86 GEOLOGY 



Peat. The dark brown or black residuum arising from the partial decom- 

 position of mosses and vegetable tissue in marshes and wet places. 



Pegmatite. A term of ill-defined usage applied to rocks whose grain 

 varies from coarser to finer, and often takes on peculiar aspects due to the 

 simultaneous crystallization and mutual inter-growths of the crystals ; graphic 

 granite is a distinct type of pegmatite in which quartz and orthoclase crystals 

 grew together along parallel axes so that cross-sections give figures resembling 

 certain Semitic letters (Fig. 12). 



Peridotite. A very basic igneous rock composed chiefly of divine with 

 augite or related ferromagnesian minerals, with magnetite and chromite as 

 accessories. 



Perlite (pearlstone). A form of glassy lava made up in part of small sphe- 

 roids formed of concentric layers which have a lustrous appearance like pearls. 



Phonolite (nephelite-trachyte, clinkstone). A compact resonant igneous 

 rock formed of sanidine and nephelite, with accessories. 



Phyllite (argillite). A variety of indurated, partly metamorphosed, clay 

 silt in which finely disseminated micaceous scales are abundant and lustrous ; 

 intermediate between typical clay slate and mica-schist. 



Pitchstone. A dark vitreous, acid, igneous rock of less perfect glassy 

 texture than obsidian, and more resinous and pitch-like. 



Plutonic rocks. Igneous rocks formed deep within the earth under the 

 influence of high heat and pressure; hypogene rocks; distinguished from 

 eruptive rocks formed at the surface. 



Porphyry. A rock consisting of distinct crystals embedded in an aphanitic 

 ground-mass, or of large crystals embedded in smaller ones. 



Pumice. A glassy form of lava rendered very vesicular through inflation 

 by steam. 



Pyroclastic rocks. Fragmental or clastic rocks produced through igneous 

 agencies, embracing volcanic ashes, tuffs, agglomerates, etc. 



Pyroxenite. An igneous rock consisting essentially of pyroxene. 



Quartzite. A rock consisting essentially of quartz, usually formed from 

 quartzose sandstone by cementation or metamorphic action. 



Regolith. A name recently suggested to embrace the earthy mantle 

 that covers indurated rocks, chiefly residuary earths; mantle-rock. 



Rhyolite. An aphanitic or partly glassy igneous rock showing flowage 

 lines, usually applied only to the acidic varieties. 



Sandstone. Indurated sand usually composed of grains of <|uartz, but 

 not necessarily so; sometimes contains grains of the various silicates. 



Schist. A crystalline rock having a foliated or parallel structure, split t ing 

 easily into slabs or flakes, less uniform than those of slate; they are composed 

 mainly of the silicate minerals. 



Scoriae. Light, cellular fragments of volcanic rock, coarser than pumice. 



Septaria. Concretions the interior cracks of which have been filled with 

 calcite or other mineral deposited from solution (Fig. 28). 



