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PHYSICAL GEOLOGY 



Classification of Igneous Rocks 



/. Coarse-grained Igneous Rocks 



The rocks included in this group are those whose mineral grains 

 are approximately of equal size and are large enough to be distinctly 

 seen. 



, Granite. — Granites are composed of quartz, feldspar, and usu- 

 ally of smaller amounts of either mica or hornblende. The grains 

 of feldspar are usually easily distinguishable because of their shiny 

 (cleavage) surfaces and their opaque white, gray, or red color. The 

 quartz grains vary in tint from colorlessness to smoky gray, and 

 can usually be recognized by their glassy luster and irregular frac- 

 ture. Mica may be either muscovite or biotite, and may be told by 

 its brilliant cleavage surface. The thin leaves, unless too small, 

 can be easily separated with the point of a penknife. Hornblende 

 occurs in green to black opaque grains or needles. Other minerals, 

 such as pyrite or garnet and other less common minerals, may also 

 be present. 



Numerous names are given to granites, some of them (commer- 

 cial) depending upon their color and their desirability for building or 

 monumental purposes, such as red, gray, yellow; while others are 

 locality names. The color of the stone depends largely upon that of 

 the feldspar, and upon the relative abundance of dark minerals. A 

 red granite owes its color to its red feldspar; a gray color may be due 

 either to the color of the feldspar alone, or to the combination of 

 black hornblende or mica, and white feldspar. 



Syenite is a rock which may be described as a granite without 

 quartz. It very closely resembles a granite, and is usually sold 

 under the latter name. 



Diorite is a granular igneous rock composed of hornblende and 

 feldspar of any kind, in which the amount of hornblende usually 

 exceeds that of the feldspar, although the two may be in equal 

 amounts. Its color is dark gray or greenish. 



Gabbro is made up of pyroxene (augite), with usually smaller 

 amounts of feldspar of any kind. Gabbro and diorite are often dis- 

 tinguished with difficulty, because of the similarity of hornblende 

 and augite to the naked eye. 



Peridotite is a dark green to black rock, composed chiefly of such 

 minerals as hornblende, olivine, and pyroxene (augite). 



