472 GEOLOGY. 



Peastone (pisolite) — a very coarse variety of oolite. 



Peat — the dark brown or black residuum arising from the partial decomposi- 

 tion 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 intergrowths 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. 345). 



Peridotite — a very basic igneous rock composed chiefly of olivine with augite 

 or related ferromagnesian minerals, with magnetite and chromite as accessories. 



Pelites — a general term embracing clay rocks. 



Perlite (pearlstone) — a form of glassy lava made up in part of small spheroids 

 formed of concentric layers which have a lustrous aj)pearance like pearls. 



Petrosilex — an old name for felsite or halleflinta. 



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; inter- 

 mediate 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 in- 

 fluence of high heat and pressure; hypogene rocks; distinguished from eruptive 

 rocks formed at the surface. 



Porphyrite — a term sometimes used for an altered form of andesite, usuall}^ 

 porphyritic in structure. 



Porphyry — a rock consisting of distinct crystals embedded in an aphanitic 

 ground-mass. 



Propylite — an altered form of andesite and similar igneous rocks. 



Protogine — a hydrated micaceous or chloritic variety of granite or gneiss. 



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 quartz- 

 ose sandstone by cementation or metamorphic action. 



Regolith — a name recently suggested by Merrill to embrace the earthy mantle 

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



Rhyolite — an aphanitic or glassy igneous rock showing flowage lines, usually 

 applied only to the acidic varieties. 



Sandstone — indurated sand usually composed of grains of quartz, but not 

 necessarily so; sometimes formed of calcareous grains or of grains of the various 

 silicates. 



Schist — a crystalline rock having a foliated or paraUel structure, splitting 



