KINDS OF ROCKS. 71 



2. Itacolumyte. — Schistose, consisting of quartz grains with some hydrous mica. 

 On account of the mica, it is sometimes flexible, and is called flexible sandstone. 



3. Siliceous Slate. — Schistose, flinty, not distinctly granular in texture. Some- 

 times passes into mica slate or schist. 



4. Chert. — An impure flint or hornstone occurring in beds or nodules in some strati- 

 fied rocks. It often resembles felsyte, but is infusible. Colors various. Sometimes 

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



5. Jaspek Rock. — A flinty siliceous rock, of dull red, yellow, or green color, or some 

 other dark shade, breaking with a smooth surface like flint. It consists of quartz, with 

 more or less clay andoxyd of iron. The red contains the oxyd of iron in an anhy- 

 drous state, the yellow in a hydrous; on heating the latter it turns red. 



6. Buhhstone. — A cellular siliceous rock, flinty in texture. It is used for mill- 

 stones. Found mostly in connection with Tertiary rocks, and formed apparently from 

 the action of siliceous solutions on preexisting fossiliferous beds. 



7. Fioryte (Siliceous Sinter, Pearl Sinter, Geyserite). — Opal-silica, in compact, 

 porous, or concretionary forms, often pearly in lustre ; 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, and about the Iceland geysers; after long exposure it crumbles 

 down and becomes changed to ordinary silica or quartz. 



2. The Mica- Orthoclase Series, or Granite Group. 



In this series the chief constituents are quartz, orthoclase (and microcline),or potash- 

 feldspar, and the two potash-micas, muscovite and biotite. The presence of much 

 potash is a marked characteristic, each of the micas containing 10 per cent, or more of 

 it, and the feldspar usually over 12. The two micas often occur together; the black, or 

 biotite, is most common. The mica is sometimes a hydrous species — margarodite or 

 damourite. The series graduates into feldspathic rocks, like granulyte, containing little 

 or no mica, and into rocks containing little besides mica. Specific gravity between 2*4 

 and 275. Accessory minerals: albite and oligoclase, hornblende, garnet, etc. Occa- 

 sionally, especially in vein granite, the orthoclase is nearly or wholly replaced by one 

 of the triclinic feldspars. Sometimes the iron mica, lepidomelane, replaces biotite. 



1. Granite. — Consists of quartz, feldspar, and mica, and has no appearance of lay- 

 ers in the arrangement of the mica or other ingredients. The quartz is usually grayish 

 or smoky white, glassy, and without any appearance of cleavage. The feldspar is com- 

 monly whitish or flesh-colored, less glassy than the quartz, and cleaves in two direc- 

 tions. The mica is 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, Biotite-granites; C, Muscovite- 

 and-biotite granites, the last much the most common. The most of the following va- 

 rieties occur under each except the hornblendic, which is usually a Biotite or Muscovite- 

 and-biotite granite. There is also, D, Hy- 

 dromica-granite. a. Common or ordinary 

 granite. The color is grayish or flesh-col- 

 ored, 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 some- 

 times the mica, feldspar, and quartz — es- 

 pecially the two former — are in large crys- 

 talline masses. An average granite (mean of 

 11 analyses of Leinster granite, by Haugh- 

 ton) consists of Silica 72*07, alumina 14*81, 



protoxyd and sesquioxyd of iron 2*52, lime 1-63, magnesia 0*33, potash 5*11, soda 2*79, 

 water 1*09 = 10035. b. Porpkyritic granite ; has the orthoclase in defined crystals, and 



