518 OVERLYING ROCKS. 



In varieties C, D, E, F, G, H, the base may 

 be simple ; or compound, and analogous to A 

 and B. Hence the varieties of aspect are infinite. 



Further, the mode of intermixture is often 

 such, that it is difficult to determine whether the 

 crystal, here called imbedded, is not rather a part 

 of the base ; and, in these cases, there is a grada- 

 tion between porphyritic and granitiform mix- 

 tures. This, in particular, frequently happens 

 in var. I. It is among the apparently ancient 

 porphyritic rocks, that this uncertainty of charac- 

 ter takes place. The porphyries, like the amyg- 

 daloids, sometimes contain empty cavities. 



It is probable that this catalogue admits of 

 considerable extension, but I have limited the 

 enumeration to those varieties which have fallen 

 under my own observation, and which occur in 

 considerable masses. A research among cabinet 

 specimens would possibly add many more, to the 

 list. 



FOURTH DIVISION. 



Supra-compounded rocks. Amygdaloids of 

 miriei^ilogists ; in this country at least, as this 



