512 OVERLYING ROCKS. 



THIRD DIVISION. 



Compounds of three or more ingredients. 



FIRST SUBDIVISION. 



Granitiform mixtures. 



A. Felspar, hornblende, and quartz. Syenite. 



This is the substance which alone rigidly 

 accords with the common definition of syenite ; 

 although, in practice, mineralogists have not held 

 it necessary to be strict on this subject ; feeling, 

 rather than acknowledging, the inconvenience of 

 such refinements in the geological description of 

 rocks. The felspar may be either compact, or 

 common. The former variety occurs among the 

 overlying rocks only ; never, as far as I have ob- 

 served, very extensively or perfectly, among the 

 granites. The latter compound forms also a 

 well-known and very prevalent variety of granite. 

 Where these rocks occur with the traps, the fel- 

 spar is often of a grey or greenish hue ; and, in 

 this case, they have frequently been called green- 

 stone, the quartz being neglected or overlooked. 



