The Jurassic system is represented in Ireland by the Lower Lias alone, and it is probable that no marine beds higher than the Upper Lias were deposited during this period. From Permian times onward, in fact, the Irish area lay on the western margin of the seas that played so large a part in determining the geology of Europe. The Lower Lias appears at intervals under the scarp of the basaltic plateaus, and contributes, as in Dorsetshire and Devonshire, to the formation of landslips along the coast. The alteration of the fossiliferous Lias by dolerite at Portrush into a flinty rock that looked like basalt served at one time as a prop for the “Neptunist” theory of the origin of igneous rocks. Denudation, consequent on the renewed uplift of the country, affected the Jurassic beds until the middle of Cretaceous times. The sea then returned, in the north-east at any rate, and the first Cretaceous deposits indicate the nearness of a shore-line. Dark “green-sands,” very rich in glauconite, are followed by yellow sandstones with some flint. These two stages represent the Upper Greensand, or the sandy type of the English Gault. Further sands represent the Cenomanian. The Turonian is also sandy, but in most areas was not deposited, or has been denuded away during a local uplift that preceded Senonian times. The Senonian limestone itself, which rests in the extreme north on Trias or even on the schists, is often conglomeratic and glauconitic at the base, the pebbles being worn from the old metamorphic series. The term “Hibernian Greensand” was used by Tate for all the beds below the Senonian; the quarrymen know the conglomeratic Senonian as “Mulatto-stone.” The Senonian chalk, or “White Limestone,” is hard, with numerous bands of flint, and suffered from denudation in early Eocene times. Probably its original thickness was not more than 150 ft., while now only from 40 to 100 ft. remain. This chalk appears to underlie nearly the whole basaltic plateaus, appearing as a fringe round them, and also in an inlier at Templepatrick. The western limit was probably found in the edge of the old continental land in Donegal. Chalk flints occur frequently in the surface-deposits of the south of Ireland, associated with rocks brought from the north during the glacial epoch, and probably also of northern origin. It is just possible, however, that here and there the Cretaceous sea that spread over Devonshire may have penetrated the Irish area.
After the Irish chalk had been worn into rolling downs, on which flint-gravels gathered, the great epoch of volcanic activity opened, which was destined to change the character of the whole north-west European area. The critical time had arrived when the sea was to be driven away eastward, while the immense ridges due to the “Alpine” movements were about to emerge as the backbones of new continental lands. Fissure after fissure, running with remarkable constancy N.W. and S.E., broke through the region now occupied by the British Isles, and basalt was pressed up along these cracks, forming thousands of dikes, from the coast of Down to the Dalradian ridges of Donegal. One of these on the north side of Lough Erne is 15 m. long. The more deep-seated type of these rocks is seen in the olivine-gabbro mass of Carlingford Mountain; but most of the igneous region became covered with sheets of basaltic lava, which filled up the hollows of the downs, baked the gravels into a layer of red flints, and built up, pile upon pile, the great plateaus of the north. There was little explosive action, and few of the volcanic vents can now be traced. After a time, a quiet interval allowed of the formation of lakes, in which red iron-ores were laid down. The plant-remains associated with these beds form the only clue to the post-Cretaceous period in which the volcanic epoch opened, and they have been placed by Mr Starkie Gardner in recent years as early Eocene. During this time of comparative rest, rhyolites were extruded locally in county Antrim; and there is very strong evidence that the granite of the Mourne Mountains, and that which cuts the Carlingford gabbro, were added at the same time to the crust. The basalt again broke out, through dikes that cut even the Mourne granite, and some of the best-known columnar masses of lava overlie the red deposits of iron-ore and mark this second basaltic epoch. The volcanic plateaus clearly at one time extended far west and south of their present limits, and the denudation of the lava-flows has allowed a large area of Mesozoic strata also to disappear.
