84 STRUCTURAL GEOLOGY. 



Htdromica Schist or Slate. — Metamorphic. Thin schistose, consisting either 

 chiefly of hydrous mica, or of this mica with more or less quartz ; having the surface 

 nearly smooth ; feeling greasy to the fingers ; pearly to faintly glistening in luster ; 

 whitish, grayish, pale greenish in color, and also of darker shades. This rock used to be 

 called talcose slate, but, as first shown by C. Dewey, it contains no talc. It includes 

 parophite schist, damourite slate, and sericite slate (glanz-schiefer, sericit-schiefcr, and 

 part of the glimmer-schiefer of the Germans). 



Varieties. — a. Ordinary; more or less silvery in luster, b. Chloritic; contains 

 chlorite, or is mixed with chlorite slate, and has therefore spots of olive-green color ; 

 graduates into chlorite slate, c. Garnetiferous. d. Pyritiferous ; contains pyrite in dissemi- 

 nated grains or crystals, e. Magnetitic ; contains disseminated magnetite, f . Quartzytic ; 

 consists largely of quartzyte, or is a quartzyte rendered schistose and partly pearly by the 

 presence of a hydrous mica. Includes the argillyte or clay-slate which has the composi- 

 tion nearly of a hydrous mica, like that of the White Mountam Notch, where much 

 of it is andalusitic. 



Agalmatoltte {Gieseckite, Finite). — Compact; cut with a knife ; composition that 

 of the hydrous mica, damourite. Derived mostly from the alteration of nephelite. — 

 From the Archaean of Lewis County, N.Y. (Dysintrybyte), China, etc. 



Paragonite Schist. — Metamorphic. Consists largely of the hydrous soda mica 

 called paragonite ; but in other characters resembles hydromica slate. 



Felsyte (Euryte, Porphyry, Petrosilex). — Eruptive and metamorphic. Compact 

 orthoclase with often some quartz intimately mixed, flint-like in fracture. Opaque. 

 Colors grayish white to red and brownish red. G = 2 '56-2 -7. 



Varieties. — a. Non-porphyritic ; of various colors, b. Black; rare. c. Porphyritic 

 Felsyte, or Porphyry, Orthophyric ; containing the feldspar in small crystals distributed 

 through the compact base ; color red, and of other shades, d. Q^iartzophyric ; containing 

 quartz in grains ; often called Quartz-porphyi-y. e. Quartzless. f. Spherophyric, the 

 Pyromeride of Corsica. 



PoRCELANTTE OR PORCELAIN Jasper. — Mctamorphic. Baked clay, having the 

 fracture of flint, and a gray to red color : it is somewhat fusible before the blowpipe, and 

 thus differs from jasper. Formed by the baking of clay-beds, when they consist 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. 



MicA-TRACHYTE. — Eruptive. Consisting of orthoclase and black mica, with some 

 orthoclase augite, chrysolite, and glass. Dark grayish green. Mount Catini. 



Trachyte (Sanidin-trachyte). — Eruptive. Ash-gray, brownish, bluish, rarely red- 

 dish. G = 2'6-2-7. Consists mainly of orthoclase, often with disseminated crystals of 

 the glassy tabular variety called sanidin. Named from the Greek for rough, in allusion 

 to the rough surface of fracture. Differs from felsyte in containing some glass, and a 

 rougher surface. Graduates into the following. 



Rhyolyte, Quartz-trachyte. — Eruptive. Like the preceding in colors, but con- 

 taining quartz, and sometimes passing into a coarsely crystallized variety called Nevadyte 

 (from Nevada). Common in the Rocky Mountain region and west of it. Pearlyte and 

 Lithoidyte are more or less glassy varieties — between glass and stone ; and pitchstone is 

 another similar variety, pitch-like in luster. These graduate into the following. 



Obsidian {Volcanic glass). — Eruptive. A true volcanic glass, but more or less 

 microlitic. Colors grayish black, gray, purplish to red, brown. Sometimes orthophyric ; 

 often contains spherulites, which are 70-75 per cent silica. Pumice is a scoriaceous 

 variety with linear cells. Constitutes a high bluff in the northwest part of the Yellowstone 

 Park, north of Beaver Lake, which has a top of pumice, and also a large area east of the 

 bluff ; cavities in Obsidian bluff often lined with crystals of sanidin, tridymite, quartz, and 

 sometimes of fayalite. 



