EOCKS. 71 



146. Granite. Ordinary granite is a mixture of quartz 

 or silex, mica, and feldspar. There is great variety in 

 this rock, according to the varying proportions of these 

 minerals, and each variety is termed feldspathic, mica- 

 ceous, or quartzose, as one or the other mineral predom- 

 inates. There are other variations also. Sometimes 

 hornblende is present in place of mica, and then the rock 

 is called syenite. This is even more durable than com- 

 mon granite. Its name comes from Syene, in Upper 

 Egypt, the locality from which most of the stones of the 

 ancient Egyptian monuments were obtained. These are 

 not, however, true syenite, but a red granite, containing 

 considerable dark-colored mica. The rock of Mount Si- 

 nai is real syenite. When albite is in the place of feld- 

 spar, the rock is called albite granite ( 129). When 

 talc replaces it, it is called protogine. When the feldspar 

 appears in the granite in large crystals, it is called por- 

 phyritic granite. There is a peculiarity in the crystal- 

 line arrangement of one kind of granite which produces 

 figures over its surface like small Oriental characters, 

 and hence this variety is styled graphic granite. In Fig. 

 25 is given the surface of a slab of this granite, and in 

 Fig. 24 its end. The colors of granite vary much, but it 



Fig. 25. 



is usually grayish, white, or flesh-colored. 

 Rocks of the granite family are widely disseminated in 

 the earth, often forming extensive mountain ranges. In 

 some parts of the Andes granite rises to the height of 

 12,000 feet. In the Alps, the Aiguille de Dree is a solid 

 spire of granite 4000 feet high. There is much granite 



