OVERLYING ROCKS. 493 



This must not be confounded with the ferru- 

 ginous clays which are often found among the 

 trap rocks, and which pass into jasper, 

 b. Cellular. 



Like Var. A, b, it is rarely cellular in large 

 masses, without also containing amygdaloidal 

 nodules ; when it ranks under another division. 

 The colours of this variety are, either ash colour, 

 or grey of different hues, or modifications of red, 

 or browns, or purplish black. 



C. Claystone. The fracture is dull and earthy, 

 and may be smooth and even, or rough and somewhat 

 granular, or imperfectly splintery, or conchoidal. It 

 differs from the preceding substances in hardness ; but, 

 as in many other cases, it is scarcely possible to convey 

 an idea of such distinctions by any mode of definition. 

 Claystone, independently of its geological differences, 

 is distinguishable from argillaceous schist, by the ab- 

 sence of the schistose structure. 



a. Massive, irregular. 



b. Prismatic, or columnar. 



c. Laminar. 



d. Cellular. 



The laminar structure is scarcely to be detect- 

 ed, except on the surface, and after exposure to 

 weather. It is sometimes combined with the pris- 

 matic configuration. It cannot be confounded, 



