m 



being formed under similar conditions farther in. It is also clear 

 that, with the recession of the shale under subaerial waste, the 

 swallow holes would recede too, so that, if these conditions are 

 represented here, we ought to find elongated swallow holes, or 

 gullies, extending continuously across the whole width of outcrop 

 of the limestone terrace. Moreover, seeing that vertical erosion 

 of limestone by chemical agencies proceeds at so rapid a rate, 

 there should be evidence of a considerable reduction in the thick- 

 ness of the limestone at the outer edge of the terrace, commensurate 

 with the amount of subaerial denudation of the shales resulting in 

 theit recession from this point to the point where they are now. 

 It is hardly necessary to reiterate the statement that the facts, in 

 the present case, are entirely opposed to any such view of the 

 subaerial origin of the features in question. There are compara- 

 tively few swallow holes in these Wensleydale limestone terraces, 

 except those along their inner margins ; all the rest of the limestone 

 remaining almost intact ; or, what signs of weathering there are, 

 affect the whole of the surface alike. The inference therefore is 

 that every part of the whole terrace, from the scar at its outer 

 edge, to the outcrop of the shale on its edge next the hill, has 

 been exposed to denudation the same length of time. In other 

 words, the weather has, manifestly, attacked the whole surface 

 equally, and has left its marks just as distinctly upon the innet 

 edge of the terrace as upon the parts, sometimes a hundred yards 

 or more off, that now form the scar at its edge ; and the terrace, 

 has^ therefore, clearly not been formed by the weathering back of 

 the shale. 



Another point to notice, hardly less significant, is that hardly 

 any of these limestone terraces shew more than traces of the vast 

 accumulation of blocks of sandstone that musthavebeen undermined 

 and detached by the waste of the beds of shale beneath them. If 

 a bed of shale lying between a bed of sandstone above and a bed 

 of limestone beneath, be wasted back fifty or a hundred yards, it 

 is obvious that some traces of the sandstone so undermined should 

 be found at that spot. But this is not the case except to a very 

 limited extent. 