Volcanic activity may have extended into Miocene times; but the only fossiliferous relics of Cainozoic periods later than the Eocene are the pale clays and silicified lignites on the south shore of Lough Neagh, and the shelly gravels of pre-glacial age in county Wexford. Both these deposits may be Pliocene. Probably before this period the movements of subsidence had set in which faulted the basalt plateaus, lowered them to form the basin of Lough Neagh, and broke up the continuity of the volcanic land of the North Atlantic area. As the Atlantic spread into the valleys on the west of Ireland, forming the well-known marine inlets, Europe grew, under the influence of the “Alpine” movements, upon the east; and Ireland was caught in, as it were, on the western edge of the new continent. It seems likely that it was separated from the British region shortly before the glacial epoch, and that some of the ice which then abutted on the country travelled across shallow seas. The glacial deposits profoundly modified the surface of the country, whether they resulted from the melting of the ice-sheets of the time of maximum glaciation, or from the movements of local glaciers. Boulder-clays and sands, and gravels rearranged by water, occur throughout the lowlands; while the eskers or “green hills,” characteristic grass-covered ridges of gravel, rise from the great plain, or run athwart valleys and over hill-sides, marking the courses of sub-glacial streams. When the superficial deposits are removed, the underlying rocks are found to be scored and smoothed by ice-action, and whole mountain-sides in the south and west have been similarly moulded during the Glacial epoch. In numerous cases, lakelets have gathered under rocky cirques behind the terminal moraines of the last surviving glaciers.
There is no doubt that at this epoch various movements of elevation and subsidence affected the north-west of Europe, and modern Ireland may have had extensions into warmer regions on the west and south, while the area now left to us was almost buried under ice. In post-Glacial times, a subsidence admitted the sea into the Lagan valley and across the eastern shore in several places; but elevation, in the days of early human occupation, brought these last marine deposits to light, and raised the beaches and shore-terraces some 10 to 20 ft. along the coast. At Larne, Greenore and in the neck between Howth and Dublin, these raised beaches remain conspicuous. To sum up, then, while the main structural features of Ireland were impressed upon her before the opening of the Mesozoic era, her present outline and superficial contours date from an epoch of climatic and geographical change which falls within the human period.
See maps and explanatory memoirs of the Geological Survey of Ireland (Dublin); G. Wilkinson, Practical Geology and Ancient Architecture of Ireland (London, 1845); R. Kane, Industrial Resources of Ireland (2nd ed., Dublin, 1845); G. H. Kinahan, Manual of the Geology of Ireland (London, 1878); E. Hull, Physical Geology and Geography of Ireland (2nd ed., London, 1891); G. H. Kinahan, Economic Geology of Ireland (Dublin, 1889); A. McHenry and W. W. Watts, Guide to the Collection of Rocks and Fossils, Geol. Survey of Ireland (2nd ed., Dublin, 1898).
(G. A. J. C.)
Economics and Administration
Population.—Various computations are in existence of the population of Ireland prior to 1821, in which year the first government census was taken. According to Sir William Petty the number of inhabitants in 1672 was 1,320,000. About a century later the tax-collectors estimated the population at a little over 2,500,000, and in 1791 the same officials calculated that the number had risen to over 4,200,000. The census commissioners returned the population in 1821 as 6,801,827, in 1831 as 7,767,401, and in 1841 as 8,196,597. It is undoubted that a great increase of population set in towards the close of the 18th century and continued during the first 40 years or so of the 19th. This increase was due to a variety of causes—the improvement in the political condition of the country, the creation of leaseholds after the abolition of the 40s. franchise, the productiveness and easy cultivation of the potato, the high prices during the war with France, and probably not least to the natural prolificness of the Irish people. But the census returns of 1851 showed a remarkable alteration—a decrease during the previous decade of over 1,500,000—and since that date, as the following table shows, the continuous decrease in the number of its inhabitants has been the striking feature in the vital statistics of Ireland.
Decrease per cent. of Population 1841-1901.
| 1841-1851. | 1851-1861. | 1861-1871. | 1871-1881. | 1881-1891. | 1891-1901. | |
| Leinster | 15.25 | 12.86 | 8.11 | 4.49 | 6.8 | 3.5 |
| Munster | 22.47 | 18.53 | 7.93 | 4.98 | 11.8 | 8.4 |
| Ulster | 15.69 | 4.85 | 4.23 | 5.11 | 7.07 | 2.4 |
| Connaught | 28.81 | 9.59 | 7.33 | 3.43 | 12.4 | 9.7 |
| Ireland | 19.85 | 11.50 | 6.67 | 4.69 | 9.08 | 5.3 |