already been pointed out that the metamorphosed Lower Silurian rocks of the Scottish Highlands are prolonged into the north of Ireland, whence they range south-westwards to Galway Bay. In the picturesque tract between Loch Mask and the mouth of Killary harbour these metamorphosed rocks are unconformably overlaid by masses of sandstones, conglomerates, and shales more than 7000 feet thick, and containing Llandovery and Wenlock fossils with a mixture of Caradoc forms. In the midsj; of the greatly metamorphosed Lower Silurian platform, portions are to be found still little altered and full of fossils. The overlying Upper Silurian strata have not been metamorphosed, but contain pebbles of the altered rocks on the upturned edges of which they lie. It is evident therefore, 03 Mr Hull has remarked, that the metamorphism must have occurred between the close of the Lower and the commencement of the Upper Silurian period. In connexion with this question it should be remarked that abundant volcanic activity accompanied the deposit of these Upper Silurian rocks in the west of Ireland, successive sheets of lava (eurite) and beds of tuff forming conspicuous bands among the stratified rocks, and reaching a collective thickness of 800 feet and upwards. Between Brandon Head and Dingle Bay a thick mass of strata on the coast, must, from the comparatively few fossils obtained from it, be held to represent Upper Silurian formations.
Continental Europe. —The broad hollow which, running from the mouth of the English Channel across the plains of northern Germany into the heart of Russia, divides the high grounds of the north and north-west of Europe from those of the centre and south, separates the European Silurian area into two distinct tracts. In the northern of these we find the Lower and Upper Silurian formations attaining an enormous development in Britain, but rapidly diminishing in thickness towards the north-east, until in the south of Scandinavia and the Gulf of Finland they reach only about 3 ⅛th of that depth. In these tracts, "too, they have on the whole escaped so well from the dislocations, crumplings, and metamorphisms so conspicuous to the south-west, that to this day they remain over wide areas nearly as horizontal and soft as at first. In the southern tract Silurian rocks appear only here and there from amidst later formations, and almost everywhere present proofs of intense subterranean movement.
In the south of Scandinavia (Christiania, Mjosen See, Malmö, Gothland) the Lower and Upper Silurian rocks attain a united thickness of not more than about 1200 feet, yet are said to contain representatives of all the leading subdivisions of the British series. The following table exhibits the Silurian succession in the south of Norway and Sweden, with the supposed English equivalents:— [table] [table]
Though the general resemblance of the succession of fossils in Scandinavia and in Britain is singularly close, there are, as might have been anticipated, differences in the range of species, some forms having appeared earlier or having survived later in the one region than in the other. Thus the Pentamerus oblongus ascends in Scandinavia into rocks full of Wenlock corals, but does not occur in the Wenlock group of Britain. On the other hand, among Scandinavian strata containing such characteristically Lower Silurian genera of trilobites as Asaphus i Trinucleus, and Ogygia, there occur organisms which in Britain are typically Upper Silurian, such as Orthoceras dimidiatum and 0. distans, two fossils of the Ludlow rocks. This is a point of considerable importance in its bearing upon the value of palaeontological evidence in correlating the formations of different countries. It shows that the order of succession found to hold good in one region cannot be rigidly applied to others, and that in such cases it is not from individual species so much as from the general facies of the fossils that we must draw geological parallels. The first appearance and duration of a species have doubtless greatly varied in different regions.
In Russia Silurian rocks must occupy the whole vast breadth of territory between the Baltic and the flanks of the Ural Mountains, beyond which they spread eastwards into Asia. Throughout most of this extensive area they lie in horizontal undisturbed beds, covered over and concealed from view by later formations. Along the flanks of the Urals they have been upheaved, and placed on end or at a high angle against the central portions of that chain, and have been partially metamorphosed into chlorite-schist, micaschist, quartz-rock, and other crystalline masses. But along the southern margin of the Gulf of Finland they appear at the surface as soft clays, sands, and unaltered strata, which, so far as their lithological characters go, might be supposed to be of late Tertiary date, so little have they been changed during the enormous lapse of ages since Lower Palaeozoic time. The great plains between the Ural chain on the east and the rising grounds of Germany on the south-west have thus from a remote geological antiquity been exempted from the terrestrial corrugations winch have affected so much of the rest of Europe. They have been alternately, but gently, depressed as a sea-floor, and elevated into steppes or plains. The following subdivisions have been established by F. Schmidt among the Silurian rocks of north-west Russia :— [table]
[table]
Iu _ the centre and south of Europe by far the most important Silurian area is the basin of Bohemia, so admirably worked out by Μ. Barrande in his great work already cited (p. 323), wherein the formations are grouped as in the subioined table :— [table]
The lower two étages {K, B) correspond probably to some of the older parts of the British Cambrian series, and perhaps in part to still older rocks. Etage C, or the Primordial Zone, is the equivalent of the Upper Cambrian rocks of Wales, possibly also partly of the Arenig series. Etage D, subdivided into five groups (dl, d2, cZ3, cZ4, and (75), appears to be, on the whole, representative of the Lower Silurian formations of the British area, though it is impossible to make the minor subdivisions in the two countries agree. The remaining four formations answer to the English and Welsh Upper Silurian series,—the highest zone of all (H) iudicating by its organic remains the approach of the Devoniau series. °
Small though the area of the Silurian basin of Bohemia is (for it measures only 100 miles in extreme length by 44 miles in its greatest breadth), it has proved extraordinarily rich in organic remains. Μ. Barrande has named and described above 2000 species from that basin alone, the greater number being peculiar to it. Some aspects of its organic facies are truly remarkable. One of these is the extraordinary variety and abundance of its straight and curved cephalopods. Μ. Barrande has determined 18 genera and 2 sub-genera, comprising in all no fewer than 1127 distinct species. The genus Orthoceras alone contains 554 species, and Cyrtoceras has 330. The trilobites likewise appear in great numbers, the same indefatigable explorer having detected as many as 42 distinct genera, comprising 350 species. Of these the most prolific genus is Bronteus, which includes 46 species entirely confined to the 3d fauna or Upper Silurian; Acidaspis has 40 species, of which 6 occur in the 2d and 34 in the 3d fauna. Proëtus also numbers 40 species, which all belong to the 3d fauna, save 2 found in the 2d. Other less prolific but still abundant genera are Dalmanites, Phacops, and llloenus. The 2d fauna, or Lower Silurian rocks, contain in all 32 genera and 127 species of trilobites, while the 3d fauna, or Upper Silurian formations, contain 17 genera and 205 species, so that generic types are more abundant in the earlier and specific varieties in the later rocks.
In Germany Silurian rocks appear in a few detached areas, but present a great contrast to those of Bohemia in their comparatively unfossiliferous character, and the absence of any one continuous succession of the whole SilurLn system. They occur in the Thüringer Wald, where a series of fucoidal-schists (perhaps Cambrian) passes up into slates, greywackes, &c., with Lingula, Discina, Calymcnc, numerous graptolites, and other fossils. These strata (from 1600 to 2000 feet thick) may represent the Lower Silurian formations. They are covered by some graptolitic alum-slates, shales, flinty slates, and limestones {Favositcs Gothlandica, &c. ), which no doubt represent the Upper Silurian, and pass into the base of the Devonian formations. Among the Harz Mountains certain greywackes and shales containing land-plants (lycopods, &c.), trilobites {Dalmanites, &c.), graptolites, &c., are regarded as of intermediate age between true Upper Silurian and Lower Devonian rocks. In the north-west of France an impoverished series of sandstones and slates represents the succession of formations up to the top of the Silurian or base of the Devonian system, and contains a suite of trilobites which indicate more analogy with the Silurian fauna of Bohemia and of Spain than with that of the British Islands. I n the western half of the Spanish peninsula Silurian rocks are found flanking the older schists and crystalline masses, and spreading over a vast area of the tableland. They appear to belong chiefly if not wholly to the lower division of the system, and they include representatives of Barrande’s primordial zone, containing 19 species of organisms, of which 9 are primordial trilobites.
North America. —In the United States and Canada the Silurian formations spread continuously over a vast territory, from the mouth of the St Lawrence southwestwards into Alabama and westwards by the great lakes. They almost encircle and certainly underlie all the later Palaeozoic deposits of the great interior basin. The rocks are most typically developed in the state of New York, where they have been arranged as in the subjoined table. [table]
[table]
The most recent researches of Mr Selwyn and his colleagues of the Geological Survey of Canada have shown that in the so-called Quebec group have been included a number of formations of very different stratigraphical positions. He recognizes in it three distinct groups (1) at the base a group of crystalline schists; (2) a group of sandstones and slates with abundant volcanic rocks, probably Cambrian; and (3) Lower Silurian slates, shales, limestones, quartzites, sandstones, and conglomerates. He objects to the introduction of new names to denote systems of rocks, and thinks that Laurentian, Huronian, Cambrian, and Silurian meet all present requirements.
Devonian and Old Red Sandstone.
In Wales and the adjoining counties of England, where so full a development of the Silurian system was originally discovered and worked out by Murchison, the abundant marine fauna comes to an abrupt close when the red rocks set in at the top of the Ludlow group. From that horizon upwards in the geological series we have to pass through some 10,000 feet or more of barren red sandstones and marls, until we again encounter a copious marine fauna in the Carboniferous Limestone. It is evident that between the disappearance of the Silurian and the arrival of the Carboniferous fauna very great geographical changes must have occurred over the area of Wales and the west of England. The sea must have been excluded from the area, or at least must have been rendered unfit for the existence and development of marine life over the area in question. From the striking contrast between the general facies of life in the Silurian and that in the Carboniferous system it is manifest that the interval between them must have been of long duration.
The geological records of this vast interval are still only very partially unravelled and interpreted. At present the general belief among geologists is that, while in the west and north-west of Europe the Silurian sea-bed was upraised into land in such a way as to enclose large inland basins, in the centre and south-west the geographical changes did not suffice to exclude the sea, which continued to cover more or less completely that region. In the isolated basins of the north-west a peculiar type of deposits termed the Old Red Sandstone is believed to have accumulated, while in the shallow seas to the south and east a series of marine sediments and limestones was formed to which the name of Devonian has been given. It is thus supposed that the Old Red Sandstone and the Devonian represent different geographical areas, with different phases of sedimentation and of life, during the long lapse of time between the Silurian and Carboniferous periods.
That the Old Red Sandstone does really represent this prolonged interval can be demonstrated by innumerable sections in Britain, where its lowest strata are found graduating downward into the top of the Ludlow group and its highest beds are seen to pass up into the base of the Carboniferous system. But the evidence is not everywhere so clear in regard to the true position of the Devonian rocks. That these rocks lie between Silurian and Carboniferous formations is indeed sufficiently clear. But it is a curious fact that where the Lower Devonian beds are best developed the Upper Silurian formations are scarcely to be recognized, or if they occur, they can hardly be separated from the so-called Devonian rocks. It is therefore quite possible that the lower portions of what has been termed the Devonian series may in certain regions to some extent represent what are elsewhere recognized as undoubted Ludlow or even perhaps Wenlock rocks. We cannot suppose that the rich Silurian fauna died out abruptly at the close of the Ludlow epoch. We should be prepared for the discovery of younger Silurian rocks than the latest of those in Britain, such as M. Barrande has shown to exist in his Étage H. The rocks termed Lower Devonian may partly represent some of these later phases of Silurian life, if they do not also mark peculiar geographical conditions of a still older period in Upper Silurian time. On the other hand,, the upper parts of the Devonian system might in several respects be claimed as fairly belonging to the Carboniferous system above.
The late Mr Jukes proposed a solution of the Devonian problem, the effect of which would be to turn the whole of the Devonian rocks into Lower Carboniferous, and to place them above the Old Red Sandstone, which would thus become the sole representative in Europe of the interval between Silurian and Carboniferous time. In the following descriptions an account will first be given of the Devonian type and then of the Old Red Sandstone.
A. Devonian.
This name was first applied by Sedgwick and Murchison to the rocks of North and South Devon and Cornwall, whence a suite of fossils was obtained which Lonsdale pronounced to be intermediate in character between Silurian and Carboniferous. The relations of these strata to Silurian rocks cannot be determined from any section. They pass upward into Carboniferous strata. They have been arranged into three divisions, as in the subjoined table. [table]
The total fauna of the British Devonian rocks numbers about 400 species. The middle group is the chief repository of fossils.
Lower.— Among the lower gritty slates and limestone bands of North Devon there are found, according to the detailed census compiled by Mr Etheridge, 18 species of fossils, comprising Favosites cervicornis, Fenestella antiqua, Orthis arenata, Spirifer cαnαliferus, S. loevicostus, àc. Of these organisms 7 species are also found among the Devonian rocks of the Rhine, Belgium, or France. Mr Etheridge remarks that it is singular that the British Lower Devonian rocks should only have yielded 1 gasteropod {Pleurotomaria aspera), 4 lamellibranchs, 1 cephalopod {Orthoceras gracile), and 1 nucleobranch {Bellerophon bisulcatus). They have furnished only 10 brachiopods. Traces of fish remains have been obtained among them in the form of bones and coprolitic debris. So far as observation has gone not a single Silurian species has been certainly detected in the Devonian rocks of Britain, with, according to Mr Etheridge, the sole exception of the long-lived and universally diffused Atrypa reticularis, which occurs in the Ilfracombe group. There can be no doubt, however, from the meagre list of fossils from the Lower Devonian rocks of Devon and Cornwall, that either the conditions for the existence or those for the fossilization of the early Devonian fauna must have been singularly unfavourable in the south-west of England. It would be exceedingly rash to argue as to the extinction of the Silurian fauna from the unsatisfactory evidence of these rocks.
Middle.— As above remarked, this is the great storehouse of Devonian fossils in the south-west of England. In this fauna, as tabulated by Mr Etheridge, there are 8amorpho-zoans, including 5 species of Stromatopora ; 23 genera and 50 species of coelenterates, among which the corals Acervularia, Alveolites, Cyathophyllum, Favosites, Pleuro-dictynm, and Petraia are conspicuous; 4 genera and 8 species of crinoids {Hexacriηus, Cyathocrinus, &c.): 1 annelide {Tentaculites annulatus); 5 genera and 13 species of crustaceans, which are all trilobites {Phacops granulatus, P. latifrons, Ρ. punctatus, Bronteus jiabellifer, Cheirurus articulatus, Harpes macrocephalus, &c.); and 12 species of polyzoans. The brachiopods are abundant; 68 species have been yielded by the rocks of South Devon, including Athyris concentrica, A. lachryma, Atrypa reticularis, A. desquamata, Camarophoria rhomboidea, Cyrtina Lemarlii, Orthis striatula, Rhynchonella acuminata, R. pugnus, Pentamerus brevirostris, Spirifer disjunctus, Stringocephalus Burtini, Uncites gryphus, &c. The lamellibranchs are poorly represented, 11 genera only occurring, and many of them represented by only 1 species. The gasteropoda are likewise present in but small numbers and variety; 12 genera and 36 species have been enumerated. Of these species, 4 {Acroculia vetusta, Euomphalus loevis, Macrocheilus imbricatus, and Murchisoηia spinosa) survived into the Carboniferous period. The cephalopods are represented by 5 genera, the most abundant specifically being Cyrtoceras and Ortho ceras ; Goηiatites, Clymenia, and Hautilus also occur. Of the total list of fossils a large proportion is found in the Middle Devonian rocks of the continent of Europe. Very few species agree with those of the Silurian or with those of the Carboniferous system.
Upper.— From the calcareous portions of the Pether- ∙win and Pilton beds of Cornwall and Devon a considerable number of fossils has been obtained. Among the more characteristic of these we find 11 species of the coiled cephalopod Clymenia {C. undulata, C. loevigata, C. striata), the trilobites Phacops granulatus and P. latifroηs, the small ostracod Cypridina serrato-striαta, the brachiopods Sρirifer disjunctus or Verneullii, Strophomena rhomboidalis, Chonetes Hardrensis, Productus subaculeatus, and the lamellibranch Cuanloea Harding ii. The Marwood and Baggy Point beds have also yielded traces of land plants, such as Knorria dichotoma and Adiantites Hibernians, the latter fern being common in some parts of the Upper Old Red Sandstone of Ireland.
The higher red and yellow sandy portions of the Upper Devonian rocks shade up insensibly at Barnstaple in North Devon into strata which by their fossils are placed at the base of the Carboniferous Limestone series. But in no other series save the south-western district of Britain can such a passage be observed. In all other places the Carboniferous system, where its true base can be seen, passes down into the red sandy and marly strata of the Upper Old Red Sandstone without marine fossils.
Continental Europe. —Devonian rocks occupy a large area in the centre of Europe, extending from the Ardennes through the south of Belgium across Rhenish Prussia to Darmstadt. They are best known from the picturesque gorges which have been cut through them by the Rhine below Bingen and by the Moselle below Treves. They have been arranged into the following groups in the Eifel region, where their true geological position was first indicated by Sedgwick and Murchison. [table]
This threefold subdivision, with a central mass of calcareous strata, is traceable westwards through Belgium (where the Calcaire de Givet represents the Stringocephalus limestone of the Eifel) and eastwards into the Harz. The rocks reappear with local petrographical modifications, but with a remarkable persistence of general palaeontological characters, in Eastern Thuringia, Franconia, Saxony, Silesia, the north of Moravia, and East Galicia. Devonian rocks have been detected among the crumpled rocks of the Stjτian Alps by means of the evidence of abundant corals, clymenias, gasteropods, lamellibranchs, and other organic remains. Perhaps in other tracts of the Alps, as well as in the Carpathian range, similar shales, limestones, and dolomites, though as yet unfossιli-ferous, but containing ores of silver, lead, mercury, zinc, cobalt, and other metals, may be referable to the Devonian system.
In the centre of Europe, therefore, the Devonian rocks consist of a vast thickness of dark-grey sandy and shaly rocks, with occasional seams of limestone, and in particular with one thick central calcareous zone. These rocks are characterized in the lower zones by numerous broad-winged spirifers and by peculiar trilobites {Ρhαcοps, Ηοmalοηοtus, &c.), which, though generically like those of the Silurian system, are specifically distinct. The central calcareous zone abounds in corals and erinoids as well as in numerous brachiopods. In the highest bands a profusion of coiled cephalopods {Clymenia) occurs in some of the limestones, while the shales are crowded with a small but characteristic ostracod crustacean {Cypridina). Here and there traces of fishes have been found, more especially in the Eifel, but seldom in such a state of preservation as to warrant their being assigned to any definite place in the zoological scale. More recently, however, E. Beyrich has described from Gerolstein in the Eifel an undoubted species of Pterichthys, which, as it cannot be certainly identified with any known form, he names P. Rhenarnιs. A Cοccοsteus has been described by F. A. Roemer from the Harz, and more recently one has been cited from Bicken near Herborn by V. Koenen; but, as Beyrich points out, there may be some doubt as to whether the latter is not a Pterichthys. A Ctcnacanthus, seemingly undistinguishable from the C. Bohémiens of Barrande’s Étage G, has also been obtained from the Lower Devonian “Nereitenschichten” of Thuringia. The characteristic Holoptychius nobilissimus has recently been detected in the Psammite de Condroz, which in Belgium forms a characteristic sandy portion of the Upper Devonian rocks. These are interesting facts, as helping to link the Devonian and Old Red Sandstone types together. But they arc as yet too few and unsupported to warrant any large deduction as to the correlations between these types.
It is in the north-east of Europe that the Devonian and Old Red Sandstone appear to be united into one system, where the limestones and marine organisms of the one are interstratified with the fish-bearing sandstones and shales of the other. In Russia, as was shown in the great work Russia and the Ural Mountains by Murchison, De Verneuil, and Keyserling, rocks intermediate between the Upper Silurian and Carboniferous Limestone formations cover an extent of surface larger than the British Islands. This wide development arises not from the thickness but from the undisturbed horizontal character of the strata. Like the Silurian formations above described, they remain to this day nearly as flat and unaltered as they were originally laid down. Judged by mere vertical depth, they present but a meagre representative of the massive Devonian greywacke and limestone of Germany, or of the Old Red Sandstone of Britain. Yet vast though the area is over which they form the surface rock, it is probably only a small portion of their total extent; for they are found turned up from under the newer formations along the flank of the Ural chain. It would thus seem that they spread continuously across the whole breadth of Russia in Europe. Though almost everywhere undisturbed, they afford evidence of some terrestrial oscillation between the timeof their formation and that of the Silurian rocks on which they rest, for they are found gradually to overlap Upper and Lower Silurian formations.
The chief interest of the Russian rocks of this age lies in the fact, first signalized by Murchison and his associates, that they unite within themselves the characters of the Devonian and the Old Red Sandstone types. In some districts they consist largely of limestones, in others of red sandstones and marls. In the former they present mollusks and other marine organisms of known Devonian species; in the latter they afford remains of fishes, some of which are specifically identical with those of the Old Red Sandstone of Scotland. The distribution of these two palaeontological types in Russia is traced by Murchison to the lithological characters of the rocks, and consequent original diversities of physical conditions, rather tlιan to differences of age. Indeed cases occur where in the same band of rock Devonian shells and Old Red Sandstone fishes lie commingled. In the belt of the formation which extends southwards from Archangel and the White Sea, the strata consist of sands and marls, and contain only fish remains. Traced through the Baltic provinces, they are found to pass into red and green marls, clays, thin limestones, and sandstones, with beds of gypsunu In some of the· calcareous bands such fossils occur as Grthis striatula, Sρiriferina prisca, Leptaena productoidcs, Spirifcr calcaratus, Spir- orbis omphaloides, and Orthoccras subfusiforme. In the higher beds Holoptychius an 1 other well-known fishes of the Old Red Sandstone occur. Followed still further to the south, ns far as the watershed between Orel and Woronesch, the Devonian rocks lose their red colour and sandy character, and become thin-bedded yellow limestones, and dolomites with soft green and blue marls. Traces of salt deposits are indicated by occasional saline springs. It is evident that the geographical conditions of the Russian area during the Devonian period must have closely resembled those of the Rhine basin and central England during the Triassic period.
The Russian Devonian rocks have been classified as folloλvs :— [table]
There is an unquestionable passage of the uppermost Devonian rocks of Russia into the base of the Carboniferous system. The Devonian rocks of North America are noticed at the end of the next section.
B. Old Ped Sandstone.
Under this name is comprised a vast and still imperfectly described series of red sandstones, shales, and conglomerates, intermediate in age between the Ludlow rocks of the Upper Silurian formations and the base of the Carboniferous system in Britain. These rocks were termed “Old” to distinguish them from a somewhat similar series overlying tho Coal-measures, to which the name New Red Sandstone was applied. When the term Devonian was adopted, it speedily supplanted that of Old Red Sandstone, inasmuch as it was founded on a type of marine strata of wide geographical extent, whereas the latter term described what appeared to be merely a British and local development. For the reasons already given, however, it is desirable to retain the title Old Red Sandstone as descriptive of a remarkable suite of formations to which there is nothing analogous in typical Devonian rocks. It is in Great Britain that the Old Red Sandstone of. Europo is almost entirely developed. This interesting series of deposits must from the first have been deposited in separate areas or basins, the sites of some of which can still be traced. Their diversities of sediment and discrepance of organic contents point to the want of any direct communication between them. It was maintained many years ago by Mr Godwin Austen, and has been more recently enforced by Γrofessor Ramsay, that these basins were lakes or inland seas. The character of the strata, the absence of unequivocally marine fossils, the presence of landplants and of numerous ganoid fishes which have their modern representatives in rivers and lakes, suggest and support this opinion, which has been generally adopted by geologists. The red arenaceous and marly beds which, with their fish remains and land-plants, occupy a depth of many thousand feet between the top of the Upper Silurian and the base of the Lower Carboniferous formations, are regarded as the deposits of a series of lakes or inland seas formed by the uprise of portions of the Silurian sea-floor. The length of time during which these lacustrine basins must have existed is shown, not only by the thickness of the deposits formed in them, but by the complete change which took place in the marine fauna between the close of the Silurian and the commencement of the Carboniferous period. The prolific fauna of the Wenlock and Ludlow rocks was extirpated over the British area by the physical changes which produced the lake-basins of the Old Red Sandstone. When a marine population—crinoids, corals, and shells— once more overspreads the area it is found to be completely different. So thorough a change must have demanded a long interval of time.
Murchison, who strongly advocated the opinion that the Old Red Sandstone and Devonian rocks represented different geographical conditions of the same period, and who had with satisfaction seen the adoption of the Devonian classification by Continental geologists, endeavoured to trace in the Old Red Sandstone of Britain a threefold division like that which had been accepted for the Devonian system. He accordingly arranged the formations as in the subjoined table. [table]
It is important to observe that in no district can these three subdivisions be found together, and that the so-called “middle” formation occurs only in one region—the north of Scotland. The classification, therefore, does not rest upon any actually ascertained stratigraphical sequence, but on an inference from the organic remains. The value of this inference will be estimated a little further on. All that can be affirmed from stratigraphical evidence in any Old Red Sandstone district in Britain is that a great physical and palaeontological break can generally be traced in the Old Red Sandstone, dividing it into two completely distinct series—a Lower, which graduates downward into the Upper Silurian, and an Upper, which passes upward into the base of the Carboniferous system.
As a whole, the Old Red Sandstone, where its strata are really red, is like other masses of red deposits, singularly barren of organic remains. The physical conditions under which the precipitation of iron oxide took place waere evidently unfavourable for the development of animal life in the same waters. Professor Ramsay has connected the occurrence of such red formations with the existence of salt lakes, from the bitter waters of which not only iron-oxide but often rock-salt, magnesian limestone, and gypsum were thrown down. He points also to the presence of landplants, footprints of amphibia, and other indications of terrestrial surfaces, while truly marine organisms are either found in a stunted condition or are absent altogether. Where the strata of the Old Red Sandstone, losing their red colour and ferruginous character, assume grey or yellow tints and pass into a calcareous or argillaceous condition, they not infrequently become fossiliferous. At the same time it is not unworthy of remark that some of the red conglomerates, which might be supposed little likely to contain organic remains, are occasionally found to be full of detached scales, plates, and bones of fishes.
Along the border of the Silurian region from Shropshire into South Wales the uppermost parts of the Silurian system graduate into a mass of red strata not less than 10,000 feet thick, which in turn pass up conformably into the base of the Carboniferous system. This vast accumulation of red rocks, termed the Old Red Sandstone, consists in its lower portions of red and green shales and flagstones, with some white sandstones and thin cornstones; in the central and chief division, of red and green spotted sandy marls and clays, with red sandstones and cornstones; in the higher parts of grey, red, chocolate-coloured, and yellow sandstones, with bands of conglomerate. No unconformability has yet been detected in any part of this series of rocks, though, from the observations of De la Beche, it may be suspected that the higher strata which graduate upward into the Carboniferous formations are separated from the underlying portions of the Old Red Sandstone by a distinct discordance.
Although, as a whole, barren of organic remains, these red rocks have here and there, more particularly in the calcareous zones, yielded fragments of fishes and crustaceans. In their lower and central portions remains of the ganoids Ceρlιalaspis, Dûlymaspis, Scaphaspis, Pteraspis, and Cyathaspis have been found, together with crustaceans of the genera Stylomιrus, Pterygotus, and Prearcturus, and obscure traces of plants. The upper yellow and red sandstones contain none of the cephalaspid fishes, which are there replaced by Pterichthys and Iloloptychius, associated with distinct impressions of land-plants. In some of the higher parts of the Old Red Sandstone of South Wales and Shropshire, Serpula aud Conularia occur; but these are exceptional cases, and point to the advent of the Carboniferous marine fauna, which doubtless existed outside the British area before it spread over the Old Red Sandstone basins.
It is in Scotland that the Old Red Sandstone shows the most complete and varied development, alike in physical structure and in organic contents. Throughout that country the system is found everywhere to present a division into two well-marked groups of strata, separated from each other by a strong unconformability and a complete break in the succession of organic remains. It occurs in distinct areas which appear to mark the site of separate basins of deposit. One of these occupies the central valley between the base of the Highland mountains and the uplands of the southern counties. On the north-east it is cut off by the present coast-line from Stonehaven to the mouth of the Tay. On the south-west it ranges by the island of Arran across St George’s Channel into Ireland, where it runs almost to the western sea-board, flanked on the north, as in Scotland, by hills of crystalline rocks and on the south chiefly by a Lower Silurian belt. Another distinct and still larger basin lies on the north side of the Highlands, but only a portion of it comes within the present area of Scotland. It skirts the slopes of the mountains along the Moray Firth and the east of Ross and Sutherland, and stretches through Caithness and the Orkney Islands as far as the south of the Shetland group. It may possibly have been at one time continued as far as the Sognefjord and Dalsfjord in Norway, where red conglomerates like those of the north of Scotland occur. There is even reason to infer that it may have ranged eastwards into Russia, for some of its most characteristic organisms are found also among the red sandstones of that country. A third minor area of deposit lay on the south side of the southern uplands over the east of Berwickshire and the north of Northumberland, including the area of the Cheviot Hills. A fourth occupied a basin on the flanks of the south-west Highlands, which is now partly marked by the terraced hills of Lome. There is sufficient diversity of lithological and palaeontological characters to show that these several areas were distinct basins, separated both from each other and from the sea.
In the central basin of Scotland between the Highlands and the southern uplands, the twofold division of the Old Red Sandstone is typically seen. The lower series of deposits attains a maximum depth of upwards of 20,000 feet. In Lanarkshire it is found to pass down conformably into the Upper Silurian rocks; elsewhere its base is concealed by later formations, or by the unconformability with which different horizons rest upon the older rocks. It is covered unconformably by every formation younger than itself. It consists of reddish-brown or chocolate-coloured, grey, and yellow sandstones, red shales, grey flagstones, coarse conglomerates, and occasional bands of limestone and cornstone. The grey flagstones and thin grey and olive shales and “calmstones” are almost confined to Forfarshire, in the north-east part of the basin, aud are known as the Arbroath flags. One of the most marked lithological features in this central Scottish basin is the prodigious masses of interbedded volcanic rocks. These, consisting of porphyrite-lavas, felsites, and tuffs, attain a thickness of more than 6000 feet, and form important chains of hills, as in the Pentland, Ochil, and Sidlaw ranges. They lie several thousand feet above the base of the system, and are regularly interstratified here and there with bands of the ordinary sedimentary strata. They point to the outburst of numerous volcanic vents along the lake or inland sea in which the Lower Old Red Sandstone of central Scotland was laid down; and their disposition shows that these vents ranged themselves in lines'or linear groups parallel with the general trend of the great central valley. The fact that the igneous rocks are succeeded by thousands of feet of sandstones, shales, and conglomerates, without any intercalation of lava or tuff, proves that the volcanic episode in the history of the lake came to a close long before the lake itself disappeared.
