LESSONS IN GEOLOGY. 323 



tion. Felsite consists of the ground mass without the crystals, 

 forming a tough compact rock. 



Liparite has a similar composition but shows more evidence 

 of a former molten or glassy state in its structure. 



Syenite, a rock often associated with granite, is composed of 

 orthoclase mixed with hornblende and occasionally some quartz 

 and mica, and sometimes triclinic feldspar occurs. There is sel- 

 dom any difficulty in identifying this rock after one learns to 

 recognize feldspar and hornblende. 



Trachyte is a volcanic rock composed largely of sanidine, a 

 variety of orthoclase, with some triclinic feldspar, and usually 

 hornblende, mica and iron, and sometimes with pyroxene, apa- 

 tite, etc. It has much the same composition as syenite but has 

 not become fully crystallized. 



Obsidian is a volcanic glass, varying considerably in its com- 

 position, but in general it is rather like trachyte. It shows no 

 trace of crystallization and may represent the first stage in the 

 development of crystalline rocks, trachyte and syenite represent- 

 ing others. 



Diorite, or Green Stone, is composed of triclinic feldspar and 

 hornblende usually with apatite and iron. 



Basalt is a dark-colored volcanic rock, composed of triclinic 

 feldspar, pyroxene, olivine, iron and apatite. It often has a 

 ground mass of glass through which crystals are scattered. 



Schists are crystalline rocks of varied composition in which 

 the materials are arranged in nearly parallel layers. Schist rocks 

 may be composed of but one mineral, but they usually consist of 

 two or more minerals in alternate layers or mingled in the same 

 layer. They are usually named from the principal ingredient, as, 

 mica schist, chlorite schist, and talcose schist, usually containing 

 quartz, feldspar and other minerals besides the one naming the 

 rock. 



Gniess. This rock is composed of quartz, feldspar and mica 

 but the proportions and the arrangements are different from 

 those in granite. It is schistose in structure but coarser than 

 schist. There is a larger proportion of mica than in granite. It 



