28 PHYSICAL GEOLOGY 



than do those containing much silica. Hence the former 

 lavas may produce coarse-grained rocks under conditions 

 where the latter would give fine-grained ones. 



We are now prepared to describe a few of the more impor- 

 tant igneous rocks. The different kinds grade into each other 

 without hard and fast lines. l 



Distinctly grained rocks. These rocks have a solid tex- 

 ture, are wholly crystalline, and the grains can be distinguished 

 with the unaided eye. The grains may be of uniform size 

 (large, medium, or small), or large crystals may be scattered 

 through a ground mass of smaller ones. In the latter case, the 

 rock is called porphyry, and is said to have a porphyritic texture. 

 These terms are applied also to rocks in which distinct crys- 

 tals are scattered through a glassy or stony ground mass 

 (Fig. 8). One way in which porphyritic texture develops has 

 been explained (p. 26). The distinctly grained rocks, 

 whether porphyritic or nonporphyritic, may be further classi- 

 fied on the basis of the minerals they contain. While there are 

 a great many kinds, only a very few of the more important 

 ones can be described here. All the different varieties shade 

 gradually into one another. 



Granite (Fig. 7) is perhaps the most common of the dis- 

 tinctly grained rocks. It always contains feldspar (as the 

 predominant mineral) and quartz, and frequently has subordi- 

 nate amounts of other minerals, especially mica. Descriptive 

 names are often employed, which indicate the leading sec- 

 ondary minerals ; thus one may speak of a mica-hornblende 

 granite. Granites have different colors, depending largely on 

 that of the feldspar, and on the abundance of dark minerals. 

 Gray and red varieties are especially common. Granite is an 

 acidic rock. Large crystals of feldspar (less often quartz) 

 may be scattered through a granitic ground mass of smaller 

 (but distinguishable) grains, giving a granite-porphyry. 



1 For this reason there is no general agreement concerning the classifica- 

 tion of igneous rocks. The classification used here differs from that employed 

 in many other books. 



