Mackintosh — On the Cliffs and Valleys in Wales. 391 



theory that a narrow connecting gorge is the expression of an 

 original river-channel, as its waters flowed transversely off a table 

 land, and that the longitudinal vales or plains were worn down by 

 the river and its tributary streams in easily denuded rooks during 

 the time that the river alone was gnawing out a narrow passage or 

 outlet in a zone of tough rocks which eventually become a ridge or 

 strip of elevated ground. It is admitted that this explanation will 

 only hold good where the rocks of the wide depression are of a kind 

 more easily eroded than those composing the ridge : that is, sup- 

 posing a gorge one-sixth of a mile in width, and a vale three miles 

 in width, the materials of the vale must have been eighteen times 

 more easily washed away than those in which the gorge was ex- 

 cavated. I do not think that the assumed correspondence between 

 the atmospheric denudability of the rocks of the respective areas has 

 been proved as regards many if not most of the districts where the 

 denudation puzzle is exhibited. It would require an extensive series 

 of observations and experiments to determine which rocks most 

 readily give way to atmospheric, fluviatile, and oceanic action. 

 Rivers affect rocks, as regards their relative composition and 

 structure, much in the same way as oceanic currents, though they 

 do not give rise to precisely the same form of effects. The atmo- 

 sphere, apart from running water charged with abrading matter, 

 as I have already shown, accomplishes its taslc chiefly by chipping 

 or splintering, which, strictly speaking, does not come under the 

 denomination of denudation until the fragments are carried away 

 by streams. Mere rain has so little influence on hard rocks that, in 

 considerations of this kind, it may be left out of the question.^ 



1 This I have endeavoured to prove as regards millstone grit and other rocks (see 

 article on Brimham Rocks— Geol. Mag. April, 1865). But I think it can he shown 

 that limestone is more or less exempt from any process of dissolution caused by the 

 atmosphere apart from the grinding action of streams. Many limestone cliffs would 

 appear to be both rain-proof and air-proof, as they still retain the smooth grooves 

 and undercuts imprinted on them by the sea. These marks may be distinctly traced 

 on those parts of the Eglwyseg rocks, in Denbighshire, which are not undergoing 

 fragmentary dilapidation. The most striking proofs, however, of the resistance offered 

 by Caboniferous Limestone to mere rain may be seen on a table land to the north- 

 west of Minera, and about eight miles from Llangollen. A number of nearly square 

 flags of limestone, separated by a very regular system of joints, lie flat on the surface. 

 From a distance they look like a vast assemblage of grave-stones. Here and there 

 whole flags or ranges of flags have been carried bodily away, without leaving the slight- 

 est trace of their existence, and that, most assuredly, by no kind of atmospheric action. 

 Nearly every remaining stone presents a series of peculiar marks, consisting of 

 smooth, semicircular grooves, from an inch to eight or nine inches in depth. These 

 grooves are generally straight, but sometimes winding, and generally, though not 

 always, roughly parallel ; they often turn round at nearly right angles, and run into 

 each other or vanish at the border of the flag. They sometimes terminate in circular 

 perforations. No one would say that glaciers could have formed such a number of 

 deep and complicated grooves. Eain is out of the question, its effect being evidently, 

 as one may see on the spot, to roughen, and that to no very great extent, the smooth 

 curvilinear outlines of the grooves. There is no appearance of any former river- 

 channel, and rivers, had they been here, could not have given rise to such indentations. 

 There is only one explanation left, and it may be seen exemplified on the sea-coast of 

 Wales — (I have seen this at Aberystwyth) — namely, the backward and forward motion 

 of pebbles driven by the advancing and receding waves of the sea. Similar wave-worn 

 flags may be seen farther to the south, and I have no doubt in other localities. Their 

 position must be at least 1000 feet above the present level of the sea. 



