CLASSIFICATION OF IGNEOUS ROCKS 259 



Fig. 240. Graphic granitic (or pegmatitic) texture. Nearly natural size. 

 (Photo, by Church.) 



solution, the rock becomes an amygdaloid. Porphyry, phanerite and aphanite 

 have been denned already (p. 248). All these names are based on texture, rather 

 than on mineralogical or chemical composition. 



Most igneous rocks are wholly crystalline, and are classified on the basis of 

 their composition. Their chemical composition determines their mineral com- 

 position, and the rocks are named according to the minerals they contain. The 

 number of varieties of igneous rock is very large, but only a few of the more im- 

 portant need be mentioned here. 



The granites. The name granite was originally used to designate a granular 

 rock (a phanerite, p. 248), and it is still popularly and properly so used. In scientific 

 treatises it usually has been confined to a rock composed chiefly of crystals of 

 quartz, feldspar (especially orthoclase) (p. 254), and mica. Recently it has been 

 proposed to give it again a more general application by including under it all 

 phanerites composed chiefly of quartz and feldspar of any kind, with mica, 

 hornblende, or other minerals in subordinate amount. In normal granite, the 

 crystals are distinct and in some cases large (Fig. 237), and more or less intimately 

 interlocked. Granites are among the most common and easily recognized of the 

 phanerites. Their color is determined largely by the feldspar, the red and pink 

 varieties of the mineral giving rise to red and pink granite, and the whitish varie- 

 ties to gray granite. Granites vary widely from their type by the addition and 

 substitution of other minerals. Whenever one of these replacing or accessory 



