82 STRUCTURAL GEOLOGY. 



6. Soda-liine-Feldspar Series, with or without Ilornbleude or Pyroxene, — the feldspar 

 a triclinic species of the series from albite to anorthite. 



III. Saussurite kocks. — Alkali (soda) bearing, but containing saussurite in place 

 of a feldspar. 



IV. Without Feldspar, or avith very little. 



1. Garnet, Epidote, and Tourmaline Rocks. 



2. Hornblende, Pyroxene, and Chrysolite Rocks. 



V. Hydrous magnesian and aluminous rocks. 



I. Siliceous Rocks. 



Quartzyte, Granular Quartz. — A siliceous sandstone, usually very firm, occurring 

 in regions of metamorphic rocks. It does not differ essentially from the harder siliceous 

 sandstones of other regions. Conglomerate beds are sometimes included. 



Varieties. — a. Massive, b. Schistose, c. 3Iicaceous ("Greisen"). d. Hydromica- 

 ceous. e. Feldspathic, and sometimes Porphyritic (then called by some, ArJcose^. 

 f. Friable, g. Flexible {Itacolumyte) . h. Andalusitic. i. Ottrelitic. j. TourmaliiiiCy 

 containing tourmaline, k. Cineissic, it occasionally graduating into gneiss. 



Siliceous Slate (Phthanyte). — Schistose, flinty, not distinctly granular in texture. 

 Sometimes passes into mica or hydromica schist. 



Chert. — An impure flint or hornstone occurring in beds or nodules in some stratified 

 rocks. It often resembles /eZsy^e, but is infusible. Colors various. Sometimes oolitic. 

 Kinds containing iron oxide graduate into jasper and clay-ironstone. 



Jasper Rock. — Dull red, yellow, brown, or green, or of some other dark shade, 

 breaking with a smooth surface like flint. Consists of quartz, with more or less clay and 

 oxide of iron. The red contains the oxide of iron in an anhydrous state, the yellow in a 

 hydrous ; on heating the latter, it turns red. 



Buhrstone. — A cellular siliceous rock, flinty in texture. Used for millstones. 

 Pound mostly in connection with Tertiary rocks, and formed apparently from the action 

 of siliceous solutions removing fossils and so making the cavities. The best is from near 

 Paris, Prance. 



PiORYTE (Siliceous Sinter, Pearl Sinter, Geyserite). — Opal-silica, in compact, porous, 

 or concretionary forms, often pearly in luster ; made by deposition from hot siliceous 

 waters, as about geysers {geyserite), or through the decomposition of siliceous minerals, 

 especially about the fumaroles of volcanic regions. Geyserite is abundant in Yellowstone 

 Park, about the Iceland geysers, and in the New Zealand geyser region. 



II. Rocks having Alkali-bearing Minerals as Chief Constituents. 

 1. The Potash-feldspar and Mica Series. 



Granite. — Metamorphic and eruptive. Consists of feldspar, mica, and quartz ; has 

 no appearance of layers in the arrangement of the mica or other ingredients. The 

 quartz usually grayish or smoky, glassy, and without any appearance of cleavage. The 

 feldspar commonly whitish or flesh-colored, less glassy than the quartz, and cleavable in 

 two directions ; the mica in very cleavable scales. 



Metamorphic granite is common in Connecticut and other parts of New England, 

 where it may be often seen graduating into gneiss, or in alternating layers with it. 



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

 C, Biotite-granites ; D, Hydromica granite. The most of the following varieties occur 

 under each except the hornblendic, which is usually a Biotite or Muscovite and Biotite 

 granite, a. Common or ordinary granite. Color, grayish or flesh-colored, according as 

 the feldspar is white or reddish, and dark gray when much black mica is present. 

 Varies in texture from fine and even to coarse ; sometimes the mica, feldspar, and quartz 



