PART I. CHAPTER VII. 101 



Mineral Composition of Volcanic Rocks. 



HYPERSTHENE ROCK, a mixture of grains of Labrador-felspar and hypersthene, 

 (Hose, Ann. des Mines, torn. 8. p. 13.) having the structure of syenite or 

 granite ; abundant among the rocks of Sky. In a geological view, it has 

 been called a greenstone, in which hypersthene takes the place of horn- 

 blende. 



MELAPHYRE. A variety of black porphyry, the base being black augite with 

 crystals of felspar ; from ne\as, melas, black. 



OBSIDIAN. Vitreous lava like melted glass, nearly allied to pitchstone. 



OPHIOLITE, sometimes same as Diallage rock (Leonhard, p. 77.) ; sometimes a 

 kind of serpentine. 



OPHITE. A green porphyritic rock, composed chiefly of hornblende, with crys- 

 tals of that mineral in a base of the same, mixed with some felspar. It 

 passes into serpentine by a mixture of talc. (BuraCs D'Aubuisson, torn. 2. 

 p. 63. 



PEARLSTONE. A volcanic rock haying the lustre of mother of pearl ; usually 

 having a nodular structure ; intimately related to obsidian, but less glassy. 



PEPERINO. A form of volcanic tuff! composed of basaltic scorias. See p. 98. 



PETROSILEX. See Clinkstone and Compact Felspar. 



PHONOLITE. Syn. of Clinkstone, which see. 



PITCHSTONE ; vitreous lava, less glassy than obsidian ; a blackish green rock 

 resembling glass, having a resinousjustre and appearance of pitch ; compo- 

 sition various, usually felspar and augite ; passes into basalt ; occurs in 

 veins, and in Arran forms a dike thirty feet wide, cutting through sand- 

 stone ; forms the outer walls of some basaltic dikes. 



PORPHYRY. Any rock in which detached crystals of felspar, or of one or more 

 minerals, are diffused through a base. See p. 96. 



POZZOLANA. A kind of tuff. See p. 53. 



PUMICE. A light, spongy, fibrous form of trachyte. See p. 97. 



PYROXENIC-PORPHYRY, same as augitic-porphyry, pyroxene being Haiiy's name 

 for augite. 



Syn. 

 See p. 97. 



SERPENTINE. A greenish rock, in which there is much magnesia ; usually con- 

 tains diallage, which is nearly allied to the simple mineral called serpentine. 

 Occurs sometimes, though rarely, in dikes, altering the contiguous strata ; is 

 indifferently a member of the trappean or hyppgene series. 



SYENITIC-GREENSTONE ; composition, crystals or grains of felspar and hornblende. 



TEPHRINE, synonymous with lava. 



TOADSTONE. A local name in Derbyshire for a kind of wacke, which see. 



TRACHYTE, chiefly composed of glassy felspar, with crystals of glassy felspar. 



See p. 96. 



TRAP TUFF. See p. 98. 

 TRASS. A kind of tuff or mud poured out by lake-craters during eruptions : 



common in the Eifel, in Germany. 



TUFACEOUS CONGLOMERATE. See p. 98. 



TUFF. Syn. Trap-tuff, volcanic tuff See p. 98. 



VITREOUS LAVA. See Pitchslone and Obsidian. 

 VOLCANIC TUFF. See p. 98. 



WACKE. A soft and earthy variety of trap, having an argillaceous aspect. It 

 resembles indurated clay, and when scratched, exhibits a shining streak. 



WHINSTONE. A Scotch provincial term for greenstone and other hard trap 

 rocks. 



