514 OVERLYING ROCKS. 



common greenstone and basalt among the gra- 

 nites, present the two strongest examples of the 

 analogy between these rocks, so different in geo- 

 logical connections. In both cases, there is no 

 distinction to the mere mineralogist, or collector 

 of specimens ; but the arrangement here adopted 

 renders it necessary thus to separate substances 

 which are, mineralogically, identical. 



No example of a mixture of quartz, felspar, 

 and mica only, has, as far as I know, yet occurred 

 among the overlying rocks. 



As in granite, the constituent minerals are 

 sometimes crystallized in these syenites. Crystals 

 of brown quartz, in particular, occur of a consi- 

 derable size in St. Kilda. 



The columnar form of the syenite of Ailsa 

 was mentioned under A, b, Second Div. First 

 Subdiv., and in that rock quartz is sometimes 

 present so as to bring it equally under the var. 

 A just noticed. 



Occasionally these varieties, like others, are 

 also laminar. 



F. Augit, felspar, and mesotype. 



G. Augit, felspar, and prehnite. 



