SYENITE FAMILY 1 99 



II. The Syenite Family 



In this family the magma much resembles that of the granite 

 group, except that the quantity of silica is less (55 to 65 %); hence 

 it is nearly or quite taken up in the formation of silicates, leaving 

 little or none to crystallize out separately as quartz, and orthoclase 

 is thus the chief mineral. The two families are connected by 

 many transitional rocks. 



Syenite obsidian is indistinguishable, except by chemical analy- 

 sis, from the glasses of the preceding family, but it is much less 

 common. 



Trachyte is a volcanic rock, consisting of phenocrysts of sani- 

 dine in a ground mass of minute felspar crystals, but having little 

 or no glass, together with more or less biotite, amphibole, or 

 pyroxene, according to which we get the varieties mica, a?nphi- 

 bole, or pyroxe?ie trachyte. In America the trachytes are very 

 much less abundant than the rhyolites. 



Phonolite differs from trachyte in the higher percentage of 

 soda which it contains, and in the presence of the felspathoid 

 nepheline or leucite, or both. The name is derived from the 

 ringing sound which thin plates of the rock give out when struck 

 with a hammer. Phonolites are quite rare rocks, and in this 

 country the best-known locality for them is the Black Hills region 

 of South Dakota. 



Syenite is a thoroughly crystalline rock, without ground mass, 

 and much resembling granite in appearance, but having no quartz. 

 It is composed typically of orthoclase and hornblende, with plagio- 

 clase, apatite, and magnetite as accessories. When the hornblende 

 is replaced by biotite, the rock is called mica syenite, and when by 

 augite, augite syenite. The name syenite is sometimes given to 

 the rock we have called "hornblende granite " (p. 198). 



Nepheline Syenite is marked by the presence of nepheline, and 

 bears the same relation to phonolite as syenite does to trachyte, 

 being the granitoid crystallization of the same magma. 



The syenites occur just as do the granites, but are not nearly 

 so frequent. 



