COAL
IN its most general sense the term coal includes all varieties of carbonaceous minerals used as fuel, but it is now usual in England to restrict it to the particular varieties of such minerals occurring in the older Carboniferous formations. On the continent of Europe it is customary to consider coal as divisible into two great classes, depending upon differences of colour, namely, brown coal, corre spending to the term “lignite” used in England and France, and black or stone coal, which is equivalent to coal as understood in England. Stone coal is also a local English term, but with a signification restricted to the substance known by mineralogists as anthracite. In old English writings the terms pit-coal and sea-coal are commonly used. These have reference to the mode in which the mineral is obtained, and the manner in which it is transported tò market.
The root koi is common to all the Teutonic nations, while in French and other Romance languages derivatives of the Latin carbo are used, e.g., charbon de terre. In France and Belgium, however, a peculiar word, houille, is generally used to signify mineral coal. This word is supposed to be derived from the Walloon hoie, corresponding to the mediaeval Latin hullae. Littré suggests that it may be related to the Gothic haurja, coal. Anthracite is from the Greek avθρaξ, and the term lithanthrax, stone coal, still survives, with the same meaning in the Italian litantrace.
It must be borne in mind that the signification now attached to the word coal is different from that which formerly obtained when wood was the only fuel in general use. Coal then meant the carbonaceous residue obtained in the destructive distillation of wood, or what is known as charcoal, and the name collier was applied indifferently to both coal-miners and charcoal-burners.
The spelling “cole” was generally used up to the middle of the 17th century, when it was gradually superseded by the modern form, “coal.” . The plural, coals, seems to have been used from a very early period to signify the broken fragments of the mineral as prepared for use.
Coal is an amorphous substance of variable composition, and therefore cannot be as strictly defined as a crystallized or definite mineral can. It varies in colour from a light brown in the newest lignites to a pure black, often with a bluish or yellowish tint in the more compact anthracite of the older formations. It is opaque, except in exceedingly thin slices, such as made for microscopic investigation, which are imperfectly transparent, and of a dark brown colour by transmitted light. The streak is black in anthracite, but more or less brown in the softer varieties. The maximum hardness is from 2 - 5 to 3 in anthracite and hard bituminous coals, but considerably less in lignites, which are nearly as soft as rotten wood. A greater hardness is due to the presence of earthy impurities. The densest anthracite is often of a semi-metallic lustre, resembling somewhat that of graphite. Bright, glance, or pitch coal is another brilliant variety, brittle, and breaking into regular fragments of a black colour and pitchy lustre. Lignite and cannel are usually dull and earthy, and of an irregular fracture, the latter being much tougher than the black coal. Some lignites are, however, quite as brilliant as anthracite; cannel and jet may be turned in the lathe, and are susceptible of taking a brilliant polish. The specific gravity is highest in anthracite and lowest in lignite, bituminous coals giving intermediate values (see Table I.) As a rule the density increases with the amount of carbon, but in some instances a very high specific gravity is due to intermixed earthy matters, which may be separated by mechanical treatment.
Coal is perfectly amorphous, the nearest approach to any thing like crystalline structure being a compound fibrous grouping resembling that of gypsum or arragonite, which occurs in some of the steam coals of S. Wales, and is locally known as “cone in cone,” but no definite form or arrangement can be made out of the fibres. The impressions of leaves, woody fibre, and other vegetable remains are to be considered as pseudomorphs in coaly matter of the original plant structures, and do not actually represent the structure of the coal itself. There is generally a tendency in coals towards cleaving into cubical or prismatic blocks, but sometimes the cohesion between the particles is so feeble that the mass breaks up into dust when struck. These peculiarities of structure may vary very considerably within small areas; and the position of the divisional planes or cleats with reference to the mass, and the proportion of small coal or slack to the larger fragments when the coal is brokfen up by cutting-tools, are points of great importance in the working of coal on a large scale.
The divisional planes often contain small films of other minerals, the commonest being calcite, gypsum, and iron pyrites, but in some cases zeolitic minerals and galena have been observed. Salt, in the form of brine, is sometimes present in coal. Some years ago a weak brine occurring in this way was utilized at a bathing establishment at Ashby-de-la-Zouche. Hydrocarbons, such as petroleum, bitumen, paraffin, <fcc., are also found occasionally in coal, but more generally in the associated sandstones and limestones of the Carboniferous formation. Gases, consisting principally of light carburetted hydrogen or marsh gas, are often present in considerable quantity in coal, in a dissolved or occluded state, and the evolution of these upon exposure to the air, especially when a sudden diminution of atmospheric pressure takes place, constitutes one of the most formidable dangers that the coal miner has to encounter.
The classification of the different kinds of coal may be considered from various points of view, such as their chemical composition, their behaviour when subjected to heat or when burnt, and their geological position and origin. They all contain carbon, hydrogen, oxygen, and nitrogen, forming the carbonaceous or combustible portion, and some quantity of mineral matter, which remains after combustion as a residue or “ash.” As the amount of ash varies very considerably in different coals, and stands in no relation to the proportion of the other constituents, it is necessary in forming a chemical classification to compute the results of analysis after deduction of the ash and hygroscopic water. Examples of analyses treated in this manner are furnished in the last column of Table I., from which it will be seen that the nearest approach to pure carbon is furnished by anthracite, which contains above 90 per cent. This class of coal burns with a very small amount of flame, producing intense local heat and no smoke. It is especially used for drying hops and malt, and in air or blast furnaces where a high temperature is required, but is not suited for reverberatory furnaces. The American anthracite is largely used in iron smelting, as is also that of South Wales, but to a less extent, the latter having the disadvantageous property of decrepitating when first heated.
The most important class of coals is that generally known as bituminous, from their property of softening or undergoing an apparent fusion when heated to a temperature far below that at which actual combustion takes place. This term is founded on a misapprehension of the nature of the occurrence, since, although the softening takes place at a low temperature, still it marks the point at which destructive distillation commences, and hydrocarbons both of a solid and gaseous character are formed. That nothing analagous to bitumen exists in coals is proved by the fact that the ordinary solvents for bituminous substances, such as bisulphide of carbon and benzole, have no effect upon them, as would be the case if they contained bitumen soluble in these re-agents. The term is, however, a convenient one, and one whose use is almost a necessity, from its having an almost universal currency among coal miners. The proportion of carbon in bituminous coals may vary from 80 to 90 per cent.—the amount being highest as they approach the character of anthracite, and least in those which are nearest to lignites. The amount of hydrogen is from 4¾ to 6 per cent., while the oxygen may vary within much wider limits, or from about 3 to 14 per cent. These variations in composition are attended with corresponding differences in qualities, which are distinguished by special names. Thus the semi-anthracitic coals of South Wales are known as “dry” or “steam coals,” being especially valuable for use in marine steam-boilers, as they burn more readily than anthracite and with a larger amount of flame, while giving out a great amount of heat, and practically without producing smoke. Coals richer in hydrogen, on the other hand, are more useful for burning in open fires—smiths’ forges and furnaces— where a long flame is required.
The excess of hydrogen in a coal, above the amount necessary to combine with its oxygen to form water, is known as “disposable” hydrogen, and is a measure of the Table I.—Elementary Composition of Coal (the figures denote the amounts per cent). [table]
fitness of the coal for use in gas-making. This excess is greatest in what we know as cannel coal, the Lancashire kennel or candle coal, so named from the bright light it gives out when burning. This, although of very small value as fuel, commands a specially high price for gasmaking. Cannel is more compact and duller than ordinary coal, and can be wrought in the lathe and polished. These properties are most highly developed in the substance known as jet, which is a variety of cannel found in the lower oolitic strata of Yorkshire, and is almost entirely used for ornamental purposes, the whole quantity produced near Whitby, together with a further supply from Spain, being manufactured into articles of jewellery at that town.
When coal is heated to redness out of contact with the air, the more volatile constituents, water, hydrogen, oxygen, and nitrogen are expelled, a portion of the carbon being also volatilized in the form of hydrocarbons and carbonic oxide,—the greater part, however, remaining behind, together with all the mineral matter or ash, in the form of coke, or, as it is also called, “fixed carbon.” The proportion of this residue is greatest in the more anthracitic or drier coals, but a more valuable product is yielded by those richer in hydrogen. Very important distinctions—those of caking or non-caking—are founded on the behaviour of coals when subjected to the process of coking. The former class undergo an incipient fusion or softening when heated, so that the fragments coalesce and yield a compact coke, while the latter (also called free-burning) preserve their form, producing a coke which is only serviceable when made from large pieces of coal, the smaller pieces being incoherent and of no value. The reason of this difference is not clearly made out, as non-caking coals are often of very similar ultimate chemical composition as those in which the caking property is very highly developed. As matter of experience, it is found that caking coals lose that property when exposed to the action of the air for a lengthened period, or by heating to about 300 C., and that the dust or slack of non-caking coal may, in some instances, be converted into a coherent cake by exposing it suddenly to a very high temperature.
Lignite or brown coal includes all varieties which are intermediate in properties between w ood and coals of the older formations. A coal of this kind is generally to be distinguished by its brown colour, either in mass or in the blacker varieties in the streak. The proportion of carbon is comparatively low, usually not exceeding 70 per cent., while the oxygen and hygroscopic water are much higher than in true coals. The property of caking or yielding a coherent cake is usually absent, and the ash is often very high. The specific gravity is low when not brought up by an excessive amount of earthy matter. Sometimes it is almost pasty, and crumbles to powder when dried, so as to be susceptible of use as a pigment, forming the colour known as Cologne earth, which resembles umber or sepia.
In Nassau and Bavaria woody structure is very common, and it is from this circumstance that the term lignite is derived. The best varieties are black and pitchy in lustre, or even bright and scarcely to be distinguished from true coals. These kinds are most common in Eastern Europe. Lignites, as a rule, are generally found in strata of a newer geological age, but there are many instances of perfect coals being found in such strata.
By the term “ash” is understood the mineral matter remaining unconsumed after the complete combustion of the carbonaceous portion of a coah This represents part of the mineral matter present in the plants from which the coal was originally formed, with such further addition by infiltration and mechanical admixture as may have arisen during consolidation and subsequent changes. The composition of the ashes of different coals is subject to considerable variation, as will be seen by the following list of analyses :—
Table II.—Composition of the Ashes of Coals. [table]
The composition of the ash of true coal approximates to that of a fire-clay, allowance being made for lime, which may be present either as carbonate or sulphate, and for sulphuric acid. The latter is derived mainly from iron pyrites, which yields sulphate by combustion. An indication of the character of the ash of a coal is afforded by its colour, white ash coals being generally freer from sulphur than those containing iron pyrites, which yield a red ash. There are, however, several striking exceptions, as for instance in the anthracite from Peru, given in Table I., which contains more than 10 per cent, of sulphur, and yields but a very small percentage of a white ash. In this coal, as well as in the lignite of Tasmania, known as white coal or Tasmanite, the sulphur occurs in organic combination, but is so firmly held that it can only be very partially expelled, even by exposure to a very high and continued heating out of contact with the air. An anthracite occurring in connection with the old volcanic rocks of Arthur’s Seat, Edinburgh, which contains a large amount of sulphur in proportion to the ash, has been found to behave in a similar manner. Under ordinary conditions, from ¾ to ¾ of the whole amount of sulphur in a coal is volatilized during combustion, the remaining I to ¾ being found in the ash.
The amount of water present in freshly raised coals varies very considerably. It is generally largest in lignites, which may sometimes contain 30 per cent, or even more, while in the coals of the coal measures it does not usually exceed from 5 to 10 per cent. The loss of weight by exposure to the atmosphere from drying may be from ¾ to ¾ of the total amount of water contained.
Coal is undoubtedly the result of the transformation of vegetable matter, mainly woody fibre, by the partial elimination of oxygen and hydrogen giving rise to a substance richer in carbon than the original wood,—the mineral matter being modified simultaneously by the almost entire removal of the alkalies and lime, and the addition of materials analagous in composition to clay, as will be seen by comparing the analyses in Table IL The following table, given by Percy, show's the relative pro portions of the different components of mineral fuels.
