Geology.— The central part of Ireland is occupied by a great undulating plain, whose highest elevation is 300 and average elevation about 200 feet. In the centre of the country, from Dublin Bay on the east to Galway Bay on the west, this plain stretches from shore to shore, but towards the south and north it is enclosed by an irregular semicircular belt of mountainous country. The surface of the plain is broken occasionally by isolated hills. Throughout nearly the whole of its extent it rests on the Carboniferous Limestone, and in several places there are remains of the Upper Carboniferous strata or Coal-measures, by which the Carboniferous Limestone was at one time overlaid, and which have been carried away during a vast period of denudation chiefly by the action of subaerial agents. The strata of limestone are nearly horizontal, except where they are contorted by local disturbances. In the central plain it is only occasionally that the limestone crops to the surface, as it is generally overlaid by boulder clay, the result of glacial action, by the middle sands and gravels formed on the bed of the shallow sea by which the plain was at one time occupied, or by the peat bogs resting on the beds of previous lakes. At one period the Carboniferous beds must have extended widely beyond their present limits, and have formed the surface strata of the uplands to the north-west and south-east. In the north-western highlands of Sligo, Leitrim, and Fermanagh they still form a lofty table-land, which occasionally rises into peaks about 2000 feet in height.

The mountain masses of Ireland are generally traversed by deep and narrow valleys running both north and south and east and west, and frequently giving rise to high and isolated peaks. The districts of Donegal and Derry in the north-west, and those of Galway and Mayo in the west, consist chiefly of metamorphosed Lower Silurian rocks, and are believed to form part of the same geological system as that of the Highlands of Scotland. Those of Donegal and Derry, lying between Donegal Bay and Lough Foyle, consist of granite, gneiss, and hornblendic and other schists, with crystalline limestones and quartzites. Their principal peaks are the isolated summit of Errigal (2466 feet) and Blue Stack (2219 feet). In West Galway and Mayo the rocks consist chiefly of quartzite, or of alternating beds of quartzite and granite or gneiss schist. They include the Twelve Pins of Connemara (2395 feet), Croagh Patrick on the shores of Clew Bay (2510 feet), the Nephin Beg mountains, and the Ox mountains. The range of hills between Killary Harbour and Lough Mask—the highest summit of which, Muilrea, has an elevation of 2688 feet—belongs to the Upper Silurian formation. The fact that these rocks do not share in the metamorphism of the Lower Silurian beds shows that the alteration must have taken place at some time between the Lower Silurian and Upper Silurian periods. Rocksof Cambrian age occur in Wexford, Wicklow, and Dublin. The principal elevations of these districts are formed of granite, and belong to an earlier epoch than that of the Old Red Sandstone. The highest summit is Lugnaquilla (3039 feet), composed of altered Silurian rocks lying on the granite. In the south-western districts of Kerry, Cork, and Waterford, the mountains are composed of broad bands of Old Red Sandstone, the valleys being formed of narrow bands of Carboniferous rocks. The lie of the strata is nearly east and west, since they have been plicated by forces acting in a transverse direction, the time of disturbance being some unknown date between the Carboniferous and the Permian periods. This district includes the rugged range of Macgillicuddy’s Reeks, which rise abruptly from the Carboniferous Limestone surrounding the Killarney lakes, and, occupying the greater part of the peninsula between Dingle Bay and Kenmare River, attain at Carntual a height of 3414 feet above sea-level. Many of the most conspicuous mountain groups in the southern half of the island consist of central cores of Silurian strata wrapped round with thick folds of Old Red Sandstone. Such are Slieve Aughty, Slieve Bernagh, the Silver Mine mountains, and Slievenaman in Clare and Tipperary, Galtymore in Limerick, which has an elevation of 3015 feet, and the Slieve Bloom mountains in Queen’s county. The principal mountain groups in the north-east are the Carlingford mountains, composed of felspathic and pyroxenic rocks, and attaining in Slieve Foy a height of nearly 2000 feet; the Mourne mountains to the north of Carlingford Lough, composed of granite, with veins of basalt, felstone, mica trap, and porphyry, the highest summit being Slieve Donard (2796 feet); and the Slieve Croob mountains, to the north of the Mourne range, composed of granite of a much earlier origin. Both the Mourne and Carlingford mountains are of intrusive origin, and probably belong to the Permian period. A considerable extent of low country in Down, Armagh, and Cavan is occupied by Silurian rocks in the form of grits and slates. The estuary of Belfast Lough lies in Triassic rocks, containing beds of rock salt. Rocks of similar age extend west of Lough Neagh, and thence, north to Lough Foyle. They are overlaid by Jurassic and Cretaceous strata, which, however, are almost wholly concealed under the great basalt flood of the northeastern counties. The greater part of Antrim and the eastern portion of Derry are occupied by basalt rock forming an elevated plateau, for the most part bounded towards the sea by precipitous escarpments, consisting of Upper Greensand and Chalk, surmounted by the black basalt which, often crowning their summits, stands in striking contrast Table I.— Area and Distribution of Land in 1880, and Population in 1881. [table]

