IRELAND, an island to the west of Great Britain, extending from 51° 26' to 55° 2I' N., and from 5° 25' to o° 3o' W. The zoo fathom line lies a short distance beyond its western coast and leaves it in structural affinity with western Europe. The north channel between the Mull of Kintyre (Scotland) and Torr head has a width of only 131 m. ; in the south, St. George's chan nel has a width of 69 m. between Dublin and Holyhead (Wales), and of 47 m. at its southern extremity; in the centre the Irish sea attains a width of 130 miles. The area is 20,360,605 ac., and the population (5926), Ireland, rising from shallow seas on the margin of the sub marine plateau of western Europe, records in its structure the successive changes that the continent itself has undergone. The first broad view of the country shows us a basin-shaped island consisting of a central limestone plain surrounded by mountains; but the diverse modes of origin of these mountains, and the differ ences in their trend, suggest at once that they represent succes sive epochs of disturbance. The north-west highlands of Donegal and the Ox Mountains, with their axes of folding running north east and south-west, invite comparison with the great chain of Leinster, but also with the Grampians and the backbone of Scandinavia. The ranges from Kerry to Waterford, on the other hand, are clearly parts of an east and west system, the continua tion of which may be looked for in South Wales and Belgium. The hills of the north-east are mainly the remnants of lava plateaux, which carry the mind towards Skye and the volcanic province of the Faeroe Islands. The two most important points of contrast between the geology of Ireland and that of England are, firstly, the great exposure of Carboniferous rocks in Ireland, Mesozoic strata being almost absent ; and, secondly, the presence of volcanic rocks in place of the marine Eocene of England.
The fact that no Cambrian strata have been established by palaeontological evidence in the north and west of Ireland has made it equally difficult to establish any pre-Cambrian system. The great difference in character, however, between the Silurian strata at Pomeroy in county Tyrone and the adjacent metamor phic series makes it highly probable that the latter masses are truly pre-Cambrian. The other metamorphic areas of the north and west present even greater difficulties, owing to the absence of any overlying strata older than the Old Red Sandstone. They are generally referred to as "Dalradian," a convenient term in vented by Sir A. Geikie for the metamorphic series of the old kingdom of Dalriada. The oldest rocks in this large area are a stratified series of mica-schists, limestones and quartzites, with numerous intrusive sheets of diorite, the whole having been meta morphosed by pressure, with frequent overfolding along axes with a north-easterly or Caledonian trend.
Following on these rocks of unknown but obviously high an tiquity, we find fossiliferous Ordovician (Lower Silurian) strata near Killary harbour on the west, graduating upwards into a com plete Gotlandian (Upper Silurian) system. Massive conglomerates occur in these series, which are unconformable on the Dalradian rocks of Connemara. In the Wenlock beds of the west of the Dingle promontory there are contemporaneous tuffs and lavas. Here the Ludlow strata are followed in apparent conformity by a thick series of barren beds (the Dingle Beds), which have been variously claimed as Upper Silurian and Lower Devonian. No certain representative of the Dingle Beds has been traced else where throughout the south of Ireland, where the Old Red Sand stone succeeds the uptilted Silurian strata with striking uncon formity. The folds which distort these Silurian rocks, including the Dingle Beds, have a north-easterly trend parallel to those which affect strata of the same age in the southern uplands of Scotland and thus belong to the Caledonian system. The best example of these folds is the axis of Leinster, its core being occupied by granite which is now exposed continuously for 7o m., forming a moorland from Dublin to New Ross. On either flank the Silurian shales, slates and sandstones, which are very rare!) fossiliferous, rise with steep dips. They are often contorted, anc near the granite pass into mica-schists and quartzites.