As a rule the deposits of this lake are singularly unfos-siliserous, though some portions of them, particularly in the Forfarshire flagstone group, have proved rich in fish remains. In Lanarkshire about 5000 feet above the base of the system a thin band of shale occurs, containing a graptolite, Spir orbis Lewisit, and Orthocer as dimidiatum, — undoubtedly Upper Silurian forms. This interesting fact serves to indicate that, though geographical changes had elevated the Upper Silurian sea-floor partly into land and partly into isolated inland water-basins, the sea outside still βontained an Upper Silurian fauna, which was ready on any favourable opportunity to re-enter the tracts from which it bad been excluded. The interval of its reappearance seems to have been very brief, however, for the band of shale containing these Upper Silurian marine organisms is only a few inches thick, and the fossils have not been detected on any other horizon. With these exceptions, the fauna ®f the formation consists entirely of fishes and crustaceans. Nine or mare species of crustaceans have been obtained, «hiefly eurypterids, but including one or two phyllopods. The large pterygotus (P. Anglicus) is especially characteristic, and must have attained a great size, for some of the individuals indicate a length of 6 feet with a breadth of 1 ⅛ feet. There occur also a smaller species (P. minor), two Eurypteri, three species of Stylonurus, and abundant clusters of crustacean egg-packets termed Parkia decipiens. Seventeen species of fishes have been obtained, chiefly from the Arbroath flags. They belong to the suborders Acanthodidae and Ostracostei. One of the most abundant forms is the little Acanthodes Mitchelli. Another common fish is Diplacanthus gracilis. There occur also C limatius seidiger, C. reticulatus, and C. uncinatus, Parexus incurvus, Euthacanthus (four species), Cephalasqñs Lyellii, and Pteraspis Mitchelli. Some of the sandstones and shales are crowded with indistinctly preserved vegetation, occasionally in sufficient quantity to form thin laminae of coal. In Forfarshire the surfaces of the shaly flagstones are now and then covered with linear grass-like plants like the sedgy vegetation of a lake or marsh. In Perthshire certain layers occur chiefly made up of compressed stems of Psilophyton. The adjoining land· was doubtless clothed with a flora in large measure lycopodiaceous.
The Old Red Sandstone of the northern basin is typically developed in Caithness, where it consists chiefly of the well-known dark-grey bituminous and calcareous flagstones of commerce. It rests unconformably upon the metamorphosed Lower Silurian schists, and must have been deposited on the very uneven bottom of a sinking basin, seeing that occasionally even some of the higher platforms are found resting against the schists and granites. The lower zones consist of red sandstones and conglomerates which graduate upward into the flagstones. Other red sandstones, however, supervene in the higher parts of the system. The total depth of the series in Caithness has been estimated at upwards of 16,000 feet.
Murchison was the first to attempt the correlation of the Caithness flagstones with the Old Red Sandstone of the rest of Britain. Founding upon the absence from these northern rocks of the characteristic cephalaspidean fishes of the admitted Lower Old Red Sandstone of the south of Scotland and of Wales and Shropshire, upon the presence of numerous genera of fishes not known to occur in the true Lower Old Red Sandstone, and upon the discovery of a Pterygotus in the basement red sandy group of strata, he concluded that the massive flagstone series of Caithness could not be classed with the Lower Old Red Sandstone, but must be of younger date. He supposed the red sandstones, conglomerates, and shales at the base, with their Pterygotus, to represent the true Lower Old Red Sandstone, while the great flagstone series with its distinctive fishes was made into a middle division answering in some of its ich-thyolitic contents to the Middle Devonian rocks of the Continent. This view has been accepted everywhere by geologists. Recently, however, it has been called in question by Professor Geikie, who gives reasons for maintaining the Caithness flagstones to be Lower Old Red Sandstone, and for denying the existence of any middle division. He shows that the discrepance in organic contents between the Caithness and the Arbroath flags is by no means so strong as Murchison supposed, but that several species are common to both. In particular, he finds that the characteristically Lower Old Red Sandstone and Upper Silurian crustacean genus Pterygotus occurs, not merely in the basement zone of the Caithness flags, but also high up in the series. The genera Acanthodes and Diρlacanthus are abundant both in Caithness and in Forfarshire. Parexus incurvus occurs in the northern as well as the southern basin. It is contended that the palaeontological distinctions are not greater than the striking lithological differences between the strata of the two regions would account for, or than the contrast between the ichthyic faunas of contiguous water-basins at the present time.
Somewhere about 60 species of fishes have been obtained from the Old Red Sandstone of the north of Scotland. Among these the genera Acanthodes, Aster oleρis, Cheiracan-thus, Cheirolepis, Coccosteus, Diplacanthus, Diplopterus, Diρterus, Glyptolepis, Osteolepis, and Pterichthys are specially characteristic. Some of the shales are crowded with the little ostracod crustacean Estheria membranacea. Landplants abound, especially in the higher groups of the flagstones, where forms of Psilophyton, Lepidodendron, Stigmaria, Sigillaria, Calamite, and C i yclopteris, as well as other genera, occur. In the Shetland Islands traces of abundant contemporaneous volcanic rocks have been observed, which, with the exception of two trifling examples in the region of the Moray Firth, are the only known instances of volcanic action in the Lower Old Red Sandstone of the north of Scotland. In the other two Scottish basins, those of lhe Cheviot Hills and of Lome, volcanic action continued long vigorous, and produced thick piles of lava like those of the central basin above referred to.
The Upper Old Red Sandstone consists in Scotland of red sandstones, clays or marls, conglomerates, and breccias, the sandstones sometimes becoming yellow or even white. These strata, wherever their stratigraphical relations can be distinctly traced, lie unconformably upon the lower division of the system, and pass up conformably into the Carboniferous rocks above. If they are studied from the side of the underlying formation, they seem naturally to form part of the Old Red Sandstone, since they agree with it in general lithological character and also in containing some distinctively Old Red Sandstone genera of fishes, such as Pterichthys and IIolopty chius. But, approached from the upper or Carboniferous direction, they appear rather to form the natural sandy base of that system into which they insensibly graduate. On the whole, they are remarkably barren of organic remains, though in one locality—Dura Den in Fife—they have yielded a number of genera and species of fishes, crowded profusely through the pale sandstone as if the individuals had been suddenly killed and rapidly covered over with sediment. Among the characteristic organisms of the Scottish Upper Old Red Sandstone are Pteιichthys major, Holoptychius nobilissimus, II. Andersoni, Glyptopomus, Glyptolaemus, and Phaneropleuron.
An interesting fact deserves mention here as a corollary to what has been stated above regarding the survival for some time of an Upper Silurian fauna outside the area of the British Old Red Sandstone lakes. In the Upper Old Red Sandstone of the basin of the Firth of Ciyde, Ptericlιthys major and Holoptychvus occur at the Heads of Ayr, while a band of marine limestone lying in the heart of the red sandstone series in Arran is crowded with ordinary Carboniferous Limestone shells, such as Productus giganteus, P. semireticulatus, P. punctatus, Chonetes Hard- τensis, Spirifer lineatus, &c. None of these fossils has been detected in the great series of red sandstones overlying the limestone. They do not reappear till the limestones at the base of the Carboniferous series; yet the organisms must have been living during all that long interval outside of the Upper Old Red Sandstone area. Not only so, but they must have been in existence long before the formation of the thick Arran limestone, though it was only during the comparatively brief interval represented by that limestone that geographical changes permitted them to enter the Old Red Sandstone basin and settle for a while on its floor. Thus we see that while, on the one hand, the older parts of the Lower Old Red Sandstone were coeval with an Upper Silurian fauna which, having disappeared from the area of Britain, survived outside of that area, on the other hand, the higher parts of the Upper Old Red Sandstone were contemporaneous with a Carboniferous Limestone fauna which, having appeared beyond the British area, was ready to spread over it as soon as the conditions became favourable for the invasion. It is, of course, obvious that such an abundant and varied fauna as that of the Carboniferous Limestone cannot have come suddenly into existence at the period marked by the base of that formation. It must have had a long previous existence outside the present area of the deposits. But it is seldom that we obtain such clear evidence of the fact as in these instances from the Scottish Old Red Sandstone.
In the north of Scotland, on the lowlands bordering the Moray Firth, and again in the island of Hoy, one of the Orkney group, yellow and red sandstones, sometimes containing characteristic Upper Old Red Sandstone fishes, are found lying unconformably upon the Caithness flags. In these northern tracts the same relation is thus traceable as in the central counties between the two divisions of the system.
Turning southward across the border districts, we trace the red sandstones and conglomerates of the Upper Old Red Sandstone lying unconformably on Silurian rocks and Lower Old Red Sandstone. Some of the brecciated conglomerates have much resemblance to glacial detritus, and it has been suggested that they have been connected with contemporaneous ice-action. Such are the breccias of the Lammermuir Hills, and those which show themselves here and there from under the overlying mass of Carboniferous strata which flanks the Silurian hills of Cumberland and Westmoreland. Red conglomerates and sandstones appear interruptedly at the base of the Carboniferous rocks even as far as Flintshire and Anglesea. They are commonly classed as Old Red Sandstone, but merely from their position and lithological character. No organic remains have been found in them. They may therefore, in part at least, be taken as the basement beds of the Carboniferous system.
In Devonshire, at Barnstaple, Pilton, Marwood, and Baggy Point, certain sandstones, shales, and limestones (already referred to in the account of the Devonian rocks) graduate upward into the base of the Carboniferous system, and appear to represent the Upper Old Red Sandstone of the rest of Britain. They contain land plants and also many marine fossils, some of which are common Carboniferous forms. They thus indicate a transition into the geographical conditions of the Carboniferous period, as is still more clearly illustrated by the corresponding strata in Scotland.
North America. —The Devonian system, as developed in the northern States, and eastern Canada and Nova Scotia, presents much geological interest in the union which it contains of the same two distinct petrographical and biological types found in Europe. If we trace the range of these rocks along the Alleghany chain through Pennsylvania into New York, we find them to contain a characteristic suite of marine organisms comparable with those of the Devonian system of Europe. But on the eastern side of the great range of Silurian hills iu the north-eastern States, we encounter in New Brunswick and Nova Scotia a succession of red and yellow sandstones, limestones, and shales nearly devoid of marine organisms, yet full of land-plants, and with occasional traces of fish remains.
The marine or Devonian type has been grouped in the following subdivisions by the geologists of New York:— [table]
In the Lower Devonian series traces of terrestrial plants {Psilo∙ phyton, Caulopteris, &c.) have been detected even as far west as Ohio. Corals (cyathoρhylloid forms, with Favorites, Syringopora, &c.) abound, especially in the Comiferous Limestone, which is perhaps the most remarkable mass of coral-rock in the American Palaeozoic series. Among the brachiopods are species of Pentamerus, Stricklaτιdinia, Rhynchonella, and others, with the characteristic European form Spirifer cultrijugatus, and the world-wide Atryj>a reticularis. The trilobites include the genera Dalmanites, Proëtus, and Phacops. The earliest known traces of American fishes occur in the Corniferous group. They consist of ichthyodorulites, and teeth of cestraciont and hybodont placoids, and plates, bones, and teeth of some peculiar ganoids Ctfacropctalichthys, Onychodus).
In the Hamilton formation (embracing the Maróellus shale, the Hamilton beds, and the Genesee shale) remains of land-plants occur, but much less abundantly than among the rocks of New Brunswick. Brachiopods are especially abundant among the sandy beds in the centre of the formation. They comprise, as in Europe, many broad-winged spirifers (S. mucronahιs, &c.), with species of Productus, Chonetes, Athyris, &c. The earliest American goniatites have been noticed in these beds. Newberry has described a gigantic fish {Dinichthys) from the Black Shale of Ohio.
The Portage and Chemung groups have yielded land-plants and fucoids, also some crinoids, numerous broad-winged spirifers, with Aviculae, and a few other lamellibranchs. These strata consist in the New York region of shales and laminated sandstones, which attain a maximum thickness there of upwards of 2000 feet, but die out entirely towards the interior. They are covered by a mass of red sandstones and conglomerates—the Catskill group, which is 2000 or 3000 feet deep in the Catskill Mountains, and thickens along the Appalachian region to 5000 or 6000 feet. Those red arenaceous rocks bear a striking similarity in their lithological and biological characters to the Upper Old Red Sandstone of Europe. As a whole they are unfossiliferous, but they have yielded some ferns like those of the Upper Old Red Sandstone of Ireland and Scotland (Cyclopteris), and some characteristic genera of fish, as Holoptyckius and P>otkriolepis.
Tuniing now to the eastern side of the ancient Laurentian and Silurian ridge, which, stretching southwards from Canada, separated in later Palaeozoic time the great interior basin from the Atlantic slopes, w e find the Devonian rocks of New York, Pennsylvania, and the interior represented in New Brunswick and Nova Scotia by a totally different series of deposits. The contrast strikingly recalls that presented by the Old Red Sandstone of the north of Scotland and the Devonian rocks of North Germany. On the south side of the St Lawrence the coast of Gaspé shows rocks of the Quebec group unconformahly overlaid by grey limestones with green and red shales, attaining, according to Logan, a total thickness of about 2000 feet, and replete in some bands with Upper Silurian fossils. They are conformably followed by a vast arenaceous series of deposits termed the Gaspé Sandstones, to which the careful measurements of Logan and his colleagues of the Canadian Geological Survey assign a depth of 7036 feet. This formation consists of grey and drab-coloured sandstones, with occasional grey shales and bands of massive conglomerate. Similar rocks reappear along the southern coast of New Brunswick, where they attain a depth of 9500 feet, and again on the opposite side of the Bay of Fundy. The researches chiefly of Dr J. W. Dawson have shown that these strata contain an abundant terrestrial flora —the oldest of which any relics have yet been recovered, for the few Upper Silurian land-plants at present known hardly deserve to be reckoned as a known flora. In his recent census he enumerates no fewer than 118 species of land-plants. They are almost all acrogens, the lycopods and ferns being largely predominant. Among the distinctive forms the following may be mentioned— Psilopkyton, Artkrostigma, Leptopkleum, and Prototaxitcs. Forty-nine ferns are given, including the genera Cycloptcris, Ncuropteris; Sphenopteris, and some treeferns (Psaronms, Caulopteris). Lepi-dodendroid and sigillaroid plants abound, as well as calamites. Higher foπns of vegetation are represented by a few conifers (Dadoxylon, Ormoxylon, Prototaxites, &c.). From a locality on Lake Erie, Dr Dawson describes a fragment of dicotyledonous wood, not unlike that of some modem trees—the most ancient fragment of an angiospermous exogen yet discovered. So abundant are these vegetable remains that in some layers they actually form thin seams of coal.
The interest of these remains of the most venerable American forests is heightened by the discovery of the fact that they were not without the hum of insect life. The most ancient known relics of insect forms have been recovered from the Devonian strata of New Brunswick. They are all neuropterous wings, and have been referred by Mr Scudder of Boston to four species combining a remarkable union of characters now found in distinct orders of insects. In one fragment he observed a structure which he could only compare to the stridulating organ of some male Orthoptera. Another wing indicates the existence of a gigantic Ephemera, with a spread of wing extending to 5 inches. In the shallow pools of the period some small crustaceans lived, the remains of which have been partially preserved. Among these is a minute, shrimp-like Eurypterus, and a more highly organized form named Amphipcltis. That the sea had at least occasional access to the inland basins into which this abundant terrestrial vegetation was washed is proved by the occurrence of marine organisms, such as a small annelid (Spirorbis} adhering to the leaves of the plants, and (in Gaspé and Nova Scotia) by the occasional appearance of brachiopods, especially Lingula, Spirifer, and Chonetes. Carboniferous.
This great system of rocks has received its name from the seams of coal which form one of its distinguishing features both in Europe and in North America. In Europe it is most completely developed in the British Islands. Elsewhere on that continent it occurs in patches, either lying in hollows of older rocks, or exposed by the removal of overlying formations.
Great Britain. —The area within which the Carboniferous rocks of Britain occur is sufficiently extensive to contain more than one type of the system, and thus to cast much light on the varied geographical conditions under which these rocks were accumulated. In prosecuting the study of this part of British geology, it is soon discovered, and it is essential to bear in mind, that, during the Carboniferous period, the land whence the chief supplies of sediment were derived rose mainly to the north and north-west, as it seems to have done from very early geological time. While therefore the centre and south of England lay under clear water of moderate depth, the north of the country and the south of Scotland were covered by shallow water, which was continually receiving sand and mud from the adjacent northern land. Hence vertical sections of the Carboniferous formations of Britain differ greatly according to the districts in which they are taken. The subjoined table may be regarded as expressing the typical subdivisions which can be recognized, with modifications, in all parts of the country. [table]
In the article Coal (vol. vi. p. 49) an account has been given of the principal coal-fields of the world; likewise a diagram (p. 48) representing the chief subdivisions of the Carboniferous system in Britain, as the rocks are traced from north to south.
Base of the System.— In the south-west of England, and in South Wales, the Carboniferous system passes down conformably into the Old Red Sandstone. The passage beds consist of yellow, green, and reddish sandstones, of green, grey, red, blue, and variegated marls and shales, sometimes full of terrestrial plants. They are well exposed on the Pembrokeshire coasts, marine fossils being there found even among the argillaceous beds at the top of the Red Sandstone series. They occur with a thickness of about 500 feet in the gorge of the Avon near Bristol, but show less than half that depth about the Forest of Dean. At their base there lies a bone-bed containing abundant palatal teeth. Not far above this horizon plant-bearing strata are found. Hence these rocks bring before us a mingling of terrestrial and marine conditions. In Yorkshire, near Lowther Castle, Brough, and in Ravenstonedale, alternations of red sandstones, shales, and clays, containing Stigmaria and other plants, occur in the lower part of the Carboniferous Limestone. Along the eastern edge of the Silurian hills of the Lake district the Old Red Sandstone appears here and there, and passes up through a succession of red and grey sandstones, and green and red shales and marls, with plants, into the base of the Carboniferous Limestone.
It is in Scotland, however, that this peculiar type of the basement Carboniferous rocks is best seen. In that country the lowest subdivision of the Carboniferous system, known as the Calciferous Sandstones, consists of red, white, and yellow sandstones, blue, grey, green, and red marls or clays, blue and black shales, thin coals, seams of limestone and cement-stone, and abundant volcanic rocks. The red sandstones lie at the base, and pass down into the Upper Old Red Sandstone, in which, as has been already pointed out (ante, p. 344), true Upper Old Red Sandstone fishes are found, while there occur also bands of limestone full of true Carboniferous Limestone corals and brachiopods. Hence it is evident that the Carboniferous Limestone fauna had already appeared outside the British area before the close of the Old Red Sandstone period. It was when the peculiar geographical conditions which prevailed during that period finally ceased, and the sea began to spread over the ancient lakes and land of Britain, that the abundant Carboniferous fauna invaded the area. The Calciferous Sandstones of Scotland may therefore represent a portion of the Carboniferous Limestone of England.
Over the greater part of the south and centre of Scotland the lower red sandstones are surmounted by a series of contemporaneous volcanic rocks. Successive sheets of porphyrites and tuffs form long ranges of hills from Arran and Bute on the west to the mouth of the estuary of the Forth on the east, and from the Campsie Fells on the north to the heights of Liddesdale and the English border. These volcanic sheets sometimes reach a thickness of 1500 feet. That they belong to the Carboniferous system is shown by the occurrence of shales and sandstones (with Carboniferous plants) at their base. They show that the early part of the Carboniferous period in Scotland was marked by a prodigious volcanic activity, which, on its cessation, was followed by the prolonged subsidence required for the accumulation of the Carboniferous system. The rocks succeeding the volcanic zone are termed the cement-stone group. In Berwickshire and the west of Scotland they consist of thin bedded white, yellow, and green sandstones, grey, green, blue, and red clays and shales, with thin bands of a pale argillaceous limestone or cement-stone. Seams of gypsum occasionally appear. These strata are, on the whole, singularly barren of organic remains. They seem to have been laid down with great slowness, and without disturbance, in enclosed basins, which were not well fitted for the support of animal life, though fragmentary plants serve to show that the adjoining slopes were covered with vegetation.
In the basin of the Firth of Forth, however, the group presents different lithological aspect and is abundantly fossiliferous. It íhere usually consists of yellow, grey, and white sandstones, with blue and black shales, clay-ironstones, limestones, “cement-stones,” and occasional seams of coal. The sandstones form excellent building stones, the city of Edinburgh having been built of them. Some of the shales are so bituminous as to yield, on distillation, from 30 to 40 gallons of crude petroleum to the ton of shale; they are consequently largely worked for the manufacture of mineral oils. The limestones are usually dull, yellow, and close grained, in seams seldom more than a few inches thick, and graduate by addition of carbonate of iron into cement-stone; but occasionally they swell out into thick lenticular masses like the well-known limestone of Burdie House, so long noted for its remarkable fossil fishes. This limestone appears to be mainly made up of the crowded cases of a small ostracod crustacean {Lcpcrditia Oke r ni, var. Scoto∙Burdigal∙ cnsis'). The coal-seams are few and commonly too thin to be workable, though one of them, known as the Houston coal, has been mined to some extent in Linlithgowshire. The fossils of the cement-stone group indicate an alternation of fresh or braekishwater and marine conditions. They include numerous plants, of which the most abundant are Sphenopteris affinis, Lepidodendron (two or three species), Lcpidostrobus variabilis, Åraucarioxylon. Some of the shales near Edinburgh have afforded a few specimens of a tine monocotyledon allied to the modern Pothos (skothocites Grantonτ). Ostracod crustaceans, chiefly the Leperditia above mentioned, crowd many of the shales. With these are usually associated abundant traces of the presence of fish, either in the form of coprolites or of scales, bones, plates, and teeth. The following are characteristic species: Elonichthys striolatus, E. Robisoni, Rhadinichthys ornatissimus, Nematoptychius Greenockii, Eurynotus arenatus, Rhizodus Hibberti, Megalichthys sp., Gyracanthus tuber- calatus, Ctenoptychius pectinatus. At intervals throughout the group marine horizons occur, usually as shale bands marked by the presence of such distinctively Carboniferous Limestone species as Spirorbis carbonarius, Discina nitida, Lingula squamiformis, Bellerophon decussatus, and Orthoceras cylindraceum.
One of the most interesting features in the cement-stone group of the basin of the Firth of Forth is the prodigious number and variety of the associated volcanic masses, and the proofs which they exhibit that, at the time when that group of strata was accumulating, the region of shallow lagoons, islets, and coal-growths was dotted over with innumerable active volcanic vents. The eruptions continued into the time of the Carboniferous Limestone, but ceased before the deposition of the Millstone Grit. The lavas are chiefly varieties of basalt-rocks, sometimes coarsely crystalline and even granitoid in texture, and graduating through intermediate stages to true close-grained compact basalts, which neither externally nor in microscopic structure differ from basalt of Tertiary date
The basement group of the Carboniferous system in Ireland is evidently a prolongation of the Scottish cement-stone group. In the south of this island, however, a very distinct and peculiar development of the Lower Carboniferous rocks is to be remarked. Between the top of the Old Red Sandstone and the base of the Carboniferous Limestone there occurs in the county of Cork an enormous mass (fully 5000 feet) of black and dark-grey shales, impure limestones, and grey and green grits and true cleaved slates. To these rocks the name of Carboniferous Slate was given by Griffith. They contain numerous Carboniferous Limestone species of brachiopods, echinoderms, &c., as well as traces of land-plants in the grit bands. Great though their thickness is in Cork, they rapidly change their lithological character, and diminish in mass as they are traced away from that district. In the almost incredibly short space of 15 miles, the whole of the 5000 feet of Carboniferous Slate of Bantry Bay have disappeared, and at Kenmare the Old Red Sandstone is followed immediately and conformably by the Limestone with its underlying shale. Mr
Jukes held that the Carboniferous Slate is the equivalent of part of the Devonian rocks of Devon and Cornwall.
Carboniferous Limestone.— The Lower Limestone shale is overlaid conformably by a thick mass of limestone, one of the most distinctive members of the British Carboniferous system. On referring to a geological map of England it will be seen that from Northumberland southwards to the low plains in the centre of England there runs a ridge of high ground, formed by a great anticline, along which the Carboniferous Limestone appears at intervals from underneath higher members of the system. In this northern Carboniferous area, of which the axis is known as the Pennine Chain, the limestone attains its maximum development. In one portion of the district it reaches a depth of 4000 feet, and yet its actual base is nowhere seen. This Pennine region appears to have been the area of maximum depression during the early part of the Carboniferous period in Britain. Traced towards the south west, the limestone diminishes to sometimes not more than 500 feet in South Wales. Northwards, losing its character as a massive calcareous formation, it is split up by intercalations of sandstone, shale, coal, &c., until actual limestone becomes a very subordinate member of the series in central Scotland.
In the Carboniferous areas of the south-west of England and South Wales, the limits of the Carboniferous Limestone are well defined by the Limestone Shale below, and by the Farewell Rock or Millstone Grit above. In the Pennine area, however, the massive limestone is succeeded by a series of shales, limestones, and sandstones, known as the Yoredale group. These cover a large area and attain a great thickness. In North Staffordshire they are 2300 feet, -which, added to the 4000 feet of limestone below, gives a depth of 6300 feet for the whole Carboniferous Limestone series of that region. In Lancashire the Yoredale rocks attain still more stupendous dimensions, Mr Hull having found them to be no less than 4500 feet thick. Both the lower or main (Scaur) limestone and the Yoredale group pass northwards into sandstones and shales, with coal-seams, and diminish in thickness. It is not impossible, as already suggested, that the cement-stone group of the Calciferous Sandstones of Scotland may represent, not only the thin Lower Limestone shale, but also some of the older parts of the English Carboniferous Limestone.
Where typically developed, the Carboniferous Limestone is a massive well-bedded limestone, chiefly light bluish? grey in colour, varying from a compact homogeneous to a distinctly crystalline texture, and rising into ranges of hills, whence its original name “Mountain Limestone.” It contains occasional scattered irregular nodules and nodular beds of dark chert. It is abundantly fossiliferous. The fossils commonly stand out on weathered surfaces of the rock, but microscopic investigation shows that even those portions of the mass which appear most structureless consist of the crowded remains of marine organisms. The limestone may be regarded as derived entirely from the organic debris of a sea-floor. Diversities of colour and lithological character occur, whereby the bedding of the thick calcareous mass can be distinctly seen. Here and there a more marked crystalline structure has been superinduced; but the most distinct examples of metamorphic change are those where the rock has been converted into what is termed “dun-stone.' ’ This alteration is frequent in the mining districts of Yorkshire and the neighbouring counties. It consists in the dolomitization of the rock along either the lines of bedding or the joints. Thus changed, it becomes a yellowish or brown crystalline dolomite, which runs vertically through the mass of the limestone along some definite joint, in courses of 20 or 30 fathoms in width. Such a metamorphism must have been effected by water percolating along the line of the joint, and affecting the rock for some distance on either side. In Derbyshire sheets of contemporaneous lava, locally termed “toadstone,” are interpolated in the Carboniferous Limestone.
The fauna of the Carboniferous Limestone of England is abundant and characteristic. Numerous foraminifers occur, Saccamina Carteri being often very characteristic. The corals are numerous, embracing upwards of 30 genera and about 100 species. These include both simple cup corals, as Zaphrentis, Clisiophyllum, and compound forms, like Alveolites and Phillipsastrasa. Crinoids are individually in enormous numbers, many beds of limestone appearing to consist of little else than their fragmentary stems and cups; Actinocrinus, Platycrinus, Poter iocrinus, Cyathocrinus, are common genera. Three species of sea-urchins are known. Θf the trilobites, so characteristic of the older Palaeozoic rocks, the last lingering forms are here found in three small genera —Brachymetopus, Phillipsia, and Griffithid.es . Polyzoa abound, some portions of the limestone being almost entirely composed of them, the genera Fenestella, Sulcoretepora, Vïncularia, Poly- para, Diastopora, and Glauconome being frequent. The brachiopods comprise 18 genera and 160 species, some of the most common forms being Productus, Spirifor, Rhynchonella, Athyris, Chonctes, Orthis, Lingula, and Discina. But the higher mollusks now begin to preponderate over the brachioρods. The lamellibranchs number 49 genera and 334 species, including forms of Aviculopecten, Leda, Nucula, Sanguinolites, Leptodomus, Schizodus, Edmondia, Modiola, and Conocardium. The gasteropoda amount to 206 species belonging to 29 genera, among which Euomphalus, Natica, Pleur otomaria, Macrocheilus, and Lexonema are frequent. The genus Bellerophon is represented by 23 species, among which B. Urei and 7?. dec wísoíwí are frequent. The cephalopods number 148 species, belonging among other genera to Orthoceras, Nautilus, Discites, and Goniatites. About 100 genera of fishes, chiefly from teeth and spines, have been described, as Psammodus, <Cochliodus, Cladodus, Petalodus, Rhizodus, Ctenoptychius, &c. Some of these were no doubt placoids which lived solely in the sea, but many, if not all of the ganoids, probably migrated between salt and fresh water; at least their remains are found in Scotland in strata full of land-plants, cyprids, and other indications of estuarine or fluviatile conditions.
