76 



LITHOLOGICAL GEOLOGY. 



is usually grayish white, glassy, and without any appearance of cleavage ; 

 the feldspar is commonly whitish or flesh-colored, less glassy than 

 the quartz, and showing a flat, polished cleavage-surface in one or 

 two directions. 



a. Common Granite. — A granite in which the feldspar is common feldspar 

 (the species orthoclase, $ 55), or potash-feldspar, the most common kind. The 

 color is grayish or flesh-colored, according as this feldspar is white or reddish. 

 The texture varies from a fine and even-grained to a coarse granite in which the 

 mica, feldspar, and quartz — especially the two former — are in large crystalline 

 masses. Porphyritic granite has the feldspar distributed in distinct crystals, 

 which appear as rectangular whitish blotches on a surface of fracture. Syenitic 

 granite contains black scales or grains of hornblende besides the mica. Miascite 

 consists of cleavable white feldspar (orthoclase), black mica, and grayish or 

 yellowish-white elseolite, with some hornblende, and occasionally albite or quartz. 

 It is associated with syenite. 



b. Albitic Granite. — A granite in which part or all of the feldspar is albite, or 

 soda-feldspar ($ 55, b). This feldspar is usually white, and when both albite 

 and orthoclase are present, the latter may often be distinguished by having a 

 more grayish or reddish color, and by not having the cleavage-surface finely 

 striated in one direction, like albite. 



(2.) Pegmatite, or Graphic Granite. — A very coarse granitic rock, consisting 



Fig. 59. 



©f common feldspar and quartz with 

 but little whitish mica; in the graphic 

 variety the quartz is distributed 

 through the feldspar in forms look- 

 ing like Oriental characters (fig. 59). 

 (3.) Granulite. — A fine-grained 

 granitic rock, consisting mainly of 

 granular feldspar with little quartz, 

 and often imperfectly schistose in 

 structure from the arrangement of 

 the quartz. It is also called eurite 

 and leptynite ; and the flinty kind, petrosilex. (See beyond, $ 85.) 



(4.) Gneiss. — Like granite, but with the mica more or less dis- 

 tinctly in layers. A gneissoid granite is a rock intermediate between 

 granite and gneiss, or showing some tendency to the gneiss struc- 

 ture. As the mica is in scales as well as easily cleavable, a gneiss 

 rock breaks most readily in the direction of the mica layers, — 

 'thus affording slabs. This structure, causing a tendency to break 

 into slabs, is called a schistose structure. 



Porphyritic Gneiss has distinct feldspar crystals disseminated through it, like 

 porphyritic granite. Gneiss may abound in garnets, or be gametiferous ; or 

 contain an excess of mica, when it is called micaceous gneiss; or much epidote, 

 becoming an epidotic gneiss. Gneiss graduates into — 



(5.) Mica Schist. — The same constituents as granite and gneiss, 