with the light-coloured strata below. In some places, however, as at the Giant’s Causeway, the Cretaceous rocks disappear, and the basalt slopes gradually to the sea, displaying a series of terraces formed of hexagonal pillars, and occasionally separated by bands of volcanic ash.

Coast-Line.— Along the present coast-line there are to be seen in several places traces of ancient sea margins, the most continuous being those on the northern and eastern coasts, especially in county Antrim. Of still more ancient sea margins there are evidences in the terraces at the base or on the flanks of the mountains. The present coast-line, especially on the west and south, is very much indented by bays and inlets, which Hull, in his Geology of Ireland, attributes in many cases to the chemical action of the sea-water on the limestone rocks. On the south coast they most commonly run in a northerly direction with a westerly inclination at the upper end, and on the west coast the direction of the larger inlets is easterly, although several of the smaller ones run north, south, and north-east. Their troughs have mostly been excavated in the synclinal folds of the rocks, which therefore frequently project far into the sea in the form of high and bold headlands. On the northern coast the inlets generally run in a southerly or south-westerly direction. Most of those on the east coast have by the accumulation of sand been either wholly or partly formed into lagoons; and on the south east coast the sea has made considerable encroachments on the land.

The principal inlets are—on the east coast Belfast Lough, Strangford Lough, Carlingford Lough, Dundalk Bay, Dublin Bay, and Wexford Harbour; on the south coast Waterford Harbour, Dungarvan Harbour, Youghal Bay, and Cork Harbour; on the south-west coast Roaring Water Bay, Dunmanus Bay, Bantry Bay, Kenmare River, and Dingle Bay; on the west coast Tralee Bay, the mouth of the Shannon, Galway Bay, Clew Bay, Blacksod Bay, Killala Bay, Sligo Bay, and Donegal Bay; and on the north coast Sheep Haven, Lough Swilly, and Lough Foyle. In all, Ireland possesses fourteen harbours suitable for the largest ships, seventeen for frigates, and over thirty for coasters, besides an immense number suitable for fishing boats.

The islands of Ireland are small in size, and are situated near the mainland, most of them being formed of rocks from which, according to Hull, the adjoining Carboniferous strata had been denuded by the action of the sea-water. They are most numerous on the west coast, especially opposite Galway, Mayo, and Donegal. Off the Donegal coast the largest is Neish Aran. Separated from the mainland of Mayo by a narrow isthmus is Achill, the largest island of Ireland, and in Clew Bay there are an immense number of islets all formed of drift. An archipelago of granite rocks off the coast of Galway is formed of continuations of the mountains, and at the mouth of Galway Bay are the three islands of Aran, composed of Carboniferous Limestone. Among the picturesque stacks of rocks off the coast of Kerry the most notable are the Skellings. The largest islands opposite Cork are Dursey Island, Bear Island, and Clear Island, south of which is a picturesque rock called the Fastnet, on which there is a lighthouse. On the east coast the principal are Lambay Island, Innis-patrick, and Ireland’s Eye off county Dublin, and Copeland Island at the mouth of Belfast Lough. On the northern coast the principal are Rathlin Island off Antrim, and Tory Island off Donegal.