The Carboniferous Limestone series of Scotland presents a striking contrast to that of the typical formation in England. It consists mainly of sandstones, shales, fire-clays, and coal-seams, with a few comparatively thin seams of encrinal limestone. Its lower portions include the chief limestone bands. 'The thickest of these seams, known as the Hurlet or main limestone, is usually about 6 feet in thickness, but in the north of Ayrshire swells out to 100 feet, which is the most massive bed of limestone in any part of the Scottish Carboniferous system. It lies upon a seam of coal, and is in some places associated with pyritous shales, which have been largely worked as a source of alum. This superposition of a bed of marine limestone on a seam of •coal is of frequent occurrence in the Carboniferous Limestone series of Scotland. Above the group of Lower Limestones comes a thick mass of strata containing many valuable coal-seams and ironstones. Some of these strata are full of terrestrial plants shepidodendron, Sigillaria, Stigmaria, Sphenopteris, Alethopteris) ; others, particularly the ironstones, contain marine shells, such as Lingula, Discina, Leda, Myalina, Euomphalus. Numerous remains of fishes have been obtained, more especially from some of the ironstones and coals (Gyracanthus formosus and other placoid fin-spines, Megalichthys Hibberti, Rhizodus Hibberti, with species of Elonichthys, Acanthodes, Ctenoptychius, &c.). Remains of labyrinthodonts have also been found in this group of strata, and have been detected even down in the Burdie House limestone. The highest division of the i Carboniferous Limestone series consists of a group of sandstones and shales, with a few coal-seams, and three, sometimes more, bands of marine limestone. Although these limestones are each seldom more than 3 or 4 feet thick, they have a wonderful persistence throughout the coal-fields of central Scotland. As already mentioned, they can be traced over an area of at least 1000 square wiles, and they probably extended originally over a considerably greater region. The Hurlet limestone with its underlying coal can also be followed across a similar extent of country. Hence it is evident that during certain epochs of the Carboniferous period a singular uniformity of conditions prevailed over a large region of deposit in the centre of Scotland.
Tiιe difference between the lithological characters of the Carboniferous Limestone series, in its typical development, as a great marine formation, and in its arenaceous and argillaceous prolongation into the north of England and Scotland, has long been a familiar example of the nature and application of the evidence furnished by strata as to former geographical conditions. It shows that the deeper and clearer water of the Carboniferous sea spread over the site of Yorkshire, Derbyshire, and Lancashire; that the land lay to the north; and that, while the whole area was undergoing subsidence, the maximum movement took place over the area of deeper water. It was from the northern land that the sand and mud were derived, but the sediment during the time of the Carboniferous Limestone seems to have sunk to the bottom before it could reach the great basin in which foraminifers, corals, crinoids, and mollusks were building up the great calcareous deposit. Yet the thin limestone bands, which run so persistently among the Lower Carboniferous rocks in Scotland, prove that there were occasional episodes during which the sediment ceased to arrive, and when the same species of shells, corals, and crinoids spread northwards towards the land, forming for a time over the sea-bottom a continuous sheet of calcareous ooze like that of the deeper water further south. These intervals of limestone growth no doubt point to times of more rapid submergence, perhaps also to other geographical changes whereby the sediment was for a time prevented from spreading so far.
Viewed as a whole, therefore, the Carboniferous Limestone series of Northumberland and Scotland contains the records of a long-continued but intermittent process of subsidence. The numerous coal-seams with their underclays were undoubtedly surfaces of vegetation that grew in rank luxuriance on the wide marine mud-flats, and mark pauses in the subsidence. Perhaps we may infer the relative length of these pauses by the comparative thicknesses of the coal-seams. The overlying and intervening sandstones and shales indicate a renewal of the downward movement, and the gradual infilling of the depressed area with sediment, until the water once more shoaled, and the vegetation from adjacent swamps spread over the muddy flats as before. The occasional limestones serve to indicate the epochs of more prolonged or more rapid subsidence, when marine life was enabled to flourish over the site of the submerged forests. But that the sea, even though tenanted in these northern parts by a limestone-making fauna, was not so clear and well suited for the development of animal life during some of these submergences as it was further south, seems to be proved by the paucity and dwarfed forms of the fossils in the thin limestones, as well as by the admixture of clay in the stone.
In Ireland the Carboniferous Limestone swells out to a vast thickness, and covers a large part of the island. It attains a maximum in the west and south-west, where, according to Kinahan, it consists in Limerick of the following subdivisions :— [table]
The chert bands which form such marked horizons among these limestones are counterparts of others found abundantly in the Carboniferous Limestone of England and Scotland. They have been recently studied by Hull and Hardman, who have found them full of siliceous replacements of calcareous foraminifers, crinoids, &c., and who regard them as due to a chemical alteration on the floor of the Carboniferous sea. Portions of the limestone have a dolomitic character, and sometimes are oolitic. Great sheets of melaphyre, felstone, and tuff, representing volcanic eruptions of contemporaneous date, are interpolated in the Carboniferous Limestone of Limerick and other parts of Ireland. As the limestone is traced northwards it shows a similar change to that which takes place in the north of England, becoming more and more split up with sandstone, shale, and coal-seams, until, at Ballycastle, it presents exactly the characters of the coal-bearing part of the formation in Scotland.
Millstone Grit.— This name is given to a group of sandstones and grits, with shales and clays, which runs persistently through the centre of the Carboniferous system from South Wales into the middle of Scotland. In South Wales it has a depth of 400 to 1000 feet; in the Bristol coalfield, of about 1200 feet. Traced northwards it is found to be intercalated with shales, fire-clays, and thin coals, and, like the lower members of the Carboniferous system, to swell out to enormous dimensions in the Pennine region. In North Staffordshire, according to Mr Hull, it attains a thickness of 4000 feet, which in Lancashire increases to 5500 feet. These massive accumulations of sediment were deposited on the north side of a barrier of Silurian and Cambrian rocks, which, during all the earlier part of the Carboniferous period, seems to have extended across central England, and which was not submerged until part of the Coal-measures had been laid down. North of this great area of deposit the Millstone Grit thins away to not more than 400 or õOO feet. It continues a comparatively insignificant formation in Scotland, attaining its greatest thickness in Lanarkshire and Stirlingshire, where it is known as the Moor Rock. In Ayrshire it does not exist, unless its place be represented by a few beds of sandstone at the base of the Coal-measures.
The Millstone Grit is generally barren of fossils. When they occur they are either plants like those in the coalbearing strata above and below, or marine organisms of Carboniferous Limestone species. In Northumberland, indeed, it contains a band of limestone undistinguishable from some of those in the Yoredale group aud Scaur limestone.
Coal-Measures.— This division of the Carboniferous system consists of numerous alternations of grey, white, yellow, sometimes reddish sandstone, dark-grey and black shales, clay-ironstones, fire-clays, and coal-seams. In South Wales it attains a maximum depth of about 12,000 feet; in the Bristol coal-field it is 5090 feet. But in these districts, as in the rest of the Carboniferous areas of Britain, we cannot be sure that all the Coal-measures originally deposited now remain, for they are always unconformably covered by later formations. In some localities, indeed, the denudation must have been very great, for the next overlying system of deposits (Permian) is found resting even on the Carboniferous Limestone. In North Staffordshire the depth of the Coal-measures is about 5000 feet, which in South Lancashire increases to 8000. These great masses of strata diminish as we trace them eastwards and northwards. In Derbyshire they are about 2500 feet thick, in Northumberland and Durlιam about 2000 feet, and about the same thickness on the west side of the island in the Whitehaven coal-field. In Scotland they attain a maximum of about 2000 feet.
The Coal-measures are susceptible of local subdivisions indicative of different and variable conditions of deposit. The following tables show the more important of these :— [table]
The numerous beds of compressed vegetation form the most remarkable feature of the Coal-measures. Each of them is usually underlaid by a seam of fire-clay, representing the soil on which the plants grew. Hence they mark successive terrestrial surfaces, which, after enduring for a longer or shorter period, were carried down beneath the water and covered over with sand and silt. There is no modern formation that affords a close parallel to that of the ancient coal-growths. The nearest analogy is furnished by the mangrove swamps alluded to already on p. 290. These masses of arborescent vegetation grow out into the sea as a belt or fringe on low shores, and form a matted soil which adds to the breadth of the land. Their roots spread in the salt water where marine organisms are abundant. The coal-growths no doubt also flourished in salt or at least brackish water; forsuch shells as Aviculopecten,Anthrawsia, and Goniatites are found lying on the coal or in the shales attached to it.
The vegetation of the Carboniferous period must have been luxuriant and varied. Upw ards of 500 species of plants have been obtained from the Carboniferous rocks of Britain, yet these must represent but a small portion of the original flora.
The most abundant forms were fems and lycopods. Numerous reed-like plants likewise played a conspicuous part in the general vegetation of the low grounds, while apparently on the drier and more elevated tracts (for their remains are less frequently met with) large coniferous trees flourished. The ferns strikingly resemble many modern forms. Among the more frequent genera were Sphenopteris, Pccopteτis, Alethopteris, Neur opter is, and Cyclopteris. The lycopodiaceous plants attained the size of forest trees. Among the more typical forms were many species of Lcpidodcndron, the stems and seed-cones of which are scattered through the Carboniferous strata. Some of the coal-seams are largely composed of the spores of these plants. Sigillaria, regarded by some as allied to the cycads, by others as more akin to the lycopods, is represented by numerous species. The roots known as Stigmaria abound in most fire-clays, showing how largely the coal consists of trees having roots of this kind. The Equisctaccae show themselves in the form of numerous Calamites which, though resembling in general aspect our modern horse-tails, differed in many points of structure, and notably in their gigantic size. Coniferous trees occur chiefly in the form of stems and branches. They include the genera Dadoxylon, Λraucarioxylon, and Pinites. Small nut-like fruits ( Trigonocarpum) abound in some beds, and are now believed to be the fruits of yew-like conifers. The presence of monocotyledonous plants is proved by the Pothocites referred to above (p. 347).
The animal remains of the Coal-measures are comparatively few 7 . In certain bands, particularly of ironstone, numerous mollusca occur,, and similar forms are to be occasionally met with in the shales.
Among the more frequent species are Anthracomya modiolaris, Authracosia ovalis, A. robusta, A. acuta, Anthracoptcra Broumiana, Aviculopccten papyraceus , and GoniaUtcs Listeri. But at the top of the Middle Coal-measures, near Manchester, a band of true marine shells occurs, including Ctcnodonta, Nautilus, Discitcs, Goniatitcs, and Orthoceras. The little Spirorbis carbonarius abounds in some shales. The crustaceans are chiefly represented by Beyrichia and Esthcria, but large eurypterid forms likewise occur. Fishes occur frequently, remains of the larger kinds usually appearing as scales, teeth, fin-spines, or bones, while the smaller ganoids are often preserved entire. Common species are Megalichthys Hibbcrti (?), Rhizodus granulatus, Caelacanthus lepturus, Palaeoniscus Egertoni, Pleuracanthus gibbosus. The British Carboniferous rocks have yielded 13 genera of labyrinthodonts {Anthracosaurus, Loxomma, Oρhidcrpeton, Pholiderpeton, Pteroplax, Urocordylus, &c.). These were probably fluviatile animals of predaceous habits, living on fish, Crustacea, and other organisms of the fresh or salt waters of the coal lagoons. The larger forms are believed to have measured 7 or 8 feet in length; some of the smaller examples, though adult and perfect, do not exceed as many inches. That the woods of the Carboniferous period were not devoid of insect life is known from the discovery of some beetles and neuroptera at Coalbrookdale.
Continental Europe. —As in Britain so on the Continent the Carboniferous system occupies many detached areas or basins—the result partly of original deposition, partly of denudation, and partly of the spread and overlap of more recent formations. There can be no doubt that the English Carboniferous Limestone once extended continuously eastward across the north of France, along the base of the Ardennes, through Belgium, and across the present valley of the Rhine into Westphalia. From the western headlands of Ireland this calcareous formation can thus be traced eastward for a distance of 750 English miles into the heart of Europe, It then begins to pass into a series of shales and sandstones, which no doubt represent the same proximity to shore as the similar strata in the north of England and Scotland. In Silesia, and still much further eastwards in central and southern Russia, representatives of the Carboniferous Limestone appear, but interstratified, as in Scotland, with coal-bearing strata. Traces of the same blending of marine and terrestrial conditions are found also in the north of Spain. But over central France, and eastwards through Bohemia and Moravia into the region of the Carpathians, the Coal-measures rest directly upon the older formations, most commonly upon gneiss and other crystalline rocks. It would appear that these tracts had remained above water during the time of the Carboniferous Limestone, but were gradually depressed during that of the Coal-measures.
In the north of France and. Belgium the British type of the Carboniferous system is well shown. At the base lies a group of green, grey, and reddish shales and yellow sandstones, precisely similar in lithological character to parts of the Calciferous Sandstones of Scotland. They are well seen in some recent railway cuttings to the north of Boulogne, and also in the valley of the Meuse above Namur, lying upon the Psammites de Condroz or Upper Devonian beds. They are succeeded by the Carboniferous Limestone, which is subdivided into eight formations, having an aggregate thickness of 800 metres, and each characterized by its own assemblage of fossils. The horizon of the Millstone Grit is marked by the occurrence of certain alum-schists. The Coal-measures of this area have been referred to in the article Coal.
The Saarbruck coal-field furnishes a good example of that type of the Carboniferous system where the lower marine series is absent. It lies unconformably on Devonian rocks, and attains a thickness of more than 10,000 feet. It is divided into the following groups of strata :—
2. Upper series, consisting at the top of red sandstones, below which come shales, sandstones, &c., with a few coal-seams— Anthracosia, Esthcria.
1. Lower series, consisting of an upper group of red conglomerates, sandstones, clays, resting upon the great coal-bearing division. Abundant plants, with labyrinthodonts and insect remains.
Geinitz, drawing attention to the distribution of plants in the Saxon Coal-measures, remarks that a certain order can be observed in their appearance. He divides the strata accordingly into three zones, each marked by a characteristic assemblage of plants, and he believes that the classification can be applied in other countries.
3. The Fern Zone, marked by the profusion of its ferns OSphenop∙ teris, Hymenossihyllites, Schizoptcris, Odontoptcris, Ncuropteris, Cycloptcris, Alethoptcris, Caulopteris). This is underlaid by
2. The Sigillaria Zone, containing many species of Sigillaria, also Lepidodendron, Calamites, Asterophyllitcs, and a few ferns.
1. The Lycopod Zone, abounding in Sagenaria Vcltheimiana, with Sphcnoptcris distans, Calamites transitionis, &c.
The lowest of these zones (1) is compared by Geinitz with the Culm, that is, the sandy, shaly, and coal-bearing representative of the Carboniferous Limestone. To the east of the Rhine valley, as already mentioned, the true Carboniferous Limestone loses its normal character and assumes that of the Culm—a series of shales, sandstones, greywackes, and conglomerates, in which the abundant fauna of the limestone is reduced to a few mollusks {Productus antiquus, P. latissimus, P. scmircticulatus, Posidonomya Becheri, Goniatitcs sphaericus, Orthoceras striatulum, kc. ). The Posidonomya particularly characterizes certain dark shales known as Posidonia schists. About 50 species of plants have been obtained from the Culm, typical species being Calamites transitionis, Sagenaria Vel- theimiana, Stigmaria ficoidcs, Sphenopteris distans, Cydoptcris tenuifolia. This flora bears a strong resemblance to that of the Calciferous Sandstones of Scotland.
North America. —Rocks corresponding in geological position and the general aspect of their organic contents with the Carboniferous system of Europe are said to cover an area of more than 200,000 square miles in the United States and British North America. The following table shows the subdivisions which have been established among them :— [table]
Permian. Britain. —In England the Coal-measures are uncon-fυrmably overlaid by a series of red sandstones, conglomerates, breccias, and marls, which at one time were grouped in one great formation as the New Red Sandstone, in contradistinction to the Old Red Sandstone lying below the Carboniferous system. They were likewise known as the Poikilitic series, from their mottled or variegated colours. They are now divided into two systems or groups of formations, the lower half being included in the Palaeozoic series under the name of Permian (after Perm, a Russian province where they are well displayed), and the upper half being relegated to the Secondary series, aud termed Trias.
The Permian system in England consists of the following subdivisions :— [table]
From the thicknesses here given, it is evident that the Permian rocks have a very different development on the two sides of England. On the east side, from the coast of Northumberland southwards to the plains of the Trent, they consist chiefly of a great central mass of limestone. But on the west side of the Pennine Chain, and extending southwards into the central counties, the calcareous zone disappears, and we have a great accumulation of red, arenaceous, and gravelly rocks.
Lower.— This subdivision attains its greatest development in the vale of the Eden, where it consists of brick-red sandstones, with some beds of calcareous conglomerate or breccia, locally known as “brockram,” derived from the waste of the Carboniferous Limestone. These red rocks extend across the Solway into the valleys of the Nith and Annan, in the south of Scotland, where they lie unconformably on the Lower Silurian rocks. Their breccias consist of fragments of the adjacent Silurian greywackes and shales, but near Dumfries some calcareous breccias or “brockrams” occur. These brecciated masses have evidently accumulated in small lakes or narrow fjords. Much further south, in Staffordshire, and in the districts of the Clent and Abberley Hills, the brecciated conglomerates in the Permian series attain a thickness of 400 feet. They have been shown by Ramsay to consist in large measure of volcanic rocks, grits, slates, and limestones, which can be identified with rocks on the borders of Wales. Some of the stones are 3 feet in diameter and show distinct striation. The same writer has pointed out that these Permian drift-beds cannot be distinguished by any essential character from modern glacial drifts, and he has no doubt that they were ice-borne, and, consequently, that there was a glacial period during the accumulation of the Lower Permian deposits of the centre of England.
Like red. rocks in general the Lower Permian beds are almost barren of organic remains. Such as occur are indicative chiefly of terrestrial surfaces. Plant remains occasionally appear, such as Caulerpites (supposed to be of marine growth), Lcpidodendron dilatatum, Calamites, Steτribergia, and fragments of coniferous wood. The cranium of a labyrinthodont (Dasyccps) has been obtained from the Lower Permian rocks at Kenilworth. Footprints referred to members of the same extinct order have been observed abundantly on the surfaces of the sandstones of Dumfriesshire, and also in the vale of the Eden.
Middle.— This subdivision is the chief repository of fossils in the Permian system. Its strata are not red, but consist of a lower zone of hard brown shale with occasional thin limestone bands (Marl Slate) and an upper thick mass of dolomite (Magnesian Limestone). The latter is the chief feature in the Permian development of the east of England. It corresponds with the Zechstein of Germany, as the Marl Slate does with the Kupfer-schiefer. It is a very variable rock in its lithological characters, being sometimes dull, earthy, finegrained, and fossiliferous, in other places quite crystalline, and composed of globular, reniform, botryoidal, or other irregular concretions of crystalline and frequently internally radiated dolomite. Though the Magnesian Limestone runs as a thick persistent zone down the east of England it is represented on the Lancashire and Cheshire side by bright red and variegated sandstone covered by a thin group of red marls, with numerous thin courses of limestone, containing Schizodus, BalceυelUa, and other characteristic fossils of the Magnesian Limestone.
The middle Permian division has yielded about 100 species belonging to 46 genera of fossils—a singularly poor fauna when contrasted with that of the Carboniferous system below. The brachiopods (9 genera, 21 species) include Prod.uctus horridus, Camarophoria multiplicata, C. Schlotheimii, Strophalosia Goldfussi, Lingula Crcdneri, and Tcrébratula elongata. The lamellibranchs number 16 genera and 31 species, among which Schizodus Schlotheimii, Βakevellia tumida, B. antiqua, B. ceratophaga, Mytilus squamosus, and Arca striata are characteristic. The univalves are represented by 11 genera and 26 species, including Pleurotomaria and Turbo as common genera. Fishes have been obtained chiefly in the marl slate to the number of 21 species belonging to 8 genera, of which Palaeoniscus is the chief. These small ganoids are closely related to some which haunted the lagoons of the Carboniferous period.
Upper.— Murchison and Harkness have classed as Upper Permian certain red sandstones with thin partings of red shale, and an underlying band of red and green marls and gypsum. These rocks, seen at St Bees, near Whitehaven, resting on a magnesian limestone, have not yet yielded any fossils.
Continental Europe. —The two types of the Permian system presented by the east and west sides of England reappear in different areas on the mainland of Europe. The eastern or Durham type is found in enormous masses of strata flanking the Harz Mountains, also in Thuringia, in Saxony, and in Bohemia. The western or Salopian type is found over many thousands of square miles in the north and east of Russia.
The German geologists, recognizing the remarkable twofold character of its rocks, have called this system “Dyas,” and have proposed to retain the term Permian to express the more simple type, such as is found in Russia and western England. They group the members of their Dyas as follows :— [table]
The name “Rothliegende” or “Rothtodtliegende' ’ (red-layer cr red-dead-layer) was given by the miners because their ores dis< appeared in the red rocks below the copper-bearing Kupfer-schiefer. The coarse conglomerates have been referred by Ramsay to a glacial origin, like those of the Abberley Hills. One of the most interesting features of the formation is the evidence of the contemporaneous outpouring of great sheets of quartz-porphyry, granite-porphyry, porphyrite, and melaphyre, with abundant interstratifications of various tuffs, not infrequently enclosing organic remains. From the very nature of its component materials, the Rothliegende is comparatively barren of fossils, a few ferns, calamites, and remains of coniferous trees are found in it, particularly towards the base, where indeed they form, in the Mansfeld district, a coal-seam about 5 feet thick.
The plants, all of terrestrial growth, on the whole resemble generically the Carboniferous flora, but seem to be nearly all specifically distinct. They include forms of Calamites (C. gigas), Astero- ρhyllites, and ferns of the genera Sphenopteris, Alethopteris, Neuropteris, Odontoptcris, with well-preserved silicified stems of tree-ferns (Psaronius, Tubicaulis). The conifer Walchia ( W. ρiniformis) is specially characteristic. Fish remains occur sparingly (Amblypterus, Palaeoniscus, Acanthodes), and traces of labyrinthodonts (Archegosaurus Dccheni) have been met with.
The Zechstein group is characterized by a suite of fossils like those of the Middle Permian of England. The Kupfer-schiefer contains numerous fish (Palaeoniscus Freislebeni, Platysomus gibbosus, &c. ) and remains of plants (coniferous leaves and fruits and sea-weeds). This deposit is believed to have been laid down in some enclosed sea-basin, the waters of which, probably from the rise of mineral springs connected with some of the volcanic foci of the time, were so charged with metallic salts in solution as to become unfit for the continued existence of animal life. The dead fish, plants, &c., by their decay, gave rise to reduction and precipitation of these salts as sulphides, which thereupon enclosed and replaced the organic forms, and permeated the mud at the bottom. This old sea-floor is now the widely extended band of copper-slate which has so long and so extensively been worked along the flanks of the Harz. After the formation of the Kupfer-schiefer the area must have been once more covered by clearer water, for the Zechstein contains a number of organisms of which Productus horridus, Spirifer undulatus, Strophalosia Goldfussi, Schizodτιs obscurus, and Fenestella retiformis are common. Renewed unfavourable conditions are indicated by the dolomite, gypsum, and rock-salt which succeed. Professor Ramsay, reasoning upon such phenomena as developed in England, has connected them with the abundant labyrinthodont footprints and other evidences of shores and land, as well as the small number and dwarfed forms of the shells in the Magnesian Limestone, and has speculated on the occurrence of a long continental period in Europe, during one epoch of which a numbeι of salt inland seas existed wherein the Permian rocks were accumulated. He compares these deposits to what may be supposed to be forming now in parts of the Caspian Sea.
North America. —The Permian system is hardly represented at all in this part of the globe. In Kansas certain red and green clays, sandstones, limestones, conglomerates, and beds of gypsum lie conformably on the Carboniferous system, and contain a few genera and species of molluscs {Bakeυellia, Myalina, <fcc,), which occur in the European Permian rocks.
III. SECONDARY OR MESOZOIC.
Triassic.
It has been already mentioned that the great mass of red rocks, which in England overlie the Carboniferous system, were formerly classed together as New Red Sandstone, but are now ranged in two systems. We have considered the lower of these under the name of Permian. The general facies of organic remains in that division is still decidedly Palaeozoic. Its brachiopods and its plants connect it with the Carboniferous rocks below. Hence it is placed at the close of the long series of Palaeozoic formations. When, however, we enter the upper division of the red rocks, though the general lithological characters remain very much as in the lower group, the fossils bring before us the advent of the great Mesozoic flora and fauna. This group therefore is put at the base of the Mesozoic or Secondary series. It is called Trias, because in Germany it consists of three well-marked subdivisions. But the old name, New Red Sandstone, is familiarly retained for it by many geologists in England. The term Trias, like Dyas, is unfortunately chosen, for it elevates a mere local character into an importance which it does not deserve. The threefold subdivision, though so distinct in Germany, disappears elsewhere.
Great Britain. —Triassic rocks occupy a large area of the low plains in the centre of England, ranging thence northwards along the flanks of the Carboniferous tracts to Lancaster Bay, and southwards by the head of the Bristol Channel to the south-east coast of Devonshire. They have been arranged in the following subdivisions :— [table]
Like the Permian red rocks below, the sandstones and marls of the Triassic series are almost barren of organic remains. Hence the subdivisions in the foregoing table are based on mineral characters, and could not therefore be relied on as a guide in districts outside of the English area. Indeed, extraordinary differences in the development of the different members of the series occur, even within that area, as may be seen from the subjoined table, which shows the variations in thickness from north-west to south-east :— [table]
Hence we observe that, while towards the north-west the Triassic rocks attain a maximum depth of 5200 feet, they rapidly come down to a fifth or a sixth of that thickness as they pass towards the south-east. Recent borings in the south-eastern counties show that the Triassic rocks are there absent altogether. It is evident that the source of supply of the sediment lay towards the north. This is further borne out by the character of the pebble-beds. These are coarsest towards the north, and, besides local materials, contain abundant rolled pebbles of quartz which have evidently been derived from some previous conglomerate, probably from some of the Old Red Sandstone masses now removed or concealed. The Trias everywhere rests unconformably on the rocks underneath it, so that, although the general physical conditions as regards climate, geography, and sedimentation, which prevailed in the Permian period still continued, great terrestrial movements had, in the meanwhile, taken place, whereby the Permian sediments were upraised and exposed to denudation. Hence the Trias rests now on Permian, now on Carboniferous, and sometimes even on Cambrian rocks. Moreover, the upper parts of the Triassic series overlap the lower, so that the Keuper groups come to rest directly on Permian or Carboniferous rocks.
One of the most interesting features in the English Trias is the occurrence of beds of rock-salt which have long been profitably worked. The uppermost subdivision of the Keuper, consisting of red marls, has a wide distribution, for it can be traced from the coast of Lancashire to the Bristol Channel, and covers a larger area of surface in the central counties than the rest of the Trias and the whole of the Permian sandstones combined. Even as far south as the coast of Devonshire, it contains casts of the cubical spaces once occupied by crystals of common salt. But in Cheshire the salt occurs in two or more beds, of which the lower is sometimes upwards of 100 feet thick. It is a crystalline substance, usually tinged yellow or red from intermixture of clay and peroxide of iron, but is tolerably pure in the best part of the beds, where the proportion of chloride of sodium is as much as 98 per cent. Through the bright red marls with which the salt is interstratified there run bands of gypsum, somewhat irregular in their mode of occurrence, sometimes reaching a thickness of 40 feet and upwards. Thin seams of rock-salt likewise occur among the red marls. These facts point to the concentration and evaporation of salt lakes or inland seas.
The organic remains of the Trias are comparatively few, as the conditions for at least animal life must have been extremely unfavourable in the waters of the ancient Dead Sea wherein these red rocks were accumulated. The land possessed a vegetation which, from the few fragments yet known, seems to have consisted in large measure of cypress-like coniferous trees (Voltzia, Walchia), with calamites on the lower more marshy grounds. The red marl group contains in some of its layers numerous valves of the little crustacean Estheria minuta, and a solitary species of lamellibranch, Pullastra arenicola. A number of teeth, spines, and sometimes entire skeletons of fish have been obtained (Dipteronotus cyphus, Palaeoniscus superstes, Hybodus Keupcri, Acrodus minimus, Sphenon- chus minimus, Lophodus, &c.). The bones, and still more frequently the footprints, of labyrinthodont and even of saurian reptiles occur in the Keuper beds— Ldbyrinthodon (4 species), Cladyodon Lloydii, Hyperodapcdon, Palaeosaurus, Teratosaurus, Thecodontosaurus, Rhyncosaurus, and footprints of Cheirotherium. The remains of a small marsupial (Microlestes) have likewise been •discovered.
Rhaetic.— At the top of the Red Marl certain thin-bedded strata form a gradation upwards into the base of the Jurassic system. As their colours are grey and blue, and contrast with the red marls on which they repose conformably, they were formerly classed without hesitation in the Jurassic series. Egerton, however, showed that, from the character of their included fish remains, they had more palaeontological affinity with the Trias than with the Lias. Subsequent research, particularly among the Rhaetic Alps and elsewhere on the Continent, brought to light a great series of strata of intermediate characters between the Trias and Lias. These results led to renewed examination of the so-called beds of passage in England, which were’ found to be truly representative of the massive formations of the Tyrolese and Swiss Alps. They are therefore now classed as Rhaetic, and considered as the uppermost member of the Trias, but offering evidence of the gradual approach of the physical geography and characteristic fauna and flora of the Jurassic period.
The Rhaetic beds extend as a continuous though very thin band at the top of the Trias, from the coast of Yorkshire across England to Lyme Regis on the Dorsetshire shores. They occur in scattered patches even up as far as Carlisle, and westwards on both sides of the Bristol Channel. Their thickness, on the average, is probably not more than 50 feet, though it rarely increases to 150 feet. They consist of thin-bedded grey and dark shales and clays, with bands of light-coloured limestone. One of their most important subdivisions is the so-called Bone-bed—a pyritous, micaceous, and occasionally rippled sandstone, sometimes in several bands, abounding in fish bones, teeth, coprolites, and other organic remains. The grey marly beds in the lower portion of the series have yielded remains of the Microlestes Rhaeticus. Among the reptilian fossils are some precursors of the great forms which distinguished the Jurassic period (Ichthyosaurus and Plesiosaurus). The fishes include Acrodus minimus, Ceratodus altus, Hybodus minor, Ncma-canthus monilifer, &c. Some of the lamellibranchs are specially characteristic; such are Cardium Rhaeticum, Avicula contorta, Pecten Valoniensis, and Pullastra arenicola.
Professor Ramsay has drawn attention to the probable geographical changes recorded by the Triassic rocks of England. Connecting them with the earlier and similar Permian sandstones and marls he points out that the great Continental period which began with the Old Red Sandstone closed with the New Red Marl, and was characterized by the existence of great lakes, many of which must have been salt, and by the abundance of labyrinthodont life. The Triassic rocks were, doubtless, laid down in one of these salt lakes round the margins of which the labyrinthodonts left their footprints on the soft sand. In the Rhaetic series we see how these inland basins were gradually invaded by the sea, which brought into the region of Britain the rich fauna of the Jurassic period.
