4T4: MINERAL COMPOSITION [Ch. XXV III 



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



OpHioLZ'JrX A name given by Al. Brongniart to serpentine. 



Ophite. A name given by Palasson to certain trap rocks of the Pyrenees, very 

 variable in composition, usually composed of Labrador-felspar and horn- 

 blende, and sometimes augite, occasionally of a green color, and passing 

 into serpentine. 



Palagonite Tuff. An altered volcanic tuff containing the substance termed pa- 

 la gonite. 8ee p. 470. 



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 tujBF, composed of basaltic scorise. S>ee p. 470. 



PetrosileXo Bee Clinkstone and Compact Felspar. 



PiiONOLiTE. 8yn. of Clinkstone, which see. 



Pitchstone, or Retinite, of the French. Vitreous lava, less glassy than obsidian; 

 a blackish green rock resembling glass, having a resinous lustre and ap- 

 pearance of pitch ; composition usually of glassy felepar (orthoclase) with 

 a little mica, quartz, and hornblende ; in Arran it forms a dike thirty feet 

 wide, cutting through sandstone. 



Pumice. A light, spongy, fibrous form of trachyte. 8ee p. 469. 



Pi'ROXENic-PORPHYRY, samc as augitic-porphyry, pyroxene being Haiiy's name for 

 augite. 



Scoria. 8yn. volcanic cinders ; reddish brown or black porous form of lava. 

 See p. 469. 



Serpentine. A greenish rock in which there is much magnesia. Its composition 

 always approaches very near to the mineral called "noble serpentine" 

 (see Table of Ayalj'-ses, p. 475), which forms veins in this rock. The mine- 

 rals most commonly found in Serpentine are diallage, garnet, chlorite, 

 oxydulous iron, and chromate of iron. The diallage and garnet occurring 

 in serpentine are richer in magnesia than when they are crystallized in 

 other rocks. [Belesse Ami. des Mines, 1851, torn, xviii. p. 309). Occurs 

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

 differently a member of the trappean or hypogene series. Its absence 

 from recent volcanic products seems to imply that it belongs properly to 

 the metamorphic class; and, even when it is found in dikes cutting 

 through aqueous formations, it may be an altered basalt, which abounded 

 greatly in olivine. 



Tephrine, synonymous with lava. Name proposed by Alex. Brongniart 

 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. 466. 

 Trap Tuff. See p. 470. 

 Trass. A kind of tuff or mud poured out by lake-craters during eruptions; 



common in the Eifel, in Germany. 

 Tuff. Syn. Trap-tuff, volcanic tuff. See p. 470. . 



Vitreous Lava. See Pitchstone and Obsidian. 

 Volcanic Tuff. See p. 470. 



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

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

 WniNSTONE. A Scotch provincial term for greenstone and other hard trap rocks. 



