102 



topographic geology, and not to the proper his- 

 tory of mere rocks, which is alone the subject of 

 the present enquiry. 



There is not much to remark respecting the 

 occasional rocks. As the jaspers appear to be 

 derived from claystones, or from common clay, 

 or from argillaceous sandstones, the siliceous 

 schists from argillaceous schist and shale, and 

 the cherts from limestone and from calcareous 

 sandstones, it is easy to comprehend how, from 

 the peculiar circumstances of their origin, they 

 must necessarily graduate into the various rocks 

 to which they owe their origin. 