Continental Europe. —The Trias is the most compactly distributed of all the geological formations of Europe. Its main area extends as a great basin from Basel down to the plains of Hanover, traversed along its centre by the course of the Rhine, and stretching from the flanks of the old high grounds of Saxony and Bohemia on the east across the Vosges mountains into France. This must have been a great inland sea, out of which the Harz mountains, and the high grounds of the Eifel, Hundsruck, and Taunus probably rose as islands. It may have extended up to the base of the Alps, for enormous masses of Triassic rocks now form part of these mountains. Traces of what were probably other basins occur eastward in the Carpathian district, along the southern front of the Alps, in the west and south of France, and over the eastern half of the Spanish peninsula. But these areas have been considerably obscured, sometimes by dislocation and denudation, sometimes by the overlap of more recent formations.
In the great German Triassic basin the deposits are as shown in the subjoined table. [table]
The Trias attains an enormous development in the eastern Alps, where it bears evidence of having been accumulated under very different conditions from those of the Trias in Germany. The great thickness of its limestones, and their unequivocally marine organisms, show that it must have accumulated in opener water, which remained clear and comparatively free from inroads of sandy and muddy sediment. It possesses, moreover, a high interest as being a massive formation of marine origin formed between Permian and Jurassic times, and containing a remarkable blending of true Palaeozoic organisms with others as characteristically Mesozoic. It is divided into two great series :—(1) Lower Trias, consisting of (a) Werfen Shales and Guttenstein Limestone, and (5) Virgloria Limestone or alpine Muschelkalk; and (2) Upper Trias,—a varied series of strata in three leading groups, having a united thickness sometimes of thousands of feet.
North America. —Rocks which are regarded as equivalent to the European Trias cover a large area in North America. On the Atlantic coast they are found on Prince Edward’s Island, New Brunswick, and Nova Scotia, in Connecticut, New York, Pennsylvania, and North Carolina. Spreading over an enormous extent of the western territories, they cross the Rocky Mountains into California and British Columbia. They consist mainly of red sandstones, passing sometimes into conglomerates, and often including shales and impure limestones. On the Pacific slope they contain distinctly marine organisms, which include a mingling of such Palaeozoic genera as Spirifer, Orthoceras, oxsåGoniatites, with the characteristically Secondary genus Ammonites. In the centre and east of the continent they are marked by the occurrence of terrestrial plants, and in Connecticut by abundant footprints of land animals. The fossil plants present a general facies like that of the European Triassic flora, among them cycads, including some of the European species of Pterophyllum. Ferns (Pecopteris, Neuropteris, Clatlιropterish, calamites, and conifers are the predominant forms. The fauna is remarkable chiefly for the number aud variety of its vertebrates. The labyrinthodonts are represented by footprints, from which upwards of fifty species have been described. Saurian footprints have likewise been recognized, but in a few cases their bones also have been found. These saurians had some bird-like characteristics, among others that of three-toed hind feet, which produced impressions exactly like those of birds. It is by no means certain, therefore, that what have been described as ornithichnites were not really made, by dinosaurs. A small insectivorous marsupial (Dromatherium), found in the Trias of North Carolina, is the oldest American mammal yet known.
Jurassic.
The next great period of geological time is termed the Jurassic, from the Jura Mountains, where the deposits of that age are well developed. It was in England, however, that they were first studied by William Smith, in whose hands they were made to lay the foundations of stratigraphical geology. The names adopted by him for the subdivisions he traced across the country have passed into universal use, and though some of them are uncouth English provincial names, they are as familiar to the geologists of France, Switzerland, and Germany as to those of England.
The Jurassic formations stretch across England in a varying band from the mouth of the Tees to the coast of Dorsetshire. They consist of harder sandstones and limestones interstratified with softer clays and shales. Hence they give rise to a characteristic type of scenery,—the more durable beds standing out as long ridges, sometimes even with low cliffs, while the clays underlie the level spaces between. Arranged in descending order, the following subdivisions of the English Jurassic system are recognized :— [table] [table]
Although these names appear in tabular order as expressive of what is the predominant or normal succession of the beds, considerable differences occur when the rocks are traced across the country. Thus the Forest Marble attains a thickness of 450 feet in Dorsetshire, but dwindles down to only 15 feet at Blenheim Park. The Inferior Oolite consists of marine limestones and shales in Gloucestershire, but chiefly of massive estuarine sandstones and shales in Yorkshire. These differences help to bring before us some of the geographical features of the British area during the Jurassic period.
The Lias consists of three formations, well marked by physical and palaeontological characters. In the lower member numerous thin blue and brown limestones with partings of dark shale are surmounted by similar shales with occasional nodular limestone bands, the whole being divided into seven zones, each characterized by the presence of one or more distinctive species of ammonite. From this point of the geological series up to the close of the Mesozoic formations, the ammonites play a chief part among the mollusks. The Middle Lias, consisting of argillaceous limestones (marlstones) with micaceous sands and clays, is divided into five ammonite zones. In its Yorkshire development this subdivision is remarkable for containing a bed of earthy carbonate of iron 15 to 20 feet thick, which has been extensively worked in the Cleveland district. The upper division is composed chiefly of clays and shales with nodules of limestones, among which three ammonite zones have been noted in Yorkshire.
The organic remains of the Lias comprise leaves and other remains of cycads (Palaeozamia), conifers (Pinites, Cupressus, Peuce), ferns (Otopteris, Λlethoptcris, kc.), and reeds (Equisctites). These fossils serve to indicate the general character of the flora, which seems now to have been mainly cycadaceous and coniferous, and to have presented a great contrast to the lycopodiaceous vegetation of Palaeozoic times. The occurrence of land-plants dispersedly throughout the English Lias shows also that the strata, though chiefly marine, were deposited within such short distance from shore, as to receive from time to time leaves, seeds, fruits, twigs, and stems from the land. Further evidence in the same direction is supplied by the numerous insect remains, which have been obtained principally from the Loλver Lias. These were, no doubt, blown off the land and fell into shallow water, where they were preserved in the silt on the bottom. The Neuroptera are numerous, and include several species of Líbellula. The coleopterous forms comprise a number of beetles. There were likewise representatives of the orthopterous, hemipterous, and dipterous orders. These relics of insect-life are so abundant in some of the calcareous bands that the latter are known as insect-beds. With them are associated remains of terrestrial plants, cyprids, and mollusks, sometimes marine, sometimes apparently brackish-water. The marine life of the period has been abundantly preserved, so far at least as regards the comparatively shallow and juxta-littoral waters in which the Liassic strata were accumulated. Corals, though on the whole scarce, abound on some horizons (Isastrcea, Montlivaltia, Septastraea, &c. ). The crinoids were represented by thick growths of Extracrinus and Pentacrinus. There were several kinds of starfishes, and also of sea-urchins (Cidaris, Diadema, Acrosalenia)— all generically distinct from those of the Palaeozoic periods. The contrast between the Liassic Crustacea and those of the older systems, as Phillips has pointed out, is very decided, the ancient trilobites having entirely disappeared, and having been succeeded by tribes of long-tailed ten-footed lobsters and prawns. There is a similar striking difference between the mollusks of the Lias and those of the Palaeozoic rocks, bearing witness to the great biological changes which had taken place in the long interval. The brachiopods are chiefly species of Rhynchonella and Terebratula— genera which, though occurring in Palaeozoic rocks, play there a comparatively subordinate part. They include the last Spiriferi and Lcptaenae. Of the lamellibranchs, a few of the most characteristic genera are Grypium, Lima, Pecten, Monotis, Cardinia, Hippopodium, Myacites, and Pholadomya. Gasteropods, though usually rare in such muddy strata as the greater part of the Lias, occasionally occur, but most frequently in the calcareous zones. Altogether 89 species are at present known, the chief genera being Pleurotomaria, Turbo, Eucyclus, Cerithium, and Chemnitzia. The cephalopods, however, are the most abundant and characteristic shells of the Lias; the family of the ammonites numbers in Yorkshire at least 113 species. As already stated, some species are so distinctive of special horizons that the whole of the Lias has been subdivided into zones, each denoted by the name of its characteristic ammonite. In ascending order, these zones are named as follows :—In the Lower Lias,—1st, Ammonites planorbis ; 2d, A. angulatus·, 3d, A. Budclandi∙, 4th, A. tuberculatus; 5th, A. obtusus·, 6th, A. oxynotus; 7th, A. varicos-totus. In the Middle Lias,—1st, A. Jamesoni and armátus ; 2d, A. Capricornus ; 3d, A. margaritatus ∙, 4th, A. spinatus∙, 5tb, A. annulatus. In the Upper Lias,—1st, A. serpentinus·, 2d, A. communis·, 3d, A. jur ensis. The genus Nautilus is also present. Tlιe dibranchiate cephalopods are represented by upwards of 40 species of the genus Belemnites. Numerous species of fishes have been obtained from the Lias. Some of these are placoids, known only by their teeth {Acrodus, Cerato dus), others only by their spines {Nemacanthus), and some by both teeth and spines {Hybodus). The ganoids are frequently found entire, the genera Dapedius, Pholidophorus, Aechmodus, Lepidotus, Pachycormus, and Leptolepis being among the most frequent. But undoubtedly the most important palaeontological feature of the Lias is the number and extraordinary interest of its reptilian remains. These include the extinct order of enaliosaurs or sea-lizards, uniting characters which are not found together in any living forms. Among these are the genus Ichthyosaurus, a creature with a fish-like body, two pairs of strong swimming paddles, and probably a vertical tail-fin. The head, joined to the body without any distinct neck, was furnished with two large eyes, having a ring of bony plates round the eyeball, and with teeth that had no distinct sockets. Some of the skeletons of this creature exceed 24 feet in length. Contemporaneous with it was the Plesiosaurus, distinguished by its long neck, the larger size of its paddles, the smaller size of its head, and the insertion of its teeth in special sockets, as in the higher saurians. These creatures seem to have haunted the shallow seas of the Liassic time. There were also huge winged bat-like reptiles {Dimorphodon), with large heads, having teeth in distinct sockets, eyes like the Ichthyosaurus, and one finger of each fore foot prolonged to a great length, for tlιe purpose of supporting a membrane for flight. The bones, like those of birds, were hollow and air-filled. Gigantic deinosaurs were likewise among the inhabitants of Britain during Liassic time. These were true reptiles, yet with peculiarities of structure, particularly in the hinder part of the skeleton, linking them with birds like the ostrich. To this order belongs the Megalosaurus from the Lias of Lyme Regis. Lastly, the true crocodiles had representatives in the Liassic waters and shores, in the genera Telco saur us and Stenco- sauras.
The Lower or Bath Oolites lie conformably upon the top of the Lias, with which they are connected by a general similarity of organic remains. Out of the 312 known species in the Upper Lias, about 39 or 40 pass up into the overlying formation. The lowest of the three subdivisions of the English Oolites consists in the southwest and centre of England of shelly marine limestones, with clays and sandstones; but, as these strata are traced northwards into Northampton, Rutland, and Lincolnshire, they pass into a series of strata indicative of deposit in the estuary of some river descending from the north, for, instead of the abundant cephalopods of the truly marine and typical series, we meet with fresh-water genera such as Cyrena and Unio i with marine forms such as Ostrea and Modiola, thin seams of lignite, and remains of terrestrial plants. These indications of the proximity of land become still more marked in Yorkshire, where the strata (800 feet thick) consist chiefly of sandstones, shales with seams of ironstone and coal, and occasional horizons containing marine shells. It is deserving of notice that the Cornbrash, which forms the top of the Lower Oolite in the typical Gloucestershire district, occurs likewise in the same position in Yorkshire. Though rarely more than 8 feet thick, it runs across the country from Devonshire to Yorkshire.
Thus a distinctly defined series of beds of an estuarine character, exactly representative of the marine formations of the south-west, shows us that at the close of the Lower Oolitic period the estuary of the northern tract was submerged, and a continuous sea-floor stretched across the whole of the south-east of England.
Thanks to the deposits of the Yorkshire estuarine series our knowledge of the Oolitic is much more ample than of the Liassic flora. With the exception of a few littoral fucoids all the plants are of terrestrial forms. They comprise about 60 species of ferns, among which the genera Pecopteris, Sphenoptêris, Phlebopteris, and Taeniopteris are characteristic. Next in abundance come the cycads, of which more than 20 species are known, belonging to the genera Otozamites, Zamites, Pterophyllum, and Cycadites. Coniferous remains are not infrequent in the form of stems or fragments of wood, as well as in occasional twigs with attached leaves; the genera Araucarites, Brachyphyllum, Thuytes, Peuce, Walchia, Cryptomcritcs, and Taxites have been recognized. The Inferior Oolite presents a tolerably copious suite of invertebrate remains, which resemble generically those of the Lias. The predominance of Rhynchonella and Tercbratula over the rest of the brachiopods becomes still more marked. Gryphaea, Lima, Pecten, Cardium, Myacites, Mytilus, Pholadomya, Trigonia are frequent shells, while ammonites and belemnites also occur, though much more sparingly than in the Lias below, and in some of the later subdivisions of the Oolitic series. The Fuller’s Earth, though well-marked in the Bath district, where it is about 200 feet thick, dies out in Oxfordshire, and contains only a few distinctive fossils, most of its forms being also found in the Inferior Oolite.
The Great or Bath Oolite consists, in Gloucestershire and Oxfordshire, of three groups-of strata. At the base comes a series of thin-bedded limestones with sands, knownas the Stonesfield Slate; in the centre lies a mass of shelly and yellow or cream-coloured often Oolitic limestones, with partings of marl or clay—the Great Oolite; while at the top lies a set of clays and shelly limestones, including the Bradford Clay, Forest Marble, and Cornbrash. The Stonesfield Slate, the lowest of these three zones, is a local but exceedingly important subdivision, which has furnished a large number of reptilian and some mammalian remains. It must have been deposited in shallow water close to thickly wooded shores.
About a dozen species of ferns have been found in the Stonesfield Slate, the genera Pecopteris, Sphenopteris, and Taeniopteris being still the prevalent forms. The cycads are chiefly species of Palaeozamia, the conifers of Thuytcs. With these drifted fragments of a terrestrial vegetation there occur remains of beetles, dragon-flies, and other insects which had been blown or washed off the land. The waters were tenanted by a few brachiopods {Rhynchonella and Terebratula), by lamellibranchs {Gervillia, Lima, Ostrea, Pecten, Astarte, Modiola, Trigonia, &c.), by gasteropods {Natica, Nerita, Patella, Trochus, &c. ), by a few ammonites and belemnites, and by placoid and ganoid fishes, of which about 50 species aτe known. The reptiles comprise representatives of turtles, with peculiar species of Ichthyosaurus and Plesiosaurus. The genus Teleosaurus, which occurs in the Yorkshire Lias, is among the organisms of the Stonesfield Slate. It was a true carnivorous crocodile, measuring about 18 feet in length, and is judged by Phillips to have been in the habit of venturing more freely to sea than the gavial of the Ganges and the crocodile of the Nile. The huge deinosaur Megalosaurus frequented the shores of the Stonesfield lagoons, walking probably on its massive hind legs, and feeding on the mollusks, fishes, and perhaps the small mammals of the district. It is estimated to have had a length of 25 feet, and to have weighed 2 or 3 tons. The flying reptiles were likewise represented by the Rhamphorhynchus,— a harpy-like creature which was able to fly, to shuffle on land, or perch on rocks, perhaps even to dive in search of its prey. But the most important organic remains of the Stonesfield Slate are undoubtedly its mammalia, of which three genera Amphitherium, Phascolotherium, and Stereognathus have been determined. Only portions of lower jaws have yet been found, pointing doubtless to the fact that, as the animals were drifted from land, the lower jaws, unprotected by outer skin, were separated in decomposition from the rest of the body. These interesting relics were the first traces of mammalian life found in strata of such high antiquity. They are regarded as having belonged to small marsupial animals, to which living analogues exist in Australia. In the Great Oolite the remains of a gigantic saurian Ceteosaurus have been found. According to Phillips it was probably, when standing, not less than 10 feet in height and 50 feet in length, a marsh-loving or river-side animal, living on the ferns, cycads, and conifers among which it dwelt.
In the Middle or Oxford Oolites, the Oxford Clay, so called from the name of the county through which it passes in its course from the coast of Dorsetshire to that of Yorkshire, consists mainly of layers of stiff blue aud brown clay. In its lower portion lies a marked zone of calcareous abundantly fossiliferous sandstone, known, frðm a place in Wiltshire, as the Kelaways Rock, which, after dying out in the midland counties, reappears on the Yorkshire coast. This zone contains about 150 species of fossils, of which nearly a half are found in lower parts of the Jurassic series, aud about the same number pass upward into higher zones.
Among its characteristic forms is Ammonites Calloviensis. The Oxford, clay, from the nature of its material and the conditions of its deposit, is deficient in some forms of life which were no doubt abundant in neighbouring areas of clearer water. Thus there are hardly any corals, few echinoderms, polyzoa, or brachiopods. Some lamellibranchs are abundant, particularly Gryphaea and Ostrca (both forming sometimes wide oyster-beds), Lima, Avicula, Pecten, Astarte, Trigonia — the whole having a great similarity to the assemblages in the Lower Oolite formations. The gasteropods are not so numerous as in the calcareous beds below, but belong mostly to the same genera. The ammonites are numerous,— A. I)uncani, A. Jason, A. Lamberti, and A. oculatus being characteristic. Of the belemnites, which also are frequent, B. hastatus is found all the way from Dorsetshire to Yorkshire. Spines and teeth of placoid fishes and entire specimens of Lepidotus are occasionally to be met with. The reptiles, besides Ichthyosaurus, Megalosaurus, Plesiosaurus (4 species), Steneosaurus, and Bhamphorhynchus, comprise also Pleiosaurus— a marine saurian with large head, short neck, paddles similar to those of Plesiosaurus, approaching the type of the ichthyosaurians, but even surpassing them in size.
The Coralline Oolite can likewise be traced, with local modifications and partial interruptions, across England from Yorkshire to Dorsetshire. It is named from its beds full of masses of coral. It consists of three zones,—a lower calcareous grit, a central rubbly limestone with corals (the true “coral rag” of William Smith), and an upper calcareous grit, which, though feebly represented further south, attains importance in Yorkshire. It is frequently entirely made •up of comminuted shells, urchins, corals, and other marine organisms. The corals include the genera Isastraea, Thamnastraea, and Thecosmilia. The urchins belong to Cidaris, Hemiddaris, Pigurus, Pigaster, and other genera. There are likewise Ammonites, Belemnites, and Nautili.
The Upper or Portland Oolites bring before us the closing epochs of the long Jurassic period in England, with the records of some of the physical revolutions which led to this change. At their base lies the Kimeridge Clay, so named from the locality on the coast of Dorsetshire where it is so well exhibited, and whence it is traceable continuously, save where covered by the Chalk, into Yorkshire. Like the Oxford .Clay below, it is distinguished by its thickness, persistence, and peculiar organic remains.
Mollusca appear in greatly diminished variety; Gryjìbaea virgula, Ostraea deltoidea, Astarte Hartwellensis, and Cardium striatulum are characteristic species. The reptiles are the most important of the palaeontological contents of this zone. They include remains of turtles, 5 species of Ichthyosaurus, 5 of Pleiosaurus, 8 of Plesiosaurus, Ceteosaurus, Megalosaurus, and the crocodilians Steneosaurus, Teleosaurus, and Goniopholis.
The Portland beds are so named from the isle of Portland, where they directly succeed the Kimeridge Clay. A feeble representative of them is believed to overlie that clay on the Yorkshire coast, but it is in the southern counties that they attain their chief development. They consist, at Portland, of a lower sandy set of beds about 150 feet thick, and of an upper calcareous zone (containing the well-known limestone so largely used for building purposes under the name of Portland stone) about 70 feet thick.
The fossils, which very commonly occur as mere empty casts, include as characteristic species Isastroca oblonga, Cardium dissimile, Trigonia gibbosa, and Terebra Portlandica. There occur also remains of some of the great Oolitic saurians.
The Purbeck beds, so named from the Isle of Purbeck, where they are best developed, are usually connected with the foregoing formations as the highest zone of the Jurassic series of England. But they are certainly separated from the rest of that series by many peculiarities, which show that they were accumulated at a time when the physical geography and the animal and vegetable life of the region were undergoing a remarkable change.
They have been arranged in three groups. The lowest consists of fresh-water limestones and clays, with layers of ancient soil containing stumps of the trees which grew in them. The middle group comprises about 130 feet of strata with marine fossils, while the highest division shows a return of fresh-water conditions. Among the indications of the presence of the sea is an oyster bed (Ostrea distorta') 12 feet thick, with Pecten, Modiola, Avicula, Thracia, &c. The fresh-water bands contain still living genera of lacustrine and fluviatile shells— Paludina, Limnaea, Planorbis, Physa, Valvata, Unio, and Cyclas. Numerous fishes, both placoid and ganoid, haunted these Purbeck waters. Many insects, blown off from the adjacent land, sank and were emtombed and preserved in the calcareous mud. These include coleopterous, orthopterous, hemipterous, ueuropterous, and dipterous forms. Remains of several reptiles, chiefly chelonian, but including the Jurassic crocodile Goniopholis, have also been discovered. But the most remarkable organic remains of the Purbeck beds are those of 10 genera and 25 species of marsupial mammals, from the size of a mole to that of a polecat. They are believed to have been mostly insectivorous. One of them (Triconodon major) is regarded by Owen as carnivorous, and probably about the size of the existing Dasyurus maugei of Australia. These mammalian remains occur, almost wholly as lower jaws, in a stratum about 5 inches thick lying near the base of the Middle Purbeck group.
Continental Europe. —Jurassic rocks cover a vast area in central Europe. They rise from under the Cretaceous formations in the north-east of France, whence they range southwards down the valleys of the Saone and Rhone to the Mediterranean. They appear as a broken border round the old crystalline nucleus of Auvergne. Eastwards they range through the Jura Mountains up to the high grounds of Bohemia. They appear in the outer chains of the Alps on both sides, and on the south they rise along the centre of the Apennines, and here and there over the Spanish peninsula. Covered by more recent formations they underlie the great plain of northern Germany, whence they range eastwards and occupy large tracts in central and eastern Russia. According to Neumayr, three distinct geographical regions of deposit can be made out among the Jurassic rocks of Europe. (1.) The Mediterranean province, embracing the Pyrenees, Alps, and Carpathians, with all the tracts lying to the south. One of the biological characters of this area was the great abundance of ammonites belonging to the groups of Heteroshhylli (Phy Hoc er ash and Fimbriati (Lytoceras). (2.) The central European province, comprising the tracts lying to the north of the Alpine ridge, and marked by the comparative rarity of the ammonites just mentioned, which are replaced by others of the groups Inflati (Aspido-cerash and Oppellia, and by abundant reefs and masses of coral. (3.) The boreal or Russian province, comprising the middle and north of Russia, Spitzbergen, and Greenland. The life in this area was much less varied than in the others, showing that in Jurassic times there was a perceptible diminution of temperature towards the north. The ammonites of the more southern tracts here disappear, together with the corals.
In France the following arrangement has been made of the Jurassic rocks, the subdivisions nearly corresponding to those first proposed in England :— [table]
In north-western Germany the subjoined classification has been adopted. [table]
North America. —So far as yet known rocks of Jurassic age play but a very subordinate part in North Amercian geology. Perhaps some of the red strata of the Trias belong to this division, for it is difficult, owing to paucity of fossil evidence, to draw a satisfactory line between the two systems. Strata containing fossils believed to represent those of the European Jurassic series have been met with in recent years during the explorations in the western domains of the United States. They occur among some of the eastern ranges of the Rocky Mountains, as well as on the western side of the watershed. They have been recognized also far to the north beyond the great region of Azoic and Palaeozoic rocks in the arctic portion of the continent. They consist of limestones and marls, which appear seldom to exceed a few hundred feet in thickness. The fossils include species of Pentacrinus, Monotis i Trigonia, Lima, Ammonites, and Belemnites. Cretaceous.
The next great series of geological formations is termed the Cretaceous system, from the fact that in England and western Europe one of its most important members is a thick band of white chalk (creta). Britain. —The Purbeck beds bring before us evidence of a great change in the geography of England towards the close of the Jurassic period. They show how the floor of the sea in which the thick and varied formations of that period were deposited came to be gradually elevated, and how into pools of fresh and brackish water the leaves, insects, and small marsupials of the adjacent land were washed down.
These evidences of terrestrial conditions are followed in the same region by a vast delta-formation, that of the Weald, which accumulated over the south of England, while the older parts of the Cretaceous system were being deposited in the north. Hence there are two types of that system, one where the strata are fluviatile or estuarine, termed the Wealden type, the other where they are marine, known as the Neocomian type. Arranged in descending order the following are the subdivisions of the English Cretaceous rocks:— [table]
Lower Cretaceous or Neocomian.— The fluviatile development of this series in the south of England consists of a great depth of sands and clays known generally as the Wealden series, from the Weald of Sussex and Kent, where they are best displayed.
They precisely resemble the deposits of a delta, and this is borne out by their organic remains, which consist partly of terrestrial plants (Equisetum, Sphenopteris, Alethopteris, Thuytes, cycads, and conifers), and fresh-water shells (Uhio, 10 species; Cyrena, 5 species; with Cyclas, Paludina, Melania, ke.), with a few estuarine or marine forms as Ostrea and Mytilus, and ganoid fishes (Lepidotus} like the gar of American rivers. Among the spoils of the land floated down by this river were the carcases of huge deinosaurian reptiles (Iguanodon, Hylaeosaurus, Megalosaurus), of the long necked plesiosaurs, and of winged ptérodactyles. The deltoid formation in which these remains occur extends in an east and west direction for at least 200 miles, and from north to south for at least 100. Hence the delta must have been not less than 20,000 square miles in area. It has been compared with that of the Quorra; in reality, however, its extent must have been greater than its present visible area, for it has suffered from denudation, and is to a large extent concealed under more recent formations. The river probably descended from the north-west, draining a vast area, of which the existing mountain groups of Britain are perhaps merely fragments.
The marine type of the Lower Cretaceous rocks is now commonly termedNeocomian, from Neufchâtel(iVeoco»i«?n), where it is well developed. In the south of England only the upper division appears, overlying conformably the Wealden series, and showing the gradual depression of the old delta and the advance of the sea. In Yorkshire, however, a thick deposit known as the Speeton Clay has been ascertained by Mr Judd to pass down into the Jurassic system, and to contain a representation of the upper parts of the Neocomian of the Continent.
The lower division of the Speeton Clay contains, among other fossils, Ammonites Noricus. The central zone is marked by Pecten cinctus, Ancyloceras Duvallii, and Meyeria ornata. The upper division is characterized by Perna Mιιlleti, Ammonites Deshayesii, Pecten orbicularis. It is the fossils of this upper division which occur in the Lower Greensand of Kent. They amount to about 300 species, of which only 18 or 20 per cent, pass up into the Upper Cretaceous. This marked palaeontological break, taken in connexion with traces of unconformability between the Lower Greensand and the Gault, shows that a definite geological boundary-line can be drawn between the lower and upper parts of the Cretaceous system.
Upper Cretaceous.— At the base of this series lies the Gault—a dark blue stiff clay or marl, sometimes sandy and calcareous. It overlaps the older parts of the Cretaceous series, and in Wiltshire lies on Kimeridge Clay. Among the characteristics fossils of this division are Cyclocyatlιus Fittoni, Caryophilia Bowerbankii, Nucula pectinata, Inoceramus sulcatus, Natica Gaultina, Rostellaria carinata, Ammonites dentatus, and Hamites attenuatus. In all, about 200 species of fossils occur, of which about 46 per cent, pass into up into the Upper Greensand.
The Gault is overlaid by a group of sands and sandstones often of a greenish tint from the presence of glauconite grains. Hence the name Upper Greensand which is applied to them. These strata can be traced westwards into Devonshire, and eastwards to the headlands of Kent, but they die out towards the north. Their mineralogical characters and variable thickness seem to point to them as deposits of the shore of the sea in which the chalk was subsequently laid down upon them.
Among their characteristic fossils are the sponges Sip>honia pyri- fσrmis and S. costata ; urchins of the genera Cidaris, Echinus and Salcnia; numerous Tercbratulae and Rhynchoncllcc∙, many lamellibranchs, particularly of the genera Exogyra, Ostrea, Gryphaea, Lima, Pecten, and Trigonia ; and gasteropods of the genera Natica, Turritella, and others. The cephalopods abound and comprise many forms of Ammonites (40 species), Hamitcs, Scaphitcs, Baculitcs, Nautilus, and Belemnites. Chalk.— This conspicuous member of the Cretaceous system has at its base a white or pale yellow marl with green grains of glauconite, phosphatic nodules, and iron pyrites (Chloritic Marl), which is succeeded sometimes by a kind of argillaceous chalk (Chalk Marl) forming the base of the true Chalk. It can be traced from Flamborough Head in Yorkshire across the south-eastern counties to the coast of Dorset. Throughout this long course its western edge usually rises somewhat abruptly from the plains as a long winding escarpment, which from a distance often reminds one of an old coast line. The upper half of the Chalk is generally distinguished by the presence of many nodular layers of flint. With the exception of these enclosures, however, the whole formation is a remarkably pure white pulverulent dull limestone, meagre to the touch, and soiling the fingers. It is composed mainly of crumbled foraιninifera, with the mingled debris of urchins, corals, and mollusks. It must have been accumulated in a sea of some depth and tolerably free from sediment, like some of the foraminiferal ooze of the existing sea-bed. There is, however, no evidence that the depth of the water at all approached that of the abysses in which the present Atlantic globigerina-ooze is being laid down. Indeed, the character of the foraminifera, and the variety and association of the other organic remains, are not like those which have been found to obtain now on the deep floor of the Atlantic.
