Page:EB1911 - Volume 14.djvu/778

GEOLOGY] from Enniskillen to the Silurian beds at Pomeroy, and some contemporaneous andesites are included, reminding us of the volcanic activity at the same epoch in Scotland. The numerous “felstone” dikes, often lamprophyric, occurring in the north and west of Ireland, are probably also of Devonian age. The conglomerates appear at intervals through the limestone covering of central Ireland, and usually weather out as conspicuous scarps or “hog’s-backs.” The Slieve Bloom Mountains are thus formed of a dome of Old Red Sandstone 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. The Old Red Sandstone is most fully manifest in the rocky or heather-clad ridges that run from the west of Kerry to central Waterford, rising to 3414 ft. in Carrantuohill in Macgillicuddy’s Reeks, and 3015 ft. in Galtymore. In the Dingle Promontory the conglomerates of this period rest with striking unconformity on the Dingle Beds and Upper Silurian series. Here there may be a local break between Lower and Upper Devonian strata. The highest beds of Old Red Sandstone type pass up conformably 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 Coomhola Grits occur. Its lower part represents the Lower Carboniferous Shales and Sandstones of the central and northern areas, while its upper part corresponds with a portion of the Carboniferous Limestone. The Carboniferous Limestone, laid down in a sea which covered nearly the whole Irish area, appears in the synclinal folds at Cork city and Kenmare, and is the prevalent rock from the north side of the Knockmealdown Mountains to Enniskillen and Donegal Bay. On the east it spreads to Drogheda and Dublin, and on the west to the heart of Mayo and of Clare. Loughs Mask and Corrib are thus bounded on the west by rugged Silurian and Dalradian highlands, and on the east appear as mere water-filled hollows in the great limestone 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 contains black flint (chert), and at some horizons conglomerates occur, the pebbles being derived from the unconformable 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 table-lands in county Sligo and other western regions. Subterranean rivers and water-worn caves provide a special type of scenery below the surface. 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, sometimes reaching the true Coal-Measures, but rarely younger than the English Millstone Grit. They are well seen in the high ground about Lough Allen, where the Shannon rises on them, round the Castlecomer and Killenaule coalfields, and in a broad area from the north of Clare to Killarney. Some coals occur in the Millstone Grit horizons. The Upper Coal-Measures, as a rule, have been lost by denudation, much of which occurred before Triassic times. South of the line between Galway and Dublin the coal is anthracitic, while north of this line it is bituminous. The northern coalfields are the L. Carboniferous one at Ballycastle, the high outliers of Millstone Grit and Coal-Measures round Lough Allen, and the Dungannon and Coalisland field in county Tyrone. The last named is in part concealed by Triassic strata. The only important occurrences of coal in the south are in eastern Tipperary, near Killenaule, and in the Leinster coalfield (counties Kilkenny and Carlow and Queen’s County), where there is a high synclinal field, including Lower and Middle Coal-Measures, and resembling in structure the Forest of Dean area in England.

The “Hercynian” earth-movements, which so profoundly affected north-west and north-central Europe at the close of Carboniferous times, gave rise to a series of east and west folds in the Irish region. The Upper Carboniferous beds were thus lifted within easy reach of denuding forces, while the Old Red Sandstone, and the underlying “Caledonian” land-surface, were brought up from below in the cores of domes and anticlines. In the south, even the Carboniferous Limestone has been so far removed that it is found only in the floors of the synclinals. The effect of the structure of these folds on the courses of rivers in the south of Ireland is discussed in the paragraphs dealing with the geology of county Cork. The present central plain itself may be regarded as a vast shallow synclinal, including a multitude of smaller folds. The earth-wrinkles of this epoch were turned into a north-easterly direction by the pre-existing Leinster Chain, and the trend of the anticlinal from Limerick to the Slieve Bloom Mountains, and that of the synclinal of Millstone Grit and Coal-Measures from Cashel through the Leinster coalfield, bear witness to the resistance of this granite mass. The Triassic beds rest on the various Carboniferous series in turn, indicating, as in England, the amount of denudation that followed on the uplift of the Hercynian land. Little encouragement can therefore be given in Ireland to the popular belief in vast hidden coalfields.

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 plateaus of the north-east, and extend for some distance into county Down. An elongated outlier south of Carrickmacross indicates their former presence over a much wider area. Rock-salt occurs in these beds north of Carrickfergus.

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