157 



compare the relative proportion borne to eacii other by these rocks 

 as they occur in the local drifts, we find them about as : — lime- 

 stone four, sandstone four, and shale only two; the proportion of 

 shale diminishing, and that of limestone increasing, the farther we 

 trace the boulders from their parent sources. It is therefore 

 evident that whatever was the agent that transported the materials 

 of the drift, that agent certainly crushed the shale into mud first, 

 reduced the sandstone to sand at a slower rate, while the limestone 

 boulders were suffered to travel great distances with a minimum 

 reduction of their bulk. It may here be mentioned again that the 

 drifts of these dales contain a percentage of clay small out of all 

 proportion to that of the argillaceous members of the local rocks. 

 Indeed large masses of drift occur here with hardly any clay in 

 them at all. (See Quart. Journ. Geol. Soc, xxxi. p. 97.) What 

 became of the crushed shale was to me a long time a mystery, 

 until I found my colleague Mr. Clement Reid wondering whence 

 had come all the clay that he had been mapping as boulder clay 

 on the chalk wolds of Yorkshire. It was then evident that, by 

 some means or other, much of the clay that was missing in north- 

 west Yorkshire had found its way to the south-eastern parts of the 

 same county, and formed the matrix of the drifts there. 



It is, then, clear, that under glacial erosion the relative destructa- 

 bility of these rocks stands as — shale, sandstone, limestone; the 

 limestone being, under this kind of agency, the last to be reduced. 

 Now let us compare the relative degrees of resistance to subaerial 

 waste presented by these rocks in sitH, and note the result. 



[Under the action of the weather each of these kinds of rock 

 behaves differently. Where the outcrop is of shale, and at the 

 same time forms a steep bank alongside a stream, the numerous 

 divisional planes help to make the rock go to pieces in a very 

 short time ; so that, in such a case, whatever the overlying beds 

 may be like, the bank is not long in being cut back. But where 

 the outcrop forms a gentle slope out of the way of constantly- 

 running water, such shale as is not more than usually sandy, 

 decomposes into a tough and nearly impervious clay, much of 

 which remains at the surface, and thereby greatly helps to lessen 