Somewhere about 800 species of fossils are known from the English Chalk. Occasional rare fragments of teπestrial wood occur, perforated by the teredo, and telling of a transport of some distance from land. Sponges are numerous. They have usually been silicified and preserved in the flint nodules. Among the more characteristic genera are Choanitcs, Cliona, Ventriculites, Brachìolitcs, Spongia, and Siphonia. Careful preparation of a fragment of chalk usually brings to light remains, sometimes •well preserved, of foraminifera.(Rotalina ornata, Cristellaria rotulata, Globigerina bulloides). Corals are represented by about 15 species {Parasmilia, Ccelosmilia, Caryophyllia, &c. ). The echini form one of the most conspicuous features among the Chalk fossils, from their individual numbers and their variety of forms. Among the more common genera the following may be named— Ananchytcs, Echinoconus {Galerites), Cardiaster, Micraster, Cyphosoma, Cidaris, Pseud,odiadema, Discoidca, and Salenia. Among other star-fishes the genus Goniaster occurs in numerous species in the uppeι division of the Chalk. The crinoids were represented in the sea of the period by a Comatula, one or two Pentacrinites, Marsupites, and Bourgucti-crinus. Polyzoa abound in the Upper Chalk {Homaeosolen, Pus-txdopora, Holostoma, kc.). The bracniopods appear in the form of great numbers of Rhynchonella, Tcrebratxιla, and Terebratulina, with Crania, Thecidea, and Kingcna. Among the lamellibranchs the genera Ostrea, Pecten, Inoccramus, and Lima are particularly frequent. Gasteropods are comapratively few, Pleurotomaria perspectiva being one of the few forms found both in the lower and upper division of the Chalk. Cephalopods, however, abound; characteristic species are Belenιnitella plena, B. mucronata, Nautilus Bcslong-champsianus, Ammonites navicularis, A. varians, A. Rothomagcnsis, Turrilites costatus, Baculitcs baculoides, Scaphites ecqualis, and Hamitcs armatus. Upwards of 80 species of fish have been discovered. These include chimaeroids (Edaphodon, Ischyodus), sharks {Hybodus, Ptychodus, Lamna, Otodus), ganoids (Macropoma, Pycnodus), and teleosteous or bony fishes {Beryx, Enchodus, Saurocc∙ phalus). Numerous reptilian remains have been found, more particularly in a bed about 1 foot thick lying at the base of the Chalk of Cambridge, and largely worked for phosphate of lime derived from reptilian coprolites and bones. Among the known forms are several chelonians, the great deinosaur Acanthopholis, several species of Plesiosaurus, 5 or 6 species of Ichthyosaurus, 10 species of Pterodactylus from the size of a pigeon upwards, one of them having a spread of wing amounting perhaps to 25 feet, 3 species of Mosasaurus, a crocodilian {Polyptychodon), and some others. At Cambridge also the bones of one or two species of birds have been found, probably belonging to Natatores allied to the living gulls.
Continental Europe. —The Cretaceous system in many detached areas covers a large extent of the Continent. From the south of England it spreads southward across the north of France up to the base of the ancient central plateau of that country. Eastwards it ranges beneath the Tertiary and post-Tertiary deposits of the great plain, appearing on the north side at the southern end of Scandinavia and in Denmark, on the south side in Belgium and Hanover, round the flanks of the Harz, in Bohemia and Poland, eastwards into Russia, where it covers many thousand square miles up to the southern end of the Ural chain. To the south of the central axis in France, it underlies the great basin of the Garonne, flanks the chain of the Pyrenees on both sides, spreads out largely over the eastern side of the Spanish table-land, and reappears on the west side of the crystalline axis of that region along the coast of Portugal. It is seen at intervals along the north and south fronts of the Alps, extending dow n the valley’of the Rhone to the Mediterranean, ranging along the chain of the Apennines into Sicily and the north of Africa, and widening out from the eastern shores of the Adriatic through Greece, and along the northern base of the Balkans to the Black Sea, round the southern shores of which it ranges in its progress into Asia, where it again covers an enormous area.
A series of rocks covering so vast an extent of surface must needs present many differences of type, alike in their lithological characters and in their organic contents. They bring before us the records of a time when one continuous sea stretched over all the centre with most of the south of Europe, covered the north of Africa, and swept eastwards to the far east of Asia. There were doubtless many islands and ridges in this wide expanse of water, whereby its areas of deposit and biological provinces must have been more or less sharply defined. Some of these barriers can still be traced, as will be immediately pointed out.
The accompanying table contains the subdivisions of the Cretaceous system which have been adopted in a few of the more important areas of Continental Europe.
It will be seen from this table that while there is sufficient palaeontological similarity to allow a general parallelism to be drawn among the Cretaceous rocks of western Europe, there are yet strongly marked differences pointing to very distinct conditions of life, and probably, in many cases, to disconnected areas of deposit. Nowhere can these contrasts be more strikingly seen than in crossing from the Cretaceous basin of the Loire to that of the Garonne. In the north of France the Upper Cretaceous beds are precisely like those of England, the soft white Chalk forming a conspicuous feature in both countries; but, on the south side of the great axis of crystalline rocks, the soft chalk is replaced by hard limestones. There is a prevalence of calcareous matter, often sparry, throughout the whole series of formations, with comparatively few sandy or clayey beds. This mass of limestone attains its greatest development in the southern part of the deparl ment of the Dordogne, where it is said to be about 800 feet thick. But the lithological differences are not greater than those of the fossils. In the north of France, Belgium, and England, the singular molluscan family of the Hippuritidae or Rudistcs appears only occasionally and sporadically in the Cretaceous rocks, as if a stray individual had from time to time found its way into the region, but without being able to establish a colony there. In the south of France, however, the hippuri tes occur in prodigious quantity. They often mainly compose the limestones, hence called hippurite limestones (Rudisten-Kalke). They attained a great size, and seem to have grown on immense banks like our modern oyster. They appear in successive species on the different stages of the Cretaceous system, and can be used for marking palaeontological horizons, as the cephalopods are elsewhere. But while these lamellibranchs played so important a part throughout the Cretaceous period in the south of France, the numerous ammonites and belemnites, so characteristic, of the Chalk in England, were absent from that region. This very distinctive type of hippurite limestone has so much wider an extension than the English type of the Cretaceous system that it should be regarded as really the normal development. It ranges through the Alps into Dalmatia, and round the great Mediterranean basin far into Asia. G umbel has proposed to group the European Cretaceous rocks into three great regions :— (1) the northern province, or area of white chalk with Belcmnitella, Table showing the Subdivisions of the Cretaceous System in the W est of Euroy>e. [table]
comprising England, northern France, Belgium, Denmark, Westphalia; (2) the Hercynian province, or area of Exogyra columba, embracing Bohemia, Moravia, Saxony, Silesia, and central Bavaria; and (3) the southern province, or area of hippurites, including the regions south of the crystalline axis of France, the Alps, and southern Europe.
The Wealden beds, with the Hastings Sands and Weald Clay, are found in north-west Germany. They contain abundant remains of terrestrial vegetation, which is sometimes aggregated into thin seams of black glancing coal, occasionally even as much as G⅛ feet thick. The marine or typical Neocomian series attains a great development among the eastern Alps, where it consists mainly of massive white and grey limestones, divided into zones according to their characteristic fossils. Some geologists place in it a part of the massive Vienna sandstone (Wiener Sandstein) which enters so largely into the structure of the outer Alps. The massive arenaceous formation formerly massed together under the general name of Quader-sandstein, but now found to be the equivalent of the calcareous bands of other regions, and capable of subdivision into the chief normal groups, forms a conspicuous feature in Saxony and Bohemia, as in the great gorge of the Elbe and the picturesque crags and pinnacles of Saxon Switzerland. From the Upper Cretaceous beds, in the neighbourhood of Aix-la-Chapelle, consisting of white sands and laminated clays 400 feet thick, a large number of terrestrial plants have been obtained. The number of species is estimated at more than 400. Of these 70 or 80 are cryptogams, chiefly ferns (Gleichenia, Lygodium, Asplénium, &c.); there are numerous conifers (some akin to Sequoia), and three or four kinds of screw-pine (Pandanus). This flora has a much more modern aspect than any other yet found in Secondary formations. But its most important feature is the occurrence of numerous true exogenous plants—the earliest yet found in Europe. The prevalent forms are Proteaceae, many of them being referred to genera still living in Australia or at the Cape of Good Hope. There occur also species of oak, bog-myrtle, &c. These interesting fragments serve to indicate the modern character of the flora of Europe towards the close of the Cretaceous period, and to show that the climate, doubtless greatly warmer than that which now prevails, nourished a vegetation like that of some parts of Australia or the Cape. Further information has been afforded regarding the extension of this flora by the discovery in North Greenland of a remarkable series of fossil plants. From certain Lower Cretaceous beds of that Arctic region, Heer has described 30 species of ferns, 9 cycads, and 17 conifers; while, from the Upper Cretaceous rocks of Noursoak, he enumerates species of poplar, fig, sassafras, credneria, and magnolia.
North America. —The recent surveys of the western territories of the United States have greatly increased our knowledge of the Cretaceous system on the American continent, where it is now known to cover a vast expanse of surface, and to reach a thickness of sometimes 10,000 feet. Sparingly developed in the eastern States, from New Jersey into South Carolina, it spreads out over a wide area in the south, stretching round the end of the long Palaeozoic ridge from Georgia through Alabama and Tennessee to the Ohio; and reappearing from under the Tertiary formations on the west side of the Mississippi over a large space in Texas and the south-west. Its maximum development is reached in the western States and Territories of the Rocky Mountain region—Wyoming, Utah, and Colorado. Cretaceous rocks have likewise been detected in Arctic America near the mouth of the Mackenzie River.
Much controversy has been carried on among American geologists regarding the upper limit of the Cretaceous system, some maintaining, from the character of the plants and of the shells, that the great plant-bearing series termed the Lignitic is of Tertiary age, others insisting, from the occurrence of true Cretaceous shells in and above the Lignitic series, that it must belong to the Cretaceous system. In the Upper Missouri region Hayden and Meek established the following subdivisions :— [table]
American Cretaceous fossils include the earliest dicotyledonous plants yet found on this continent, upwards of 100 species having been found, of which one-half were allied to living American forms. Among them are species of oak, willow, poplar, beech, maple, hickory, fig, tulip-tiee, sassafras, sequoia, American palm (Sabal), and cycads. The more characteristic mollusca are species of Terebratula, Ostrca, Gryphaea, lnoccramus, Hipymrites, Radiolites, Ammonites, Scaphites, Hamites, Baculites, Belemnites, Ancyloceras, and Turrilitcs. Of the fishes of the Cretaceous seas 97 species are known, comprising large predaceous representatives of modem or osseous types like the salmon and saury, though cestracionts and ganoids still flourished. But the most remarkable feature in the American Cretaceous fauna, as at present known, is the great number, variety, and size of the reptiles. According to the enumeration of Cope, who includes, however, in his list the Lignitic group here placed among the Tertiary formations, there are known at present 18 species of deinosaurs, 4 pterosaurs, 14 crocodilians, 13 sauropterygians or sea-saurians, 48 testudinates (turtles, &c.), and 50 pythonomorphs or sea-serpents. One of the most extraordinary of these reptilian forms was the Elasmosaurus—a huge snake-like form 40 feet long, with slim arrow-shaped head on a swan-like neck rising 20 feet out of the water. This formidable sea-monster “probably often swam many feet below the surface, raising the head to the distant air for a breath, then withdrawing it and exploring the depths 40 feet below without altering the position of its body. It must have wandered far from land, and that many kinds of fishes formed its food is shown by the teeth and scales found in the position of its stomach” (Cope). But the real rulers of the American Cretaceous waters were the pythonomorphic saurians or sea-serpents. Some of them attained a length of 75 feet or more. They possessed a remarkable elongation of form, particularly in the tail; their heads were large, flat, and conic, with eyes directed partly upwards. They swam by means of two pairs of paddles, like the flippers of the whale, and the eel-like strokes of their flattened tail. Like snakes they had four rows of formidable teeth on the roof of the mouth, which served as weapons for seizing their prey. But the most remarkable feature in these creatures was the unique arrangement for permitting them to swallow their prey entire, in the manner of snakes. Each half of the lower jaw was articulated at a point nearly midway between the ear and the chin, so as greatly to widen the space between the jaws, and the throat must, consequently, have been loose and baggy like a pelican’s. Nine species of birds have been obtained from the American Cretaceous rocks. Three of these belonged to the order of Natatores or swimmers, which includes our modern gulls, ducks, and geese; four were Grallae or waders; while two belonged to a long extinct order, and united certain ichthyic and reptilian characters with those of birds. (See Cope, Report of U.S. Geol. Surv. of Territories, vol. ii., 1875; Marsh, American Journ. Science, 3d ser., i. to iv.; Leidy, Smithsonian Contributions, 1865, No. 192; Lesquereux, Cretaceous Flora, Report of U.S. Geol. Surv. of Territories, vol. vi.)
IV. TERTIARY OR CAINOZOIC.
The close of the Secondary periods was marked in the west of Europe by great geographical changes, during which the floor of the Cretaceous sea was raised partly into land and partly into shallow marine and estuarine waters. These events must have occupied a vast period of time, so that, when sedimentation once more began in the region, the organic remains of the Secondary ages had (save in a few low forms of life) entirely disappeared and given place to others of a distinctly more modern type. In England, the interval between the Cretaceous and the next geological period represented there by sedimentary formations is marked by the abrupt line which separates the top of the Chalk from all later accumulations, and by the evidence that the Chalk seems to have been in some places extensively denuded before even the oldest of what are called the Tertiary beds were deposited upon its surface. There is evidently here a considerable gap in the geological record. We have no data for ascertaining what was the general march of events in the south of England between the eras chronicled respectively by the Upper Chalk and the overlying Thanet beds.
Here and there on the Continent a few scraps of evidence are obtainable which help to fill up this gap. Thus, on the banks of the Meuse at Maestricht, a series of shelly and pυlyzoan limestones with a conglomeratic base {Système Maestrichtien of Dumont, who places it above his Senonian system in the Upper Cretaceous series) contains a mingling of true Cretaceous organisms with others which are characteristic of the older Tertiary formations. It contains, for example, the characteristic Upper Chalk crinoid, Bourgeti-crinus ellipticus, in great numbers; also Ostrea vesicularis, Βacιιlitaes Faυjasii, Belemnitella mucronata, and the great reptile Mosasaurus; but with these occur such Tertiary genera as Voluta, Fasciolaria, and others. At Faxoe, on the Danish island of Seeland, the uppermost member of the Senonian series contains in like manner a blending of well-known Upper Chalk organisms with the Tertiary genera Cypraea, Oliva, and Mitra. In the neighbourhood of Paris also, and in scattered patches over the north of France, a formation known as the pisolitic limestone occurs, which was formerly classed with the Tertiary formations, seeing that its fossils had more affinities with later than with older rocks. But the discovery in it of numerous distinctively Upper Cretaceous forms has led to its being placed at the top of the Senonian series, from which, however, it is marked off by a decided unconformability, for it rests on a denuded surface of the White Chalk. These fragmentary formations are interesting, in so far as they help to show that, though in western Europe there is a tolerably abrupt separation between Cretaceous and Tertiary deposits, there was nevertheless no real break between the two periods. The one merged insensibly into the other; but the chronicles of the intervening ages have been in great measure destroyed.
In entering upon the Tertiary series of formations, we find ourselves upon the threshold of the modern type of life. The ages of lycopods, ferns, cycads, and yew-like conifers have passed away, and that of the dicotyledonous angiosperms—the hard-wood trees and evergreens of to-day—now succeeds them, but not by any sudden extinction and recreation, for, as we have seen (ante, p. 359), some of these trees had already begun to make their appearance even in Cretaceous times. The ammonites, baculites, and other cephalopods, which had played so large a part in the molluscan life of the Secondary periods, now cease. The great reptiles, too, which in such wonderful variety of type were the dominant animals of the earth’s surface, alike on land and sea, ever since the commencement of the Lias, now wane before the increase of the mammalia, which advance in ever-augmenting diversity of type until man appears at their head.
The name Tertiary, given in the early days of geology before much was known regarding fossils and their history, has retained its hold on the literature of the science. It is sometimes replaced by the term Cainozoic (recent Ufa), which expresses the great fact that it is in the series of strata comprised under this designation that most recent species and genera have their earliest representatives. Taking as the basis of classification the percentage of living species of mollusca found in the different groups of the Tertiary series, Lyell proposed a scheme of arrangement which has been generally adopted. The older Tertiary formations, in which the number of still living species of shells is very small, where, in fact, we seem to see as it were the first beginnings of the modern life, he named Eocene (dawn of the recent), including under that title those parts of the Tertiary series of the London and Paris basins wherein the proportion of existing species of shells was only 3⅛ per cent. The middle Tertiary beds in the valleys of the Loire, Garonne, and Dordogne, containing 17 per cent, of living species, were termed Miocene (less recent). The younger Tertiary formations of Italy were included under the designation Pliocene (more recent), because they contained a majority or from 35 to 95 per cent, of living species. This newest series, however, was further subdivided into Older Pliocene (35 to 50 per cent, of living species) and Newer Pliocene (90 to 95 per cent.). This classification, with various modifications and amplifications, has been adopted for the Tertiary group not of Europe only but of the whole globe.
As the North American development of the Tertiary series differs in so many respects from that of Europe, it will be most conveniently considered by itself after the European classification has been described.
Eocene. Great Britain. —The Eocene rocks of Britain are entirely confined to the south-east of the island, where they occupy two great depressions of the chalk, known respectively as the London and Hampshire basins. They have been arranged into the groups shown in the subjoined table. [table]
[table]
Grouped in relation to the physical changes which they record, these strata naturally stand in three divisions. At the base lies a series of beds laid down in fresh, estuarine, and sea-water, on an upraised denuded surface of Chalk. Then comes a central group bearing witness to the deepening of these shallow waters, and to the advance of the sea far up the former estuary. The upper group brings before us proof of the eventual retreat of the sea, and the conversion of the area once more into fresh-water lakes and rivers. This arrangement does not quite tally with that which is based on a comparison of the fossils with existing forms, and with those of other Tertiary districts, for it places the London Clay in the middle series, though the fossil evidence distinctly shows that formation to belong to the older Eocene groups.
Lower Eocene.— The Thanet Sand at the base of the London Tertiary basin consists of pale yellow and greenish sand, sometimes clayey, and containing at its bottom a layer of green-coated flints resting directly on the Chalk. According to Mr Whitaker, it is doubtful if any proof of actual erosion of the chalk can anywhere be seen under the Tertiary deposits in England, and he states that the Thanet Sands everywhere lie upon an even surface of chalk with no visible unconformability. Professor Philips, on the other hand, describes the chalk at Reading as having been “literally ground down to a plain or undulated surface, as it is this day on some parts of the Yorkshire coast,” and having likewise been abundantly bored by lithodomous shells. The fossils of the Thanet Sand comprise about 70 known species (all marine, except a few fragments of terrestrial vegetation). Among them are several foraminifera, numerous lamellibranchs (Astarte tenera, Cyprina Morrisii, Ostrea Bellovacina, <fcc.), a few species of gasteropods (Natica subdepressa, Aporrhais Sowerbii, <fcc.), a nautilus, and the teeth or palatal bones of fishes (Lamna, Pisodus).
The Woolwich and Reading beds, or Plastic Clay of the older geologists, consist of lenticular sheets of plastic clay, loam, sand, and pebble beds. The organic remains show that the sea of the Thanet Sand era gradually shallowed into an estuary. They amount to more than 100 species, and include a few plants of terrestrial growth such as Ficus Forbesi, Greυillea Heeri, and Laurus Hookeri. The lamellibranchsare partly estuarine or fresh-water (Cyclas, Cyrena, Dreissena, Unio), partly marine. Of the latter a characteristic species is Ostrea Bellovacina, which forms a thick oyster bed at the base of the series. Ostrea tenera is likewise abundant. The gasteropods include a similar mixture of marine with fluviatile species (Cerithium funatum, Melania inquinata, Natica subdepressa, Fusus latus, Paludina lenta, &c.) The fish are chiefly sharks (Lamna). Bones of turtles and scutes of crocodiles have been found. The highest organisms are bones of mammalia, one of which, the Coryphodon, was allied to the modern tapir.
The Oldhaven beds forming the base of the London Clay, though of trifling thickness, have yielded upwards of 150 species of fossils. Traces of Ficus, Cinnamomum, and Coniferae have been obtained from them; but the organisms are chiefly marine and partly estuarine shells, the gasteropods being particularly abundant. The London Clay, as its name implies, is a mass of clay, stiff, brown, or bluish-grey, with septarian nodules. It extends through both the London and Hampshire basins, attaining a maximum thickness in the south of Essex. It has yielded a long and varied suite of organic remains, from which we can see that it must have been laid dcwn in the sea beyond the mouth of a large estuary, into which abundant relics of the vegetation, and even sometimes of the fauna of the adjacent land were swept. Its fossils are mainly marine mollusca, and, taken in connexion with the flora, indicate that the climate was somewhat tropical in character. The plants include the fruits or other remains of palms (Nipadites), custardapple, acacia, gourds, melons, Proteaceae, and Coniferae. Crustacea abound (Xanthopsis, Hoploparia). Gasteropods are the prevalent mollusks, the common genera being Pleurotoma (45 species), Fusus (15 species), Cypraea, Murex, Cassidaria, Pyrula, and Voluta. The cephalopods are represented by 6 or more species of Nautilus, by Belosepia sepioidea, and Beloρtera Levesquei. Nearly 100 species of fishes occur in this formation, the rays (Mylio-bates, 14 species) and sharks (Lamna, Otodus, &c.) being specially numerous. A sword-fish (Tetrapterus priscus), and a saw-fish (Pristis bisulcatus) about 10 feet long, have been described by Agassiz from the London Clay of Sheppey, whence almost the whole of the fish remains have been obtained. The reptiles were numerous, but markedly unlike, as a whole, to those of Secondary times. Among them are numerous turtles and tortoises, two species of crocodile, and a sea-snake (Palaeophis), estimated to have been 13 feet long. Remains of birds have also been met with; one of these (Lithornis vulturinus) appears to have been allied to the vulture, another (Halcyomis toliapicus) to our modern king-fisher, besides waders and other types. The mammals numbered among their species a hog (Hyra- cotherium), several tapirs (Coryphodon, <tc.), an opossum (Didelphys), and a bat. The carcases of these animals must have been boιne seawards by the great river which transported so much of the vegetation of the neighbouring land.
Middle.— The Bagshot group consists of sand and bands of clay which, in the Hampshire basin, are tolerably fossiliferous. As developed in the Isle of Wight and at Bournemouth, their lower members have yielded a large number of terrestrial plants, among which the Proteaceae are still numerous, together with species of fig, cinnamon, fan-palm (Sabal), oak, yew, cypress, laurel, lime, senna, and many more. Crocodilian forms still haunted the waters, and have left their bones with those of sea-snakes and turtles and of the tapirs and other denizens of the land, which still, as in the time of the London Clay, continued to be washed out to sea. Among these strata we now find one of the most typical organisms of the Eocene rocks of the Mediterranean basin, a foraminifer termed Nummulites (N. laevigata, N. scabra, N. variolaria). Characteristic fossils are Voluta athleta, V. luctatrix, Murex asper, Fusus longaevus, Cardita planicosta, and Chama squamosa. Upper. — The strata of this division of the English Eocene formations are entirely confined to Hampshire and the Isle of Wight. They consist of sands, clays, marls, and limestones, in thin-bedded alternations. These strata were accumulated partly in the sea, partly in brackish, and partly in fresh water. They were hence named by Edward Forbes the fluvio-marine series. Among the marine fossils are— Fusus porrectus, Oliva Branderi, Natica labellata, Ostrea callifera, and Nummulites laevigata. The genera Cerithium, Potamomya, Paludina, Planorbís, Limnaea, and Cyclas are abundant, showing the brackish and fresh-water conditions in which many of the strata were deposited. Remains of turtles, snakes, crocodiles, and alligators continue to occur in these Upper Eocene beds. With these are found the bones of several very characteristic mammals also met with in the Paris basin,—the three-toed Palaeotherium, resembling the living tapir; Anoρlotherium, a more graceful animal with long tail, and two-toed feet, forming a kind of intermediate type between a hog and a deer or antelope; Dichobune, allied to the last named: also Dichodon, Hyopo- tamus, Chaeropotamus, Hyaeuodon, <fcc. The top of the Eocene series in the Isle of Wight has been removed by denudation, so that we have no evidence in Britain of what took place after the close of the Eocene period.
Continental Europe. —Geologists on the continent of Europe, finding it impossible to carry out the principle of percentage of recent species, as originally formulated by Lyell in his terminology of the Tertiary series, have made various modifications of this nomenclature. By some the three terms Eocene, Miocene, and Pliocene are retained, but, following Beyrich, they subdivide the Miocene into two, keeping that term for the upper half and calling the lower Oligocène, which corresponds with Lyell’s Lower Miocene. Others would consider the whole Tertiary and post-Tertiary series as divisible into three groups, the Eocene or Older Tertiary, corresponding pretty closely to the Lyellian use of the term, the Neogene or Younger Tertiary, embracing both Miocene and Pliocene, and the Diluvial and Alluvial.
In the Paris basin the Eocene formations assume a somewhat different type from that which they present in England, though the occurrence of a number of the same species of fossils in both allows of their being paralleled in a general way. The lower Eocene consists there of sand and clay answering in lithological character to the Thanet Sand and Plastic Clay of the London basin. The common species in that basin {Ostrca Bellovacind) occurs there in great numbers, while the brackish water-beds contain some of the common species at Woolwich, such as Cyrena cuneiformis and Melania inquinata. Beds of lignite occur in this division, likewise bones of Coryphodon Eocaenus, Viverra gigantea, and the bird Gastomis. The Middle Eocene is made up of the characteristic “Calcaire grossier”—a mass of limestone, sometimes tender and crumbling, in other places so compact as to be largely quarried as a building stone. Some portions are entirely composed of minute foraminifera (miliolitic limestone). Among the characteristic fossils of this division are Nummulites, Cerithium giganteum, with bones of I)ichobune, Lophiodon, Paloplotherium, &c. The Upper Eocene consists of sand (Sables moyens) overlaid by the great gypsum and gypseous marl group of Montmartre. This is the deposit from which so many of the mammals of the Eocene period have been recovered. It is divided into three zones, and among its fossils are upwards of 50 species of quadrupeds, including many Palaeotheres, Anoplo-theres, Paloplotheres, with Xiphodon, Dichobune, Adapis, Chaeropotamus, Myoxus, Canis Parisiensis, Viverra Parisiensis, Vespertilio, Didelphys Cuvieri, and about 17 species of birds.
The Eocene formations of the north-λvest of Europe occupy but a few detached basins, and consist for the most part of soft clays, sands, marls, and thin limestones. They were laid down partly in estuaries, rivers, or lakes, partly in shallow seas near land. They contain abundantly the vegetation, with some remains of the quadrupeds and birds, of that land, and show that still in older Tertiary times, as during the long Palaeozoic and Secondary ages, the chief area of land lay to the north-west. But when we turn to the corresponding formations in central and southern Europe, they present a totally different aspect. In the first place, they at once impress us with the idea of their antiquity, for they consist chiefly of massive, hard, crystalline, and sometimes even marble-like limestones, which suggest some of the Palaeozoic rocks rather than those of so modern a date as the London Clay and Calcaire Grossier. Again, instead of being confined to a few local basins, they cover an enormous geographical area and play a notable part in the structure of some of the great mountain chains of the globe. Crowded as they are with nummulites, they must have been deposited not in estuaries and shallow bays but in a wide and clear sea, which, traced by the area of these limestones, must have ranged across the whole of the south of Europe and north of Africa, through Greece, Turkey, Asia Minor, and the heart of Asia, to the far shores of China and Japan. Since the time when this wide channel connected the Atlantic and the Pacific across the heart of the Old World, the great mountain ranges of the Pyrenees, Alps, Apennines, Carpathians, and of Central Asia, have been upheaved to their present altitude. Some of the prominent peaks along their flanks consist of the hardened and crumpled calcareous mud of the Eocene sea.
In the northern and southern Alps the Eocene formations consist of nummulitic limestone—a grey, yellow, sometimes reddish compact rock, usually containing and often made up of nummulites; nummulite sandstone; Vienna sandstone—an enormous mass of arenaceous rock almost destitute of organic remains, and referred partly to the Cretaceous and partly to the Eocene series; and Flysch —a massive development of dark shales or schists, sandstones, and argillaceous limestone, sometimes charged with the remains of fucoids and (at Matt, Glarus) of fish. Tlιe nummulitic series of southern Europe is divided into zones characterized by fossils, and brought into a kind of broad parallelism with the subdivisions of the English and French Eocene basins. In the eastern Alps, near Vienna and elsewhere, some of the nummulitic sandstones contain enormous blocks of granite, gneiss, and other crystalline rocks, which are believed to have been ice-borne, and therefore to prove the existence of Alpine glaciers even in Eocene times. These mountains already existed, as it were, in embryo, even far back in the Secondary and Palaeozoic ages. During the later part of the Eocene period they seem to have been clothed with an abundant flora, among which the fan-palm, Banksia, Dryandria, and other plants remind one of the living vegetation of tropical America, the East Indies, and Australia. Out of these plants the important coal-seams of Haring in Tyrol were formed.
Miocene.
According to the original nomenclature proposed by Lyell, this subdivision of the Tertiary series was meant to include those strata in which 17 per cent, or thereabouts of the. shells belong to still living species. As the system of nomenclature was adopted at a time when our knowledge both of living and fossil species was still very defective, it could not but require modification with the progress of science. Some strata, classed at one time as Miocene from their proportion of recent forms, might, on more extended research, prove to contain a much larger percentage, and therefore to be referable to a later part of the Tertiary series. The term, however, is used as a convenient and long-established designation for a series of strata younger than the Eocene, which they seem to have succeeded, though in some parts of the European area after enormous geographical changes.
