98 JF. Gunn — Suhaerial Denudation v. Glacial Erosion. 



whole, extremely partial, and are often entirely wanting in districts 

 composed of argillaceous and sandy formations, which must, never- 

 theless, have been upheaved at the same time, and by the same 

 intermittent movements, as the adjoining calcareous rocks." ^ I cannot 

 help thinking from the foregoing paragraphs that Lyell thought the 

 argillaceous and sandy formations would disappear faster by the 

 agencies of the atmosphere than would calcareous rocks. Prof. 

 Ramsay says that the soft clay of the Gault has been more easily 

 worn away than the comparatively hard Chalk,^ and that some of the 

 limestones of the Oolites form minor escarpments ;^ yet the Chalk is 

 more easily wasted than the Oolitic escarpment.* Elsewhere it is 

 stated that the plateaux of Carboniferous Limestone have suffered 

 waste by solution equal to that of the Chalk ; ^ but this cannot mean 

 that Carboniferous Limestone wastes as fast as Chalk, which is easily 

 worn mechanically by rain, as well as chemically^ (or we should 

 have to infer that Carboniferous Limestone is softer or more easily 

 acted on by rain than Oolitic Limestone) , but that it has undoubtedly 

 been exposed for a far longer period to atmospheric action. 



Mr. Plant,^ Mr. Mackintosh,^ and Mr. Pengelly ^ have adduced 

 evidence to show the slow rate at which limestone weathers. 



If Subaerial Denudation acted so quickly on limestone that were it 

 not for Glacial Erosion it would everywhere save near the streams 

 be dissolved clean out of sight (p. 327), surely a scar would result; 

 for if the limestone were eaten back so as to be out of the reach of 

 rain, it would still stand as a vertical or overhanging crag, as repre- 

 sented in the accompany- 

 ing diagram (Figure 1), 

 and the more prominent i 

 features would still be 



of limestone, and not of l ^-^]l'HI) \\ l\\\ |]|1 il I/I Iji 1\ ! /)[']" " ' a 

 sandstone, as supposed. -^ 



I should like to know Fig. 1. Diagram to illustrate tlie formation of a 

 how Mr. Goodchild re- limestone ' scar.' 



conciles his supposed «• Limestone. *. Shale. ..Sandstone. 



rapid wasting away of limestone by subaerial agencies with the fact 

 that glacial strias exist close to the inner margin of some of his 

 widest limestone terraces, within a few feet of the outcrop of beds of 

 shale or sandstone next above (see pp. 325, 326, 359) ; and that in 

 the case of some of the more compact and thickly-bedded rocks, the 

 highest strata of the hard bed even yet form the upper surface of the 

 terrace a hundred yards or more from the outcrop of the overlying 

 soft bed (p. 360). It is true there may have been a covering of 

 glacial drift, the recent removal of the last traces of which has bared 

 the glacial markings ; but if so, would not this also account for the 

 shale slope above having been only slightly denuded since the Glacial 



1 Op. cit. p. 79. 



2 Phys. Geol. and Geog. of Great Britain, third ed., p. 117. 



3 Op. cit. p. 96. 4 Op. cit. p. 221. ^ Op. cit. p. 263. 

 s Lyell's Elements, p. 363. 



7 Geol Mag. Vol. IV. pp. 81, 82. 8 Qp. cit. pp. 138, 296. 



8 Mentioned hy Mackintosh in op. cit. p. 139. 



