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272 PITCHSTONE FAMILV. bulk: hence, it is conjectured that some gaseous sub- 1 stance escapes; and Von Humboldt is inclined to believe, that the gas evolved during the fusion of obsidian in the interior of the earth, may give rise to the earthquakes that agitate the Cordilleras. Constituent Parts. Obsidian of Iceland. Silica, 74 Alumina, - 2 Oxide of Iron, - 14 Loss, 90 10 100 AbilgaarcL Obsidian of the Serro de las Novagas. Silica, - 78.0 Alumina, . 10.0 Lime, . l.o Natron, - 1.6 Potass, . 6.0 Oxide of Iron, 1.0 97.6 Vauqutlin. Abilgaard is of opinion, that the loss in his analysis was owing to the escape of either potass or soda. American. American. American. Silica, ... 72.0 72.0 71.0 Alumina, - . 12.5 14.2 13.4 Natron and Potass, - 10.0 3.3 6.0 Lime, . .- 0.0 1.2 1.6 Oxide of Iron & Manganese, 2.0 3.0 4.0 96.5 9A7 95.0 Collet-Dcscotilid Drappicr. Drappicr. Geognostic Situation. This mineral occurs both in jtrimitive and flcetz coun try : the primitive obsidian is said to occur forming mountain masses, in the state of porphyry, (obsidian* porphyry); also in beds alternating with elaystone, ° r felspar-porphyry, or imbedded in pearlstone-porphy r ) r ‘ . In