In south-eastern Wexford, in northern Wicklow (from Ash. ford to Bray), and in the promontory of Howth on Dublin Bay an apparently earlier series of green and red slates and quartzite: forms an important feature. The quartzites, like those of th( Dalradian series, weather out in cones, such as the two Sugar. loaves south of Bray, or in knob-set ridges, such as the crest of Howth or Carrick Mt. in county Wicklow. The radial or fan. shaped markings known as Oldhamia were first detected in this series, but are now known from Cambrian beds in other countries; in default of other satisfactory fossils, the series of Bray and Howth has long been held to be Cambrian.
Extensive denudation followed the Caledonian folding and or the land-surface thus formed the Devonian lakes gathered, while the rivers poured into them enormous deposits of sand and con glomerate. Large areas of Old Red Sandstone have been brought up from beneath the denuded Carboniferous along the Hercynian folds of the south. The conglomerates, moreover, appear at inter vals through the limestone covering of central Ireland, and usually weather out as conspicuous scarps or "hogs'-backs." The Slieve Bloom mountains are thus formed of a dome of Old Red Sand stone folded on a core of unconformable Silurian strata; while in several cases the domes are worn through, leaving rings of Old Red Sandstone hills, scarping inwards towards broad exposures of Silurian shales. Further north an extensive area of Old Red Sandstone is preserved within the rift valley, which is contin ued from central Scotland, beneath the North Channel and the basalt plateau, and comes to light again south-west of Lough Neagh. The Old Red Sandstone shows local evidence of contem poraneous volcanic activity.
The highest beds of Old Red Sandstone type pass up conform ably in the south of Ireland into the Lower Carboniferous, through the "Yellow Sandstone Series" and the "Coomhola Grits" above it. The Yellow Sandstone contains Archanodon, the oldest known fresh-water mollusc, and plant-remains ; the Coomhola Grits are marine, and are sometimes regarded as Carboniferous, sometimes as uppermost Devonian. In the south, the Carboniferous deposits open with the Carboniferous Slate, in the base of which the Coom hola Grits occur. Its lower part represents the Lower Carbonif erous Shales and Sandstones of the central and northern areas, while its upper part corresponds with a portion of the Carbonif erous Limestone of the central plain.
The Lower Carboniferous Sandstones are conspicuous in the region from Milltown near Inver Bay in southern Donegal to Ballycastle in county Antrim. In the latter place they contain workable coal-seams. The Carboniferous Limestone often con tains black flint (chert), and at some horizons conglomerates occur, the pebbles being derived from the pre-existing ridges of the "Caledonian" land. A black and often shaly type called "calp" contains much clay derived from the same land-surface. While the limestone has been mainly worn down to a lowland, it forms fine scarps and tablelands in county Sligo and other western regions. Contemporaneous volcanic action is recorded by tuffs and lavas south-east of Limerick and north of Philipstown. The beds above the limestone are shales and sandstones, the true stratigraphical horizon of which is as yet ill defined, but which certainly range in places into the Middle Coal Measures. No beds equivalent to
the Upper Coal Measures of England have yet been proved South of the line between Galway and Dublin the coal is anthra citic, while north of this line it is bituminous.
The "Hercynian" earth-movements, which so profoundly affected north-west and north-central Europe at the close of Car boniferous times, gave rise in the south of Ireland to a series of east and west folds and in the centre to a series of lesser folds deflected along Caledonian trend lines. The uplift which followed upon this folding led as in England to extensive denudation. It is however only in the north-east of Ireland that the subsequent history can be traced. Here the succeeding Permo-Triassic beds lie unconformably across the Carboniferous and older strata.
The Permian sea has left traces at Holywood on Belfast Lough and near Stewartstown in county Tyrone. Certain conglomeratic beds on which Armagh is built are also believed to be of Permian age. The Triassic sandstones and marls, with marine Rhaetic beds above, are preserved mainly round the basaltic plateaux of the north-east and extend for some distance into county Down.
