THE ORIGIN AND DESCENT OF ROCKS. 



413 



the formation of distinct minerals reach a high stage, and as a natural 

 result the varieties of rock are numerous. Taken as a group they are 

 phanerites. If they are to be more particularly characterized, it is 

 usually done on the basis of the minerals of which they are composed. 

 The following are the leading types, beginning with those which are 

 rich in siHca and poor in basic oxides, and ending wdth those which are 

 rich in basic oxides and poor in sihca. 



Fig. 344. — Granitic texture. About half natural size. (Photo, by Church.) 



The granites. — The term granite was originally used to designate a 

 granular, i.e., a distinctly crystalline, rock, and it is still popularly and 

 properly so used. In scientific treatises it has usually been confined 

 to a special aggregate of crystals of quartz, feldspar, and mica. It 

 has recently been proposed to give it again a more general apphcation, 

 though not quite its original one, by including under it all holocrystal- 