Table III.—Composition of Fuels (assuming Carbon — 100). [table]
Mohr has computed that the transformation of wood into coal is attended with a loss of about 75 per cent, in weight; and, having regard to the difference in density of the two substances, the volume of the coal can only be from - r ⅛- to ⅜ of the woody fibre from which it is derived.
The nature of the change is essentially a slow oxidation under water or any covering sufficient to protect the dead wood from the direct action of atmospheric air, as -in the latter case the vegetable mould or humus w ould be produced. The products of such decomposition vary with the length of time and the nature of the plants acted on, and in the case of anthracite the change is so great that no portion of the original plant structure can be recognized, at the same time the density and conductivity for heat and electricity are increased. This, however, is a case of metamorphosis analogous to the transformation of sedimentary into crystalline rocks, the extreme term of such metamorphosis being the production of graphite or plumbago. Daubrée has shown that wood may be converted into anthracite by exposure to the action of superheated water at a temperature of 400° C.
The plants concerned in the production of coal vary very considerably in different geological periods. In the coal measures proper, acrogens, ferns, equisetums, and similar allied forms are most abundant. It is stated by some observers that entire beds of coal are sometimes made up of the spores of ferns. This, however, appears to depend upon the inspection of microscopic sections, and may not be capable of rigorous quantitative demonstration. In the coals of newer date exogenous wood and leaves are more common than in those of the coal measures; the former also contain resins, sometimes in considerable quantity.
The number of species of land plants in the British sedimentary formations, which may be taken as a measure of the comparative prevalence of coal in the different series, is as follows :— [table]
The most generally received opinion is that much if not all coal results from the transformation of plants upon the site of their growth. The principal evidence in favour of such a supposition is afforded by the common occurrence of a bed of clay, the so-called “under-clay,” containing the roots of plants, representing the old soil, immediately below every coal seam—a fact that was first pointed out by the late Sir W. E. Logan in South Wales. In Yorkshire the same thing is observed in the siliceous rock called ganister occurring in similar positions, showing that the coal plants grew there upon sandy soils.
The action of water in bringing down drift wood may have also contributed some material, but much less than the local growth. This may probably have been concerned in the production of the very thick masses of coal of small extent found in some coal-fields in Southern Europe.
Another theory, that proposed by Dr Mohr, deserves notice, namely, that coal may be of marine origin, and derived from the carbonization of sea weeds, such as the great kelp plant of the Pacific Ocean. This has been very ingeniously elaborated by the author, and much apparently good evidence adduced in support (see his Geschichte der Erde, Bonn, 1875). But the positive evidence afforded by roots found in the under clays is sufficient to render such an hypothesis unnecessary in the majority of instances.
It must be remembered, however, that, although cellulose or wood fibre is most probably the chief material concerned in the production of coal, this substance is readily convertible into dextrine by the action of protein or analogous fermentescible matters containing nitrogen, a change that is attended with the loss of structure, the fibre being converted into a gummy mass. Some forms of cellulose, such as that in the lichens known as Iceland moss, are soluble in water, and are without fibre. The preservation of recognizable woody tissue therefore in coals can only be regarded as accidental, and any argument founded upon the relative quantity of the recognizable vegetable structures in microscopic sections is likely to be unsound, unless the relative durability of the different portions of the plants be taken into account. Thus the bark of trees is, as a rule, less perishable than the solid wood, while tissues impregnated with resinous matters are almost indestructible by atmospheric agency. Instances of this are afforded by the fossil trees found in the coal measures, which are often entirely converted into siliceous masses, the bulk of the wood having decayed and been replaced by silica, while the bark is represented by an external layer of bright coal. Fossil resins, such as amber, are of common occurrence in coals, especially those of secondary or tertiary age.
In an investigation of the coking properties of the Saarbrücken coals by Schondorff, it was found that they could be separated into three different materials, which he distinguished as glance or bright coal, dull or striped coal, and fibrous coal. The last, which is known in England as “mother of coal,” resembles a soft, dull, black charcoal, containing abundant traces of vegetable fibre, and yielding a high proportion of non-coherent coke, behaving, in fact, like charcoal. The bright or glance coal is without any apparent structure, cleaving into cubical masses, contains but little mineral matter, and yields a strong coke. The striped coal consists mainly of a dull substance, with fine alternations of bright matter, and is essentially a gas coal yielding only an inferior coke. These differences are supposed to be due to original differences in the substances from which the coals have been derived. Thus the fibrous coal may result from unaltered cellulose, the glance ‘coal from the insoluble mucilage derived from the maceration of the plants in water, and the dull coal from the soluble parts, such as gum and dextrine, either original or produced by the transformation of cellulose and starch. That something analogous to a pulping process has gone on in the production of coal is evident from the intimate intermixture of the mineral matter constituting the ash, which is quite unrecognizable before burning in the majority of instances.
F. Muck (Chemische Aphorismen über Steinkohlen, Bochum, 1873) has recorded some interesting experiments on the behaviour of the three isomeric carbohydrates,— cellulose, starch, and gum arabic,—which are all of the same ultimate composition, namely, C 6 H 10 O 5 . When subjected to the process of coking, cellulose, in the form of Swedish filter paper, gave a residue of 6∙74 per cent, of a perfectly non-coherent coke, starch 11-30 per cent, of a bright vesicular coke like that from strongly coking coal, and gum-arabic 20∙42 per cent, of a hard dull coke re sembling that produced from imperfectly coking gas coals. The volume of gas given off by cellulose and starch is much larger and of a higher illuminating power than that produced from gum under the same conditions.
The conditions favourable to the production of coal seem therefore to have been—forest growth in swampy ground about the mouths of rivers, and rapid oscillation of level, the coal produced during subsidence being covered up by the sediment brought down by the riveι forming beds of sand or clay, which, on re-elevation, formed the soil for fresh growths, the alternation being occasionally broken by the deposit of purely marine beds. We might therefore expect to find coal wherever strata of estuarine origin are developed in great mass; and this is actually the case,—the Carboniferous, Cretaceous, and Oolitic series being all coal bearing horizons, though in unequal degrees,—the first being known as the coal measures proper, while the others are of small economic value in Great Britain, though more productive in workable coals on the continent of Europe. The coal measures which form part of the Palaeozoic or oldest of the three great geological divisions are mainly confined to the countries north of the Equator, Mesozoic coals being more abundant in the southern hemisphere, while Tertiary coals seem to be tolerably uniformly distributed irrespective of latitude.
The nature of the coal measures will be best understood by considering in detail the areas within which they occur in Britain, together with the rocks with which they are most intimately associated. The general succession of these rocks is given in fig. 1 (cols. 1 to 4),which is taken from the index of strata issued by the Geological Survey. The commencement of the carboniferous period is marked by a mass of limestones known as the Carboniferous or mountain limestone, which contains a large assemblage of marine fossils, and has a maximum thickness in S.W. England and Wales of about 2000 feet. The upper portion of this group consists of shales and sandstones known as the Yoredale Rocks, which are highly developed in the moorland region between Lancashire and the north side of Yorkshire. These are also called the upper limestone βhale, a similar group being found in places below the limestone, and called the lower limestone shale, or, in the North of England, the Tuedian group. Going northward the beds of limestone diminish in thickness, with a proportional increase in the intercalated sandstones and shales, until in Scotland they are entirely subordinate to a mass of coal-bearing strata, which forms the most productive members of the Scotch coal fields. The next member of the series is a mass of coarse sandstones, with some slates and a few thin coals, known as the Millstone Grit, which is about equally developed in England and in Scotland. In the southern coal-fields it is usually known by the miners’ name of Farewell Rock, from its marking the lower limit of possible coal working. The Coal Measures, forming the third great member of the carboniferous series, consist of alternations of shales and sandstones, with beds of coal and nodular ironstones, which together make up a thickness of many thousands of feet— from 12,000 to 14,000 feet when at the maximum of development. They are divisible into three parts, the lower coal measures, the middle or Pennant, a mass of sandstone containing some coals, and the upper coal measures, also containing workable coal. The latter member is marked by a thin limestone band near the top, containing Spirorbis carbonarius, a small marine univalve.
The uppermost portion of the coal measures consists of red sandstone so closely resembling that of the Permian group, which are next in geological sequence, that it is often difficult to decide upon the true line of demarcation between the two formations. These are not, however, always found together, the coal measures being often covered by strata belonging to the Trias or upper New Red Sandstone series.
The areas containing productive coal measures are usually known as coal fields or basins, within which coal occurs in more or less regular beds, also called seams or veins, which can often be followed over a considerable length of country without change of character, although, like all stratified rocks, their continuity may be interrupted by faults or dislocations, also known as slips, hitches, heaves, or troubles (fig. 2).
The thickness of coal seams varies in this country from a mere film to 35 or 40 feet; but in the south of France and in India masses of coal are known up to 200 feet in thickness. These very thick seams are, however, rarely constant in character for any great distance, being found commonly to degenerate into carbonaceous shales, or to split up into thinner beds by the intercalation of shale bands or partings. One of the most striking examples of this is afforded by the thick or ten-yard seam of South Staffordshire, wsliich is from 30 to 45 feet thick in one connected mass in the neighbourhood of Dudley, but splits up into eight seams, which, with the intermediate shales and sandstones, are of a total thickness of 400 feet in the northern part of the coal-field in Cannock Chase. Seams of a medium thickness of 3 to 7 feet are usually the most regular and continuous in character. Cannel coals are generally variable in quality, being liable to change into shales or black-band ironstones within very short horizontal limits. In some instances the coal seams may be changed as a whole, as for instance in South Wales, where the coking coals of the eastern side of the basin pass through the state of dry steam coal in the centre, and become anthracite in the western side.
British Coal-fields.
There are about twenty principal coal-fields of Great Britain, besides several smaller ones, whose position is shown in Plate I., which may be classed under three heads:—1. Those forming complete basins, entirely circumscribed by the lower members of the carboniferous series; 2. Those in which one limb of the basin only is visible, the opposite one being obscured by Permian or other strata of newer date; and 3. Those in which the boundaries are formed by faults, which bring down the upper overlying strata into contact with the coal measures. The South Wales and Dean Forest basins are examples of the first of the above classes, the North of England and Yorkshire and Derbyshire fields of the second, and the South Staffordshire of the third. The last two classes are of the greatest geological interest, as giving rise to the important problem of their probable extension within workable limits beneath the overlying strata. Examples of the three different cases are given in Plate II.,—the first being represented by the section across the Forest of Dean, fig. 1; the second by that of the Lancashire coal-fields, fig. 2; and the third by the North Staffordshire section, fig. 3.