Great Britain. —Miocene formations, in the ordinary sense of the term, are almost entirely absent from the British Islands. In Devonshire, at Bovey Tracey, a small but interesting group of sand, clay, and lignite beds, from 200 to 300 feet thick, lies between the granite of Dartmoor and the Greensand hills, in what was evidently the hollow of a Miocene lake. From these beds Heer of Zurich, who has thrown so much light on the Miocene flora of both the Old World and the New, has described about 50 species of plants, which, he says, place this Devonshire group of strata on the same geological horizon with some part of the Lower Miocene formations of Switzerland. Among the species are a number of ferns (foastraea stiriaca, Pecopteris lignitum, &c.); some conifers, particularly a Wellingtonia called the Sequoia Couttsiae, the debris of which forms one of the lignite beds; a few grasses, water-lilies, and a palm. Leaves of oaks, figs, laurels, willows, and seeds of grapes have also been detected—the whole vegetation implying a subtropical climate. In the north of Ireland lies a great plateau of basalt, presenting along the coast of Antrim a magnificent range of mural escarpments. The basalt-beds mark successive outpourings of lava, which took place on a prodigious scale from the Antrim region northwards through the Western Islands and the Faroe Islands into Iceland, and even far up into Arctic Greenland. In Ireland the basalts attain a maximum thickness of 900 feet; in Mull about 3000 feet. They are associated with tuffs, pitchstones, trachytes, and granitoid rocks, which mark the position of the main vents of eruption. It is evident that long-continued and vigorous volcanic action took place in these north-western regions. The geological date of this activity can be approximately fixed by the fossil plants here and there to be found in leaf-beds between the sheets of basalt. They agree generally with species found in the Older Miocene beds of Switzerland; and hence the date of this marked volcanic era in the north-west of Europe and in Greenland is placed in the older part of the Miocene period. With these exceptions there are no Miocene rocks in Britain.
Continental Europe. —The records of the Miocene period in Europe bring before us a scene very different from that which the region presented during the Eocene ages. The least amount of change took place in the north-west and north. It consisted chiefly in the upraising of the floor of the shallow sea which had stretched eastwards across the north of France, and the formation of numerous fresh-water lakes, lagoons, and morasses. But over the Alpine region a vast revolution took place. The Eocene sea had its bed elevated, and the Alps rose many thousand feet above their previous height, carrying up on their flanks the hardened, upturned, crumpled, and crystallized nummulite limestone. These vast corrugations of the earth’s crust were general over the whole globe about the same geological period. Subterranean movements appear to have continued during Miocene times in the Alpine area; large lake-basins were formed over a great part of what is now Switzerland, and in these the deposits of the period took place. Hence the Miocene formations of Europe are in great measure of lacustrine and terrestrial formation. The sea, however, was allowed to pass across the south of France between the central axis and the Pyrenees. It likewise covered a wide area in eastern Europe, sweeping far up into the Vienna basin and the foot of the Austrian Alps. Traces of the occasional presence of the sea can likewise be followed across Belgium and North Germany, and even on the south side of the Eifel and Westphalian high grounds in the basin of Mainz.
In France, on the north side of the axis of old rocks, the upper parts of the Eocene series are covered by certain marls, clays, and limestones, containing Ostrea cyathula, and passing up into the thick sandstone of Fontainebleau, well known for its picturesque scenery. These are considered as the base of the Miocene (or Oligocène) series. They contain Fusus elongatus, Aporrhais speciosa, Pectunculus obovatus, Cyrena semistriata, Cerithium plicatum, &c. They are succeeded by some fresh-water limestones (Calcaire de Beauce) full of Paludina, Planorbis, Limnaea, and Helix. These strata show very instructively the gradual shallowing of the sea in the north of France, and its conversion into fresh-water lakes and land. Further to the south a subsequent depression took place, in which the shelly sands and marls (faluns) of Touraine were accumulated. These strata lie in scattered patches, and seldom exceed 50 feet in thickness. They contain chiefly marine shells (Cypraea, Oliva, Mitra, Terebra, Fasciolaria, Conus, kc.), also remains of the Dichobune and Cheer opotamus, with some of the huge early pachyderms,— Mastodon, Dinothérium, rhinoceros, hippopotamus, and species of dolphin, morse, &c.
In North Germany the older Miocene rocks are marine where they pass westwards into the Belgian area, but as they are followed southwards they bear evidence of increasing proximity to land, till at last they contain abundant seams of lignite, and hence have been termed the Brown-coal series. They appear to underlie most of the great north Germanic plain, on the south side of which they rise up into the Rhine valley, Thuringia, Saxony, and Silesia. The Brown-coal series consists of sandstones, conglomerates, and clays, with leaf-beds and seams of lignite. The last, which have been largely worked as fuel, are mainly composed of remains of conifers, ( Tarîtes, Taxoxylon, Cupressinoxylon, Sequoia), but with them are associated, in the neighbouring strata, leaves and other fragments of oak, beech, maple, fig, laurel, cinnamomum, magnolia, sassafras, palmacites, &c. These strata are overlaid by marine beds containing Ostrea, Cardita, Pleurotoma, Voluta, and numerous corals. The subterranean movements above referred to as having long affected the Alpine region were felt likewise in North Germany, for we find these marine beds succeeded by a second lignitic formation, seen at Grosser Hubenecken, on the eastern horn of the Gulf of Dantzic. But this could only have been a local uplift, for marine sands and clays are next found with a wide diffusion over the plains of northern Germany.
Nowhere in Europe do the Miocene formations play so important a part in the scenery of the land, or present on the whole so interesting and full a picture of the state of Europe when they were deposited, as in Switzerland. Rising into massive mountains, as in the well-known Righi and Rossberg, they attain a thickness of more than 6000 feet. While they include indications of the presence of the sea, they have preserved with marvellous perfection a large number of the plants which clothed the Alps, and of the insects which flitted through the woodlands. They are termed “Molasse” by the Swiss geologists, and have been divided into the following groups :—
Upper fresh-water Molasse and brown-coal, consisting of sandstones, marls, and limestones, with a few lignite-seams and fresh-water shells, and including the remarkable group of plant and insect-bearing beds of Oeningen.
Upper marine Molasse (Helvetian stage)—sandstones and calcareous conglomerates, with 37 per cent, of shells, which are to be found living partly in the Mediterranean, and partly in tropical seas.
Lower fresh-water Molasse (Mainz stage)—sandstones with abundant remains of terrestrial vegetation, and containing also an intercalated marine band with Cerithium, Venus, &c.
Lower brown-coal or red Molasse (Aquitanian stage)—the most massive member of the whole series, consisting of red sandstones, marls, and conglomerates (Nagelfluh), resting upon variegated red marls. It contains seams of lignite, and a vast abundance of the remains of terrestrial vegetation.
Lower marine Molasse (Tongrian stage)—sandstone containing marine and brackish-water shells, among which is the Ostrea cyathula, above mentioned as a characteristic shell, at the base of the Miocene series in the north of France.
By far the larger portion of these strata is of lacustrine origin. They must have been formed in a large lake, the area of which probably underwent gradual subsidence during the period of deposition, so that the sea gained occasional admission into it. We may form some idea of the importance of the lake from the fact already stated, that the deposits formed in its waters are upwards of 6000 feet thick. Thanks to the untiring labours of Professor Heer, we know more of the vegetation of the mountains round that lake than we do of that of any other ancient geological period. The woods were marked by the predominance of an arborescent vegetation, among which evergreen forms were conspicuous, the whole having a decidedly American aspect. Among the plants were palms of American type, the Californian coniferous genus Sequoia, a great variety of maples, oaks, hornbeams, poplars, planes, willows, laurels, evergreen oaks, with vines, clematis, cypresses, and many more. In the Oeningen beds, so gently have the leaves, flowers, and fruits fallen, and so well have they been preserved, that we may actually trace the alternation of the seasons by the succession of different conditions of the plants. Selecting 482 of those plants which admit of comparison, Heer remarks that 131.might be referred to a temperate, 266 to a sub-tropical, and 85 to a tropical zone. American types are most frequent among them; European types stand next in number, followed in older of abundance by Asiatic, African, and Australian. Great numbers of insects (between 800 and 900 species) have been obtained from Oeningen. Judging from the proportions of species found there, the total insect fauna may be presumed to have been then richer in some respects than it now is in any part of Europe. The woodbeetles were specially numerous and large. Nor did the large animals of the land escape preservation in the silt of the lake. We know, from bones found in the Molasse, that among the inhabitants of that land were species of tapir, mastodon, rhinoceros, and deer. The woods were haunted by musk-deer, apes, opossums, three-toed horses, and some of the strange, long-extinct Tertiary ruminants, akin to those of Eocene times. There were also frogs, toads, lizards, snakes, squirrels, hares, beavers, and a number of small carnivores. On the lake the huge Dinothérium floated, mooring himself perhaps to its banks by the two strong tusks in his under jaws. The waters were likewise tenanted by numerous fishes (of which 32 species have been described, all save one referable to existing genera), crocodiles, and chelonians.
Contemporaneously with the existence of the great Swiss Miocene lake, one or more large sheets of fresh water lay in the heart of France. In these basins a series of marls and limestones (1500 feet thick) accumulated, from which have been obtained the remains of nearly 100 species of mammals, including some palaeotheres, like those of the Paris Eocene basin, a few genera found also in the Lower Miocene beds of Mainz, crocodiles, snakes, and birds. This water basin appears to have been destroyed by volcanic explosions, which afterwards poured out the great sheets of lava, and formed the numerous cones and puys so conspicuous on the plateau of Auvergne.
The sea which during the later part of the Miocene period stretched across the south of France ran also up the present valley of the Danube to the foot of the Alps and the high grounds of Bohemia and Moravia In this wide eastern bay or Vienna basin a series of deposits was laid down, which have been grouped in four divisions,—the two lower classed as Miocene, and the two upper as Pliocene. The lowest group (Mediterranean stage or Marine Tegel) has yielded more than 1000 species of mollusca (Comis, Ancillaria, Pyrυla, Murex, Cardium, Ac.). with remains of Mastodon, Dino- thcrium, and other mammals. The second group (Sarmatian stage) consists of brackish-water beds showing the partial uprise of the bottom of the Vienna basin. It contains Ccrithium, Paludina, Rissoa, with bones of dolphins, seals, turtles, and fish. The two uppermost divisions (Congeria stage and Belvedere stage), referred to the Pliocene series, are fresh-water formations, showing the final freshening and disappearance of the Miocene sea in the south-east of Europe.
In Styria and Carinthia the lower Aquitanian or Mediterranean stage bears witness in its plants and lignites to the proximity of land during its formation, while its shells are of fresh and brackish water genera. It has subsequently been upheaved, and the later Miocene strata lie unconformably on its edges. The subterranean movements east of the Alps culminated in the outpouring of enormous sheets of trachyte, andesite, propylite, and basalt in Hungary and along the flanks of the Carpathian chain into Transylvania. In Croatia the older Miocene marls, with their abundant land-plants, insects, &c., contain two beds of sulphur (the upper 4 to 16 inches thick, the under 10 to 15 inches), which have been worked at Radoboj. At Hrastreigg, Buchberg, and elsewhere, coal is worked in the Aquitanian stage in a bed sometimes 65 feet thick. In Transylvania, and along the base of the Carpathian mountains, extensive masses of rock-salt and gypsum are interstratifìed in the Tertiary formations. The largest of these, that of Parajd, has a maximum length of about 2500 yards, a breadth of 1800 yards, and a depth nearly 200 yards, and is estimated to contain 3500 millions of cubic feet of salt.
Greenland. —One of the most remarkable geological discoveries of recent times has been that of Miocene plant beds in North Greenland. Heer has described a flora extending at least up to 70° N. lat., containing 137 species, of which 46 are found also in the central European Miocene basins. More than half of the plants are trees, including 30 species of conifers (Sequoia, Thujopsis, Salisburia, &c.), besides beeches, oaks, planes, poplars, maples, walnuts, limes, magnolias, and many more. These plants grew on the spot, for their fruits in various stages of growth have been obtained from the beds. From Spitzbergen (78° 56' N. lat.) 136 species of fossil plants have been named by Heer. But the latest English Arctic expedition brought to light a bed of coal, black and lustrous like one of the Palaeozoic fuels, from 81° 45' lat. It is from 25 to 30 feet thick, and is covered by black shales and sandstones full of land-plants. Heer notices 26 species, 18 of which had already been found in the Arctic Miocene zone. As in Spitzbergen, the conifers are most numerous (pines, firs, spruces, and cypresses), but there occur also the arctic poplar, two species of birch, two of hazel, an elm, and a viburnum. In addition to these terrestrial trees and shrubs the stagnant waters of the time bore water-lilies, while their banks were clothed with reeds and sedges. When we remember that this vegetation grew luxuriantly within 8° 15' of the North Pole, in a region which is now in darkness for half of the year, and is almost continuously buried under snow and ice, we can realize the difficulty of the problem in the distribution of climate which these facts present to the geologist.
Pliocene. Great Britain. —The Miocene period seems to have passed away without any notable portion of the British Islands being depressed under the sea. Save the great outpouring of lava in the north-west, and the rise of hundreds of “dykes” of basalt along cracks of the crust in the north of England and throughout Scotland, the area of Britain seems to have remained as a part of the mainland of Europe, little affected by the subterranean movements which, as we have seen, were so potent among the Alps and in eastern Europe. At length the south-eastern counties began to subside, and on their submerged surface some sandbanks and shelly deposits were laid down, very much as similar accumulations now take place at the bottom of the North Sea. These formations are termed the Crag, and are subdivided, according to their proportion of living species of shells, into the following groups :— [table]
The White Crag consists of shelly sands and marls. It contains 316 species of shells, of which 84 per cent, are still living. Among these are Terebratιda grandis, Lingιda Dumortieri, Pecten opercularis, Pholadomya histerna, Pyrula reticulata. The name coralline was given to the formation from the immense number of coral-like polyzoa which it contains, no fewer than 130 species having been described. The Bed Crag is also a thin and local formation, consisting of a dark-red or brown ferruginous shelly sand. Of its mollusks, 92 per cent are believed to be still living species, and, out of 25 species of corals, 14 are still natives of British seas. Some of the typical shells of this subdivision are Trophon antiquum (Fusus contrarius), Voluta Lamberti, Purpura tetragona, Peden opercularis, Pectunculus gly-cimeris, and Cyprina rustica. Numerous mammalian remainshave been obtained from these sands, including bones of Mastodon Arvernensis and Μ. tapiroides, Elephas meridionalis, Rhinoceros Schleiermachaeri, Tapiruae priscus, Sus antiquus, Equus plicideus, Hipparion, Hyaena antiqua, Felis pardoides, and Cervus anoceros. The Norwich or Fluviomarine or Mammaliferous Crag consists of a few feet of shelly sand and gravel, containing, so far as known, 139 species of shells, of which 93 per cent, are still living. About 20 of the species are land or fresh-water shells. The name of mammaliferous was given from the large number of bones, chiefly of extioct species of elephant, recovered from this deposit. These fossils comprise Mastodon Arvernensis, Elephas meridionalis, E. antiquus, a hippopotamus, horse, and deer, likewise the living species of otter and beaver. One interesting feature in this formation is the decided mixture in it of northern species of shells, such as Rhynchonella p>sittacea, Scalaria Groenlandiaea, and Astarte borealis. These may be regarded as the forerunners of the great invasion of Arctic plants and animals which, in the beginning of the Quaternary ages, came southward into Europe, together with the severe climate of the north. The Chillesford beds occur likewise as a thin local deposit in Suffolk and Norfolk. Of the shells which they contain, about two-thirds still live in Arctic waters. It is evident that, in these fragmentary accumulations of the Crag series, we have merely the remnants of some thin sheets of shelly sands and gravels laid down in the shallow waters of the North Sea, while that great lowering of the European climate was beginning which culminated in the succeeding or Glacial period.
Continental Europe. —Marine strata, sometimes of considerable thickness, were laid down over different portions of the European area during the Pliocene period. The most extensive of these occur in Italy and Sicily; in the latter island they have since been upheaved to a height of 3000 feet above the sea. They have likewise been raised into the chain of heights flanking the Apennine Mountains, where they are known as the Subapennine series. In the shore waters and estuaries of that ancient Italy some of the same huge mammals lived as were contemporaneously denizens of England,— the Auvergne mastodon. Rhinoceros Etruscus Elephas meridionalis, Hippopotamus major, with bears and hyaenas.
Eastwards we obtain evidence of the gradual exclusion of the sea from the areas of the European continent which it had covered during the Miocene period. The Congeria stage (above referred to) of the Vienna basin brings before us the picture of an isolated gulf gradually freshening by the inpouring of rivers like the modern Caspian, but with bays nearly cut off from the main body of water, and undergoing so copious an evaporation without counterbalancing inflow that their salt was deposited over the bottom as in the Karaboghaz of the Caspian (ante, p. 279).
Tertiary Series of North America.
Tertiary formations of marine origin extend in a strip of low land along the Atlantic border of the United States, from the coast of New Jersey southward round the margin of the Gulf of Mexico, whence they run up the valley of the Mississippi to beyond the mouth of the Ohio. On the western sea-board they also occur in the coast ranges of California and Oregon, where they sometimes have a thickness of 3000 or 4000 feet, and reach a height of 3000 feet above the sea. Over the Rocky Mountain region Tertiary strata cover an extensive area, but are chiefly of fresh-water origin, though containing marine interstratifications. The following are the subdivisicns into which they have been grouped, together with their supposed European equivalents :— [table]
1. Lignitic.— As already mentioned, it is still matter of dispute whether this formation should not be included wholly or in large measure in the Cretaceous system below. It consists mainly of lacustrine strata, with occasional brackish water and marine bands. Its name is derived from the beds of fossil fuel which it contains. The mollusca in some of the shell-bearing beds comprise species of Inoccramus,Anclιura,Gyrodcs, Ca†dium, Cyrena, Melampus, Ostrca, and Anomia. Other strata contain the modern lacustrine and fluviatile genera Physa, Valvata, Cyrena, Corbula, and Unio. An abundant terrestrial flora has been disinterred from the Lignitic strata. It resembles in many respects the present flora of North America, comprising as it does species of oak, poplar, maple, elm, dogwood, beech, hickory, ilex, plane, fig, cinnamon, laurel, magnolia, smilax, thuja, sequoia, and several palms. A few of the species are common to the Middle Tertiary flora of Europe, and a number of them have been met with in the Tertiary beds of the Arctic regions. Some of the seams of vegetable matter are true bituminous coals and even anthracites.
It was stated in the account of the North American Cretaceous rocks that considerable difference of opinion exists as to the line to be drawn between these rocks and those of Tertiary age. According to Cope, the Vertebrate remains of the Lignitic series bind it indissolubly to the Mesozoic formations. Lesquereux, on the other hand, insists that the vegetation is unequivocally Tertiary. The former writer, admitting the force of the evidence furnished by the fossil plants, concludes, that “there is no alternative but to accept the result that a Tertiary flora was contemporaneous with a Cretaceous fauna, establishing an uninterrupted succession of life across what is generally regarded as one of the greatest breaks in geologic time." The Lignitic series was disturbed along the Rocky Mountain region before the deposition of the succeeding Tertiary formations, for these lie uncomformably upon it. So great have been the changes in some regions that the strata have assumed the character of hard slates like those of Palaeozoic date, if indeed they have not become in California thoroughly crystalline masses.
2. Alabama Group.— The strata included in this group are believed to represent the Middle and Upper Eocene of Europe. As their name implies, they are well developed in the State of Alabama, where they consist of two sub-groups,—(1) the Clayborne beds— clays, marls, limestones, lignite, and sands, and (2) the Vicksburg beds—lignitic clays, limestones, and marls,—the whole attaining a thickness of nearly 250 feet. But the strata thicken into South Carolina. Towards the west the marine fossils give place to leaf-beds and lignites. In the Green River basin the strata attain a thickness of more than 2000 feet, lying on the disturbed Lignitic group, and containing beds of coal, with remains of fish and of tapiroid pachyderms. The fossils of the Alabama group in the eastern States comprise numerous sharks, some of which are specifically, and more are generically, the same as some of the English Eocene forms, such as Lamna elegans, and Carcharodon megalodon ; also bones of several crocodiles and snakes. In the lacustrine and fluviatile deposits of the west there have been found also the remains of several birds, and a large number of mammals, including marsupials, numerous representatives of the tapirs, with double pairs of nasal horns, an early form of horse (Orohippus) not larger than a fox, and with four toes in the fore foot, rodents, insectivores, bats, carnivores, and a number of forms allied to the living lemurs and marmosets. The richness of this fauna is remarkable, particularly in the mammals.
3. Yorktown Group.— Under this name are classed strata of sand and clay, which extend over a large area in the seaward part of the eastern States. Their organic remains (comprising mollusks, with remains of sharks, seals, walruses, whales, &c.) show them to have been chiefly laid down in a shallow sea in Miocene time. Westward, in the Upper Missouri region, and across the Rocky Mountains into California and Oregon, strata assigned to the same geological period were laid down in great lakes, and attain thicknesses of 1000 to 4000 or 5000 feet. The organic remains of these ancient lakes embrace examples of three-toed, horses (Anchitherium), of horned tapir-like animals (Titanotherium), of forms related to the hog, rhinoceros, camel, lama, deer, musk-ox, hare, squirrel, beaver, hyaena, wolf, panther, and tiger. The intermediate types indicated by Cuvier among the Paris Tertiary beds have been greatly increased from the American Tertiary groups by the researches of Marsh and Leidy.
4. Sumter Group or Pliocene.— In the Carolina States beds of loam, clay, or sand, lying in hollows of the older Tertiary deposits, and containing from 40 to 60 per cent, of living marine shells, are referred to the Pliocene age. In the Upper Missouri region, the Yorktown group is overlaid by other fresh-water beds, which are believed to be Pliocene. These strata contain land and fresh-water shells, probably of existing species. But their most remarkable organic remains are the mammalia, which continue the wonderful series in the strata underneath. Dana thus summarizes the facts :— “Leidy has determined a large number of Pliocene mammals, all now extinct. They include three species of camel (Procamelus) ; a rhinoceros as large as the Indian species; a mastodon smaller than Μ. Americanus, L., of the Quaternary; an elephant (E. Americanus), occurring also in the Quaternary; four or five species of the horse family, one of which was closely like the modern horse; a species of deer (Cervus Warreni, L.); others near the musk-deer of Asia; species of Mcrechyus, allied to Oreodon (a genius intermediate between deer, camel, and hog); a wolf larger than any living species; a small fox; a tiger (Felis augustus, L. ) as large as the Bengal tiger, besides other carnivores; a small beaver; a porcupine. The collection of animals has a strikingly Oriental character, except in the preponderance of herbivores.”
V. POST-TERTIARY OR QUATERNARY.
Under this division are included the various superficial deposits in which all the mollusca are of still living species. It is usually subdivided into two series—(1) an older group of deposits in which many of the mammals are of extinct species,—to this group the names of Pleistocene, PostPliocene, or Diluvial have been given; and (2) a later series, wherein the mammals are all of still living species, to which the name of Recent or Alluvial has been assigned. These subdivisions, however, are confessedly very artificial, and it is often exceedingly difficult to draw any line between them.
In Europe and North America a tolerably sharp demarcation can usually be made between the Pliocene formations and those now to be described. The Crag deposits of the south east of England show traces of a gradual lowering of the temperature during later Pliocene times. This change of climate continued to augment until at last thoroughly arctic conditions prevailed, under which the oldest of the PostTertiary or Pleistocene deposits were accumulated.
It is hardly possible to arrange these deposits in a strict chronological order, because we have no means of deciding, in many cases, their relative antiquity. The following table is rather an enumeration of the more important of them than an arrangement in their exact sequence :— [table]
Pleistocene or Glacial.
Under the name of the Glacial Period or Ice Age, a remarkable geological episode in the history of the northern hemisphere is denoted. We have seen in the foregoing section on the Crag deposits that there is evidence of a gradual refrigeration of the climate at the close of the Tertiary ages. This change of temperature affected the higher latitudes alike of the Old and the New World. It reached such a height that the whole of the north of Europe was buried under snow and ice, extending southwards even as far as Saxony. The Alps and Pyrenees were loaded with vast snow-fields, from which enormous glaciers descended into the plains, overriding ranges of minor hills on their way. The greater portion of Britain was similarly ice-covered.
The effect of the movement of the ice was necessarily to remove the soils and superficial deposits of the land surface. Hence in the areas of country so affected, the ground having been scraped and smoothed, the glacial accumulations laid down upon it rest abruptly, and without any connexion, on whatever underlies them. Considerable local differences may be observed in the nature and succession of the different deposits of the glacial period, as they are traced from district to district. It is hardly possible to determine, in some cases, whether certain portions of the series are coeval or belong to different epochs. But the following are the leading facts which have been established for the North-European aι>ea. First, there was a gradual increase of the cold, though with warm intervals, until the conditions of modern North Greenland extended as far south as Middlesex, Wales, the south-west of Ireland, and 50° N. lat. in central Europe. This was the culmination of the Glacial period. Then followed a considerable depression of the land and the spread of cold arctic water over the submerged tracts, with abundant floating ice; next came a re-elevation, with renewed augmentation of the snow-fields and glaciers. Very gradually, and after intervals of increase and diminution, the ice retired towards the north, and with it the arctic flora and fauna that had peopled the European plains. The existing snow-fields and glaciers of the Pyrenees, Switzerland, and Norway are remnants of the great icesheets of the glacial period, while the arctic plants of the mountains are relics of the northern vegetation which was universal from Norway to Spain.
Ice-worn Rocks.— Beginning at the base of the deposits from which this interesting history is compiled, we find the solid rocks over the whole of northern Europe to present the characteristic smoothed flowing outlines which can be produced only by the grinding action of land-ice (ante, p. 282). Where they have been long exposed, this peculiar surface is apt to be effaced by the disintegrating action of the weather, though it retains its hold with extraordinary pertinacity. Along the fjords of Norway and the sea-lochs of the west of Scotland, it may be seen slipping into the water, smooth, bare, polished, and grooved as if the ice had only recently retreated. But where the protecting cover of clay or other superficial deposits has been newly removed, the peculiar ice-worn surface is as fresh as that by the side of a modern glacier. Observations of the directions of the striae have shown that on the whole these markings diverge from the main masses of high ground. In Scandinavia they run westwards and south-westwards on the Norwegian coasts, and eastwards or south-eastwards across the lower grounds of Sweden. When the ice descended into the basin of the Baltic and the plains of northern Germany, it moved southwards and south-westwards, but seems to have slightly changed its direction in different areas and at different times. Its movements can be made out partly from the striae on the solid rock, but more generally from the glacial drift which it has left behind. Thus it can be shown to have moved down the Baltic into the North Sea. At Berlin its movement must have been from east to west. But at Leipsic, as recently ascertained by Credner, it came from N.N.W. to S.S.E., being doubtless shed off in that direction by the high grounds of the Harz mountains. Its southern limit can be traced with tolerable clearness from Jevennaar in Holland eastwards across the Rhine valley, along the base of the Westphalian hills, round the projecting promontory of the Harz, and then southwards through Saxony to the roots of the Erzgebirge. Passing next south-eastwards along the flanks of the Riesen and Sudeten chain, it sweeps across Poland into Russia, circling round by Kieff, and northwards by Nijni Novgorod towards the Urals.
It has been estimated that, excluding Finland, Scandinavia, and the British Isles, the ice must have covered not less than 1,700,000 square kilometres of the present lowlands of Europe. Its influence on the solid rocks over which it passed has not been everywhere equal. Over much of the north German plain, indeed, the rocks are concealed under drift. But in the more undulating hilly ground, particularly in the north and north-west, the ice has effected the most extraordinary abrasion. It is hardly possible, indeed, to describe adequately in words these regions of most intense glaciation. The old gneiss of Norway and Sutherlandshire, for example, has been so eroded, smoothed, and polished, that it stands up in endless rounded hummocks, many of them still smooth and flowing like dolphins’ backs, with little pools, tarns, and larger lakes lying between them. Seen from a height the ground appears like a billowy sea of cold grey stone. The lakes, every one of them lying in a hollow of erosion, seem scattered broadcast over the landscape. So enduring is the rock, that even after the lapse of so long an interval, it retains its ice-worn aspect almost as unimpaired as if the work of the glacier had been done only a few generations since.
Some idea of the massiveness of the ice-sheet is obtainable from a consideration of the way in which the striae run across important hill ranges, and athwart what might seem to be their natural direction. Whilst there was a general southward movement from the great snow-fields of Scandinavia, the high grounds of Britain were important enough to have their own independent ice, which, as the striae show, radiated outward, some of it passing westwards into the Atlantic and some of it eastward into the North Sea. So thick must it have been as it moved off the Scottish Highlands that it went across the broad plains of Perthshire, filling them up to a depth of at least 2000 feet, and passing across the range of the Ochil Hills, which at a distance of 12 miles runs parallel with the Highland mountains, and reaches a height of 2352 feet. In such cases it has been observed that the striae along the lower slopes of the hill barrier run either parallel with the trend of the ground or slant up obliquely, while those on the summits may cross the ridge at right angles to its course. This shows that there must have been a differential movement in the great icesheet, the lower parts, as in a river, becoming embayed, and being forced to move in a direction sometimes even at a right angle to that of the general advance. On the lower grounds, also, the striae, converging from different sides, unite at last in one general trend as the various ice-sheets must have done, as they descended from the high grounds on either side and coalesced into one common mass. This is well seen in the great central valley of Scotland. Still more marked is the deflexion of the striae in Caithness and the Orkney and Shetland Islands. In these districts the general direction of the striation is from S.S.E., which, in Caithness, is nearly at right angles to what might have been anticipated. This deflexion has been attributed to the coalescence of the ice from Norway and from the northern Highlands in the basin of the North Sea, and its subsequent progress along the resultant line into the Atlantic. But it may have been due to the fanshaped spreading out of the vast mass of ice descending into the Moray Firth; for the striae on the south side of that inlet run E. by S., and at last S.E., on the north-east of Aberdeenshire, showing that the ice, on the one hand, turned southwards into the North Sea, until it met the N.E. stream from Kincardineshire and the valleys of the Dee and Don, while, on the other, it moved northward so as no doubt to join the Scandinavian sheet, and march with it into the Atlantic. The basin of the North Sea must have been choked up with ice in its northern parts, if not entirely. At that time England and the north-west of France were united, so that any portion of the North Sea basin not invaded by land-ice must have formed a lake ; with its outlet by the hollow through which the Strait of Dover has since been opened. It has been suggested that during such a condition of things the widespread freshwater deposit termed Loess was formed, which covers so large a space in the lower plains of the Rhine and the north of Belgium, and appears in the valleys of the southeast of England.
