Porphyrite

Porphyrite, one of the crystalline igneous rocks. It consists principally of plagioclase. The ground-mass of the rock is composed of microlites and minute rod-like crystals of plagioclase, interspersed amongst which may occur crystalline granules of hornblende, augite, rhombic pyroxene, mica, ilmenite, magnetite, &c. Sometimes traces of a glassy or devitrified base can be detected. Throughout this ground-mass are scattered porphyritically larger crystals of plagioclase, accompanied by one or more of the following minerals: hornblende, augite, rhombic pyroxene, mica, &c. The rock shows all varieties of colour, and ranges in texture from vitreous and cryptocrystalline up to coarsely crystalline. It is often vesicular and amygdaloidal. It occurs abundantly in Scotland (where it forms many of the hill-ranges of the Lowlands), both in the form of lava-flows and intrusive sheets, dykes, and masses. Porphyrite is so closely allied to Andesite (q.v.) that it may be considered as merely an altered variety of that rock.