Porphyrite—a term sometimes used for an altered form of andesite, usually 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 quartzose 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.