Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/260

202 On the surface are marks of glaciation, which wore the rock and shaped the surface, and carried the debris southwards to Waterford, along a wide hollow which goes from one end of Ireland to the other, along the courses of the Ban and the Waterford rivers.

At the edge of the country are marks of horizontal undermining at "the destructive plane of the sea."

The marks of rivers and weather are also plainly seen (but they are insignificant about Lough Neagh) in plains and highlands. The amount of weathering is measured on quartz-veins, which retain glacial marks and stand out a couple of inches at most from the weathered surfaces. Rivers work only in their narrow beds; and most of them still flow on drift in low grounds. Since ice vanished and the land rose, these last-named engines have done little work.

VII. Limestone.—I will take another case in which the main hollow trends N.W.

The Sligo hills to the south of Donegal Bay are steeply scarped plateaux with cliffs and talus. The outline is often like that of the Antrim coast; the plan is like that of the Antrim glens. The chief difference in the forms of these two sets of hills is in the greater steepness of harder slopes in Sligo. Ridges end in sharp peaks like needles off the Atlantic coasts.

The tops of these hills and their high plateaux are made of flat beds of blue Mountain-Limestone, resting conformably upon grits and sandstones.

Cliffs are fractures; and some of these may coincide with faults. If so, I could not find them.

These beds were deposited horizontally at the bottom of a sea, long before the Antrim chalk. Elsewhere they have been greatly disturbed and bent into basins, notably about Lough Neagh, in the coal-field near Dungannon. Their geology is studied because of the coals which accompany Mountain-Limestone in Ireland and elsewhere. But about Beinn Gulban, famous in Celtic tradition, the beds are flat or slightly inclined. Like Antrim chalk, their edges appear on the sides of hills, in deep glens, at points, in "cols," and in cliffs. It is manifest on the ground that these Sligo glens have been hollowed out of a raised plateau, and that more than 2000 feet of limestone and lower beds have been carried away from large areas in this region about Lough Erne and Donegal Bay. Not one sample of Mountain-Limestone could I find in drift about Dangloe and the northern end of Ireland; but the low lands of Central Ireland are thickly covered with limestone-gravels. At Galway are sections of Boulder-clay full of scratched polished fragments of limestone; and great blocks of it have been carried on to hills about Kenmare, in Kerry, in the south-west. Like the flints, the limestone-drift travelled southwards. Measuring from the limestone in Sligo to the plain, about 2000 feet of rocks have been removed. The fragments did not go far north; but a great stream of ice certainly travelled from Lough Erne north-westwards into Donegal Bay. The marks are well preserved at Bally-Shannon on sandstone.

The Irish coal-fields now are patches scattered about the country,