JF. Giinn — Siibaerial Denudation v. Glacial Erosion. 99 



Period ? the denuding forces having had first to get rid of the drift 

 before they could act on the solid rocks below. I have often seen 

 glacial striae on limestone where it was covered with Boulder-clay, 

 but have seen very few on bare limestone rock ; and none in situa- 

 tions where it was not probable the striae had been exposed quite 

 recently. Thus I do believe in the slow wasting away of the lime- 

 stone by the action of meteoric agencies — ''hot and cold, and moist 

 and dry." But it does not necessarily follow that because limestone 

 is acted on chemically as well as mechanically by rain, that therefore 

 it would be wasted faster than shale, or even than the thinner-bedded 

 and softer sandstones. Eain, which can have but a slight mechanical 

 effect upon hard Carboniferous Limestone, easily wears away soft 

 shale on such steep slopes as we have in these Yorkshire Dales. A 

 very careful and accurate observer says : ''It should also be recollected 

 that in mountainous regions the slopes, even those supporting vege- 

 tation, are for Ihe most part highly inclined, so that mineral and 

 vegetable matter, finely divided, is readily moved downwards me- 

 chanically suspended in myriads of little threads of water, which 

 pass almost imperceptibly among the vegetation itself."^ 



And we find that where a few inches of shale is interbedded with 

 limestone in ravines, scars, or even in the very modern vertical 

 sides of an old quarry, the softer shale has weathered back and left a 

 hollow, whereas, according to Mr. Goodchild, it should project as a 

 ledge and the limestone should be wasted most. Suppose we put up 

 a tombstone of rather hard carboniferous shale, would only an inch 

 have been denuded from its surface in 500 years ? Would it even 

 last 50 years ? Would not the rain and frost rapidly act on its 

 numerous divisional planes, and cause it to crumble to pieces in a 

 short time ? We get the answer when we look at vralls in which 

 both limestone and hard shale have been used in building. The lime- 

 stone is comparatively unweathered, while the hard shale splits up 

 and falls to pieces. 



" There can be no doubt that the water of streams acts chemically 

 upon rocks, in the same way as does rain and spring water." ^ 

 Then is there not inconsistency between the supposed rapid weather- 

 ing of limestone from rain and its slow denudation from river action 

 (p. 327), which must be more powerful than that of rain ? If the 

 difference in the width of a post-glacial ravine at its lower and 

 upper ends gives the amount of subaerial denudation, a limestone 

 ravine should be wider at its lower end than a shale ravine, other 

 things being equal (where is there an example?), and river-valleys 

 should narrow where shale is the predominant rock, and widen 

 where limestone is found. I have always found a ravine narrowest 

 in the limestone : take, for examples, the ravines of the Yore at 

 Aysgarth, of the Greta at Rokeby, and of the Tees at the Abbey 

 Bridge below Barnard Castle. Contrast also the deep narrow lime- 

 stone valley of Upper Wharfdale about Kettlewell and Buckden 

 with Wensleydale, composed of alternations of limestone, sandstone, 



^ De la Beche, Researches in Theoretical Geology, p, 170. 

 2 Jukes and Geikie, p. 397. 



