161 



shales under the action of the subaerial forces now attacking them, 

 are very different from what they are under glacial erosion, and 

 are clearly such as would, in the long run, end by leaving only the 

 beds of sandstone at the surface to form all the features of promi- 

 nence, except those situated on a very steep slope. Where the 

 rocks lie in a nearly horizontal position, as they do over large areas 

 in north-west Yorkshire and the parts of Westmorland adjoining, 

 we ought, therefore, if it is true that the present surface features 

 are due to the action of subaerial forces, to find nearly all the 

 more prominent scars, and the terraces that extend inwards from 

 their edges, almost exclusively of the particular rock that wastes at 

 the slowest rate ; that is to say, of sandstone. The shale would 

 be expected to be cut back fastest, while the beds of limestone, 

 being attacked by two sets of subaerial forces, instead of by but 

 one set, would be dissolved clean out of sight all along its outcrop. 

 Besides this we ought to find, all along the outcrop of the sand- 

 stones, a talus of blocks that had been dislodged from the scar by 

 the undercutting of the softer beds beneath them. 



So far as the particular district now referred to is concerned, we 

 find the surface features as unlike this ideal as they could well be. 

 There are, it is true, many scars, and here and there a few terraces, 

 of sandstone ; but all the more conspicuous features of this kind 

 consist of the more-perishable limestone. Moreover, these lime- 

 stone terraces occur in the greatest perfection at the very points 

 where, according to the theory that they are due to the action of 

 subaerial waste, they ought to be entirely absent. Another very 

 important point in connection with these terraces cannot he too 

 much insisted upon. This is, that in many cases, the terrace of 

 limestone (see A. B. in fig. lo) consists of a shelf, or sheet, of lime- 

 stone, of very nearly its full thickness, even at the very outer edge 

 of the terrace. The limestone surface, as a whole, betrays hardly 

 any evidence of having been longer exposed to the wasting influences 

 of denudation at its outer edge A than it does at B along the line 

 where the newer beds come on above it, which is presumably 

 the part last exposed. This feature is rendered all the more 

 striking by the fact that swallow holes are almost exclusively 



