Trachyte

Trachyte, a crystalline igneous rock, generally grayish in colour, sometimes brown, yellow, green, or red. It is usually a fine-grained or compact rock, more or less markedly porphyritic. The essential constituents of trachyte are sanidine (felspar), of which the rock is chiefly composed, and one or more of the ferro-magnesian bisilicates—biotite, augite, and hornblende; various other minerals may be present. The rock has often a rough or harsh feeling (Gr. trachys, 'rough'), due to the abundant presence of minute vapour or gas pores, or to angular cavities. Trachyte occurs both in the form of lava-flows and as intrusive sheets, dykes, and masses, and ranges from early Tertiary times to the present. See IGNEOUS ROCKS.