The largest and most important of the British coal-fields is that of South Wales, which extends from Pontypool in Monmouthshire on the east, to Kidwelly in Pembrokeshire, a length of about 50 miles, and from Tredegar on the north to Llantrissant on the south, a breadth of about 18 miles, in addition to which a further narrow slip of about 20 miles long, E. and W., extends across Pembrokeshire. Excluding the latter portion, it forms a complete basin of an approximately elliptical shape, surrounded by older rocks, the Carboniferous limestone and Devonian shale dipping generally towards the centre. The basin-shaped structure is, however, modified by a central anticlinal axis, which brings the lower bed within reach of the surface. The total thickness of the coal measures is estimated at about 11,000 feet on the south, and 7000 feet on the north side in the western district. In the central portion between Britton Ferry and the River Taff, it diminishes to 4800 feet on the north side, and is still further reduced in Monmouthshire and on the eastern side generally to about 2500 feet. The coal-bearing portions are divisible into three groups, known as- [table]
The Upper Pennant series attains the maximum development of about 3000 feet on the south rise of the measures near Swansea; at Neath the thickness is reduced to about 1200 feet, and in Monmouthshire to between 500 and 700 feet. It contains all the free burning and bituminous coals of the Swansea and Neath districts, and the housecoals of Monmouthshire and the eastern districts, which latter contain 26 seams above 12 inches thick, making a total of about 100 feet of coal, an amount that increases westward to 82 seams and 182 feet. The Lower Pennant series averages from 1100 to 1500 feet between the Taff Vale and Llanelly, but on the north side of the anticlinal thickens to 3000 feet. The average total of workable coal in seams which do not exceed 3 feet is 25 feet, among which are some fair steam coals, associated in places with black-band ironstone and good manufacturing and household coals, yielding slack suitable for coking,—the most valuable among them being those of the Rhondda valley. The lowest or White Ash series contains the bulk of the valuable steam and iron making coals which have given the coal field its great reputation. It is about 500 feet thick on the eastern side, and about 1000 feet in the centre of the basin. The coals and accompanying ironstone are generally thicker and more abundant on the south than on the north coast. The workable coals in this division amount to about 50 feet, in seams varying from 3 to 9 feet in thickness. The western extension into Pembrokeshire belongs to this part of the series; it covers about 7 0 square miles, extending in a narrow east and west belt, varying from 2 to 6 miles in breadth from Tenby to St Bride’s Bay. The measures are very much disturbed, but are probably about 1500 feet, containing in the upper 1000 feet 8 seams of anthracite of about 18 feet total thickness.
The total area of the coal-field is about 1000 square miles, of which amount about 153 square miles lie beneath the sea in Swansea and Carmarthen Bays. Only one square mile is covered by newer formations.
According to the quantity of the coal produced, the area fs divided as follows :— [table]
The most valuable class of South Wales coals is the semi-anthracite or smokeless steam coal of the lower measures, which is in constant demand for the use of ocean steamers all over the world. It is principally exported from Cardiff, Neath, and Swansea.
The configuration of the ground, owing to the deep north and south valleys of the Usk, Ebbw, Taff, Rhondda, and Neath Rivers, and the longitudinal anticlinal axis, renders the coals of comparatively easy access. The surface rises, to a height of about 2000 feet above the sea-level, and in the valleys a greater vertical range is brought within working limits than is the case in any other coal-field of similar thickness.
The Forest of Dean basin is an outlying portion of that of South Wales, from which, as is shown by Ramsay, it has been separated by denudation. It is of triangular form, occupying an area of 34 square miles, between the Wye and the Severn estuary, with a total thickness of 2765 feet and 31 seams, together 42 feet thick, only 9 of which are above 2 feet in thickness. The depth from the surface to the bottom of the basin, in the centre, is about 2500 feet. The lower beds of sandstone and the Carboniferous limestone contain considerable quantities of brown hematite, in irregular deposits, which is smelted in part on the spot and partly exported to other districts. Owing to the symmetrical basin-shaped form of the measures (Plate II. fig. 1), the coals have been worked from the surface downwards along the outcrops of the seams, leaving large hollows for the accumulation of water, which render the working of the lower ground difficult, on account of the great pumping-power required to keep down the water flowing in from the old shallow mines.
North of the Malvern Hills a straggling patch of coal measures extends about 35 miles N. and S., from near Worcester to Newport in Shropshire. This is divisible into two nearly equal areas of triangular form. The southern part is known as Forest of Wyre, and the northern as Colebrookdale. The former is unimportant, having a great thickness of measures which rest directly on the Devonian rocks, but scarcely any workable coal seams. The Colebrookdale measures rest upon the Upper Silurian rocks, are about 800 feet thick, with about 50 feet of coal in 18 seams, and many beds of nodular ironstone, which has given the district a celebrity in the production of ironwork, especially high-class castings. The eastern boundary is concealed by overlying Permian strata, and it was formerly supposed that the productive measures had been removed by denudation on this side; but there is little doubt of their continuity towards South Staffordshire.
To the westward of Colebrookdale are the two small fields of Leebotwood and Shrewsbury. These lie on the Silurian rocks. The exposed area of the former extends to 12 square miles; that of the latter (which stretches in a crescent shape to the south and west of Shrewsbury) to 18. Both are partly hidden by Permian strata.
The South Staffordshire coal-field extends about 22 miles N. and S., from Rugeley to Halesowen, with a greatest breadth of about 10 miles from Wolverhampton to Oldbury. It is entirely surrounded byNewRed Sandstone rocks, which in some places are faulted against the coal measures, rendering it difficult to decide upon the chances of a profitable extension beneath the visible boundaries. The coal measures rest upon the Upper Silurian rocks, which are exposed at several points within the area, especially at Dudley and the Wren’s Nest. This district is remarkable as containing the thickest known coal seam in England, the Thick or Ten Yard Seam, which varies from 30 to 45 feet in thickness in the neighbourhood of Dudley, but splits up northwards into several thinner seams in the northern or Cannock Chase district. There are 6 principal seams, with a total of from 57 to 70 feet in 1300 feet of measures. The field was formerly very productive of clay ironstone, but the supply has now considerably diminished. The coals are also subject to curious alterations in places, from the intrusion of igneous rock, especially in the Rowley Hills, near Dudley.
The Warwickshire or Tämworth coal-field is a narrow strip of measures, with a maximum thickness of 3000 feet, extending about 12 miles in a N.W. and S.E. line from Coventry to Tamworth. It contains 5 seams, which are mainly worked for house coal and steam purposes. It is entirely surrounded by New Red Sandstone strata, except for a short distance near Atherstone, where it is seen to rest upon the millstone grit, which is altered into quartzite by intrusive igneous rocks.
The Leicestershire or Ashby coal-field is an irregular patch of 30 square miles, on the east side of Chamwood Forest, about midway between Leicester and Burton-on-Trent. It has 7 principal seams, and probably rests upon the mountain limestone, except at the eastern end, where it may lie upon the old slatey rocks of Charnwood Forest. Southward it extends under the New Red marl towards Leicester. In the centre is a patch of barren measures upon which the town of Ashby-de-la-Zouche stands, after which the coal-field is often named. The eastern side, which contains the mines of Whitwick, Snibston, and ColeOrton. contains some igneous rocks apparently connected with those of Charnwood Forest, which are not seen on the western or Moira side, which contains the more important workings, None of the seams occurring in either division can be identified with certainty in the other, although only a few miles distant. The total thickness of the coal measures is about 2500 feet, the principal seams occurring about the middle, as is also the case in Warwickshire.
North of the Trent the carboniferous strata present a more complete and regular development than is seen in the central coal-fields. The Carboniferous limestone and millstone grit formations form a central ridge of high moorlands and hills, the so-called Pennine chain, in a gently sloping anticlinal, running nearly north and south from the north of Derbyshire to the borders of Scotland. The coal measures occur on both flanks of this ridge, the largest connected mass being that of the Derbyshire and Yorkshire coalfield, which extends north and south for about 60 miles from Bradford to within a few miles from Derby, where it is covered by the New Red Sandstone formation. The exposed breadth varies from 9 miles at the south end to 22 miles at the north. The measures dip regularly at a low angle to the eastward, and pass under the Permian or magnesian limestone formation, which forms the eastern boundary continuously from Nottingham through Worksop and Doncaster to Wakefield. The total thickness of measures is about 4000 feet (with about 20 seams), belonging to the middle and lower ganister series, the upper series being absent. A generalized section of the strata in this coal-field is given in the fifth column of fig. 1. The principal seams are the Black shale, or Silkstone, from 5 to 7 feet thick, which is extensively worked as a house coal, and the Top hard, or Barnsley coal, which is much used for steam purposes. At the north end of the field, in the neighbourhood of Leeds and Bradford, two thin seams, known as the Low Moor black bed and better bed, remarkable for their exceptional purity, are used for iron-making purposes at Bowling and Low Moor. Iron ores are also found in considerable quantity on the Derbyshire side of the field, which are smelted at Butterly and other works near Chesterfield. The area covered by the magnesian limestone formation has been proved by several borings and sinkings, the first winning having bç.ðn opened at Shireoak near Worksop, where the Top hard coal was reached at 1548 feet below the surface. It is estimated that about two-thirds of the total area of this field is to be looked for within the concealed part.
On the west side of the Pennine axis, and between the same parallels as the Derbyshire and Yorkshire coal-fields, are those of North Staffordshire and Lancashire, which extend from Longton on the south to Colne on the north, the continuity being, however, broken by a small fold of the Carboniferous limestone shales, which is brought to the surface between Macclesfield and Congleton. Parallel to this group, however, and to the eastward of it, is situated the small but important coal-field of North Staffordshire, also known as the Pottery coal-field. It has an exposed area of about 94 square miles, which is very irregular in form, being 17 miles in greatest breadth E. to W., and about 13 miles from N. to S. The south-eastern portion, which is nearly detached, is known as the basin of Cheadle, or Froghall, which is chiefly remarkable for a band of calcareous iron ore formerly exported to Staffordshire, but now nearly exhausted. The main or western portion con sists of a mass of strata about 5000 feet thick, with 37 seams of coal, out of which 22, measuring together 97 feet, are over 2 feet in thickness; in addition to which there are many valuable beds of ironstone, both argillaceous and black-band. The strata, which are less regularly arranged than those of S. Lancashire, as will be seen by the transverse sections, figs. 2 and 3 in Plate IL, being bent in contrasted curves, and much broken by faults, form the eastern limb of a basin having a general westerly dip, which carries them in a short distance below the New Red marl plain of Cheshire.
The Lancashire coal-field is of an irregular four-sided: form. The greatest breadth, from Oldham on the east to ' Saint Helen’s on the west, is about 52 miles, and the length, from Burnley on the north to Ashton-under-Lyne, about 19 miles. Within the area are, however, two large islands of the millstone grit, which divide the northern or Burnley district from the main coal-field of Wigan and Manchester. This barren area is about compensated by a tongue of coal measure, which extends southward from Stockport tβ Macclesfield. The thickness of the measure is very great, and as the ground is much broken by faults, and the beds dip at a high angle, the workings have extended a greater depth than in any other district, the deepest workings being at Rose Bridge pits near Wigan, which have been sunk to 815 yards, and at Dukinfield, east of Manchester, where the Astley pit is 672 yards deep, and the coals have been wrought to a total depth of 772 yards by inclines. The greatest thickness is observed in the Manchester district, where the total section is as follows, according to Hull. [table]
There is a total of 100 feet of coal in workable seams (exceeding 2 feet), which are chiefly situated in the 3000 feet forming the bottom of the middle and top of the lower coal measures. In the Wigan district there are 18 workable seams, about 65 feet in all, the total section being:— [table]
The Wigan district is remarkable for the production of a large quantity of cannel coal.
In the Burnley district the lower and middle coal measures together are from 2500 to 3000 feet in thickness, the upper measures being unrepresented.