The ice is computed to have been at least between 6000 and 7000 feet thick in Norway, measured from the present sea-level. From the height at which its transported debris has been observed on the Harz, it is believed to have been at least 1470 feet thick there, and to have gradually risen in elevation as one vast plateau, like that which at the present time covers the interior of Greenland. Among the Alps it attained almost incredible dimensions. The present snowfields and glaciers of these mountains, large though they are, form no more than the mere shrunken remnants of the great mantle of snow and ice which then overspread Switzerland. In the Bernese Oberland, for example, the valleys were filled to the brim with ice, which, moving northwards, crossed the great plain, and actually overrode a part of the Jura mountains. Huge fragments of granite and other rocks from the central chain of the Alps are found high on the slopes of that range of heights.
Boulder-clay or Till.— Under this name is included the debris which accumulated and was pushed onward under the ice-sheet, the “grund-moräne” or “moraine profonde” (ante, p. 282). All over the low grounds of North Germany, Denmark, and Holland lies a stony or earthy clay passing into sand or gravel, in which, together with locally derived debris, there is a greater or less proportion of fragments from the north. Some of the rocks of Scandinavia, Finland, and the Upper Baltic are of so distinctive a kind that they can be recognized in small pieces in the boulder-clay. Thus the peculiar syenite of Laurwig in the south of Norway has been recognized abundantly in the drift of Denmark; it occurs in that of Hamburg, and in the boulder-clay of the Holderness cliffs in Yorkshire. The well-known Rhombenporphyr of southern Norway has likewise been recognized at Holderness. Fragments of the Silurian rocks from Gothland, or from the Russian islands Dago or Oesel, have been met with as far as the north of Holland. These transported fragments, so abundant within the line of demarcation just traced, are an impressive testimony to the movements of the northern ice-sheet and floating ice.
The boulder-clay is not spread as a uniform sheet over the ground, but varies greatly in thickness and in irregularity of surface. Round the mountainous centres of dispersion it is apt to occur in long ridges or “drums” which run in the general direction of the rock-striation.
In those areas which served as independent centres of dispersion for the ice-sheet, the boulder-clay partakes largely of the local character of the rocks of each district where it occurs. Thus in Scotland the clay varies in colour and composition as it is traced from district to district. Over the Carboniferous rocks it is dark, over the red sandstones it is red, over the Silurian rocks it is fawn-coloured. The great majority of the stones also are of local origin, not always from the immediately adjacent rocks, but from points within a distance of a few miles. Evidence of transport, however, can be gathered from the stones, for they are found in almost every case to include a proportion of fragments which have come from beyond the district. The direction indicated by the percentage of travelled stones is always the same for each region. Thus, in the loweτ part of the valley of the Firth of Forth, while most of the fragments are from the surrounding Carboniferous formations, from 5 to 20 per cent, is found to have come eastward from the Old Red Sandstone range of the Ochil Hills—a distance of 25 or 30 miles, while 2 to 5 per cent, are pieces of the Highland rocks, which must have come from the high grounds at least 50 miles to the north-west. The evidence of dispersion revealed by the stones in the boulderclay harmonizes with that of the striae on the rocks. Every main mass of elevated ground in Britain seems to have caused the ice to move outward from it for a certain distance, until the stream coalesced with that descending from some other height. In Scotland the ice was massive enough to move out into the basin of the North Sea (then doubtless in great part usurped by the glaciers) until it met that which was streaming down from Scandinavia. Hence no Scandinavian blocks have ever been found in Scotland. But the Norwegian ice which crept southwards across Denmark, may once have extended across the North Sea to the Yorkshire coast, if the Scandinavian stones of Holderness were not carried on floating ice.
The stones in the boulder-clay have a characteristic form and surface. They are usually oblong, have one or more flat sides or “soles,” are smoothed or polished, and have their edges worn round. Where they consist of a fine-grained enduring rock, they are almost invariably found to be striated, the striae running with the long axis of the stone. These markings are precisely similar to those on the solid rocks underneath the boulder clay, and like them have manifestly been produced by the friction of stones and grains of sand as the whole mass of debris was being steadily pushed on in one given direction by the resistless advance of the ice (ante, p. 282).
Interglacial Beds.— The boulder-clay is not one uniform mass of material. In a limited section, indeed, it usually appears as an unstratified mass of stiff stony clay. But it is found on further examination to be split up with various inconstant and local interstratifications. Beds of sand, gravel, fine clay, and peaty layers occur in this way in different platforms in the boulder clay. In Scotland and elsewhere these interpolated beds bear witness to intervals when the ice retired from the area, and the land, so far as uncovered, was clothed with vegetation and traversed by herds of the hairy mammoth, reindeer, and muskox. Hence the long glacial period must have been interrupted by episodes probably of considerable duration when a milder climate prevailed. Such an alternation of conditions is explained on the hypothesis discussed in previous pages (<z7zie, pp. 218-220).
Evidences of Submergence.· —After the ice had attained its greatest development, the land, which perhaps in northwestern Europe stood at a higher level above the sea than it has done since, began to subside. The ice-fields were carried down below the sea-level, where they broke up and cumbered the sea with floating bergs. The heaps of loose debris which had gathered under the ice, being now exposed to waves, ground-swell, and marine currents, were thereby more or less washed down and reasserted. Coast-ice, no doubt, still formed along the shores, and was broken up into moving floes, as happens every year now in northern Greenland. The proofs of this phase of the long glacial period are contained in the sands, gravels, erratic blocks, and stratified clays which overlie the coarse older till. It is difficult to determine the extent of the submergence, for when the land rose the more elevated portions continued to be the seats of glaciers, which, moving over the surface, destroyed the deposits which would otherwise have remained as witnesses of the presence of the sea. The most satisfactory evidence is undoubtedly that supplied by beds of marine shells. These have been observed on Moel Tryfan, in North Wales, at a height of no less than 1350 feet, but as the same kind of deposits in which they occur extend to a much greater height, the submergence probably considerably exceeded the limit at which the shells occur. In Cheshire beds of shells have been met with at a height of 1200 feet. In Scotland the highest level from which they have yet been obtained is 524 feet; and in this instance they lie in one of the interstratifications in the boulder-clay. The coast of Scotland is fringed by a succession of raised beaches which, up to that at 100 feet above the present mean tide-level, are often remarkably fresh. The !00-feet terrace forms a wide plateau in the estuary of the Forth, and the 50-feet terrace is as conspicuous on the Clyde. The elevation of the land has brought up within tide marks some of the clays deposited over the sea-floor during the time of the submergence. In the Clyde basin and some of the western fjords these clays (Clyde beds) are full of shells. Comparing the species with those of the adjacent seas, we find them to be more boreal in character; nearly the whole of the species still live in Scottish seas, though a few are extremely rare. Some of the more characteristic northern shells in these deposits are Pecten Islandicus, Tellina calcarea, Leda truncata, L. lanceolata, Saxicava rugosa, Panopaea Norvegica, Trophon clathratum, and Naiica clausa.
That ice continued to float about in these waters is shown by the striated stones contained in the fine clays, and by the remarkably contorted structure which these clays occasionally display. Sections may be seen where, upon perfectly undisturbed horizontal strata of clay and sand, other similar strata have been violently crumpled, while horizontal beds lie directly upon them. These contortions must have been produced by the horizontal pressure of some heavy body moving upon the originally flat beds. No doubt the agent was ice in the form of large stranding masses which were driven aground in the fjords or shallow waters where the clays accumulated. Another indication of the presence of floating ice is furnished by large boulders scattered over the country, and lying sometimes on the stratified sands and gravels, though no doubt many of the so-called erratics belong to the time of the chief glaciation.
One of the most puzzling members of the Drift is the series of sands and gravels which cover the low grounds in many places, and rise up to heights of 1000 feet and more. These deposits are sometimes spread out in undulating sheets, which, in the lower districts, seem to merge insensibly into the marine terraces and raised beaches. But they are also very commonly disposed in remarkable mounds and in ridges which run across valleys, along hill-sides, and even over watersheds. These ridges are known in Scotland as kames, in Ireland as eskers, and in Scandinavia as osar. They consist sometimes of coarse gravel or earthy detritus, but more usually of clean, well-stratified sand and gravel, the stratification towards the surface corresponding with the external slopes of the ground, in such a manner as to prove that the ridges are original forms of deposit, and not the result of the irregular erosion of a general bed of sand and gravel. Some writers have compared these features to the submarine banks formed in the pathway of tidal currents near the shore. Others have supposed them rather to be of terrestrial origin, due to the melting of the great snow-fields and glaciers, and the consequent discharge of large quantities of water over the country. But no very satisfactory explanation has yet been given.
Re-emergence.—Later Glaciers.— When the land began to rise again, the temperature all over central and northern Europe was still severe. Vast sheets of ice still held sway over the mountains, and continued to descend into the lower tracts and to go out to sea. To this period are ascribed certain terraces or “parallel-roads” which run along the sides of valleys in the Scottish Highlands. It is believed that the mass of ice descending from some of the loftier snowfields of this time was so great as to accumulate in front of lateral valleys, and to so choke them up as to cause the water to accumulate in them and flow out in an opposite direction by the col at the head. In these natural reservoirs the level at which the water stood for a time was marked by a horizontal ledge or platform due partly to erosion of the hill-side and partly to the arrest of the descending debris when it entered the water.
Every group of mountains nourished its own glaciers; even small islands, such as Arran in Scotland, had their snowfields, whence glaciers crept down into the valleys and shed their moraines. It would appear indeed that some of the northern glaciers of Scotland continued to reach the sea-level even when the land had risen to within 50 feet or less of its present elevation. On the east side of Sutherlandshire the moraines descend to the 50- feet raised beach; on the west side of the same county they come down still lower. The higher mountains of Europe still show the descendants of these later glaciers, but the ice has retreated from the lower elevations. In the Vosges the glaciers have long disappeared, but their moraines remain still fresh. In Wales, Cumberland, and the southern uplands and Highlands of Scotland, moraines, perched blocks, and roches moutonnées attest the abundance and persistence of the last glaciers. It is sometimes possible to trace the stages in the gradual retreat of the ice towards its parent snow-fields, for the crescent-shaped moraine mounds lie one behind another until they finally die out about the head of the valley, near what must have been the edge of the snow-field.
We know as yet very little of the fauna and flora of the land during the Glacial Period in Europe. The vegetation was doubtless in great measure the same as that of arctic Norway at the present day. The animals included the musk-ox, lemming, rein-deer, and other still living arctic forms, but included some which have become extinct, such as the hairy mammoth and woolly rhinoceros. During the milder inter-glacial periods denizens of warmer regions found their way northwards into Europe. Thus the hippopotamus haunted the rivers of the south of England. By degrees, as the climate ameliorated, the arctic vegetation was finally extirpated from the lower grounds of central and western Europe, and plants loving a milder temperature, which had doubtless been natives of Europe before the period of great cold, were enabled to reoccupy the sites whence they had been driven. On the higher mountains, where the climate is still not wholly uncongenial for them, colonies of this once general arctic flora still survive. The arctic animals have also been driven away to their northern homes, or have become wholly extinct. And thus, as imperceptibly as it began, the long and varied Ice Age came to an end as it merged into the next succeeding geological period.
Recent or Human Period.
That long succession of ages to which the name of the Glacial Period has been given shaded without abrupt change of any kind into what is termed the Human or Recent Perio¢l. Indeed it may be said with truth that the Glacial Period still exists in Europe. The snow-fields and glaciers have disappeared from Britain, but they still linger among the Pyrenees, remain in larger mass among the Alps, and spread over wide areas in northern Scandinavia. This dovetailing or overlapping of geological periods has been the rule from the beginning of time, the apparently abrupt transitions in the geological record being due to imperfections in the chronicle.
The question has often been asked whether man was coeval with the Ice Age. To give an answer, we must know within what limits the term Ice Age is used, and to what particular country or district the question refers. For it is evident that even to-day man is contemporary with the Ice Age in the Alpine valleys and in Finmark. There can be no doubt that he inhabited Europe after the greatest extension of the ice, but while the rivers were still larger than now from the melting snow, and flowed at higher levels.
The proofs of the existence of man in former geological periods are not to be sought for in the occurrence of his own bodily remains, as in the case of other animals. His bones are indeed now and then to be found, but in the vast majority of cases his former presence is revealed by the implements he has left behind him, formed of stone, metal, or bone. Many years ago the archaeologists of Denmark, adopting the subdivisions of the Latin poets, classified the prehistoric traces of man in three great divisions—those of (1) the Stone Age, (2) the Bronze Age, and (3) the Iron Age. There can be no doubt that, on the whole, this has been the general order of succession. Men used stone and bone before they had discovered the use of metal. Nevertheless, the use of stone long survived the introduction of bronze and iron. In fact, in many European countries where metal has been known for many centuries, there are districts where stone implements are still employed, or where they were in use until quite recently. It is obvious also that, as there are still barbarous tribes unacquainted with the fabrication of metal, the Stone Age is not yet extinct in many parts of the world. In this instance we again see how geological periods run into each other. The nature or shape of the implement cannot therefore be always a very satisfactory proof of antiquity. We must judge of it by the circumstances under which it was found. From the fact that in north-western Europe the ruder kind of stone weapons occurs in what are certainly the older deposits, while others of more highly finished workmanship are found in later accumulations, the Stone Age has been subdivided into an early or Palaeolithic and a later or Neolithic epoch. There can be no doubt, however, that the later was in great measure coeval with the age of bronze, and even to some extent of iron.
The deposits which contain the history of the human period are cavern-loam, river-alluvia, lake-bottoms, peatmosses, sand-dunes, and. other superficial accumulations.
Most calcareous districts abound in underground tunnels and caverns which have been dissolved by the passage of water from the surface (ante, p. 271). In a large number of cases these cavities have communicated with the outer surface, so that terrestrial animals, including man himself, have made use of them as places of retreat, or have fallen or been washed into them. The floors of some of them are covered with loam or cave-earth, resulting from the deposit of the silt carried in the water which formerly flowed through them. Very commonly also, a deposit of stalagmite has formed from the drip of the roof. Hence any organic remains which may have found their way to these floors have been sealed up and admirably preserved.
Above the present levels of the rivers there lie platforms or terraces of alluvium, sometimes to a height of 80 or 100 feet. These deposits are fragments of the river gravels and loams laid down when the streams flowed at that elevation, and therefore before the valleys were widened and deepened to their present form. River action is at the best but slow. To erode the valleys to so great a depth beneath the level of the upper alluvia, must have demanded a period of many centuries. There can therefore be no doubt of the high antiquity of these deposits. They have yielded the remains of many mammals, some of them extinct, together with the flint flakes made by man. From the nature and structure of some of the high-lying gravels, there can be little doubt that they were formed at a time when the rivers were larger than now, and were liable to be frozen and to be obstructed by large accumulations of ice. We are thus able to connect the formations of the human period with some of the later phases of the Ice Age in the west of Europe.
From the evidence of caverns and river-alluvia of Britain, Germany, and France, Dawkins enumerates about fifty mammals as among the chief inhabitants of Europe during the early human period. They include many of the animals still found wild in Europe, Asia, or Africa, such as the marmot, hare, beaver, lemming, leopard, lynx, wild cat, otter, brown bear, wolf, fox, African elephant, musk sheep, chamois, wild boar, horse, reindeer, and hippopotamus. It will be observed what a remarkable mixture there is in this list of forms now found in alpine or arctic regions, on the one hand, with others only now to be seen in warm latitudes. Probably there continued to be great alternations of climate and changes in the distribution of food, so that migrations successively from opposite quarters took place into central and western Europe. But among the denizens of these regions were some that have been long extinct, such as the mammoth, Elephas antiquus, Machairodus latidens, the woolly rhinoceros, the Irish elk, and others. That man was the contemporary of these extinct forms is shown both by the association of hi ’ j flint weapons with their remains, and by the discovery of a tusk of the mammoth with an admirable outline of the animal carved upon it. This valuable relic, with bones of the reindeer and stag admirably sculptured into likenesses of these animals, was found in one of the caves of Perigord in central France.
From the beds of lakes, and from peat-mosses, evidences of more advanced civilization have been obtained. In Switzerland and elsewhere, remains of wooden pile-dwellings have been observed during times when the water has been low. Associated with these are weapons and implements of stone, in other cases of bronze and of iron, pottery, rude kinds (of cloth, seeds of different cereals, and bones of domesticated animals. The dog, swine, goat, horned sheep, and other familiar animals appear as the companiotìs of man. But there is evidence that some of the creatures which he tamed to his use were not natives of Europe, but had their original stocks in central Asia, and that some of his grains must likewise have been introduced. Hence we have glimpses into some of the early human migrations from that eastern centre whence so many successive waves of population have invaded Europe.
The later chronicles of the geologist merge insensibly into those of the archaeologist. The latter claims as his field whatever belongs to the history of man on the globe; the former includes the history of man in that larger history of the earth of which a brief outline has been given in the foregoing pages.
Post-Tertiary Series in North America.
The general succession of events in post-Tertiary times appears to have been nearly the same over the northern hemisphere both in the New and the Old World. In North America we have the same sharply-defined line between the older post-Tertiary deposits and previous formations, due to the glacial conditions which, overspreading these regions, in great measure destroyed the superficial accumulations of the immediately preceding eras. The Quaternary or post-Tertiary formations are grouped by American geologists in the following subdivisions :— [table]
1. Glacial.— As in Europe, the glacial deposits increase in thickness and variety from south to north. The southern limit of the unstratified drift lies somewhere in the neighbourhood of the 39th parallel of north latitude, and the deposit ranges from the Atlantic westward to the meridian of 98°. It spreads, therefore, across British North America, and is found over a considerable area of the north-eastern States. It rises to a height of 5800 feet among the White Mountains. The absence of any true boulder-drift on the Rocky Mountain slopes, where it might have been looked for, is remarkable.
Underneath the boulder-clay the solid rocks, as in Europe, are often well striated. The direction of the striae is generally southward, varying to south-east and south-west according to the form of the ground. In recent years extensive ice-worn rock-surfaces have been observed among the Rocky Mountains by Hayden, King, and others, proving that these elevations formerly possessed then· glaciers, if they were not buried under the great ice-sheet.
Tlιe drift bears witness to a general southerly transport of material, and, in conjunction with the striated rocks, shows that the great icesheet moved from north to south at least as far as about the latitude of Washington. Logan mentions that in some parts of Canada the glacial drift and boulders run in ridges north and south, thus corresponding with the general direction of transport, like the “drums” in Britain. As in Europe, the coarse boulder-clay at the base of the Quaternary deposits is essentially unfossiliferous.
2. Champlain.— Under this name American geologists class the sands, gravels, and clays which overlie the lower boulder-drift. These deposits include coarse unstratified gravels, as well as finely-stratified clays. In eastern Canada they are well developed, and show the following subdivisions :— [table]
The lower subdivisions consist chiefly of clays, which rise to a height of 600 feet above the sea. They have some interstratified beds of siliceous sand, but few boulders. They contain marine organisms, such as Leda truncata, Saxicava rugosa, Tcllina Greenlandica, bones of seals, whales, &c. On the banks of the Ottawa, in Gloucester, the clays contain numerous nodules which have been formed round organic bodies, particularly the fish Mallotus villosus or capeling of the Lower St Lawrence. Dawson also obtained numerous remains of terrestrial marsh plants, grasses, carices, mosses, and algae. This writer states that about 100 species of marine invertebrates have been obtained from the clays of the St Lawrence valley. All except four or five species in the older part of the deposits are shells of the boreal or Arctic regions of the Atlantic; and about half are found also in the glacial clays of Britain. The great majority are now living in the Gulf of St Lawrence and neighbouring coasts, especially off Labrador.
3. Terrace.— This division includes the terraced deposits of alluvial material so marked along the river valleys and lake margins in the northern part of the United States and in Canada, and found also in some degree along the sea-coast These deposits occur in successive platforms or terraces, marking the contraction in volume of the lakes and rivers, consequent, probably, upon intermittent upheavals of the land. They are well developed round the great lakes. Thus in the basin of Lake Huron deposits of fine sand and clay containing fresh-water shells rise to a height of 40 feet or more above the present level of the water, ami,run back from the shore sometimes for 20 miles. Regular terraces, corresponding to former waterlevels of the lake, run for miles along the shores at heights of 120, 150, and 200 feet. Shingle beaches and mounds or ridges, exactly like those now in course of formation along the exposed shores of Lake Huron, can be recognized at heights of 60, 70, and 100 feet. Unfossiliferous terraces occur abundantly on the margin of Lake Superior. At one point mentioned by Logan, no fewer than seven of these ancient beaches occur at intervals up to a height of 331 feet above the present level of the lake. Most of the rivers are bordered with lines of terraces, as in the well-known example of the Connecticut valley described by Hitchcock. The rivers are believed to liave had their maximum volume at the beginning of the Terrace epoch, swollen doubtless by the melting of the still existing icesheets and snow-fields. Their work consisted partly in depositing fine alluvium or loess over their flood-plains, partly in scouring their channels out of the Champlain formations. Greater elevation towards the interior, by augmenting their slope, increased their excavating power.
Terraces of marine origin likewise occur both on the coast and far inland. On the coast of Maine they occur at heights of 150 to 200 feet, round Lake Champlain at least as high as 300 feet, and at Montreal nearly 500 feet above the present level of the sea. In the absence of organic remains, however, it is not always possible to distinguish between terraces of marine origin marking former seamargins, and those left by the retirement of rivers and lakes. In the Bay of Fundy evidence has been cited by Dawson to prove subsidence, for he has observed there a submerged forest of pine and beech lying 25 feet below high-water mark.
4. Recent and Prehistoric. — The deposits in this group are essen tially the same with those in Europe; and, as in that continent so in America, no definite lines can be drawn within which they should be confined. They cannot be sharply separated from the Terrace series, on the one hand, nor from modern accumulations, on the other. Besides the marshes, peat-bogs, and other organic deposits which belong to an early period in the human occupation of America, some of the younger alluvia of the river-valleys and lakes can no doubt claim a high antiquity, though they have not supplied the same copious evidence of early man which gives so much interest to the corresponding European formations. Heaps of shells of edible species occur on the coasts of Nova Scotia, Maine, &c. The large mounds of artificial origin in the Mississippi valley have excited much attention.
PART VII.—PHYSIOGRAPHICAL GEOLOGY.
In the investigation of the geological history of any country, two questions present themselves. We have first to consider the nature and arrangement of the rocks which underlie the surface, and to ascertain from them what has been the march of events, what changes in geography have successively taken place, and what races of plants and animals have come and gone. The gradual geological evolution of the earth has been sketched in the foregoing part of this article. But besides the history of the solid rocks beneath the surface of the land, there is that of the surface itself. Mountains and plains, valleys and ravines, cliffs, peaks, passes, lakes, and the many other features of a country demand attention. By what processes have these varied outlines been impressed upon the surface of the globe? Are they of different ages, and if so, how can their history be ascertained î
The branch of geological inquiry which endeavours to answer these questions has been termed Physiography or Physiographical Geology. Its investigations evidently demand an acquaintance with Stratigraphical Geology. We must be able to trace out the former geographical conditions of the globe before we can adequately reason on the origin of those now existing. Hence the consideration of this branch of the subject has necessarily been reserved for this concluding section.
The stratified formations, of which the succession and history have been traced in the previous pages, were chiefly laid down on the sea-floor in wide horizontal or gently inclined sheets. They have since been upraised into land; their horizontality has been in great part destroyed; and they have been enormously wasted by denuding agents. In con sidering therefore how they have acquired their present external forms, we have to deal with the effects of two kinds of forces, one acting from below, the other on the surface.
These stratified rocks were, on the whole, deposited in shallow water, and have been repeatedly upraised and denuded, so that the younger have been formed out of the waste of the older. They have their modern counterparts, not in the deposits of the great ocean-basins, but in those of comparatively shallow seas. The inference to be drawn from these facts is that the present continental regions, through many local oscillations, have existed as terrestrial ridges from a remote geological antiquity, and that the ocean basins in like manner have, on the whole, retained their identity. When the geologist asks himself how the present distribution of sea and land is to be accounted for, he finds that the answer to the question goes back to early Palaeozoic times, whence he can in some cases trace the gradual growth of a continent downward through the long cycles of geological time. But there still remains the problem to account for the original wrinkling of the surface of the globe, whereby the present great ridges and hollows were produced.
It is now generally agreed that these inequalities have been produced by unequal contraction of the earth’s mass, the interior contracting more than the outer crust, which must therefore have accommodated itself to this diminution of diameter by undergoing corrugation. But there seems to have been some original distribution of materials in the globe that initiated the depressions on the areas which they have retained. It has been already pointed out (ante, p. 223) that the matter underlying the oceans is more dense than that beneath the continents, and that, partly at least, to this cause must the present position of the oceans be attributed. The early and persistent subsidence of these areas, with the consequent increase of density, seems to have determined the main contours of the earth’s surface.
From what has been stated in part iv., the reader will understand that rocks which were originally horizontal, or nearly so, have been crumpled over tracts thousands of square miles in extent, so as to occupy now a superficial area greatly less than that which they originally covered. It is evident that they have been horizontally compressed, and that this result can only have been achieved as a consequence of the subsidence of such a curved surface as that of our globe. The difficulty of explaining these corrugations on the hypothesis of the contraction of a solid globe is undoubtedly great. Mr 0. Fisher, indeed, believes that the present inequalities of contour on the earth’s surface are from sixty-six to eleven and a half times as great as they would have been had they resulted from the contraction of a solid globe; and he has suggested that the earth need not have become solid throughout simultaneously, and consequently may have been considerably larger than it is now at the time when a solid crust was first formed.
The geological phenomena long ago led to a belief in the liquidity of the earth’s interior. Since this belief has been so weightily opposed by the physical arguments already adduced (ante, p. 225), geologists have endeavoured to modify it in such a way as, if possible, to satisfy the requirements of physics, while at the same time providing an adequate explanation of the corrugation of the earth’s crust. Mr Hopkins, Professor Dana, Professor Shaler, and Mr Fisher have, on different grounds, advocated the existence of a fluid or viscous substratum beneath the crust, the contraction and consolidation of which produce the corrugations of the rocks and of the surface. “The increase of temperature,” says Mr Fisher, “though rapid near the surface, becomes less and less as we descend, so that, if the earth were once wholly melted, the temperature near the centre is not very greatly above what it is at a depth which, compared to the earth’s radius, is small. Consequently, if it requires great pressure to solidify the materials at such a temperature, it is probable that the melting temperature may be reached before the pressure is sufficient to solidify.” The crust, of course, must be able to sustain itself on the corrugated surface of the supposed viscous layer without breaking up and sinking. The same writer has even suggested that the observed amount of corrugation is more than can be accounted for even on this hypothesis, and that the shrinkage may have been due not merely to cooling, but to the escape of water from the interior in the form of the superheated steam of volcanic vents. More recently Herr Siemens has been led, from observations made in May 1878 at Vesuvius, to conclude that vast quantities of hydrogen gas, or combustible compounds of hydrogen, exist in the earth’s interior, and that these, rising and exploding in the funnels of volcanoes, give rise to the detonations and clouds of steam.
Leaving the vexed question of the condition of the earth’s interior, the hypothesis of secular cooling and contraction furnishes a natural explanation of the origin of the dominant elevations and depressions of the surface, and of the intense crumpling which the rocks in many regions have undergone. Taking 0O9 as the coefficient of contraction for a supposed stratum 500 miles thick, lying beneath 25 miles of crust, and passing from a fused into a solid state, Mr Fisher found that every 100 miles measured along a. great circle on the surface would have been one mile larger before the contraction, and that this might produce a triangular elevation of “25 square miles on a base of 100 miles, which would give a range of mountains half a mile high. If only 50 miles out of the hundred were disturbed, the range would be a mile high, and so on.”
The effects of this lateral pressure may show themselves either in broad dome-like elevations, or in narrower and loftier ridges of mountain. The structure of the crust is so complex, and the resistance offered by it to the pressure is consequently so varied, that abundant cause is furnished for almost any diversity in the forms and distribution of the wrinkles into which it is thrown. It is evident, however, that the folds have tended to follow a linear direction. In North America, from early geological times, they have kept on the whole on the lines of meridians. In the Old World, on the contrary, they have chosen diverse trends, but the last great crumplings—those of the Alps, Caucasus, and the great mountain ranges of central Asia—have risen along parallels of latitude.
Mountain chains must therefore be regarded as evidence of the shrinkage of the earth’s mass. They may be the result of one movement, or of a long succession of such movements. Formed on lines of weakness in the crust, they have again and again given relief from the strain of compression by undergoing fresh crumpling and upheaval. The successive stages of uplift are usually not difficult to trace. The chief guide is supplied by un conformability, as explained on p. 318. Let us suppose, for example, that a mountain range consists of upraised Lower Silurian rocks, upon the upturned and denuded edges of which the Carboniferous Li∙ιestone lies transgressively. The original upheaval of that range must have taken place at the period of geological time represented by the interval between the Lower Silurian and the Carboniferous Limestone formations. If, in following the range along its course, we found at last the Carboniferous Limestone also highly inclined and covered unconformably by the Upper Coal-measures, we should know that a second uplift of that portion of the ground had taken place between the time of the Limestone and that of the Upper Coal-measures. By this simple and obvious kind of evidence the relative ages of different mountain chains may be compared. In most great mountain-chains, however, the rocks have been so intensely crumpled, and even inverted, that much labour may be required before their true relations can be determined.
The Alps offer an instructive example of a great mountain ehaiιi formed by repeated movements during a long succession of geological periods. As has been already stated, the central portions of the chain consist of gneiss, schists, granite, and other crystalline rocks, partly referable to the Archaean series, but many of which appear to be metamorphosed formations of Palaeozoic, Secondary, and even of older Tertiary age.
