156 



They occur also, according to Professor Green, on the east side of 

 the Derbyshire hills ; and they may be found farther from the 

 parent source even than that. 



The inference to be drawn from these facts is that, under the 

 various forms of glacial action affecting the surface, only the 

 toughest and most durable rocks survived ; all the rocks of lesser 

 toughness being sooner or later crushed and ground into small 

 fragments. Or, to put the same statement in to another form, it is 

 those rocks whose boulders have travelled farthest in the drift that 

 have suffered least under glacial action at home. The tougher the 

 rock, the farther its boulders have gone, and the more it has with- 

 stood wear and tear in sit-A : the more readily it goes to pieces 

 under the action of mechanical abrasion, the less chance is there 

 of any of its boulders surviving a lengthy transportal in the ice, 

 and the more the parent mass has suffered. 



This relation between the capacity of resistance of any given 

 rock to mechanical erosion and the development of surface 

 features, plays a much more important part in the sculpturing of 

 rock surfaces than many are disposed, even yet, to admit. In the 

 Yorkshire dales, especially, its effects are very striking and 

 suggestive ; because large areas there consist chiefly of limestone, 

 and the action of chemical forces comes prominently into play in 

 its effects upon surface features. Taking one of these districts as 

 an example, say Upper Wensleydale and the dales branching out 

 of it, we find the whole of the valleys carved out of a great pile of 

 Carboniferous rocks of marine origin, lying sheet upon sheet of rock 

 to a thickness of two thousand feet or more, and still remaining in 

 very nearly the same position of horizontality that they had when 

 originally spread out upon the old sea bottoms. These rocks 

 consist of a great number of alternations of beds of limestone, 

 sandstone, and shale : the limestone beds, as I have before pointed 

 out, nearly always being overlaid by shales ; and being succeeded 

 below by beds of sandstone. The relative proportion of these 

 three kinds of rock to each other may be roughly set down as 

 limestone two, sandstone three, shale five. That is to say, there 

 is more than twice as much shale as there is limestone. But if we 



