198 IGNEOUS ROCKS 



varying proportion of glass. Other names used for rhyolite are 

 liparite and quartz trachyte. The rhyolites are exceedingly com- 

 mon in the western part of the United States. The Felsites are 

 very dense, fine-grained, and light-coloured rocks, in which pheno- 

 crysts are absent or scanty ; they are ancient rocks which have 

 been formed in different ways, by the devitrification of obsidians 

 and rhyolites, by the recrystallization of tuffs, and by original cool- 

 ing from fusion. 



Quartz Porphyry shades imperceptibly into rhyolite or felsite 

 on the one hand, and into granite on the other ; it is made up 

 of phenocrysts of quartz, or of quartz and orthoclase, in a finely 

 crystalline ground mass of the same minerals. If the phenocrysts 

 are all of orthoclase, the rock is called a felspar porphyry. The 

 difference from rhyolite consists in the greater compactness and 

 density of the rock and the absence of glassy ground mass. 



Granite. — The granites are thoroughly crystalline rocks, of 

 typically granitoid texture, to which they have given the name, 

 and without any ground mass. The grains have not their proper 

 crystalline shape, the separate minerals interfering with each other 

 in the process of crystallization. The characteristic minerals are 

 quartz, orthoclase, some acid plagioclase, muscovite, and biotite ; 

 magnetite and apatite are always present, though in small quan- 

 tities. The variations in granite are principally in the ferro- 

 magnesian minerals. Thus we have muscovite granite, with white 

 mica only ; granitile, with biotite only ; hornblende granite, the 

 hornblende replacing the mica, or in addition to biotite ; angite 

 granite, with augite and biotite. Those in which the percentage 

 of soda is high are called soda-granites. When the dark silicates 

 and mica are all absent, the rock is called a binary granite. 



The colour of granite is dark or light in accordance with the 

 proportion of dark silicates present, while the shades of the felspar 

 determine whether the rock shall be red, pink, or white. The 

 texture of granite varies from fine to very coarse, and in some 

 cases there is found a ground mass of ordinary granite, in which 

 are embedded very large crystals of orthoclase ; this is sometimes 

 called a granite porphyry. 