It would appear therefore that the first outlines of the Alps were traced out even in Archaean times, and that after submergence, and the deposit of Palaeozoic formations along their flanks, if not over most of their site, they were re-elevated into land. From the relations of the Mesozoic rocks to each other, we may infer that several renewed uplifts after successive denudations took place before the beginning of the Tertiary formations. A large part of the range was, as we have seen, submerged during the Eocene period .under the waters of that wide sea which spread across the centre of the Old World, and in which the Nummulitic Limestone and Flysch were deposited. But about the close of that period the grand upheaval took place to which the present magnitude of the mountains is chiefly due. The older Tertiary rocks, previously horizontal under the sea, were raised up into land, crumpled, dislocated, inverted, together with all the older formations of the chain. So intense was the compression to which the Eocene clays and sands were subjected that they were converted into rocks as hard and crystalline as many of the Palaeozoic masses. It is strange to reflect that the enduring materials out of which so many of the mountains, cliffs, and pinnacles of the Alps have been formed are of no higher geological antiquity than the London Clay and other soft Eocene deposits of the south of England. After the paroxysm of elevation had ended, one or more large lakes were formed along the northern base of the mountains. In these hollows the Swiss molasse accumulated to a depth of more than 6000 feet—a great pile of slowly formed gravels, sands, and clays. That the sea gained occasional access to the region is shown by the interpolation of bands containing marine organisms, as already stated (ante, p. 363). Not improbably a gradual subsidence of the region was going on during the formation of the molasse. But towards the close of the Miocene period another great epoch of mountain-making was ushered in. The lakes disappeared, and their thick sediments were thrust up into large, broken, mountain masses. The Righi, Rossberg, and other prominent heights along the northern flank of the Alps are formed of these upturned lacustrine deposits. Since that great movement no paroxysm seems to have affected the Alpine region. Ceaseless changes, indeed, have been in progress, but they have been due not so much to subterranean causes as to those subaerial forces which are still so active.
The gradual evolution of a continent during a long succession of geological periods has been admirably worked out for North America by Dana, King, Hayden, Newberry, Powell, Dawson, and others. The general character of the structure is extreme simplicity, as compared with that of the Old World. In the Rocky Mountain region, for example, while the Palaeozoic formations lie unconformably upon the Archaean gneiss, there is, according to King, a regular conformable sequence from the Lower Silurian to the Jurassic rocks. During the enormous interval of time represented by these massive formations what is now the axis of the continent remained undisturbed save by a gentle and protracted subsidence. In the great depression thus produced all the Palaeozoic and a great part of the Mesozoic rocks were accumulated. At the close of the Jurassic period the first great upheavals took place. Two lofty ranges of mountains,—the Sierra Nevada (now with summits more than 14,000 feet high) and the Wahsatch,—400 miles apart, were pushed up from the great subsiding area. These movements were followed by a prolonged subsidence, during which Cretaceous sediments accumulated over the Rocky Mountain region to a depth of 9000 feet or more. Then came another vast uplift, whereby the Cretaceous sediments were elevated into the crest of the mountains, and a parallel coast-range was formed fronting the Pacific. ∙ Intense metamorphism of the Cretaceous rocks is stated to have taken place. During the Tertiary ages the Rocky Mountains were permanently raised above the sea, and gradually elevated to their present height. Vast lakes existed among them, in which, as in the Miocene basins of the Alps, enormous masses of sediment accumulated. The slopes of the land were clothed with an abundant vegetation, in which, as already stated (ante, p. 365), we may trace the ancestors of many of the living trees of North America. One of the most striking features in the later phases of this history was the outpouring of great floods of trachyte and other lavas from many points and fissures over a vast space of the Rocky Mountains. In the Snake River region these lavas have a depth of 700 to 1000 feet, over an area 300 miles in breadth.
These examples show that the elevation of mountains has been occasional and, so to speak, paroxysmal. Long intervals elapsed when a slow subsidence took place, but at last a point was reached when the descending crust, unable auy longer to withstand the accumulated lateral pressure, was forced to find relief by rising into mountain ridges. With this effort the elevatory movements ceased. They were followed either by a stationary period, or more usually by a renewal of the gradual depression, until eventually relief was again obtained by upheaval, sometimes along new lines, but often on those which had previously been used.
We see also how, by such enormous compression, the rocks should have acquired a cleavage structure (ante, p. 306). Soft clays have been squeezed and folded till they have become hard fissile slates. So intense have been the corrugation and compression that the strata have undergone a chemical rearrangement of their particles; they have been “metamorphosed” or changed into schists and gneisses, if indeed some portions of them have not been actually fused and intruded into the surrounding masses as igneous rocks.
The consideration of these changes enables us to realize why the strata of a great mountain chain should rise into steeper folds as they are traced away from the plains, until they are found at last folded back upon themselves, and the older are made to overlie the younger. Instead of overlying the central and more ancient masses of the range, they seem really to dip into and under them, so that a section across the region might convey the impression of a great syncline instead of a great and complicated anticline. This fan-shaped arrangement of the rocks may be observed even in the single mountains of a great chain. Mount Blanc is a familiar example.
Another piece of geological structure is sometimes brought vividly before us by the examination of these regions of disturbance. Not only have the rocks been crumpled and inverted; they have likewise been traversed by great dislocations. Those on one side of a fissure have been pushed bodily over those on the other side, or they have experienced a vertical displacement of hundreds or even thousands of feet. As a rule,however, dislocations are more easily traced, if they are not also larger and more numerous, among the low grounds than among the mountains. One of the most remarkable and important faults in Europe is that which bounds the southern edge of the Belgian coal-field. It can be traced across Belgium, has recently been detected in the Boulonnais (ante, p. 350, note), and may not improbably run beneath the Secondary and Tertiary rocks of the south of England. It is a remarkable fact that faults which have a vertical displacement of many thousands of feet produce little or no effect upon the surface. The great Belgian fault, for example, is crossed by the valleys of the Meuse, and other northerly-flowing streams. Yet so indistinctly is it marked in the Meuse valley that no one would suspect its existence from any peculiarity in the general form of the ground, and even an experienced geologist, until he had learned tha structure of the district, would scarcely detect any fault at all.
With the fractures along mountain chains we may connect the hot springs so frequently to be met with in these regions. But the most important connexion with the heated interior is that established by volcanic vents. The theory of secular contraction, while affording a rational explanation of the origin of the great terrestrial ridges, serves at the same time to show why volcanoes should so frequently rise along these ridges (ante, p. 254). The elevation of the crust, by diminishing the pressure on the parts beneath the upraised tracts, permits them to assume a liquid condition, and to rise within reach of the surface when, driven upwards by the expansion of superheated vapours, they are ejected in the form of lava or ashes.
It appears therefore that the present contours of the earth’s surface must be due in large measure to the effects of the contraction of a cooling globe. The crust has been repeatedly corrugated, sometimes suffering sudden and paroxysmal shocks, at other times undergoing slow and long-continued upheaval and depression.
But these subterranean movements form only one phase of the operations by which the outlines of the land have been produced. They have ridged up the solid crust above the sea-level, and have thus given rise to land, but the land as we now see it has acquired its features from the prolonged and varied action of the epigene agents upon rocks of very varied heights and powers of resistance.
It is evident that, as a whole, the land suffers ceaseless erosion from the time that it appears above water. It is likewise clear, from the nature of the materials composing most of the rocks of the land, that they have been derived from old denudations of the same kind. And thus, side by side with the various upheavals and subsidences, there has been a continuous removal of materials from the land, and an equally persistent deposit of these materials under water, and consequent growth of new rocks.
This degradation of the surface may be aptly compared to a process of sculpturing, which begins as soon as the land emerges from the sea, and never ceases so long as any portion of the land remains above water. The implements employed by nature in this great work are those epigene forces whose operations have already been described. Each of them, like a special kind of graving tool, produces its own characteristic impress on the land. The work of rain, of frost, of rivers, of glaciers, can be readily discriminated, though they all combine harmoniously towards the achievement of their one common task. Hence the present contours of the land must depend partly (1) on the vigour with which the several epigene agents perform their work of erosion, (2) on the original configuration of the ground, and the influence it may have had in guiding the operations of these agents, and (3) on the varying structure and powers of resistance possessed by the rocks.
1. Taking a broad view of denudation, we may conveniently group together the action of air, frost, springs, rivers, glaciers, and the other agents which wear down the surface of the land, under the one common designation of subaerial, and that of the sea as marine. The general results of subaerial action are—to furrow and channel the land, to erode valleys, to sharpen and splinter the ridges of mountains, and thus, while roughening, to lower the general surface and carry out the detritus to the sea. The action of the sea, on the other hand, is to plane down the land to the level at which the influence of breakers and ground-swell ceases to have any erosive effect; the flat platform, so often visible between tide-marks on a rocky exposed coast-line, is an impressive illustration of the tendency of marine denudation. The combined result of subaerial and marine action, if unimpeded by any subterranean movement, would evidently be to reduce the land to one general level under the sea. For, except in that upper marginal zone where the waves and tidal currents play, the waters of the ocean protect the solid rocks which they cover. And the rocks indeed can find no permanent protection anywhere else. But to reduce a large area of land such as a continent to the condition of a submarine plain, would require a longer period of time than seems to have elapsed between two epochs of upheaval· Traces of ancient plains of marine denudation are to be met with in Scandinavia and in Scotland, on but a comparatively small scale, as if there had been time for only a narrow platform to be formed before the next paroxysm of contraction and uplift completely renovated the geography of the region.
Instead of trying to estimate how much work is done by each of the subaerial agents in eroding the land, we gain a much more impressive idea of the reality and magnitude of their work as a whole by treating their operations as one great process, the effects of which can be actually measured. The true gauge of the present yearly waste of the surface of the land is furnished by the amount of mineral matter carried every year into the sea by rivers. This mineral matter is partly in mechanical suspension, partly in chemical solution, and is to no small extent pushed in the form of shingle and sand along the bottoms of the streams. Some data respecting its amount have been already given (ante, pp. 274, 278). If we take the ratios furnished by the Mississippi as a fair average, which, from the vast area and varied climatal and geographical characters of the region drained by that river, they probably are, then we learn that 7 rθ χ irσ th of a foot is worn away from the general surface of the land every year. At this rate, if the present erosion could be sustained, the whole American continent, of which, according to Humboldt, the mean height is 748 feet, would be worn down to the sea-level in about 4∣ millions of years —a comparatively short period in geological chronology. It is obvious, however, that the denudation is not equally distributed over the whole surface of the land. If rσ 1 rσ th of a foot is the mean rate from the whole surface, then some parts, including the more level grounds, must lose very much less than that amount, while other parts, such as the slopes and valleys, must lose very much more. The proportions between these extremes must continually vary throughout every country, according to angle of declivity, nature of surface, amount and distribution of rainfall, and whether the rain is spread over the year or concentrated into a short period.
The proportion between the area covered by the more level ground of a country, where the rate of denudation is least, and that of the declivities, valleys, and stream channels, where that rate is greatest, may be assumed as nine to one. The extent of the annual waste may be further taken to be nine times greater over the latter than over the former, so that, while the more level parts of the surface have been lowered 1 foot, the valleys have Jost 9 feet. Taking the mean rates of waste over the whole area to be κl ⅛ τr th of a foot per annum we find that on these data the annual loss amounts to ¾ths of a foot from the flatter grounds and 5 feet from the valleys in 6000 years. This is equal to a loss of 1 foot from the former in 10,800 years and from the later in 1200 years, or to ⅛th of an inch from the one in 7 5 and from the other in 8∣ years. At this rate of erosion, a valley 1000 feet deep may be excavated in 1,200,000 years. These estimates are only approximations to the truth, but they are valuable in directing attention to the real efficacy of the apparently insignificant subaerial denudation now in progress. Any other estimates of the relative amount of material worn away from the different parts of the surface may be taken, but the mean annual loss from the whole area, as ascertained by the river discharge, remains unaffected. If we represent too large an amount as removed from the valleys we diminish the loss from the open country, or if we make the contingent derived from the latter too great we lessen that from the former.
2. From this reasoning it follows that, apart altogether from irregularities of surface due to inequalities of upheaval, every area of land exposed to ordinary subaerial action must, in the end, be channeled into a system of valleys. Even a smooth featureless tract elevated uniformly above the sea would eventually be widely and deeply eroded. Nor would this require a long geological period, for, at the present rate of waste in the Mississippi basin, valleys 800 feet might be carved out in a million years. Undoubtedly the original features superinduced by subter ranean action would guide and modify the operations of running water, though their influence would certainly wane as the features themselves slowly disappeared. In no case probably would the aboriginal contour remain through a succession of geological periods. Traces of it might still be discernible, but they would be well-nigh effaced by the new outlines produced by the superficial agents. In the vast tablelands of Colorado and the other western regions of the United States an impressive picture is visible of the results of mere subaerial erosion on undisturbed and nearly level strata. Systems of stream-courses and valleys, river gorges unexampled elsewhere in the world for depth and length, vast winding lines of escarpment, like ranges of seacliffs, terraced slopes rising from plateau to plateau, huge buttresses and solitary stacks standing like islands out of the plains, great mountain masses towering into picturesque peaks and pinnacles cleft by innumerable gullies, yet everywhere marked by the parallel bars of the horizontal strata out of which they have been carved—these are the orderly symmetrical characteristics of a country where the scenery is due entirely to the action of subaerial agents on the one hand and the varying resistance of perfectly regular stratified rocks on the other. The Alps, on the contrary, present an instructive example of the kind of scenery that arises where a mass of high ground has resulted from the intense cor rugation and upheaval of a complicated series of stratified and crystalline rocks, subsequently for a vast period carved by rain, frost, springs, and glaciers. We see how, on the outer flanks of those mountains among the ridges of the Jura, the strata begin to undulate in long wave-like ridges, and how, as we enter the main chain, the undulations assume a more gigantic tumultuous character, until, along the central heights, the mountains lift themselves towards tlιe sky like the storm-swept crests of vast earth billows. The whole aspect of the ground suggests intense commotion. Where the strata appear along the cliffs or slopes they may often be seen twisted and crumpled on the most gigantic scale. Out of this complicated mass of material the subaerial forces have been ceaselessly at work since its first elevation. They have cut out valleys, sometimes along the original depressions, sometimes down the slopes. They have eroded lake-basins, dug out corries or cirques, notched and furrowed the ridges, splintered the crests, and have left no part of the original surface unmodified. But they have not effaced all traces of the convulsions by which the Alps were upheaved.
3. The details of the sculpture of the land have mainly depended on the nature of the materials on which nature’s erosive tools have been employed. The joints by which all rocks are traversed have served as dominant lines along which the rain has filtered, and the springs have risen, and the frost wedges have been driven. On the high bare scarps of a high mountain the inner structure of the mass is laid open, and there the system of joints is seen to have determined the lines of crest, the vertical walls of cliff and precipice, the forms of buttress and recess, the position of cleft and chasm, the outline of spire and pinnacle. On the lower slopes, even under the tapestry of verdure which nature delights to hang where she can over her naked rocks, we may detect the same pervading influence of the joints upon the forms assumed by ravines and crags. Each kind of rock, too, gives rise to its own characteristic form of scenery. The massive crystalline rocks, such as granite, yield each in its own fashion to the resistless attacks of the denuding forces. They are broadly marked off from the stratified rocks in which the parallel bands of the bedding form a leading feature in every cliff and bare mountain slope. Among the latter rocks also very distinctive types of surface may be observed. A range of sandstone hills, for example, presents a marked contrast to one of limestone.
In the physiography of any region, the mountains are the dominant features. A true mountain chain consists of rocks which have been crumpled and pushed up in the manner already described. But ranges of hills almost mountainous in their bulk may be formed by the gradual erosion of valleys out of a mass of original high ground. In this way some ancient tablelands, those of Norway and of the Highlands of Scotland, for example, have been so channeled by deep fjords and glens that they now consist of massive rugged hills, either isolated or connected along the flanks. The forms of the valleys thus eroded have been governed partly by the structure and composition of the rocks, and partly by the relative potency of the different denuding agents. Where the influence of rain and frost has been slight, and the streams, supplied from distant sources, have had sufficient declivity, deep, narrow, precipitous ravines or gorges have been excavated. The cañons of the Colorado are a magnificent example of this result. Where, on the other hand, ordinary atmospheric action has been more rapid, the sides of the river channels have been attacked, and open sloping glens and valleys have been hollowed out. A gorge or defile is usually due to the action of a waterfall, which, beginning with some abrupt declivity or precipice in the course of the river when it first commenced to flow, or caused by some hard rock crossing the channel, has eaten its way backward, as already explained (p. 276).
Lakes may have been formed in several ways. 1. By subterranean movements as, for example, during those which gave rise to mountain chains. But these hollows, unless continually deepened by subsequent movements of a similar nature would, be filled up by the sediment continually washed into them from the adjoining slopes. The numerous lakes in such a mountain system as the Alps cannot be due merely to this cause, unless we suppose the upheaval of the mountains to have been geologically quite recent, or that subsidence must take place continuously or periodically below each independent basin. But there is evidence that the upheaval of the lakes is not of recent date, while the idea of perpetuating lakes by continual subsidence would demand, not in the Alps merely, but all over the northern hemisphere where lakes are so abundant, an amount of subterranean movement of which, if it really existed, there would assuredly be plenty of other evidence. 2. By irregularities in the deposition of superficial accumulations prior to the elevation of the land or during the disappearance of the ice-sheet. The numerous tarns and lakes enclosed within mounds and ridges of drift-clay and gravel are examples. 3. By the acccumulation of a barrier across the channel of a stream and the consequent ponding back of the water. This may be done, for instance, by a landslip, by the advance of a glacier across a valley, or by the throwing up of a bank by the sea across the mouth of a river. 4. By erosion. The only agent capable of excavating hollows out of the solid rock such as might form lake-basins is glacier-ice (ante, p. 282). It is a remarkable fact, of which the significance may now be seen, that the innumerable lakebasins of the northern hemisphere lie on surfaces of intensely ice-worn rock. The striae can be seen on the smoother rock-surfaces slipping into the water on all sides. These striae were produced by ice moving over the rock. If the ice could, as the striae prove, descend into the rock-basins and mount up the farther side, smoothing and striating the rock as it went, it could erode the basins. It is hardly possible to convey in words an adequate conception of the enormous extent to which the north of Europe and North America has had its surface ground down by ice. The ordinary rough surfaces produced by atmospheric disintegration have been replaced by a peculiar flowing contour which is traceable even to below the sea-level.
In the general subaerial denudation of a country, innumerable minor features are worked out as the structure of the rocks controls the operations of the eroding agents. Thus, among comparatively undisturbed strata, a hard bed resting upon others of a softer kind is apt to form along its outcrop a line of cliff or escarpment. Though a long range of such cliffs resembles a coast that has been worn by the sea, it may be entirely due to mere atmospheric waste. Again, the more resisting portions of a rock may be seen projecting as crags or knolls. An igneous mass will stand out as a bold hill from amidst the more decomposable strata through which it has risen. These features, often so marked on the lower grounds, attain their most conspicuous development among the higher and barer parts of the mountains, where subaerial disintegration is most rapid. The torrents tear out deep gullies from the sides of the declivities. Corries are scooped out on the one hand, and naked precipices are left on the other. The harder bands of rock project as massive ribs down the slopes, shoot up into prominent aiguilles, or give to the summits the notched saw-like outlines they so often present.
Tablelands may sometimes arise from the abrasion of hard rocks and the production of a level plain by the action of the sea, or rather of that action combined with the previous degradation of the land by subaerial waste. But most of the great tablelands of the globe seem to be platforms of little-disturbed strata which have been upraised bodily to a considerable elevation. No sooner, however, are they placed in that position than they are attacked by running water, and begin to be hollowed out into systems of valleys. As the valleys sink, the platforms between them grow into narrower and more definite ridges, until eventually the level tableland is converted into a complicated network of hills and valleys, wherein, nevertheless, the key to the whole arrangement is furnished by a knowledge of the disposition and effects of the flow of water. The examples of this process brought to light in Colorado, Wyoming, Nevada, and the other western territories, by Newberry, King, Hayden, Powell, and other explorers, are among the most striking monuments of geological operations in the world.
The materials worn from the surface of the higher are spread out over the lower grounds. We have already traced how streams at once begin to drop their freight of sediment when, by the lessening of their declivity, their carrying power is diminished (p. 276-7). The great plains of the earth’s surface are due to this deposit of gravel, sand, and loam. They are thus monuments at once of the destructive and reproductive processes which have been in progress unceasingly since the first land rose above the sea and the first shower of rain fell. Every pebble and particle of their soil, once part of the distant mountains, has travelled slowly and fitfully downward. Again and again have these materials been shifted, ever moving downward and sea-ward. For centuries, perhaps, they have taken their share in the fertility of the plains and have ministered to the necessities of flower and tree, of the bird of the air, the beast of the field, and of man himself. But their destiny is still the great ocean. In that bourne alone can they find undisturbed repose, and there, slowly accumulating in massive beds, they will remain until, in the course of ages, renewed upheaval shall raise them into future land, there once more to pass through the same cycle of change. (a. ge.) Index.
Acadian formation, 331. Air, movements of, 264; geological operations of, 264.
Alps, structure of, 371,373. America, North, geology of, 328, 339, 345, 350, 352, 354, 359, 364, 369.
Andesite, 235.
Animals, geological operations of, 289.
Apatite, 228.
Archaean rocks, 327. Arenig group, 331. Atmosphere, 220, 265. Austria, geology of, 327, 364.
Augite, 228.
Bagshot group, 361.
Bala group, 332.
Basalt, 235.
Bath Oolites, 355.
Belgium, geology of, 341, 350, 358.
Blown sand, 237.
Bohemia, geology of, 330, 339.
Boulder-clay, 367. Bracklesham beds, 361. Bradford Clay, 354, 355. Breccia, 237.
Britain, geology of, 327, 329, 331, 340, 342, 346, 351, 352, 354, 357, 360, 362, 364.
Brooks, 272.
Bunter group, 352, 353. Cainozoic formations, 360. Calciferous Sandstones, 346.
Calcite, 228.
Cambrian System, 329.
Canada, geology of, 328, 339, 345, 350, 354, 369. Caradoc group, 332.
Carboniferous Limestone, 347.
„ System, 346. Carrara marble, 231, 232. . Caverns, 271, 320, 369. Chalk, 358, 359, 360. Chlorite, 228.
Clays, 237.
Clay-slate, 238.
Cleavage, 261, 306, 372. Cliff-debris, 237.
Climate, affected by earth’s movements, 218; by ocean, 285; by man, 291; indicated by fossils, 321.
Coal, 238.
Coal-measures, 349.
Colonies, doctrine of, 323. Concretions in rock, 294. Conglomerate, 237.
Contemporaneity, geological, 322.
Contraction, effects of, 261, 264, 371.
Coral islands, 257.
Coralline Oolite, 354,356. Cornbrash, 354, 355.
Cosmical aspects of geology, 213. *
Crag deposits, 364.
Cretaceous System, 357.
Currents, marine, 283.
Delcssite, 228.
Deltas, 278, 319.
Denudation, 372.
Devonian System, 340.
Diabase, 235.
Diorite, 235, 309.
Dip, 298.
Dolomite, 228, 232.
Dust-showers, 266.
Dykes, 311.
Dynamical geology, 213, 239.
Earth, planetaryrelations of, 214; form of, 215; movements of, 216; stability of axis of, 216; changes in centre of gravity of, 217; crust of, 222, 227, 371; interior of, 223, 371; internal heat of, 223; age of, 226.
Earthquakes, 254.
Ecliptic, change in obliquity of, 216.
Eocene formations, 360.
Eozoon,occurrence of,328.
Epidote, 228.
Epigene or surface action, 263.
Equinoxes, precession of, 216.
Europe, geology of, 327, 330, 338, 341, 350, 351, 353, 356, 358, 362,363, 364, 365.
Excavation by rivers, 275. Expansion, effects of, 264. False-bedding, 292.
Faults, 261, 301, 372.
Fjords, 257.
Flint, 238.
Fluor-spar, 228.
Foliation, 315.
Foraminiferal ooze, 290.
Forest marble, 355.
Formations, table of geological, 327.
Fossils, use of in geology, 296, 321, 324, 325; nature of, 319; how preserved, 319; relative value of, in geology, 320; relative age of,321.
France, geology of, 350, 356, 358, 362, 363.
Frost, 265, 280.
Gabbro, 235.
Garnet, 228.
Gas-spurts, 294.
Gault, 357, 359.
Geognosy, 213, 220.
Geological record, 213, 323, 326.
Geology, definition of,212; cosmical aspects of, 213; dynamical, 213, 239; structural, 213, 291; palaeontological, 213, 319; stratigraphical, 13, 325; physiographical, 213, 370.
Germany, geology of, 328, 338, 339, 341, 350, 351, 353, 357, 358, 363, 366.
Glacial drift, 365.
,, periods, 219.
Glaciation of Europe, 366; of America, 369.
Glaciers, 281.
Gneiss, 236.
Granite. 230, 233, 308.
Graphite, 228.
Greenland, 359, 363, 364.
Greensand, 357.
Greywacke, 231, 287.
Gulf-stream, influence of, in climate, 219.
Hail, 280.
Harlech group, 329.
Haüyne, 228.
Heat, effects of, on rocks, 258, 262.
Hippurite limestone, 358. Homotaxis, 322, 327.
Hornblende, 228.
Human period, 368.
Huron, lake, 328, 370.
Hypogene action, 240.
Ice, geological action of, 281, 366.
Ice-cap, effects of polar, 217, 219.
Igneous rocks, structures of, 307.
Interbedded rocks, 313.
Interglacial periods, 220, 367.
Intrusive rocks, 307, 310.
Iron, oxides of, 228, 232.
Joints, 297.
Jurassic System, 354.
Kellaways rock, 354, 356.
Keuper group, 352, 353.
Kimeridge Clay, 354, 356. Lakes, 279, 319, 374; frozen, 280.
Laminae of rocks, 293.
Laurentian rocks, 327. Lavas, 242, 246.
Leucite, 228.
Lias, 354.
Life, geological effects of plant and animal, 289.
Lignite, 238.
Limestones, 232-238.
Lingula Flags, 329.
Liparite, 234.
Llandeilo Flag group, 332.
Llandovery group, lower, 332.
Llandovery group, upper, 334.
London Clay, 361.
Longmynd group, 329.
Ludlow group, 335.
Man, antiquity of, 368.
Man as a geological agent, 291.
Mangrove swamps, 289.
Marlstone, 354.
May Hill Sandstones, 334.
Menevian group, 329.
Metamorphic rocks, 314.
Metamorphism, 263, 315, 372.
Mica, 228.
Mica-schist, 231-236.
Millstone Grit, 349.
Mineral veins, 317.
Miocene rocks, 362.
Molasse, 363.
Mountains, origin of, 258, 370, 371, 374.
Neocomian series, 357, 359.
Nepheline, 228.
Nosean, 228.
Obsidian, 230, 234.
Oceanic circulation, 284.
Oceans, 221, 282.
Oil-shale, 238.
Old Red Sandstone, 340, 342.
Olivine, 228.
Oolites, Lower or Bath, 355; Middle or Oxford, 356; Upper or Portland, 356.
Overlap, 295.
Oxford Clay, 354, 356.
Palaeontological geology, 213, 319.
Palaeozoicformations,32S.
Pearlstone, 23 !.
Peat, 238, 290, 319.
Permian System, 351.
Phonolite, 234.
Physiographical geology, 213, 370.
Pitchstone, 231, 234.
Planets, relative densities of, 214.
Plants, geological operations of, 289, 319.
Pleistocene deposits, 365.
Plication, 261, 298.
Pliocene deposits, 364. Porphyry, 234.
Post-Tertiary deposits, 365.
Prehistoric formations, 368, 369, 370.
Pressure, effects of, 261. Primordial zone, 329, 330. Prismatic structure, 249. Propylite, 235.
Pumice, 234.
Purbeck group, 354, 356.
Quartz, 227.
Quartz-porphyry, 233.
Quartz-rock, 237.
Quaternary deposits, 365. Rain, 267.
Rain-prints, 294.
Raised beaches, 256. Recent period, 368, 370. Rhaetic beds, 352, 353. Ripple-mark, 293.
Rivers, 272, 373; geological action of, 273; frozen, 280.
Rocks, characters of, 229; crystalline, 229, 230, 231, 307; fragmental, 229, 231, 236, 292; microscopic structure of, 220; schistose, 235, 314; volcanic fragmental, 239, 242; permeability of, 267; stratified, 292, 370; joints of, 297; inclination of,298; curvature of, 298.
Rock-salt, 228.
Rocky Mountains, structure of, 372, 373.
Russia, geology of, 338, 342, 367.
Sandstone, 237.
Scandinaria, geology of, 331, 338, 358, 366, 367.
Sea, geological operations of, 284; deposits in, 287, 290.
Sea-water, composition of, 221.
Secondary or Mesozoic systems, 352.
Serpentine, 228, 232.
Siderite, 228.
Silurian System, 331.
Snow, 280.
Soil, 265.
Speeton Clay, 357.
Springs, 270; hot, 223.
Stonesfield slate, 354,355.
Strata, 293, 37Ö; alternations of, 295; persistence of, 295; groups of, 296, 327.
Stratification, 292.
Strike, 298.
Structural geology, 213, 291.
Submerged forests, 256, 321.
Subsidence, movements of, 256, 321, 367.
Sulphates, 228, 232. Sulphides, 228.
Sulphur, 228 Sun-cracks, 294. Superposition, order of, 295, 321, 325.
Switzerland, geology of, 353, 359, 362, 363, 371.
Syenite, 234.
Table-lands, 374.
Talc, 228.
Tarannon Shale, 334. Tertiary systems, 360- Thanet Sand, 361.
Tides, 283.
Till, 367.
Tourmaline, 228. Trachyte, 234. Trees, influence of, in geology, 289.
Tremadoc Slates, 330.
Triassic System, 352.
Tuffs, 239.
Unconformability, 318.
United States, geology of, 328, 331, 339, 345, 350, 352, 354, 357, 359, 364, 369, 372, 373.
Upheaval, movements of, 255, 321.
Veins of intrusion, 811.
Volcanic action, causes· of, 253
Volcanoes, 223, 240; products of, 241; action of, 243; fissures at, 244; explosions of, 244; showers of stones from, 245; geological effects of, 249; mud from, 250; structure of, 251; submarine, 252; distribution of in space and time, 252; vapours from, 241, 251.
Water as a geological agent, 267; subterranean, 269; influence of, in hypogene change, 262.
Waves, 285.
Wealden series, 357, 359. Weathering, 265, 268. Wenlock group, 335. Woolwich and Reading beds, 361.
Zeolites, 228.
Zircon, 228.
