In this family the magma much resembles that of the granite group, except that the quantity of silica is less (50 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 analysis, 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, amphibole, or pyroxene 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".
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 50 frequent.