KINDS OF ROCKS. 77 



are spotted with feldspar crystals, and the mass looks homogeneous until closely ex- 

 amined. There is, besides, a true feldspar-porphyry, of igneous origin, differing little 

 in composition from much trachyte, but having a very compact texture and smooth 

 surface of fracture. 



(4.) Phonolyte (Clinkstone). — Compact, of grayish blue and other shades of color, 

 more or less schistose or slaty in structure; tough, and usually clinking under the ham- 

 mer like metal when struck, whence the name. Sp. gr. 2-4-2-6. Consists of glassy 

 feldspar (orthoclase or oligoclase), with nephelite and hornblende; G. Jenzsch gives, 

 for the composition of the Bohemian phonolj-te, — Sanidin (glassy orthoclase) 53-55, 

 nephelite 31-76, hornblende 9-34, sphene 3'67, pyrite 0-04. Under treatment with acids, 

 the nephelite is dissolved out. Zeolites, according to the later examinations, are not an 

 original constituent of the rock. 



(5.) Trachyte. —Color, pale grayish blue, rarely greenish, whitish, yellowish, red- 

 dish; texture peculiarly rough to the feel, and usually porous, owing to the angular 

 form of the particles. Often contains disseminated crystals of glassy feldspar (sanidin) 

 and hornblende, also mica and magnetite. G. = 2-5-2*7. Silica usually 60 to 65 per 

 cent. Decomposed by the action of muriatic acid, into a soluble and an insoluble sili- 

 cate, the former in less proportion than in clinkstone, or 10 to 14 per cent. Com- 

 position of the whole (from Drachenfels), according to Abich, -Silica 67-09, alumina 

 15-64, potash 3-47, soda 5-08, lime 2-25, oxyds of iron 4-59, magnesia 0-98, protoxyd 

 of manganese 0-15, titanic acid - 38, water, etc. 0-45. 



Trachytes sometimes contain also free quartz, and are then called quartz-trachytes, 

 in which the silica amounts to 70 per cent, or more. The feldspar in trachytic rocks 

 may be either of the species, and thus there are as many varieties of trachyte. There is 

 wide variation also in texture, from a porous pumice-like trachyte, through the usual 

 rough granular forms, to a gray syenyte-like trachyte, consisting of glassy feldspar and 

 hornblende crystals with some mica ; and also to porphyritic trachyte and feldspar- 

 porphyry. 



(6. ) Rhyolyte. A feldspathic rock containing more or less free silica, but undistin- 

 guishable by the eye. The paste is white, gray, yellow, green, red, or brown in color, 

 usually of light shades. Texture glassy to pearly; passes into lithoid and micro-crys- 

 talline kinds, which are quartz-trachytes. Contains sometimes disseminated crystals of 

 glassy feldspar. Obsidian in part, pearlstone, and pumice are here included. 



(a.) Pumice. — Very light, porous, with the pores minute, capillary, and parallel. 

 Color, oale grayish, greenish, yellowish, and sometimes of darker shades. It is a kind 

 of porous trachyte. Contains 69 to 70 per cent, of silica, and probably, therefore, 

 some free quartz. Often contains glassy feldspar, and sometimes hornblende, mica, 

 leucite. 



(b.) Obsidian. — A volcanic glass, taking its characters from the composition of the 

 volcanic lavas. The lavas cooling slowly form stony lava, and those cooling rapidly a 

 glassy, — the two being different conditions of the same substance. 



Spherulitic obsidian contains small feldspathic concretions. 



(c.) Pearlstone. — Near pitchstone, but less glassy and more pearly in lustre: 

 usaully grayish in color, also yellowish, brownish, and reddish. The peculiar pearly 

 appearance is due to an intimate mixture of a portion of the rock in the glassy state 

 with another larger portion in the stony state. It often contains spherical concretions, 

 called spherulites, which consist of feldspar with an excess of silica. The silica varies 

 from 68 to 80 per cent. 



(d.) Pitchstone (Retinyte). — An imperfectly-glassy volcanic rock, pitch-like in 

 appearance, and of various colors from graj* to black, through greenish, reddish, and 

 brownish shades. It contains 70 to 73 per cent, of silica, and, in some of the published 

 analyses, 8 to 10 per cent, of water. It is partly Rhyolyte, like the preceding. 



2. Hornblende-and-Pyroxene series. 



This series includes three sections. (1.) The Syenytic, comprising syenyte, hypo- 

 syenyte and dioryte, which also occur as metamorphic rocks; (2.) The basaltic, includ- 

 ing rnelaphyres, doleryte, and peridotyte, to which three rocks the name basalt was 



