206 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 22, 



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 simi- 

 lar 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 sur- 

 face ; but currents beneath the surface rise and fall, or move 

 sideways, following the shape of sunken rocks and stones and sand- 

 banks. 



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 ob- 

 stacle 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 loDg 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 slowli/ in di- 