The coal-field of Northumberland and Durham lies north of that of Yorkshire, on the east side of the Pennine axis. In the intermediate ground between Leeds and Darlington, about 55 miles, the lower Carboniferous rocks are directly overlain by the magnesian limestone, which preserves the north and south course observed further south until it reaches the sea at the mouth of the Tyne. The coal-field extends north and south from Darlington through Durham to the mouth of the Coquet, about 65 miles, with a greatest breadth of about 22 miles in Durham. From the Tyne to the Coquet the eastern boundary is formed by the sea, while in the remaining area, from the Tyne to the Tees, which is included in Durham, the coal measures dip beneath the magnesian limestone. The measures are, as a rule, very regular, their dip being lower than that observed in other districts.. .The total thickness is about 2000 feet, with 16 seams of coal, together about 47 feet thick. The millstone grit is continuously exposed below the coal measures along the eastern edge as far as the Tees, where it is overlapped by the magnesian limestone and Triassic rocks, so that there is a portion of the coal-field hidden beyond the exposed southern boundary, but the extent is probably not large. The seaward extension has been proved in several deep mines in the neighbourhood of Sunderland, more especially at Ryhope and Monkwearmouth which are worked at a depth of about 1850 feet to a short distance from the shore. At these points the coals are nearly flat, but at Harton, near Shields, they rise to the eastward, proving that the centre of the basin has there been passed. The best estimate gives 11 feet of coal, and about 16 square miles of area, as the probable extent of this submarine portion of the coal measures. The character of the coal produced varies in the different parts of the basin. The southern and western districts adjoining Bishop Auckland and Ferryhill produce a strongly caking coal, which is chiefly employed in the manufacture of a pure and dense coke for use in the Cleveland and Cumberland iron works, a considerable amount being also exported for foundry use. The central district, adjoining Newcastle and Sunderland, produces the best class of house coal, known in London under the name of Wallsend, from the pits on the Tyne where it was originally mined, which were close to the eastern termination of the wall built by the Romans to protect the country between the Tyne and the Solway from the incursions of the Picts. These collieries have been long since abandoned, but the name is still given in the London market to the best Durham house coals, and even to much that has been produced in other places, as indicating a coal of superlative excellence. The great merit of Wallsend coal is in its small proportion of ash, which also, being dark-coloured, is not so obtrusive on the hearth as the white ash generally characteristic of the Midland coals. The strongly caking property, and the large amount of gas given out in burning, tend to produce a bright and enduring fire. In. the district north of the Tyne the produce is principally steam coal, which is known as Hartley coal, being named after one of the principal collieries. It is largely used for sea-going steamers, and was lately in use in the Royal Navy mixed with South Wales coal, a combination which was supposed to give a higher evaporating value in raising steam than when either class was burnt alone. Although of a lower calorific power, and making more smoke than South Wales coal, the north country coal deteriorates less rapidly than the fonner when stored in hot climates. There are two small coal-fields in the mountain limestone district of the Tyne near Hexham, and another on the Solway at Can-nobie; these are, however, of small importance.
The Cumberland field extends along the coast of the South Irish Channel from Saint Bees northward for 15 miles to Maryport, where it turns eastward for about 17 miles, and is exposed with constantly diminishing breadth until it disappears under the Permian rocks of the Vale of Eden. The greatest breadth is about 5 miles at Whitehaven and Workington, but. as in Northumberland and Durham, the beds dip and the coals have been worked below the sea to a distance of 1 £ miles from the shore or 21 miles from the pit. The total thickness of the measures is 1500 feet, with three workable seams. The produce is largely consumed within the district, a considerable portion of the export being to Belfast and other Irish ports.
The coal measures of North Staffordshire and Lancashire reappear on the western side of the plain of Cheshire in the coal-fields of Denbighshire and Flintshire, which form a nearly continuous tract from the neighbourhood of Oswestry through Ruabon and Wrexham to the mouth of the Dee, and along the Welsh coast near Mostyn. The separation between them is formed by a slight roll in the mountain limestone near Gresford, corresponding to that dividing the two coal-fields on the eastern out-crop. The Denbighshire field is about 18 miles long, having 7 seams, together from 26 to 30 feet in thickness. The principal workings are near Ruabon, where there are several large collieries producing a much esteemed house coal. The Flintshire field is about 15 miles long. The greatest breadth is in the neighbourhood of Mold, whence it narrows in a N.W. direction, being covered by the estuary of the Dee. At Mostyn coal has been extensively worked under the river, but great difficulty was experienced in keeping the mines clear of water. The details of the measures in this district have not been fully worked out, but the southern portion is the most valuable. The higher measures contain six seams, including some valuable beds of cannel, the total being about 28 feet. In the northern district bordering the Dee the beds are much disturbed by faults, but the deeper coals are said to be of good quality.
The basin formed by the North Wales, Lancashire, and North Staffordshire coal-field is probably the most extensive tract of coal measures in the country, as it may be assumed to extend under the overlying Triassic strata under the Dee and the Mersey to South Lancashire and across the plain of Cheshire, an area of 800 to 1000 square miles. Much of this, however, is far beyond workable limits, the depth to the top of the coal measures being estimated at 10,000 feet below the surface at the point of greatest depression. The area within the limits of 4009 feet below the surface, which has been assumed as a possible maximum working depth, may be seen by reference to Plate I.
There is a small coal-field in the Island of Anglesea, which is interesting for its geological peculiarities, but it is of very small economic value.
The Somersetshire coal-field appears at the surface in the form of several disconnected patches, the largest of which extends northward of Bristol for about 12 miles, while the remainder stretches southward for about the same distance to the Mendip hills. The Carboniferous limestone is seen at many places along the western flank, but the connection is generally hidden by a peculiar modification of the New Red Sandstone known as the Dolomitic Conglomerate, which overlaps both formations indifferently. Towards the east the measures are further obscured by the overlap of the lias and oolitic rocks, this being the only field in which such an overlap takes place in England. The exposed area of the coal measures is only about 14 square miles, but it is estimated that they extend over 238 square miles, the remainder being concealed by overlying strata. The character of the measures is similar to those of South Wales and Dean Forest, namely an upper and lower productive series separated by a nearly barren mass of Pennant sandstones. The sections, which vary very considerably, are summarized by Prestwich as follows :— [table]
The disturbance of the strata by faults is much greater than in any other British coal-field. The whole series is squeezed into a comparatively narrow trough, which throws the bottom of the basin to about 8000 feet below the surface. The coals are in some instances tilted up vertically, or even turned over, a kind of disturbance which is usually attended with considerable shattering of the strata. In one instance the upper series of measures have been shifted horizontally by an inclined or slide fault for a distance of about 200 feet above the lower series. In spite of the difficulties caused by these disturbances, coal seams of only a foot in thickness are regularly worked in Somersetshire, which is far below the limits considered to be profitable in other districts.
The coal-bearing strata of Scotland are confined to the Carboniferous formation, the only exceptions being the little patch of Oolitic coal at Brora in Sutherland and certain thin seams which occur intercalated among the Miocene volcanic rocks of the Western Islands. The Scottish Carboniferous Formation is divisible into four series, viz.,— 1. Coal Measures; 2. Millstone Grit; 3. Carboniferous Limestone series; 4. Calciferous Sandstone series. Coal is confined chiefly to the first and third of these groups, but in West Lothian and Mid-Lothian the lowest (calciferous sandstones) yields some coals, one of which has been worked (Houston coal, 6 feet thick). These coals are associated with the well-known “oil-shales,” forming a peculiar development of the upper portion of the calciferous sandstone series which is not repeated elsewhere in Scotland. The millstone grit contains no workable coals. The coal bearing strata of the coal measures and limestone series are irregularly distributed over the central or lowland district of the country between a line drawn from St Andrews to Ardrossan, and a second line traced parallel to the first from Dunbar to Girvan. Throughout this region the strata are disposed in a series of basins, of which there are properly speaking only three, namely,—(1) The basin of Mid-Lothian and Fifeshire, which is bounded on the west by the calciferous sandstone series and some older strata, forming the Pentland hills, Arthur’s Seat, the rolling ground that extends west of Edinburgh into Linlithgowshire, and the heights behind Burntisland in Fifeshire, and in the east by the barren sandstones and igneous rocks of the calciferous sandstone series in the east of Haddingtonshire and Fifeshire; (2) The basin of Lanarkshire and Stirlingshire, the eastern boundary of which begins in the south at Wilsontown, and runs north by Bathgate and Borrowstounness to the borders of Clackmannan, extends west to the foot of the Campsie and Kilbarchan Hills, and is separated by the Paisley and Dunlop Hills from (3) the basin of Ayrshire, the main mass of which is bounded in the south and east by the valley of the Doon, the Silurian uplands behind Dalmellington and New Cumnock, and the calciferous sandstone and Old Red Sandstone heights which overlook the heads of the Ayr and Irvine valleys. Two small outlying coal-fields lie beyond these boundary lines, viz., the Girvan and Sanquhar (Dumfriesshire) coalfields, but both belong geologically to the Ayrshire basin. Although there are thus only three great basins, it is usual, nevertheless, to speak of five principal coal-fields, each of which is named after the county in which it is most abundantly developed. Thus we have the coal-fields of Ayrshire, Lanarkshire, Stirlingshire, Fifeshire, and MidLothian.
Ayrshire Coal-fields.— The Ayrshire basin, owing to undulations and faultings of the strata, comprises a number of subsidiary coal-fields, such as those of Girvan, Sanquhar (Dumfriesshire), Dalmellington, New Cumnock, Lugar and Muirkirk, Kilmarnock, Kilwinning, Dalry, &c. The coal measures of this basin are of variable thickness; they contain from 5 to 8 and 11 principal coal-seams, yielding a united thickness of from 13 ft. to 40 ft. The Carboniferous limestone series of Ayrshire sometimes contains no workable seams of coal, while occasionally its seams equal or surpass in number and thickness those of the coal measures. Thus in the Girvan field there are 7 coals with an aggregate thickness of 50 feet, while at Muirkirk the same number yield a thickness of 40 feet of workable coal. The Ayrshire coals consist chiefly of common coals, including “hard” or “splint” and “soft” varieties. In some districts the intrusion of igneous rocks has converted certain seams into “blind coal,” a kind of anthracite, much used for steam purposes. Gas or parrot coal (so called from its decrepitating or chattering when heated) is met with here aud there, chiefly near New Cumnock. Parrot coal often occurs in thin lines or bands, which, when intercalated and alternating with dark carbonaceous ironstone and coaly matter, form seams of what is called black-band ironstone. The Ayrshire black-bands occur chiefly at Dalry, Lugar, and Dalmellington.
Lanarkshire Coal-fields.— These are the most extensive in Scotland, covering an area of not less than 150 square miles. The coal measures, which attain a thickness of upward of 2000 feet, contain about 18 workable coals; but all these are not continuous throughout the whole coalfield, while some are too thin in places to pay the cost of working. At their best they yield an aggregate thickness of 70 feet or thereabout, but in many places they do not average more than 40 or 30 feet, or even less. The limestone series is well-developed in the Lanarkshire coal-fields, but it is a very variable group, as indeed is the case throughout Scotland. It consists of upper, middle, and lower groups, the coals being confined chiefly to the middle group, only one or two seams occurring in the lower, while in the upper only one seam occasionally attains a workable thickness. The principal coals of the limestone series vary in number from 1 to 9, their aggregate thickness seldom reaching more than'15 feet. The Lanarkshire coals consist chiefly of varieties of common coal, namely, hard or splint, soft,, dross, <fcc. But here and there excellent gas coal is worked, as at Auchenheath, Wilsontown, <kc., the former being considered the finest of all the Scotch gas coals. Another well-known parrot coal is that of Boghead near Bathgate, the subject of much litigation. Parrot or gas coal frequently occurs forming a part of musselband and black-band ironstones, which seams, when traced along their crop, are often seen to pass into gas coal. The best known blackbands are those wrought at Palacecraig, Airdrie, and Quarter, Bellside, Calderbraes, Bowhousebay and Braco, Goodockhill and Crofthead, Earnockmuir, Possil, Garscadden, and Johnstone.
At Quarrelton, Renfrewshire, an abnormal development of coal seams occurs below the horizon of the main or Hurlet limestone, which is usually the lowest important bed in the limestone series. The strata underlying that limestone contain here and there irregular lenticular patches of coal, never of any value. At Quarrelton, however, a number of these seams come together, and form a mass of coal more than 30 feet thick.
