1862.] JUKES — ^RIVER-VALLETS. 397 



spheric influences woiild be deepened and intensified by tbeir vertical 

 action, which may be likened to the artificial processes of carving or 

 graving deeper and deeper hnes or features once traced out. Had 

 the denudation been that of the sea, it would have tended to cut 

 down and plane off all these features to one uniform level, so far as 

 its influence extended ; the termination of that influence being marked 

 by abrupt vertical cliflfe, like those which now form so much of the 

 present coast-line. 



The rain and the weather have disintegrated and lowered to son e 

 extent even the highest of the ridges left by the original marine de- 

 nudation, and have impressed upon them the character of their own 

 action, instead of that of the sea ; but they have had still greater 

 effect on the slopes, and the greatest of all on the channels of the 

 streams tha!: first commenced to drain the land. This influence has 

 always been proportionate to the power exercised, and the nature of 

 the material on which it worked. When exercised on the quartzose 

 ridges of the Old Eed Sandstone, its greatest effect was confined 

 within narrow limits, where the water ran rapidly, and thus produced 

 glens or ravines. When the influence acted on the softer and more 

 easily disintegrated argillaceous beds of the Carboniferoas Slate, and 

 still move when brought to bear on the soluble beds of the Carboni- 

 ferous Limestone, it produced broader effects, and formed the larger 

 lon'^itudinal valleys. 



The fact that the limestone ground is now everywhere so much 

 lower than the groand formed of other rocks is evidently due to the 

 solubility of that rock. The siliceous rocks have suffered chiefly from 

 mechanical disintegration and attrition alone ; the calcareous rocks, 

 eqj-ally exposed to that action, have also been destroyed by the che- 

 micjvl action of the carbonic acid of the atmosphere, and much of 

 their mass has been carried away in solution. 



The limestone of the plains and valleys has, in fact, sunk in its bed 

 past the other rocks, like the ice of a glacier under a summer sun. 

 This image was brought to my mind (quite independently of this 

 explanation) when one day, during the summer of 1861, I was tra- 

 versing the limestone hiUs of Burren, in county Clare, with my 

 colleague, Mr. F. J. Poot. The bare floors of Kmestone in that district 

 consist often of loose blocks, each block being deeply furrowed at its 

 edge by channels formed by the rain-water running over its surface. 

 Many of them look as if they had been artificially carved all round 

 into deep indented mouldings, in order to produce an ornamental 

 pattern. In other places, the whole of the blocks that once formed a 

 bed two or three feet in thickness have been so wasted away that a 

 layer of mere cakes, an inch or two thick, is all that is now left to 

 lejresent them. AU the joints that divide the rocks into blocks, 

 instead of being mere planes of division, with their walls still clo ely 

 to 'ching, are near the surface m^^de into open fissures, several metres j 

 e^en a foot or more, in width, and frequently 5 or 6 feet in depth. 



Not only there, but in all the limestone country, the same facts 

 may be ob'^erved, wherever the rocks are sufficiently well exposed. 

 On frst baring the rock for a limestone-quarry, its surface may often 



