88 LITHOLOGICAL GEOLOGY. 



titanic acid, 0.38, water, etc., 0.45. In others, silica forms 61 to 67 per cent, 

 of the whole. 



Trachytic lava. — A very cellular trachyte. 



Dotnite. — An earthy friable trachyte, from Puy de Dome, Auvergne. 



Slaty trachyte. — Structure schistose. 



Porphyritic trachyte. — Containing disseminated crystals of glassy feldspar, 

 often without any hornblende. 



Granitoid trachyte. — A granular aggregate of glassy feldspar, hornblende, and 

 mica. Approaches diorite. 



Homblendic trachyte. — Containing much hornblende in disseminated crystals. 



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

 parallel. Color, pale 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. Composition, according to 

 Berthier, — Silica, 70.0, alumina, 16.0, sesquioxyd of iron, 0.5, lime, 2.5, pot- 

 ash, 6.5, water, 3.00 = 98.50. Abich obtained 6.21 per cent, of soda, and 3.98 

 of potash. 



Often contains glassy feldspar, and sometimes hornblende, mica, leucite. 



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

 of the volcanic lavas. The lavas cooling slowly form stony lava, and cooling 

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



Obsidian connected with feldspathic lavas is either solid or slag-like (scoria- 

 ceous), and in color brown to greenish-black and black. A Mexican variety 

 afforded Vauquelin — Silica, 78, alumina, 10, potash and soda, 6, lime, 1, sesqui- 

 oxyd of iron, 2, id. of manganese, 1.6 = 98.6. Another, from Telki-Banya, 

 afforded Erdmann — Silica, 74.80, alumina, 12.40, potash and soda, 6.40, lime, 

 1.96, sesquioxyd of iron, 2.03, id. of manganese, 1.31, magnesia, 0.90 = 99.80. 



Spherulitic obsidian. — Contains small feldspathic concretions. 



(7.) Pitchstone (Retinite). — An imperfectly-glassy volcanic rock, pitch-like 

 in appearance, and of A r arious colors from gray 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. Delesse obtained 

 (Bull Soc. Geol. de France, 1853, p. 105) for a pitchstone from Santa Natolia — 

 Silica, 62.59, alumina, 16.59, protoxyd of iron, 3.17, id. of manganese, 0.55, 

 lime, 1.15, magnesia, 2.26, potash, 6.48, soda, 3.14, water and organic matter, 

 3.90 = 99.83. 



(8.) Pearlstone. — Near pitchstone, but less glassy and more pearly in 

 lustre: usually 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 quartz. The silica varies from 68 to 80 per cent. An analysis 

 by Erdmann of a variety from Hlinick afforded — Silica, 72.87, alumina, 12.05, soda 

 and potash, 6.13, lime, 1.30, magnesia, 1.10, sesquioxyd of iron, 1.75, water, 3.00 

 = 98.20. 



(9.) Melaphyr. — Closely related in its connections to dolerite and basalt, but 

 consisting of compact labradorite, and having a specific gravity not above 2.7. 

 ■.Color, reddish gray, greenish blackish, and sometimes mottled. Different rocks 