Stirlingshire Coal-fields.— These embrace the coal-fields of Falkirk, Carron, and Grangemouth, Slamannan, Clackmannan, and Borrowstounness. In the Falkirk, Carron, and Grangemouth fields, the coal measures are about 600 feet thick, and contain 9 workable seams of coal, yielding an aggregate thickness of 30 or 31 feet; the thickest seam is only 4 feet. In the Slamannan field, the coal measures are some 720 feet thick, and show 6 workable coals, yielding an aggregate thickness of 15 or 16 feet, the thickest seam being 4¾ feet. A small outlier of coal measures at Coneypark, however, gives a depth of 1140 feet of strata, containing 12 workable coals (two of which are 7 feet thick respectively), which yield an aggregate thickness of 44 feet. The coal measures of the Clackmannan district attain a thickness of 900 feet, and yield 10 workable seams of coal (thickest seam 9 feet) with an united thickness of 41 feet. The limestone measures of the Stirlingshire basins contain, as a rule, few coal seams. Where these are best developed, they vary in number from 5 (Bannockburn) to 11 seams (Oakley); and their aggregate thickness ranges from ll¾ feet to 37 feet. The coals embrace the variety usually met with in Scotland, viz., hard (or splint) and soft coals, some of the seams being good caking coals. Good gas coal was formerly obtained at Oakley; and other coarse parrot coals occur in various parts of the fields. Oil shale and blackband ironstone are also - inet with. The coal-field of Borrowstounness is remarkable for containing thick sheets of basalt rocks, which are of contemporaneous origin, and do not alter the beds that rest upon them.
Mid-Lothian and Fifeshire Coal Fields.— The Mιd-Lothian coal field is disposed in what are for Scotland unusually symmetrical and unbroken lines. The basins lie with their principal synclinal axes from north to south. In the deepest basin the coal measures lie in a trough 2¾ miles broad and 9 miles in length, stretching from the sea at Musselburgh through Dalkeith to Carrington. The trough is underlaid by the millstone grit (Roslin Sandstone or Moor Rock), whose outcrop surrounds that of the coal measures in a band rarely more than half a mile broad. The Carboniferous limestone series rises from beneath the basin of millstone grit and coal measures on its west side, and crosses at a high angle, in a band about a mile in breadth, through Portobello, Gilmerton, and Penicuik. South of Penicuik the millstone grit forms another basin at Auchencorse Moss, but the trough is not deep enough to bring in the coal measures. West of Dalkeith the limestone series forms a shallow undulating basin with an outcrop of about 7 miles broad, extending from the sea at Cockenzie by Tranent and Pathhead. The Dalkeith basin of the coal measures has a total thickness of 1180 feet. There are 14 coal seams of a workable, thickness, with an aggregate of 43 ft. 4 in. The limestone series of MidLothian contains numerous coal seams. The total thickness of the series is 1582 feet, with 23 workable coal seams, aggregating 68 ft. 3 in. The “great seam” averages between 8 and 11 feet, and in one place is 12 ft. 6 in. thick. The coals of the Mid-Lothian basins are of the usual varieties met with in Scotland. The basins of the MidLothian coal-fields reappear on the southern coast of Fife, and are undoubtedly continuous (though somewhat denuded) beneath the Firth of Forth. A segment of the western half of the coal measures trough (the prolongation of that of Dalkeith) extends from Dysart by Markinch, Kennoway, and Largo Bay. On the north this trough is bounded by faults, and on the east and south it is covered by the sea. Measured from Coaltown to Methil (at right angles to the line of strike) the thickness of the coal measure strata exposed to view may be roughly estimated at 4600 feet; but as the centre of the basin is not reached at the coast, the total thickness of strata is not seen. There are about 11 workable seams, with an aggregate of 61 feet. The Dysart Main coal is 16 feet thick. Another little basin, comprising the lower seams of the coal measures, occurs at Kinglassie. The Dysart or Leven coal measure basin occupies about 18 square miles, and that of Kinglassie from 3 to 4. The limestone series of Fife lies in several much broken basins on the south side of the Ochils and Lomond Hills from Alloa to Earlsferry. The principal coal fields in this series are those of Dunfermline, Halbeath, Lochgelly, and Kelty; but coals have been worked in many other places, as at Ceres, Radernie, Largo Ward, Markinch, &c. The coal-bearing strata vary in thickness, but do not exceed 600 feet. In the Dunfermline coal-field there are 10 seams, with an aggregate thickness of 41 feet. Halbeath coal-field yields 8 seams, with an aggregate thickness of 29¾ feet; Lochgelly coal-field contains some 14 seams, with an aggregate thickness of about 65 feet; in the Kelty and Beath coal-field there are 12 seams, yielding an aggregate of 43⅛ feet. The workable seams in these separate fields range in thickness from about 2 feet up to 10 and 14 feet. The 14 feet coal of Lochgelly is divided by thin ribs of stone, which thicken out eventually so as to divide the coal into 5 separate workable seams, which, with the intervening strata, yield a thickness of 10 fathoms of strata. It is worth noting that, in the lower Carboniferous rocks of Fifeshire, two coals are worked at Balcarmo and elsewhere. As a rule, this series in Scotland is barren.
The carboniferous strata of Ireland consist chiefly of the Carboniferous limestone, which covers the greater portion of the island in one connected mass. The coal measures have probably been at one time nearly as extensive, but they have been almost entirely removed by denudation, the largest remaining basins being that of Castlecomer, near Kilkenny, and another in the west, between Tralee, Mallow, and Kilarney. In the north the small basin of Coal Island, on the west side of Lough Neagh, is partly covered by New Red Sandstone strata, and trials have been made to discover a possible extension of the coal measures in the valley of the Lagan, between Belfast and Lisburn.
The two coal fields of South Wales and Somersetshire differ from those of the central and northern counties in their strike or direction, their longer axes being placed east and west, instead of north and south, which is the prevailing direction of the latter,—the strata in the Somersetshire area being sharply bent and broken on a north and south line in a manner which is not seen elsewhere in this country, but is reproduced on a much larger scale in the north of France and Belgium. The most easterly point in England at which the coal measures have been worked is near Bath, where the overlying Liassic and New Red Sandstone strata are .about 360 feet thick, beneath which the coal has been followed for some 5 or 6 miles from the outcrop. From this point nothing certain is known of their extension until we reach the neighbourhood of Valenciennes, where a coal field, known as that of Hainault and Valenciennes, extends with a general east and west strike as far as Namur, a distance of 65 miles. At Namur the width is about 2 miles, near Charleroi from 7 to 8, and through the north of France from 6 to 7. Only the eastern half, between Charleroi and Namur, comes to the surface, the western portion being covered by Tertiary and Cretaceous strata. Within 30 miles of Calais the coal measures end, the shales of the Carboniferous limestone having been pierced in a boring of 1113 feet deep at the latter place. East of Namur the coal measures come in again at Liège, continuing for about 45 miles, with a width of from 3 to 8 miles to beyond Aix la Chapelle, where they are divided by a ridge of Carboniferous limestone into two parallel basins, covered by Cretaceous and newer deposits, till they appear again on the right bank of the Rhine in the valley of the Ruhr, in the great Westphalian basin, which is probably the largest in Europe.
The same general structure is apparent along the whole of this line, which, from the western end of the South Wales basin to Frome, and from the N. of France to the Ruhr, is about 470 miles long. The measures generally dip regularly from N. to S. along the northern line of outcrop where it is known, but on the southern side they are bent into sharp folds by the elevation force which has uplifted the underlying Carboniferous limestone and Devonian strata along an east and west line, extending from the old slaty rock of the Ardennes to the Mendip Hills and the western part of Pembrokeshire. The known coal fields extend for about 350 miles out of the above amount of 470, and from the similarity of their position and structure many geologists are of opinion that other basins similarly placed may be reasonably supposed to exist in the intermediate ground between Somersetshire and Belgium. This subject has been treated in great detail by Mr Godwin Austen and Prof. Prestwich in the Reports of the Royal Commission upon Coal. The probable direction of this axis is shown on the map, Plate I. The only actual determinations of the rocks made within this area have been in two borings at Kentish Town and Harwich. In the former, sandstones, supposed to be of Devonian age, were reached below the Cretaceous strata at 1113 feet, and in the latter the Carboniferous limestone shale at 1025 feet. The most likely positions for the coal measure trough are considered byPrestwich to be in Essex and Hertfordshire, while Mr Godwin Austen places them iu the valley of the Thames or under the North Downs. The latter seems to be the more probable than the line further north. The point, however, is purely speculative in the absence of any trial borings as guides; and a great number of these would certainly be required before any generalization as to the position of workable coal measures even within a wide range could be accepted. The deep boring on the southern part of the Wealden area, near Hastings, which it was supposed would have thrown a considerable amount of light on this matter, has hitherto been without other result than the proof of the existence of a totally unexpected and exceedingly great thickness of the upper Oolitic clays, similar to what is known on the French coast, near Boulogne.
On the south side of the Mendip axis a very large area in Devonshire is occupied by the lowest coal measures or culm series, which consist almost entirely of clay slates, with a few beds of anthracite in the northern portion of the district, near Barnstaple and Bideford. These are only worked to a small extent, their principal use being, not for fuel, but as a pigment for covering iron ¼’ork, which is known as Bideford black.
The coal-bearing areas of Secondary and Tertiary age in the United Kingdom are of very small importance. In Devonshire a lignite-bearing series of strata of Miocene age occurs in the flank of the granite of Dartmoor at Bovey Tracey, near Newton Abbot. This is principally remarkable for its associated clays, which are derived from the waste of the granite, and contain numerous impressions of dicotyledonous leaves and other plant remains. The coal is a lignite resembling a mere heap of tree stems drifted together and partially decomposed. It is not now worked, the original excavations being filled with water; and as the demand is restricted to supplying the wants of the local potteries, there is no opening for profitable mining. -
In the Great Oolite of Yorkshire, some thin seams of coal or lignite were formerly worked at numerous points upon the moors between the Cleveland Hills and the Vale of Pickering. The most important product of this district, however, is the jet which is obtained from the waste of coal-bearing strata of the same age along the cliffs near Whitby, where it is manufactured into ornaments. The largest Oolitic coal deposit in this country is that of Brora in Sutherland, where a seam of about 3j feet in thickness has been worked at intervals for a considerable period, but never to any considerable extent except during the prevalence of high prices in the coal trade.
Another area in which coal is found in strata of Secondary age is that of Scania, near Helsingborg, in southwestern Sweden, in the three coal-fields of Hoganas,Stabbarp, and Róddinge. These are situated in the uppermost Triassic or Rhaetic series. At the first, which is the most important locality, the strata vary from 100 to 800 feet in thickness, with two seams of coal respectively 1 and 4∣ feet iu thickness. There is a good fire-clay associated with the lower seam, which is extensively worked for fire bricks and pottery, a large proportion of the coal being used on the spot. In the Danish Island of Bornholm similar coal-bearing strata, probably of Liassic age, form a narrow belt along the south and south-west coast, which it is supposed may continue under the alluvial plain of the Baltic into Pomerania.
The Coal-fields of the Continent of Europe.
The coal-fields of the continent of Europe, though more scattered and disturbed than those of England, may be similarly divided into two groups according to their geological structure, the first being those in which the series is complete, the coal measures being symmetrically arranged upon the Carboniferous limestone and Devonian strata. Examples of this structure are afforded by the long line of coal-fields extending through the north of France and Belgium to the Rhine valley on the north side of the Ardennes, and those of the more easterly district of Silesia and of the north of Spain. The remaining and far more numerous European coal-fields are either contained in hollows in crystalline schists, or rest on the older Palaeozoic rocks, e.g., the central and southern French basins, and those of Saxony and Bohemia. Further east, in central and southern Russia, the order observed in Scotland is reproduced, there being a large development of coal in Carboniferous limestone strata, and something of the same kind seems to be probably the case in China.
The best developed portions of the Franco-Belgian coalfield are seen within the territory of Belgium, the westerly extension into France being entirely covered by a' great thickness of newer strata. Commencing at the eastern side, the first field or basin is that of Liège, which extends from the Prussian frontier near Verviers in a S.W. direction for about 45 miles, the greatest breadth being about 9 miles near Liège. The principal working points are concentrated on the western edge, where the lower beds rest on the Carboniferous limestone, the eastern portion being partly covered by Cretaceous and Tertiary strata. The number of coal seams is 83, the upper series of 31 being so-called fat coals, suitable for coking and smiths’ fires; the middle series of 21 seams are semi-dry-or flaming coals; and the remainder or lower series of 31 are dry, lean, or semi-anthracitic coals. The upper series, which are the most valued, are found only in a small area near the cèntre of the basin at Ougrée, near Liège. The seams vary from 6 inches to 5¾ feet iu thickness, the average being barely 3 feet. This order of succession is observed in the whole of the districts along this axis. The same general structure also prevails throughout the strata which have a comparatively small slope on the northern crop, and are very sharply contorted, faulted, or broken along on the south side of the basins. The local terms platteurs and dressants are used to distinguish the flat and steep portions of the coals respectively.
