438 DESCRIPTIONS OF ROCKS. 



of cleavage. The feldspar is commonly whitish or flesh- 

 colored, and may be distinguished from the quartz by its 

 cleavage surfaces, which reflect light brilliantly when the 

 specimen is held in the sunlight. The mica is usually in 

 small bright scales, either silvery, brownish-black, or black 

 in color, and the point of a knife carefully used will easily 

 split them into thinner scales ; the silvery mica is muscovite, 

 but sometimes of the allied hydrous kinds, margarodite or 

 damourite, and the black mica is usually biotite, though 

 occasionally the allied, more iron-bearing, species, lepidome- 

 lane. Oligoclase or albite is very often present. 



Occurs both metamorphic and eruptive. Metamorphie 

 granite may often be seen graduating into gneiss, or lying 

 in beds alternating with gneiss. 



Varieties. — There are, A. Muscovite granites ; B. Biotite granites ; 

 C. Muscovite-and- biotite granites, the last much the most common. 

 The most of the following varieties occur under each except the horn- 

 blendic, which is usually a Biotite, or Muscovite-and-Biotite, granite. 

 There is also, D. Hydromica-granite. a. Common or Ordinary granite ; 

 the color is grayish or flesh-colored, according as the feldspar is white 

 or reddish, and dark gray when much black mica is present. Granite 

 varies in texture from fine and even, to coarse ; and sometimes the 

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

 crystalline masses. An average granite (mean of 11 analyses of Lein- 

 ster granite, by Haughton) affords Silica ?2"07, alumina 14-81, iron 

 protoxide and sesquioxide 2'52, lime 1*63, magnesia 0'33, potash 5-11, 

 soda 2*79, water 1-09 =100 -35. b. Porphyrinic granite ; has the ortho- 

 clase in defined crystals, and maybe yd) small porphyritic, or (fj) large 

 porphyritic, and have the base (y) coarse granular, or (8) fine, and 

 even subaphanitic. c. Albitic granite; contains some albite, which is 

 usually white, d. Oligoclase granite (Miarolite) ; contains much oligo- 

 clase. e. Microcline granite ; contains the potash triclinic feldspar, 

 microcline. f. Hornblendic granite; contains black or greenish-black 

 hornblende, along with the other constituents of granite, g. Black 

 micaceous granite ; consists largely of mica, with defined crystals 

 of feldspar (porphyritic), and but little quartz, h. lolitic ; con- 

 taining iolite. i. Globv.lifc rons granite; contains concretions which 

 consist of mica, or of feldspar and mica. j. Gneissoid granite; a 

 granite in which there are traces of stratification ; graduates into 

 gneiss, k. Pcgmatyte, or Graphic granite; consists mainly of ortho- 

 clase and quartz, with but little mica ; but the quartz is distributed 

 through the feldspar in forms locking like oriental characters. 



A porphyritic granite, occurring at the junction of the andalusite 

 mica-argillyte (page 441) of the White Mountain Xotch, X. H., with 

 the Mt. Willard granite, on the west side of Mt. Willard, conformable 

 with the bedding of the argillyte, has the argillyte for its base ; and 

 in it the orthoclase is in large well-defined crystals, and the quartz 

 in double six-sided pyramids, both easily separable from the matrix ; 

 the layer is six to twentv feet thick. 



