Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/264

 indifferently. The undulating surface above cuts through them irregularly. The original surface of these crumpled quartz-beds has been entirely destroyed by denudation. Slieve Liag, with its grand cliffs and caves on one side and its glacial striae on the other, demonstrates that the shape of Ireland in this region is chiefly due to glacial and marine " denudation." But all other Irish cliffs and coast-sections, and all the cliffs I have ever seen, tell the same story.

I leave it to professed geologists to measure the quartz-beds which are marked Old Red Sandstone on the map, and to calculate how much has been destroyed above since hard horizontal beds were crushed laterally and folded together like potter's clay. Upon similar worn surfaces later geological formations are piled, so far as I know any thing about them. It is enough for my present purpose to show that the upper surface of Ireland is a worn surface, in which the hardest parts usually are the highest, and that other old surfaces of the same kind are under newer formations, as shown at Slieve Gallion and at Cushendal in Antrim.

XII. Ice-marks. — In order to read Ogham we must learn that alphabet. In order to read older Irish records inscribed upon rocks by ice, we must learn the meaning of these signs. Snow is water. A snowball is plastic water ; for it can be squeezed into moulds or pushed through narrow tubes. Glaciers are but plastic water. Water flows downhill. If it meets an obstacle while flowing, it runs up hill over it, or splits and flows round it. If a stream is stopped, it gathers behind the dam till deep enough to flow over it. A deep stream, like an ebb-tide, flows over all obstacles beneath the surface ; but currents beneath the surface rise and fall, or move sideways, following the shape of sunken rocks and stones and sandbanks.

Plastic water in large glaciers moves like fluid water, and for the same reasons, but more slowly. A sheet of ice split upon the upper end of a ridge joins at the lower end. Two glaciers unite at a fork, as rivers do. Ice which has tumbled over a rock, like water over a fall, "regelates." It is plastic and it welds; so it mends like a broken snowball, and flows on till it melts. A lump of putty gives a ready illustration of the movements of glacier -ice ; for it is plastic and heavy, it moves slowly, tears and mends, and moulds itself upon the surface beneath it, as glaciers do. "When heavy glaciers press upon or against rocks under them, strength must decide which is to yield. If a rock is crushed, fragments help to grind rocks too hard for crushing. If the ice yields, it leaves a track on the obstacle which turned it from its course. When a glacier melts so as to leave the bed of it for inspection, it drops the upper angular moraine upon beds of clay and stones which were under the glacier, and these upper and under beds of drift rest upon rocks which were crushed or ground by the ice and the stones. So long as these tracks endure, the last movements of the melted glacier are recorded by drift and by glaciated rocks.

Whatever theories may be formed as to glacial periods and the motion of glaciers, it is certain that ice now moves slowly in di-