SHALE. 459 



position. They sometimes also contain larger 

 fragments or nodules of the rocks, from the de- 

 composition of which they have been formed. 



Lastly, they contain organic remains both 

 vegetable and animal. In the limestones, they are 

 often the principal repository of the shells which 

 appertain to these groups of beds They are also 

 the occasional repositories, like marie and coal, 

 of the remains of vegetables. From analogous 

 causes, they are also often impregnated with 

 bitumen, even in such excess, as to be imper- 

 fectly combustible. 



A general conception of their distinguishing 

 characters as forming a species of rock, may be 

 deduced from these remarks. More particular 

 notions can only be conveyed by the description 

 of varieties. 



