68 LITHOLOGICAL GEOLOGY. 



Metamorphic granite is common in Connecticut and other parts of 

 New England, where gneiss may be often seen graduating into granite, 

 or in alternating layers with it. 



a. Common Granite. — A granite in which the feldspar is chief)}' orthoclase or 

 potash feldspar, the most common kind ; oligoclase also is often present. The color 

 is grayish or flesh-colored, according as the 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. There are 

 often two kinds of mica present, a light-colored (muscovite or else margarodite), and 

 a black (biotite, sometimes lepidomelane). An average granite Uriean of 11 analyses 

 of Leinster granite, by Haughton), consists of — Silica 72"07, alumina 14-81, protoxyd 

 and sesquioxyd of iron 2 "52, lime l - 63, magnesia 0"33, potash 5 "11, soda 2-79, water 

 1-09 = 100-35. 



b. Porphyritic Granite has the feldspar distributed in distinct crystals, which appear 

 as rectangular whitish blotches on a surface of fracture. Hornblendic Granite contains 

 black scales or grains of hornblende besides the mica. 



c. Albitic Granite contains albite in place of part of the orthoclase ; and, in — 



d. Oligoclase Granite (a much more common kind), oligoclase replaces part of the 

 orthoclase. 



2. Pegjiatyte, or Graphic Granite. — A very coarse granitic rock, consisting of 

 common feldspar and quartz, with but lit- -p. .„ 



tie whitish mica ; in the graphic variety, -■— ^~r — -— 



the quartz is distributed through the f eld- /^\. I' A^k^i'v 

 spar in forms looking like Oriental char- $ A 



acters (Fig. 59). f > ^ \ 



3. Gkanulyte. — A fine-grained gran- l\ n fo> 

 itic rock, consisting mainly of granular J* \^ \ ^ <f\ ^ ^ a ^ \ 

 feldspar with little quartz, and often im- < n ^ 1L . W / } J ^ J\ 



perfectly schistose in structure, from the i a l£k > /x L^ tt^ -ffl \ 



arrangement of the quartz. It is also r*T" k^ \ jf_ ^ >>A A j) 



called" Euryte and Leptynyte ; and the \ — 2. \ "^ ^ ^ ~ ^ ^^ 



flinty kind, Petrosilex or Felsyte. (See beyond, p. 71.) 



(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. Gneiss breaks most readily in the direction of 

 the mica layers, and thus affords slabs, or is schistose in structure. 



Porphyritic Gneiss has distinct feldspar crystals disseminated through it, like por- 

 phyritic granite. Gneiss may abound in garnets, or be garnetiferous ; 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, 

 but with more quartz, less feldspar, and much more mica — therefore 

 glistening in lustre ; slaty, or very schistose, in structure, breaking 

 into thin slabs ; often friable, or wearing easily. 



Mica schist often abounds in garnets and staurolite, and sometimes in 

 tourmaline. It passes at times into hornblende schist. 



The variety plumbaginous schist contains plumbago (p. 59) in its layers. Calcareous 

 mica schist contains, disseminated through it, beds of carbonate of lime or calcite. 

 Hornblendic, anthophyllitic, and concretionary varieties occur. 



