442 DESCRIPTIONS OF ROCKS. 



much of it is andalusitlc. Analysis of this White Mountain 

 rock, by Hawes, afforded Silica "46 '01, alumina 30*56, iron 

 sesquioxide 1*44, iron protoxide 6*85, manganese protoxide 

 0-10, magnesia 1*42, soda 1*12, potash 6'66, titanium dioxide 

 1*91, water 4*13 = 100\22. (Compare with analyses of hy- 

 drous muscovite, or margarodite.) Metamorphic. 



11. Felsyte. Quartz-Felsyte. (Eiiryte, Petrosilex.) — Com- 

 pact orthoclase, with often some quartz intimately mixed; 

 fine granular to flint-like in fracture; sometimes contains 

 oligoclase. Colors white, grayish-white, red, brownish-red 

 to black. G-. = 2*6-2*7. Both metamorphic and eruptive. 



Varieties. — There are two sections, I. Felsyte, and II. Quartz- 

 Felsyte, and under each occur the following varieties, a. Porphyritic 

 Felsyte, or Porphyry ; containing the feldspar in small crystals distri- 

 buted through the compact base ; color red and of other shades ; called 

 sometimes Quartz-porphyry , when the base is a quartz-felsyte. b. Con- 

 glomerate felsyte; containing pebbles, as at Marblehead, Mass., and in 

 the White Mountains, c. Oligoclase-oearing ; containing this triclinic 

 feldspar intimately blended with the orthoclase. d. Cellular or amyg- 

 daloidal. e. Ehanyte ; essentially a quartzose felsyte, of gray, bluish- 

 gray to brown and red colors, and often containing disseminated grains 

 or crystals of quartz and feldspar, and some oligoclase ; some compact 

 slate-rock has the same composit on. Occurs in Cornwall. 



The metamorphic and eruptive kinds are not easily distinguished. 

 The former occurs associated with sedimentary strata, and often con- 

 tains pebbles or other evidence of fragmental origin ; while the lat- 

 ter is frequently in dikes, that is, fills the fissures through which it 

 was ejected. Some of the eruptive felsyte has nearly the aspect of 

 trachyte, with which rock it is identical in composition. Much of the 

 red porphyry contains hornblende with the feldspar of the base, and 

 has the constitution of dioryte (p. 447). 



12. Porcelanyte. {Porcelain Jasper.) — A baked clay, hav- 

 ing the fracture of flint, and a gray to red color: it is some- 

 what fusible before the blowpipe, and thus differs from 

 jasper. Formed by the baking of clay-beds, when they con- 

 sist largely of feldspar. Such clay-beds are sometimes baked 

 to a distance of thirty or forty rods from a trap dike, and 

 over large surfaces by burning coal beds. Metamorphic. 



13. Trachyte. Quartz-Trachyte. —Consists mainly of feld- 

 spar, which is partly in glassy crystals, either sanidin or 

 oligoclase ; and, owing to the angular forms of the glassy 

 feldspar and the porosity of the rock, the surface of frac- 

 ture is rough, whence the name froni the Greek traclius, 

 rough. Sometimes contains disseminated quartz, and is 

 then quartz-trachyte. Color ash-gray, greenish-gray, brown- 

 ish-gray, but sometimes yellowish and reddish. G. =2*5- 

 2'7. Besides the feldspar there are distributed, somewhat 



