516 OVERLYING ROCKS, 



I have found this rock, like N, forming a mass 

 of considerable extent, near Glasgow, and have 

 quoted them both as examples of very complicated 

 varieties of what must be included with the green- 

 stones. Doubtless many other varieties, which 

 have not come under my notice, exist ; and these 

 examples will serve to direct the attention of 

 mineralogists to a minute examination of these 

 rocks, should it be thought necessary to add to 

 this catalogue, all the varieties that may be found 

 in a class of rocks which include so many mi- 

 neral substances. 



SECOND SUBDIVISION. 



One species of crystals imbedded in a com- 

 pound base, or two species of crystals imbedded 

 in a base that is either simple or compound. This 

 includes all the remaining porphyries. 



A. Felspar crystals in a base of greenstone. 



This is the greenstone porphyry of minera- 

 logists, and it may exist under many different 

 aspects according to the quality of the base. 

 The student may distinguish these by referring 

 to the preceding parts of this catalogue, as the 



