Melaphyre. A variety of black porphyry, the base being black augite with crystals of felspar; from μελας, melas, black.
[Obsidian]. Vitreous lava like melted glass, nearly allied to pitchstone.
Ophiolite, sometimes same as Diallage rocks (Leonhard, p. 77.); sometimes a kind of serpentine.
Ophite. A green porphyritic rock composed chiefly of hornblende, with crystals of that mineral in a base of the same, mixed with some felspar. It passes into serpentine by a mixture of talc. (Burat's d'Aubuisson, tom. ii. 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 scoriæ. See [p. 374.]
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; composition various, usually felspar and augite; passes into basalt; occurs in veins, and in Arran forms a dike thirty feet wide, cutting through sandstone; 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. 372.]