PART 1. CHAPTER VII. 101 



Mineral Composition of Volcanic Rocks. 



Hypersthene rock, a mixture of grains of Labrador-felspar and hypersthene, 

 {Rose, 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 lakes 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. {Burafs D'Aubuisson, torn. 2. 

 p. 63. 



Pearlstone. a volcanic rock having 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 scorise. 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 resinous lustre 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 ciystals of felspar, or of one or more 

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



Pozzolana. a kind of tufE 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. 



ScoRiiE. Syn. volcanic cinders ; reddish brown or black porous form of lava. 

 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 hypogene series. 



Syenitic-greenstone ; composition, crystals or grains of felspar and hornblende. 

 See p. 96. 



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 Pitchstone 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. 



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