KINDS OF ROCKS. 75 



quartzite, hard, slates like roofing-slate, and partially crystallized 

 limestones, at the other ; so that a distinct line between them and 

 the sedimentary beds cannot always be drawn. They are some- 

 times called plutonic rocks, to distinguish them from the true 

 igneous rocks. 



The common ingredients are quartz, feldspar of different kinds, 

 mica, hornblende, pyroxene, talc, epidote, chlorite, serpentine; to which 

 garnet, andalusite, staurotide, tourmaline, topaz, graphite, may be added as 

 characterizing a number of varieties. The rocks are aggregates in 

 general of two or more of the above-mentioned minerals ; and, as 

 the proportions may vary indefinitely, the kinds of rocks are not 

 in air cases well defined ; they graduate into one another through 

 imperceptible shades. 



Metamorphic rocks may, for the most part, be distributed into 

 three series parallel with one another. These are the mica-bearing 

 series, containing granite, gneiss, mica schist, etc. ; the hornblendic, 

 characterized by the presence of hornblende or the allied pyroxene, 

 as in syenite, hornblendic gneiss, etc. ; and the hydrous magnesian 

 series, containing talc, chlorite, and serpentine rocks. Besides 

 these, there are other groups, which with the foregoing are de- 

 scribed beyond in the following order : — 



1. Mica-bearing series. 



2. Hornblendic series. 



3. Feldspathic, epidotic, and garnet rocks, having the mass or body of the 

 rock compact (cryptocrystalline). 



4. Hydrous magnesian series. 



5. Hydrous aluminous series, or rocks consisting essentially of agalmatolite 

 or pyrophyllite. 



6. Quartz rocks. 



7. Iron-ore rocks. 



1. The Mica-bearing Series. 



The mica-bearing series commences with granite, the most highly 

 crystalline, and descends through gneiss and mica schist to argillite or 

 roofing-slate, and also to quartzite, which is but little removed from a 

 sandstone. Quartz is a constant ingredient, as well as mica. The 

 series branches off into crystalline feldspathic rocks like granulite, 

 containing little or no mica. The specific gravity is between 2.4 

 and 2.8. 



83. (1.) Granite. — A granular crystalline rock, consisting of 

 quartz, feldspar, and mica, having no appearance of layers in the 

 arrangement of the mica or other ingredients. The mica is in 

 scales, usually white, black, or brownish, easily separable into 

 thinner elastic scales by means of the point of a knife ; the quartz 



