28 



GEOLOGY 



of their composition. Their chemical composition determines their mineral 

 composition, and the naming of the rocks is based on the minerals they con- 

 tain. It will be seen, however, that the chemical composition is the more 

 fundamental. The number of varieties of igneous rock is very large, but 

 only a few of the more important need be mentioned here. 



The granites. This name 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 rock composed 



*^ 





Fig. 12. Graphic granitic (or pegmatitic) texture. Nearly natural size, 

 (Photo, by Church.) 



chiefly of crystals of quartz, feldspar (especially orthoclase, (p. 79), and 

 mica. It has recently been proposed to give it again a more general appli- 

 cation, though not quite its original one, by including under il all phanerites 

 (p. 59) 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 sometimes large (Fig. 9). They are intimately 

 mingled with one another, and in growing, interfered more or le>-; with each 

 other and so became interlocked. The granites are among the most common 

 and easily recognized of the phanerites. Their color is determined largely 