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 epicontinental seas that played so large a part in determin ing the geology of Europe. Denudation, consequent on the re newed 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 glau conite, 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. Even the succeeding Senonian chalk, where it rests in the extreme north on Trias or even on the schists, is often conglom eratic and glauconitic at the base, the pebbles being derived from the old metamorphic series. This chalk appears to underlie nearly the whole basaltic plateaux, appearing as a fringe round them, and also in an inlier at Templepatrick.
No records of late-glacial submergence such as characterize Scotland and Fennoscandia are known in Ireland. The earliest post-glacial deposits are the submerged forests and the marls at the base of the peat bogs. In Early Neolithic (Campignian) times a slight submergence with a maximum of some 20 ft. affected the north-east of Ireland, and the fauna of the deposits dating from this period shows a temperature several degrees higher than that of the present day. Wide-spread growth of mountain pelt has occurred since the bronze age.
After the Irish chalk had been elevated and worn into gently rolling downs, on which flint-gravels gathered, the great epoch of volcanic activity opened, which was destined to change the char acter of the whole north-west European area. Fissure after fissure, running with remarkable constancy north-west, 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 z 5 m. long. 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 plateaux of the north. There was little explosive action, and few volcanic vents can now be traced, so that it is thought that the lavas originated in fissure eruptions like those of Iceland. After a time, a quiet interval allowed of the formation of a deep zone of lateritic weathering represented by 5o to 90 ft. of lithomarge capped by pisolitic iron ore and bauxite. 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. A renewal of the basaltic eruptions produced the Upper Series of plateau lavas, beneath which this old subtropical land surface is preserved. There was then a second long period of weathering which pro duced a soil comparable in thickness with that of the Interbasaltic zone. Warping led to the denudation of this soil, and its con centration in the centre of the basin now occupied by Lough Neagh. Thus were formed the Lough Neagh Clays, the plant remains of which are identical with those of the Interbasaltic Zone, and which must therefore be regarded as also of Eocene age. The whole plateau was subsequently greatly modified by faulting during the diastrophism of the Miocene period, but no further volcanic activity supervened, and no deposits of this period or of the later Pliocene are preserved to us. A well de veloped peneplain and still later deep cut fjords are all that re main as records of the history of Ireland between the end of the Eocene and the beginning of the Glacial periods.
Along the whole south coast of Ireland occurs a preglacial shoreline buried beneath the glacial drifts and lying some so ft. above present sea-level. This runs up the sides of the fjords and proves their submergence in preglacial times. Overlying scree deposits from the degradation of the sea-cliff, descend below sea-level and prove emergence before the advent of the land ice.
During the Glacial period Ireland was covered by ice-sheets, which extended from sea to sea and with the exception of some of the higher hills, which no doubt had their own snow-fields there was no actually unglaciated area. The Newer Drift probably does not cover as wide an area as the older, but its limits are not yet known. Boulder clay and glacial gravels cover large regions in the central plain and moraines occur around the mountains. The main centres of snow accumulation lay in the hill areas of the north-west and west but the whole eastern coast was at one time invaded by ice, which came south from Scotland along the basin of the Irish sea and carried with it the highly characteristic riebec kite-eurite of Ailsa Craig in the Firth of Clyde. Wide-spread moraines and esker ridges mark the stages of retreat of the ice, and systems of marginal drainage are developed round the hills.
BIBLIOGRAPHY.—See maps and explanatory memoirs of the Geological Survey of Ireland (Dublin) ; G. Wilkinson, Practical Geology and Ancient Architecture of Ireland (1845) ; R. Kane, Industrial Resources of Ireland (2nd ed., Dublin, 1845) ; G. H. Kinahan, Manual of the Geology of Ireland (1878) and Economic Geology of Ireland (Dublin, 1889) ; E. Hull, Physical Geology and Geography of Ireland (2nd ed., 1891) ; A. McHenry and W. W. Watts, Guide to the Collection of Rocks and Fossils, Geol. Survey of Ireland (2nd ed., Dublin, 1898) ; and G. A. J. Cole and T. Hallissy, Handbook of the Geology of Ireland