Peat—the dark brown or black residuum arising from the partial decomposition 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 appearance like pearls.
Petrosilex—an old name for felsite or hälleflinta.
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.