606 MINERAL COMPOSITION [Oh. XXVIII. 



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 traps of Skye. It is extremely tough, grayish, and 

 greenish black. Some geologists consider it a greenstone, in which hyper- 

 sthene replaces hornblende ; and this opinion, says Delesse, is borne out by 

 the fact that hornblende- usually occurs in hypersthene rock, often envelop- 

 ing the crystals of hypersthene. The latter have a pearly or metalhc-pearly 

 lustre. 



Laterite. A red, jaspery, brick-like rock, composed of silicate of alumina and 

 oxide of iron, or sometimes consisting of clay colored with red ochre. See 

 above, p. 603. 



Melaphyre. A variety of black porphyry composed of Labrador-felspar and a 

 small quantity of augite. Its black color was formerly attributed to dissemi- 

 nated microscopic crystals of augite, but M. Delesse has shown that the 

 paste is discolored by hydrochloric acid, whereas this acid does not attack 

 the crystals of augite, which are seen to be isolated, and few in num- 

 ber. {Ann. des Mines, 4th. ser. torn. xii. p. 228.) From \izkaq, melas, 

 black. 



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



Ophiolite. A name given by Alex. Brongniart to serpentine. 



Ophite. A name given by Palassou 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 Tuee. An altered volcanic tuff containing the substance termed pala- 

 gonite. See p. 603. 



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 scoria?. See p. 602. 



Petrosilex. See Clinkstone and Compact Pelspar. 



Phonolite. Syn. of Clinkstone, which see. 



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

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

 ance of pitch ; composition usually of glassy felspar (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. See p. 602. 



Pyroxenic-porphyry, same as augitic-porphyry, pyroxene being Haiiy's name for 

 augite. 



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

 p. 602. 



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 Analyses, p. 608), which forms veins in this rock. The minerals 

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

 iron, and chromate of iron. The diallage and garnet occurring in serpen- 

 tine are richer in magnesia than when they are crystallized in other rocks. 

 (Belesse x Ann. des Mines, 1851, torn. xviii..p. 309.) Occurs sometimes, 

 though rarely, in dikes, altering the contiguous strata; is indifferently a 

 member of the trappean or hypogene series. Its absence from recent vol- 