Divers and Canals.— Several of the rivers of Ireland, including the largest of them, have had their channels determined by a previous physical condition of the land surface, and must have been formed during a long period of denudation. Many of the valleys are dried-up river beds, and along various of the present river valleys traces of old river terraces may still be seen. In some cases the alterations of the courses by breaks and dislocations of the strata are of very recent occurrence. In the districts of Sligo and Fermanagh, as well as of Galway, Clare, Kerry, and Cork, the rivers and streams have frequently cut out subterraneous passages through the limestone, in some cases altogether disappearing; and along their courses turloughs or blind lakes, and abrupt deep holes called sluggas are frequently formed.

Owing to the moistness of the climate and the lie of the surface of the country, Ireland is more intersected by large rivers than England or Scotland, and it is a remarkable circumstance that in several cases there are groups of rivers with closely contiguous sources, which, however, flow in widely different directions. The largest river is the Shannon, which has its source in the Carboniferous mountains of Fermanagh and Leitrim, and flows southward through Lough Allen, Lough Ree, and Lough Dearg to Limerick, where it opens out into a wide estuary and takes a westerly course to the ocean. Up to Limerick, where it becomes tidal, it is navigable for large vessels, and for vessels of small tonnage it is navigable within 5 miles of Lough Allen. Its course above Lough Dearg is very sluggish, but from that lake to Limerick its descent is very rapid. Its total length is 240 miles, and it drains an area of 4544 square miles. The Suir, the Nore, and the Barrow, which have their sources not far from each other in the Slieve Bloom mountains, and unite at Waterford, drain together an area of 3400 square miles. The Suir is navigable for boats as far as to Clonmel, and the Nore to Innistioge. The Barrow, by means of a branch of the Grand Canal, forms a line of 120 miles of inland navigation between Dublin and Waterford. The other principal rivers, all of which are to some extent navigable, are—debouching on the west coast the Erne, the Moy, and the Corrib; on the south coast the Blackwater and the Lee; on the east coast the Lagan, the Boyne, the Liffey, and the Slaney; and on the north coast the Bann and the Foyle.

The Grand Canal, which with its various branches has a length of 165 miles, connects Dublin with the Shannon at Shannon Harbour; and the Royal Canal, with a length of 76 miles, connects Dublin with the Shannon at Cloondora. Lines of inland navigation, partly natural and partly artificial, connect Lough Neagh with Belfast, Newry, and Lough Erne. From the sea at Galway there is communication by Lough Mask and Lough Corrib to Lough Carra. Since the introduction of railways the passenger traffic on the canals has wholly ceased, but the goods traffic is still considerable.

Lakes.— Many lakes of considerable extent exist both in the mountainous and lowland districts of Ireland, and the number of small lakes is very great. Altogether the area covered by lakes amounts to 711 square miles, of which 287 are in Ulster, 305 in Connaught, 69 in Munster, and 50 in Leinster. Lough Neagh in Ulster is the largest inland lake in the United Kingdom, and has an area of 153 square miles, with a general depth of from 20 to 40 feet. Lough Erne in Fermanagh has a length of upwards of 40 miles, but a breadth of only 8 miles. Properly speaking, it consists of two lakes 5 miles apart and connected by a river, the upper lake being 13 miles in length with an area of 9278 acres, and the lower 24 miles in length with an area of 28,000 acres. Both lakes are dotted with numerous islets, and the lower one is famed for its picturesque beauty. Lough Corrib and Lough Mask have respectively an area of 43,484 and 22,219 acres. The country to the west of Lough Corrib contains about 130 lakes, 25 of which are more than a mile in length. The lakes of Killamey in Kerry, which are three in number and closely adjoin each other, are situated in the midst of wild and picturesque mountain scenery. The area of the lower lake is 5001 acres, of the middle one 680, and of the upper one 430, Lough Dearg, a small lake in the south of Donegal, has been resorted to from time immemorial as a place of penance by Roman Catholics. The other lake of the same name in the course of the Shannon has an area of 29,570 acres. The other principal lake districts are Sligo, Cavan, Westmeath, and Longford. Hull, in his Geology of Ireland, makes a classification of the lakes, according to their modes of formation, under the three heads of—(1) lakes of mechanical origin, (2) lakes of glacial origin, and (3) lakes of chemical solution. Of the first group, which are those whose formation is due chiefly to faults or dislocation of the strata, he mentions as special examples Lough Neagh and Lough Allen, both of which originated before the Glacial period, and probably between the Miocene and Pliocene periods. Those of glacial origin occur in the mountainous districts, and are due either to the scooping out of the rocks by the passage of ice over their surface, or to the accumulation of embankments at the end of the valleys or hollows. Those due to chemical solution are situated either on the limestone plain or in districts where the limestone formation has penetrated, and have been produced by the solution of the limestone through the action of water containing carbonic acid gas. , Coal.— Of the Upper Carboniferous beds which at one time overspread the central plain of Ireland, only small patches remain in isolated spots, serving chiefly as an indication of the immense loss that has been sustained in an important element of material prosperity. The principal coal-fields are the Leinster, the Munster, the Connaught, and the Tyrone.