The next basin, that of the Sambre, extends for about 30 miles from Namur to Charleroi, the greatest exposed breadth being about 9⅛ miles. The western and a greater part of the northern side are covered by Tertiary strata, which are very heavily watered. At Montceau, near Charleroi, there are 7 3 seams, which pass through the various conditions of fat, flaming, and dry coals, from above downwards, according to the order already described.
The most important development of the coal measures in Belgium is in the basin of Mons, which extends from Mons to Thulin, a length of about 14 miles, with a breadth of about 7 or 8 miles, a large portion of the area being covered by newer strata. The number of known coal seams is 157, out of which number from 117 to 122 are considered to be workable, their thickness varying generally between 10 and 28 inches, only a very few exceeding 3 feet. These are classified, according to position, into the following groups, which are taken as a standard for the whole of the north of France and Belgium:—
1. Upper series (charbon flenu), 47 seams. These, which occur chiefly in the neighbourhood of Mons, are very rich bituminous coals, especially adapted for gasmaking.
2. Hard coal series (charbon dur), 21 seams. These are, in spite of their name, soft caking coals, less rich in volatile matter than the flenu, but excellent for coking purposes.
3. Forge coal series, 29 seams. These are chiefly used for smithy purposes and iron works, but the lower members approximate to dry steam coals.
4. Dry or lean coals, 20 to 25 seams, forming the bottom series. They are of small value, being chiefly used for brick or lime burning.
The amount of compression to which the strata have been subjected in these coal-fields, has caused them to be sharply contorted into zig-zag folds. In the neighbourhood of Mons a single seam may be passed through six times in a pit of 350 yards vertical depth, and the strata, which if flat would be 9 miles broad, are squeezed into a space 7 miles across and about 8200 feet deep to the bottom of the basin. At Charleroi the compression is still greater, a breadth of 8⅜ miles Of flat strata being narrowed to rather less than half that quantity by contortion into 22 zig-zag folds.
The thickness of the overlying Tertiary and Cretaceous strata in the neighbourhood of Mons is from 500 to 900 feet; towards the French frontier the thickness is between 200 and 400 feet, and at Valenciennes about 250 feet. At Aniche these overlying measures, or terrains morts, are 400 feet thick, below which the coal measures are found to contain 23 feet of coal in 12 seams. At Anzin, near Denain, there are 18 seams, together 39 feet, which is about the maximum development in the north of France. This coal-field, which was unknown before 1734, has reached a very high state of production in spite of great difficulties interposed by the water bearing strata covering the coal measures. It extends for about 45 miles, diminishing in extent and value to the westward. The structure is very similar to that of the Belgian, one of the most remarkable features being the inclined fault called the cran de retour, which brings the lower or dry coal series of the north side against the higher coking coals of the south side, as shown in the section, Plate II. fig. 4.
At Hardinghen, near Boulogne, a small patch of disturbed coal strata was formerly worked. These are now supposed to be of the age of the Carboniferous limestone.
The coal-fields of central and southern France are mostly small in area and irregular in structure, with at times remarkable single accumulations of coal of enormous thickness, which do not, however, extend for any distance. The most important basin is that of Saint Etienne and Rive de Gier, south of Lyons, on the right bank of the Rhone. It is of triangular form, about 28 miles long, with a base of 8 miles. The thickness of the three principal seams at the latter place is about 33 feet, but at Saint Etienne there are from 15 to 18 seams, making together about 112 feet in a total depth of measures of about 2500 fept.
The basin of the Saône et Loire, near Chalons and Autun, is about 25 miles long in a S.W. and N.E. line. At Creusot, on the north crop, the coals, which are in places extremely thick (the main seam averaging 40 feet, but occasionally swelling out to 130 feet), dip at a high angle below a covering of New Red Sandstone strata, and appear in a modi tied form,bothas regards thickness and position, on the south side at Blanzy, An attempt has been made to prove the continuity of the series in the bottom of the basin by a deep boring, which was, however, abandoned at a depth of over 3000 feet without passing through the overlying strata. At Montchanin a remarkable seam or mass of coal was found extending for about 650 yards, with a thickness varying from 60 to 200 feet at the surface, which, however, diminished to one half 60 yards down, and wedged out at 140 yards deep. Another coal field of considerable importance is that of Alais and Grand Combe near Nîmes, which is partly covered by Liassic strata, and has a total maximum thickness of 80 feet of coal.
In addition to these must be mentioned the anthracitic series of the Alps, which extend along the flanks of that chain from Savoy and the Tarentaise into Styria and Carinthia. They are of small economic importance.
The Secondary and Tertiary coals of France are of comparatively small importance. Lignite is worked, among other places, near Dax in the Pyrenees, and at Trets and Fuveau near Marseilles.
The coal-fields of Prussia, situated on the extension of the Franco-Belgian axis, are the two small basins of the Inde and Worm, east of Adelnau, near Stolberg and Eschweiler, which are included in single sharply sloped folds of the mountain limestone, and the great Westphalian basin east of the Rhine, in the valley of the Ruhr. The latter, which is one of the most important in Europe, extends for about 30 miles east and west from Essen to Dortmund. The breadth is unknown; the beds are exposed for about 15 miles at the broadest part, but the actual boundaries to the north and north-east are hidden by Creta-taceous rocks. The greatest depth from the surface to the bottom of the basin is probably about 5000 feet. It is divided lengthways by transverse axes of elevation into four principal basins, besides several smaller ones. The total thickness of measures already proved is from 6000 to 8000 feet, with about 130 seams of coal, together about 300 feet thick. These are divided into three series by two bands of barren measures. The thickness of the individual coal seams varies from 8 inches to 7 feet. Seventy-six are considered to be workable, having a combined thickness of 205 feet, and 54 are unworkable, containing 42 feet of coal. The proportion of workable coal to the whole thickness of strata is as 1 to 33. The order of succession as regards quality is similar to that observed in Belgium, the most highly valued gas and coking coals being at the top of the series, and the dry semi-anthracitic seams at the bottom. On the south side of the axis of the Rhenish Devonian strata, which is the high ground known as the Eifel and Hunsruck, carboniferous strata reappear in what is known as the Pfalz-Saarbrücken basin, occupying a rectangular area between Bingen, Donnersberg, Saarbrücken, and Mettlach, about 60 miles long and 20 miles broad, the productive coal measures being restricted to a triangular space of about 175 square miles in the S.W. corner. The Carboniferous limestone is absent, but the thickness of the coal measures is very great, the upper or Ottweiler series measuring from 6500 to 11,700 feet, with about 20 feet of coal in different parts of the district, and the lower or Saarbrücken series from 9000 to 5200 feet, with 82 workable and 142 unworkable coal seams, making a total of about 350 to 400 feet of coal. The greatest thickness of the upper strata is found in those localities where the lower are thinnest, but the total thickness is computed to be about 20,000 feet in the thickest known section. The coals of the lower division are divided into groups by certain well-marked horizons, usually prominent seams, which have this peculiarity that the best coking and gas coals are found in, the bottom of the series, and the drier ones at the top, thus reversing the order observed in the basins on the northern slope. The amount of hygroscopic water in the coal is also found to diminish downwards.
In the district between the Ems and the Weser, are situated the small coalfields of Ibbenbüren, on the easterly extension of the Westphalian basin, and the Piesberg, near Osnabrück, which are of true Carboniferous age. Besides these, there is a curious development of coal in the Wealden strata which extend in a narrow discontinuous band E. and W. for about 150 miles. The coals are or have been worked at Tecklenburg and Borgloh in the Teutoburger Wald, at Bückeburg in Schaumburg, and in the Osterwald south of Hanover. The coal seams are small and of inferior quality, but are interesting as showing how nearly the conditions prevailing at the time of the older coal measures were repeated over a part of the same area in Cretaceous times. There are traces of thin discontinuous coal-beds in the Wealden strata of Sussex, but nowhere approaching to the extent of those in the Wealden strata of N. Germany.
In the low ground north of Halle, small and irregular patches of coal measures are found at Wettin, Löbejün, and Plötz. These are probably the remains of a single coal-field which has been disturbed and broken up at the time of the eruption of a great mass of igneous rocks which is found in a nearly central position between them. The coal measures are also found in the Thüringer Wald, the Schwarzwald, on the south side of the Harz, and in the Bavarian Oberpfalz, but none of these localities are important as centres of production. In Saxony there are two principal coal-fields, the first being that of the Plauens’che Grund, near Dresden, which is chiefly interesting for the very disturbed condition of the measures, and the consequent difficulty in working; and the other that of Zwickau, which is one of the most important in Europe. It forms an elliptical basin, about 20 miles long, between Zwickau and Chemnitz, and from 6 to 7 miles in maximum breadth, the greater portion being covered by New Red Sandstone strata. The coal measures, which rest upon old argillaceous schists, are about 1700 feet thick ata maximum, containing 12 principal seams of coal, besides several smaller ones. The most important is the so-called soot coal (Russkohle), which at times attains to a thickness of 25 feet. ‘The series is divided by Geinitz into groups, according to the prevailing character of the associated fossil plants, as follows :—
1. Zone of Ferns, corresponding to the upper group.
2. Zone of Annularia and Calamites, or middle group.
3. Zone of Sigillaria, or lower group.
A fourth, or Sagenaria zone, found in Silesia, corresponding to the culm measures of Devonshire, completes this classification.
The most important coal-fields of Eastern Europe are those of Silesia. The Carboniferous limestone series and the lowest coal measures or culm strata reappear in these basins, and are associated with numerous valuable mineral deposits, mainly of zinc and lead ore. The coal-field of Lower Silesia and Bohemia forms a basin between Glatz, Waldenburg, Landshüt, and Schatzlar, about 38 miles long and 22 miles broad. The number of seams from 3¾ to 5 feet thick is very considerable (from 35 to 50); but it is difficult to trace any one continuously for any great distance, as they are liable to change suddenly in character. The lower seams usually lie at a higher angle than those above them. There does not appear to be any relation between the coking power of the coals and their geological position, and the same seam often varies in quality in neighbouring mines.
The upper Silesian coal district extends in several disconnected masses from Mährisch-Ostrau in Moravia, in a N.W. direction, by Rybnik and Gleiwitz in Prussia, and Myslowitz in Poland, being held partly by Austria, Prussia, and Russia, the Prussian portion between Zrabze and Myslowitz being the most important, extending over 20 miles in length, by nearly 15 in breadth. The greatest thickness of coal in workable seams (from2¾ to 60 feet thick) is estimated at a total of 333 feet, the thickness of the measures beingabout 10,000 feet. A very large proportion of this coal-field is hidden by New Red and Cretaceous strata.
The Tertiary coals or lignites of Germany are of considerable importance, being distributed over large areas, the seams often attaining a great thickness, although rarely continuous for any great distance. The principal deposits are situated in the lower parts of the valleys of the Rhine and the Elbe, in Nassau, and in the high ground of the Rhon in Bavaria. The lignite district of the Rhine extends from near Bonn down to Deutz and Bensberg below Cologne. The pigment known as Cologne earth is a sepiacoloured lignite, which can be ground to a fine powder when dried. In Nassau the so-called bituminous wood, a variety of lignite containing flattened masses of wood of a light brown colour, is very common. The produce of these districts is mainly consumed for house fuel and steam boilers, some small quantity having been used for the production of paraffin and photogen oil.
