100 W. Gunn — Suhaerial Denudation v. Glacial Erosion. 



and shale. I refrain from referring to the numerous limestone 

 ravines to be found in many other places. 



We get evidence of the comparative rates of weathering of lime- 

 stone and sandstone in buildings. Mr. A. B. Wynne, in a very 

 suggestive paper on Denudation, states that " atmospheric action 

 is sometimes slighter upon limestone than upon other rocks, as 

 may be witnessed in ruins and old buildings, the masonry of 

 which includes both limestone and sandstone." ^ Sir Henry De la 

 Beche says the Carboniferous Limestone of the South-west of England 

 is both a hard and durable material, while few of the sandstones 

 of the carbonaceous series can be considered durable except those 

 in which the siliceous matter so abounds as to render them almost 

 cherty.^ Prof. Eamsay says: ''The Carboniferous Limestone also 



is an exceedingly durable stone In Caernarvon Castle the 



preservation of this limestone is well shown. The Castle is built of 

 layers of limestone and sandstone, the sandstone having been chiefly 

 derived from the Millstone Grit, and the limestone from quarries in 

 Anglesey. ..... The limestone has best stood the weather." ^ 



I think I have now shown that Mr. Goodchild is mistaken about 

 the comparative rates at which limestone, sandstone, and shale are 

 denuded under purely atmospheric conditions, and that the order is 

 the same for rain and frost as that he gives for mechanical erosion in 

 a stream, viz. : "Shale goes fastest; next to this come the thinner- 

 bedded sandstones ; and longest of all in being worn away are the 

 blocky sandstones and the purer kinds of limestone" (p. 327). I 

 think that some of ttie hard blocky sandstones of these dales would 

 stand out longer against atmospheric denudation than even the 

 hardest limestone. 



I do not see that the regular form of the scars and tlieir parallelism 

 to others above and below is a special difficulty for subaerialists, for 

 however first formed, they would be pretty evenly cut back all along 

 by rain and frost, as mentioned by Messrs. Bonney and Greenwood, 

 and so roughly preserve their parallelism to the stream and to one 

 another. And if we suppose the scars had no pre-glacial existence, 

 the beds which now form scars must have outcropped along the 

 sides of the valleys pretty regularly and roughly parallel to each 

 other, unless it is supposed that the ice acted on a set of very irre- 

 gular outcrops and denuded some very much more than others, in 

 order to produce the parallelism we see — a supposition which seems 

 very unlikely ; so that the parallelism of the scars, if they have been 

 formed by ice-action, is not directly due to such action. 



Nor does the correspondence of the scars on one side of a dale 

 with those on the other present any special difficulty. Where the 

 valley is pretty straight, we should expect the scars to be roughly 

 straight on both sides ; where there is a curve in the valley, we 

 should expect the outlines to be concave on one side and convex on 

 the other. But the curves of the valley as a whole may not exactly 



' Geol. Mag. Vol. IV. p. 7. 



2 Geological Eeport on Cornwall and Devon, p. 490. 



^ Phys. Geol. and Geog. of Great Britain, third ed., pp. 316, 317. 