The Leinster or Castlecomer field, situated between the Nore and Barrow, consists of a range of hills varying from 800 to 1000 feet high, and extending over portions of Kilkenny, Queen’s county, and Carlow. It lies in the form of a basin, its most productive beds occupying the centre. These are the Middle Measures, but in the field both the Middle and Lower are represented. The coal is anthracite. The most common fossils are either terrestrial or freshwater, marine fossils being found chiefly in some of the upper beds. Above the Barrow coal in county Kilkenny several remarkable reptilian remains have been found. The Lower Measures consist of gannister beds resting upon Carlow flags, and contain some beds of shale and a few thin seams of coal, with several beds of marine fossils. This field is the most important in Ireland, and yields a larger weight of coal than all the others together.

The West Munster coal-field occupies portions of Clare, Limerick, Kerry, and Cork, and consists of a series of low hills extending from near Galway Bay in the north to Killarney in the south. All the three measures are represented, but there are only a very few workable seams, as most of the coal is very thin, and the strata very much inverted. The principal collieries are at Dunhallow in Cork, and the coal, which is anthracite, is used chiefly for lime-burning. Many of the Lower Measures are very rich both in terrestrial and in marine fossils.

The East Munster coal-field consists of a low range of hills in Tipperary, closely adjoining the Carboniferous hills in Leinster, from which they are separated by the river Nore. Thence it extends to near Cashel, a distance of about 20 miles, and its average breadth is about 5 miles. All the measures are represented. The productive portion of the field is at Killenaule, and consists of two thin seams in the Upper Measure. In the Lower Measures the principal fossils are marine; and plant impressions, especially those of ferns, are very numerous.

The Connaught coal-field embraces the mountainous district round Lough Allen, and includes portions of Sligo, Roscommon, and Leitrim in Connaught, and of Fermanagh and Cavan in Ulster. Both the Middle and Lower Measures are represented. They are composed chiefly of yellow sandstone and shale, and are overlaid by beds of grit. The coal is bituminous, and a large portion of it is workable.

The Tyrone coal-field includes the district between Dungannon and Lough Neagh, having a length of 6 and a breadth of 1 to 2 miles, and the small basin of Annaghone a little to the north. All the measures are represented. The coals have been worked only near the surface, but it is believed that very extensive and valuable seams of workable coal exist at lower depths.

The average quantity of coal raised in Ireland annually is about 130,000 tons, of which about 80,000 are raised in Leinster, 30,000 in Munster, 16,000 in Antrim and Tyrone, and 6000 in Leitrim and Roscommon. As the annual importation of coal exceeds 2,000,000 tons, it is evident that the coal supply obtained from Ireland’s own mines is quite a minor element in its prosperity; but the industry might be much more largely developed, the total available amount of coal being estimated at 180,000,000 tons.

Peat.— For the absence of coal the country is to some extent compensated by the supply of peat fuel obtained from the red bogs situated in the central plain, and occupying a large tract included within two lines drawn across the island, the one from the Hill of Howth to Sligo, and the other from Wicklow to Galway. Originally this district was occupied by a forest, principally of oak trees, which after being gradually killed by the growth of mosses and other peat-producing plants, were succeeded by a forest of firs, these also in turn perishing. The average depth of the bog is 25 feet, but in some cases it is over 40 feet. According to its...