strata, they must be considered strictly primary ; 

 and this may be true, as already suggested, re- 

 specting the porphyries now connected with the 

 primary strata : or with some at least of these ; 

 since it is very certain that the double connection 

 just pointed out does actually exist in many of 

 them. 



It has been seen that the unstratified division 

 of the primary class comprises only one rock, 

 namely granite. But under the same head, in the 

 secondary, is included a great variety of sub- 

 stances, differing in their characters, and consti- 

 tuting many distinct rocks. As all these, how- 

 ever, pass into each other by imperceptible gra- 

 dations, and are connected by one common geo- 

 logical bond, it has been judged convenient to 

 adopt one common term for the whole. This term 

 is that of the overlying rocks. It must be con- 

 sidered as generic and geological. No single 

 term derived from any individual of the whole 

 family, could have been applied to them ; as is 

 conveniently done in the case of granite, where 

 the same word suffices to express both the geolo- 