The coal-fields of the empire of Austria-Hungary are of very considerable interest, from the great diversity in their geological position. Coals of Carboniferous age are mainly confined to the northern provinces of Bohemia, Moravia, and Silesia; but in Hungary and the Alpino lands, especially in Styria, coals of Tertiary age are found, which approach very closely in composition and quality to those of the coal measures.
First in importance among the former class, is the basin of Pilsen in Bohemia, which covers an area of about 300 square miles. It rests upon Silurian shale, and is covered unconformably by Permian conglomerate and sandstone. The coals vary considerably in different localities; the total thickness of the workable seams, from 3 to 5 in number, does not exceed 20 feet. There is a remarkable bed of slaty cannel in the upper part of the series, which contains animal remains of Permian types associated with the ordinary coal flora. Another important basin, that of Schlan-Kladno, E. of Prague, appears along the north edge of the Silurian strata, extending for about 35 miles E. and W. At Kladno, where it is best developed, it contains two principal seams, of which the upper is from 10 to 20 feet, and the lower or main seam from 19 to 40 feet thick.
At Rossitz, near Brünn, in Moravia, a belt of coal measure, resting upon crystalline rocks, has been considerably worked. There are three seams, together from 27 to 30 feet thick. These beds are said to be the equivalent of the upper seams of Pilsen and Kladno.
In Moravia, Silesia, and Poland the coal measures are associated with the mountain limestone, which in Central Germany, east of Westphalia, is generally absent. The upper Silesian coal-field is situated in Prussia, Austria, Silesia, and Russian Poland, the largest portion being in the first country. The area of this basin is about 1700 square miles, a considerable portion of it being hidden by Secondary and Tertiary strata. In the Austrian portion at Ostrau in Moravia there are 370 seams, of which 117 are workable, with a thickness of about 350 feet of coal. The largest seams are situated in the upper series, the principal one being about 13 feet thick. The coals of the neighbourhood of Ostrau are very full of gas, which occasionally finds its way into the cellars of the houses in the town, besides giving off large quantities of fire damp in the workings. A bore hole put down 150 feet to a seam of coal in 1852, gave off a stream of gas which was ignited at the surface, and has continued to burn, with a flame many feet in length, to the present time. The same coal-field extends into the district of Cracow, where it contains numerous seams of great thickness, which, however, have been but partially explored. In the Austrian Alps anthracitic coals occur at various points along the northern slopes, in strata of the age of the culm measures, but nowhere in any great quantity. In the Carpathian countries true coal measures are not largely developed, the principal locality being near Reschitza in the Banat, where 4 seams, from 3 to 10 feet in thickness, are worked to a certain extent.
At Steyerdorf, near Oravicza on the Danube, a remarkable coal-field is found in the Lias. There are 5 seams, from 3 to 7 feet in thickness, which are bent into an anticlinal, besides being disturbed by numerous faults. The coal is of a very good quality, yielding a coke suitable for iron-smelting. The annual production is about 260,000 tons. Similar coals occur in the Lias at Drenkowa, and near Fünfkirchen, where there are 25 workable seams, together about 80 feet thick, also of a good coking quality, but very tender in working, making a great deal of slack.
Secondary coals occur in the Trias and Oolitic strata at various points in the Alps, but are only of local interest.
In the Gosau strata belonging to the chalk, coal is worked at various points in the Alpine lands, the average annual production being about 25,000 tons. Eocene coals occur in Dalmatia, and Miocene lignite in the Vienna basin in Southern Moravia, one seam, about 10 feet thick, covering an area of about 120 square miles. In the Styria-Hungarian Tertiary basin, Tertiary coals are developed on a very great scale, especially in Styria, at Salgo Tarjan in N. Hungary, and in the depression between the Matra and the crystalline rocks of Upper Hungary. These localities represent only those best known by workings, many more being undeveloped. The lignite beds are often of great thickness, e.g., 70 feet at Hrastnigg, and 130 feet at Trifail. The production of Tertiary coal in Styria is about 500,000 tons annually. At Leoben and Pohnsdorf, lignites are worked of a quality closely approaching to that of Carboniferous coal, and are largely consumed in the production of iron and steel, having almost entirely replaced charcoal in the local forges. In Bohemia, Miocene brown coal strata cover a very large area, the principal basins being those of Eger, Carlsbad, and Teplitz, together about 600 square miles, the main seam occasionally attaining a thickness of over 100 feet. The trade in this coal is very considerable along the entire valley of the Elbe.
The coal-fields of Russia have been but imperfectly known until a comparatively recent period, when the demand for fuel caused by the extension of railways and the increase in manufacturing industries has stimulated explorations, which have resulted in the discovery of coalbearing strata of considerable magnitude and extent. These belong to the period of the Carboniferous limestone, like the lower coals of Scotland.
In Central Russia the coal-bearing area belonging to the Carboniferous limestone is said to cover about 13,000 square miles, the centre of the basin being at Tula, S. of Moscow. There are two principal seams, 3 ft. 6 in. and 7 feet thick, in the bottom of the series near the top of the Old Red Sandstone. The coal is of inferior quality, containing about 12 to 16 per cent, of ash, and from 2 to 5 per cent, of sulphur.
In Southern Russia, between the river Donetz and the head of the sea of Azoff, a more important coal-field occurs, also in the Carboniferous limestone, covering an area.of 11,000 square miles. There are sixty seams of coal, forty-four being workable, with a total thickness of 114 feet. The best is a dry or semi-anthracitic coal, resembling that of South Wales. At Lugan and Lissitchia Balka, a thickness of 30 feet of coal is found in 900 feet of strata.
In the Ural, coal is found in sandstones, interstratified in the Carboniferous limestone in the district north of Perm, between the parallels of 57° and 60° N. latitude. The strata dip at a high angle to the west, under the Permian strata. The thickest coals are at Lithwinsk at the northern end, where there are three seams worked, measuring from 30 to 40 feet each; further south they become thinner. The coals appear to be similar in quality to those of the central coal-field.
In Poland, about Bendzin and Lagorze, N. of Myslowitz, an extension of the Upper Silesian coal-field covers an area of about 80 square miles, being partly covered by Permian strata. Nine seams of coal are known, varying from 3 to 20 feet in thickness; but they do not occur together, except in a small part of the centre of the basin. The aggregate thickness of coal is about 60 feet. This is the only district in which true coal measure strata are found in European Russia.
Among the southern countries of Europe, the first place must be given to the coal-fields of Spain, but even these are of comparatively small importance, when measured by a northern standard, consisting of a few small and scattered basins, in which both Carboniferous and Secondary coals are represented. The Carboniferous limestone acquires a considerable development in the Cantabrian chain along the north coast, and is associated with overlying coal measures near Oviedo and Leon. In the former area the coals are often considerably disturbed, becoming anthracitic at the same time. The best seams are from 5 to 8 feet thick. In the Satero valley near Sotillo, N,E, of Leon, a seam called El Carmen, averaging 60 feet, is sometimes 100 feet thick, and is said to be in places associated with another which is occasionally 180 feet thick. Another basin of importance is that of Belmez and Espiel, occupying a narrow valley in older Palaeozoic strata, about 20 miles north of Cordova, which has recently been traversed by a railway connecting it with the main lines from Lisbon and Cadiz. This produces coking and gas coals of good quality, which are in considerable demand for smelting in the lead and other mineral districts in the neighbourhood. The other principal localities are at Villaneuva del Rio near Seville, and San Juan de la Abaderas in Catalonia. Coals of Neocomian age are found at Montalban, in the province of Teruel, and lignites of Miocene age, among other places, at Alcoy in Valencia, and Calas in Catalonia.
In Portugal a small tract of lower Carboniferous strata, containing anthracite, occurs at San Pedro de Cova, near Coimbra, but the produce is very small.
In Italy there is very little Carboniferous coal, what does occur being mainly of an anthracitic character in very dis turbed strata in the Piedmontese Alps. Tertiary lignites are worked at several places in Tuscany and in Naples, but the total output is inconsiderable when measured by the standards of more northern countries.
Extτa-Euroρean Coalfields.
In Turkey, Carboniferous coal is found at Heraclea in Asia Minor, and has been worked from time to time, but hitherto without much influence upon the coal produce of Europe. Lignites are known to occur near Smyrna, and in the Lebanon and various other points in Syria.
It is doubtful whether any Carboniferous coal exists in Africa. Coal-bearing strata, probably of the age of the New Red Sandstone, the so-called Karoo beds, cover a considerable area, both in the Cape Colony and Natal, but little is known of the details of the coal-beds beyond statements of the excellence of the quality of the coals. Lignite occurs in the high lands of Abyssinia, and probably at numerous other points in the interior.
The coal-bearing strata of India occur in numerous detached basins, which are widely distributed over the whole peninsula, their aggregate area, however, being but small. The principal development is in the valley of the Damodar river, one of the southern tributaries of the Hugli, the largest coal field being that of Raniganj, on the line of the East Indian Railway, about 140 miles W. of Calcutta, which covers an area of about 500 square miles. It is a basin resting upon crystalline schists, and partly covered by Triassic sandstones in the centre, and by jungle and alluvium, to the eastward, so that the real area is not yet known. The strata are divisible into three series as follows :—
Upper or Raniganj series—coal-bearing. Middle or Ironstone series—no coals.
Lower or Barrakur series—coal-bearing.
The total thickness may be from 3000 to 4000 feet; the ironstone series is a group of shales containing nodular ironstone about 1500 feet thick, but diminishing westward. Numerous coal seams are worked at different points, but they cannot be traced continuously for more than a short distance without change. In the upper series an average of 11 seams,’ together about 120 feet thick, are known in the eastern or Raniganj district, and 13 seams, together 100 feet, on the western side. Occasionally single seams acquire a great thickness (from 20 to 80 feet), but the average of those worked locally is from 12 to 18 feet. In the lower series, 4 seams, together 69 feet, are known. The coals are generally of inferior quality, containing a considerable amount of ash, and are non-coking in character. The coals of the lower series are better, yielding fairly good coking and gas coal at Sanktoria, near the Barrakur River.
A small coal-field at Kurhurbali, near Luckeeserai, on the East Indian Railway, has recently been developed to a considerable extent for locomotive purposes. It covers about 11 square miles, with an aggregate of 3 seams, varying from 9 to 33 feet in thickness. They are of better quality than those of any other Indian coal-field at present known, and are of great value to the railway, which is now supplied with fuel at a lower rate than probably any other railway company in the world.
There are several other coal-fields in Bengal, especially that at Jherria, near the sacred mountain of Parisnath, those south of Hazaribagh, and those on the Sone River, but none are as yet developed to any extent, being away from the great lines of communication. On the western side of India the principal workings are at Mopani, on the Nerbudda, on the line of the Great Indian Peninsular Railway, the coal being used by the railway. It is of inferior quality, and the strata are inclined at a considerable angle, rendering the working difficult.
In the Central Provinces a new coal-field of considerable extent has been recently discovered, almost entirely by boring, on the Wardha and Chanda districts, on the upper tributaries of the Godaveri, a considerable portion being within the Nizam’s province of Berar. It is probable that this may become one of the most important sources of coal supply for Central and Western India, but no great amount of work has as yet been done upon it.
Besides the above, there are several other known coalfields, for details of which the reader is referred to the Reports of the Geological Society of India.
The age of the Indian coals is generally supposed to be Permian, the only fossils that have been found in them being plants which are referred to Permian types in Europe. If, however, the overlying sandstones, containing reptilian fossils, generally reputed to be of Triassic age, should, as seems likely, prove to be Permian, it is not improbable that the coal-bearing strata may actually belong to the period of the upper coal measures, and the Indian coal-fields would then be strictly analogous to the deep irregular basins of Southern France and Central Europe, with which they have many structural points in common. No marine strata, or anything approximating to the character of the Carboniferous limestone, are known anywhere on the plains of India, although they are found in the salt range of the Punjab and in the Himalayas.
