OVERLYING ROCKS: 



So many substances, of external characters 

 and compositions so different, are included in 

 this family, that it is necessary to separate them 

 into divisions before attempting to enumerate the 

 general characters by which a student may be 

 induced to search for his specimen among them. 

 They may, for this purpose, be conveniently di* 

 vided into the simple, the granitic, the porphy- 

 ritic, and the amygdaloidal. 



The simple rocks of this family, consist 

 either of compact felspar, clinkstone, or indu- 

 rated clay. 



The characters of compact felspar have al- 

 ready been described, and as this substance 

 occurs both in the primary and secondary class, 

 although under slight differences of aspect, it is 

 evident that these can only be distinguished by 

 their respective geological positions. 



The characters of clinkstone are the follow- 

 ing. It is a simple rock, massive, but occasion- 