The coal-fields of China are known, from the researches of Baron von Richthofen, Prof. Pumpelly, and other travellers, to cover a very large area, comparable only with those of North America; but, as may be imagined, no very detailed information has as yet been obtained concerning them. According to the first-named authority, there are no newer formations than the Trias in China other than alluvial deposits of enormous thickness, but Palaeozoic strata, from the Silurian ∏owards, are developed on a very large scale. Coal of Carboniferous age exists in Manchuria, mostly in inaccessible mountain valleys, and further west all along the Great Wall. Near Peking there are beds 95 feet thick, which supply the city with fuel. The most extensive development is to the west and north-west, on the south of the great mountain range which stretches across Western China, where there is an area of Carboniferous strata of 100,000 square miles. The great plain of China is bounded by a limestone escarpment from 2000 to 3000 feet high, which is capped by a plateau covered by 30,000 square miles of coal measures, in which the coal seams, 30 feet thick, lie perfectly horizontal for 200 miles, and are reported to extend beyond the frontier into Mongolia. Most of the localities are, however, far in the interior. The coal of Shantung, though not near good harbours, is the most accessible of all Chinese coal from the sea. It also occurs in the other maritime provinces, but in districts offering fewer facilities for export. It is obvious, from the enormous dimensions given to these coal-fields, that it will be a long time before anything like a moderately accurate estimate of their value can be obtained.
In Japan coal is worked at several points, but no detailed account of the mode of its occurrence has been published. At the island of Takasima, near Nagasaki, a colliery is worked by the Japanese Government for the supply of their steamers on a tolerably large scale.
In the great islands of the Indian and South Pacific Oceans, coal-bearing strata are known at many different points; but in the absence of systematic investigation, no general estimate can be formed of their position, extent, or value. In the Dutch settlements, coal has been found in Sumatra and Borneo, the best known deposit being that of Pengaron, on the south-east of the latteτ island, where a mine has been worked by the Dutch authorities for several years. The section of the strata, as proved by a level, shows a series of 15 seams above 1 foot in thickness, together about 36 feet, in about 520 feet of measures, 6 of these having been worked. The best appear to be somewhat similar to the steam coal of the North of England. In the British settlement of Labuan, off the north coast of Borneo, 5 workable seams, together about 27 feet thick, are estimated to cover the whole island. This is probably of Tertiary age, but approximates in composition to many of the noncoking coals of the coal measures. The Labuan coal is also remarkable for containing large masses of fossil resin.
The most important southern coal deposits, however, are those of Australia, which extend, with short intervals, from the Gulf of Carpentaria to Bass’s Straits. In the northern districts, the distribution appears to be somewhat similar to that seen in South America, Secondary and Tertiary basins occupying the ground near the sea, while true Carboniferous coal is found further inland; but in New South Wales, where their development is greatest, older coal-bearing strata extend along the eastern slope of the continent, between the parallels of 29 and 35 degrees S. latitude, covering a very large area in several detached portions, the largest probably exceeding 12,000 miles, and come down to the sea. The principal workings are situated near Newcastle, at the mouth of the Hunter River, at Wollongong, 60 miles south of Sydney, and at Hartley, about 90 miles inland. The coal seams vary from 3 to 30 feet in thickness in the Newcastle district, 16 seams above three feet thick being known. The coals are mainly of a free-burning class, but some are bituminous, giving a good coke. In the uppermost part of the series oil shales and cannel are found. The age of the Australian coal measures has been the subject of considerable controversy. Formerly it was supposed that they were Oolitic, from the supposed affinities of the fossil plants; but it has since been shown that the coal-bearing portions of the series are interstratified with marine strata, containing fossils of Carboniferous and Devonian types. 1 he same association is observed in the coal series of Bowen River in Queensland, and on those of the Mersey River in Tasmania, showing the extension of the Carboniferous strata in a chain of detached basins from the 20th to the 40th parallel of S. lat., or about 1400 miles. In Queensland the strata are estimated to cover an area of 24,000 square miles, without taking into account possible extension under the Cretaceous strata of the interior. Up to the present time, however, very little has been done towards their development, the districts in which they occur being too far from the settled portions of the country. The principal mines now open are on newer strata of Cretaceous age nearer the sea, at Ipswich, in the neighbourhood of Brisbane. Some of these coals are remarkably like those of South Durham, and yield a good hard coke, suitable for blast-furnace purposes.
True coal measures are not known to exist in New Zealand, but coal-bearing strata of two different periods have been described by Dr Hector, Dr Haast, Captain Hutton, and other geologists. The newer series yield a lignite, which is described in the reports as hydrous coal; while the older, which is probably of Cretaceous or Jurassic age, yields a superior class of combustible, known as anhydrous coal. These minerals occur at many different points in the two larger islands, and although nö systematic detailed account of them is as yet available, a considerable amount of information on this subject is contained in the various geological reports published by the New Zealand surveyors.
In North America, the Carboniferous strata are divided by geologists into two principal groups,—the lower or sub-Carboniferous, which correspond to the Carboniferous limestone of Europe, and the Carboniferous, which includes the millstone grit and coal measures.
The first of these is about 5000 feet thick in Pennsylvania, consisting mainly of shales and sandstones; but in the Mississippi valley, in Illinois, Iowa, and Missouri, a considerable thickness of limestone is developed in this part of the series. In the former region some thin coal seams are found, the relation between the two areas being in this respect similar to that of the Carboniferous limestone in England to the coal-bearing formations of similar age in Scotland.
The millstone grit forms a mass of sandstones and conglomerates from 1200 to 1400 feet thick in Eastern Pennsylvania, but thins rapidly to the westward, being only from 100 to 250 feet thick in Ohio and Tennessee. In Arkansas, the compact siliceous rock known as novaculite, or Arkansas hone stone, occurs in this member of the Carboniferous series.
The coal measures proper cover a very large area, both in the United States and in Canada. First in importance is the Appalachian coal-field, covering about 60,000 square miles, extending through parts of Pennsylvania, Ohio, ∖ irginia, eastern Kentucky, Tennessee, and Alabama. The maximum thickness of strata is from 2500 to 3000 feet; that of included coal is 120 feet near Pottsville, 62 feet at Wilkesbarre, and about 2o feet at Pittsburg, showing a gradual diminution to the westward. The most persistent coal is the Pittsburg seam, which is known over an area measuring 225 miles by 100 miles, but with a thickness varying from 2 to 14 feet.
The anthracite district of central Pennsylvania occupies an area of about 650 miles on the left bank of the Susquehanna River. The strata between Pottsville and Wyoming, which belong to the lowest portion of the coal measures, are probably about 3000 feet thick, but it is difficult to arrive at an exact estimate, owing to the numerous folds and contortions. There are from ten to twelve seams above 3 feet in thickness; the principal one, known as the Mammoth or Baltimore vein, is 29j feet thick at Wilkesbarre, and in places even exceeds 60 feet.
The Illinois and Missouri basin covers a considerable part of these States, as well as of Indiana and Kentucky, Iowa, Kansas, and Arkansas. Its area is estimated at 60,000 square miles, the thickness varying from 600 feet in Missouri to 3000 feet in western Kentucky. The aggregate thickness of coal is about 70 feet. A good furnace coal is obtained in Indiana, the so-called block coal of Brazil near Indianopolis, which, like the splint coals of Scotland and those of Staffordshire, can be used in the blast furnace without coking.
In Michigan a nearly circular area of coal measures, of about 5000 square miles, occurs in the lower peninsula between lakes Huron and Erie. The thickness is only 120 feet, and the coals unimportant.
Other coal-bearing areas of less value are known in Texas and Rhode Island.
The Carboniferous strata are largely developed in the eastern provinces of the Dominion of Canada, notably in New Brunswick and Nova Scotia. The lower Carboniferous group here consists of about 6000 feet of red sandstones and green marls, with thick beds of fossiliferous limestones, accompanied by gypsum. The limestones increase in thickness southward. In this series occurs the peculiar pitch-like or asphaltic coal of the Albert mine in New Brunswick, of which an analysis is given in Table I., supra. The overlying coal measures, including the millstone grit, occupy an area estimated at 18,000 square miles. The whole thickness of this group at South Joggins is about 14,750 feet, with 76 included coal seams, together 45 feet in thickness, which are contained in the middle division of the series. At Pictou there are six seams, together measuring 80 feet in thickness. The coal measures in this area approach more near to the great coalfields of Europe in thickness than those of the other American Carboniferous districts. Rocks of Carboniferous age occur in various places on both flanks of the Rocky Mountains, and in the Arctic Archipelago, but have not yet been explored.
Lignite-bearing strata of Cretaceous and Tertiary age occupy a very considerable area in the central and western portions of North America, especially in the upper Missouri and Saskatchewan valleys, in Utah and Texas, and in California, Oregon, and Vancouver Island. In the last locality coal has been extensively mined near Nanaimo, on the east coast, for several years past, in strata of Cretaceous age. Tertiary lignites are worked in Bellingham Bay, at Goose Bay in Oregon, and at Monte Diabolo, near San Francisco. The lignitic formations of the eastern flank of the Rocky Mountains, which are considered by Hayden to occupy a position between the Cretaceous and Eocene Tertiary strata, occupy an area estimated at about 50,000 square miles within the United States, and extend both northward into Canada and southward into Mexico.
In South America coal, probably of Carboniferous age, is found in the Brazilian provinces of São Pedro, Rio Grande do Sul, and Santa Catharina, and in the neighbouring state of Uraguay. The largest area is that known as the Candiota coal-field, which is exposed for about 50 miles in the valley of the river of the same name. The sections exposed show 5 seams from 9 to 25 feet each, or together about 65 feet of coal. Other basins are known at S. Sepé and S. Jeronimo, on the Jacahahay River. The latter is the only point at which mines are worked, as the coals, though thinner than those of the other localities mentioned, are situated within the reach of navigable waters, having only to bear a land carriage of 8 miles to the river.
On the west coast of South America, Cretaceous coals are worked at Lota, in Chili, and at Sandy Point, in the Straits of Magellan. In Peru both Secondary and Carboniferous coals are known at various points in the interior, the former occupying a position on the first rise of the table land of the Andes, while the latter occur in higher ground, at a greater distance from the coast. Good coal is also found at many points in the Santa valley.
Much of the Peruvian coal has undergone considerable disturbance and metamorphism subsequent to its deposition. At Porton, 45 miles east of Truxillo, a ridge of coal-bearing sandstones has been changed into a hard quartzite, with an interstratified seam of anthracite in a nearly vertical position. The coal is remarkable as containing a large amount of sulphur (see analysis Table I.). The hitherto inaccessible position of these places, which are usually more than 10,000 feet above the sea-level, has prevented the development of coal-mining in Peru; but the extension of railways into the mountains will probably bring them into importance, by stimulating a local demand for fuel.
Extent of existing Workable Coal.
The following summary of the amount of coal estimated as workable remaining in the different districts, which is taken from the report of the Royal Commission on coal, and founded upon investigations made in the years 1866-71, furnishes an approximate measure of the comparative value, present and prospective, of the different coal-fields of the United Kingdom. The quantities represent the probable aggregate yield of all seams above 1 foot thick. [table]
The quantity estimated as lying above the workable limit of 4000 feet under the Permian and other formations, in the central and northern counties of England, is 56,248,000,000 tons, covering an area of 2044 square miles, in addition to which, in the flat ground between the Mersey, Denbighshire, the North Staffordshire hills, Cannock Chase, and Colebrookdale, a further area of 843 square miles at inaccessible depths is computed to contain— [table]
the quantity of workable coal, as made up of the two amounts, 90,206,240,387 and 56,248,000,000 tons, given above. From this it follows that, out of the probable total quantity of coal in the British coal measures, rather more than three-fourths may become available for consumption, or about 1170 times the amount of the present annual out put of 125 million tons.
Similar estimates have been formed for the coal-fields of other countries, especially in France and Germany, but it is doubtful whether the necessary structural details are sufficiently well known to admit of more than a tolerably rough guess being made.
