DACITE (from

See also:

• DACIA

Dacia, mod. Transylvania)  , in petrology, volcanic rocks which may be considered a quartz-bearing variety of andesite . Like the latter they consist for the most part of plagioclase felspar with biotite, hornblende, augite or enstatite, and have generally a porphyritic structure, but they contain also quartz as rounded, corroded phenocrysts, or as an element of the ground-mass . Their felspar ranges from oligoclase to andesite and labradorite, and is often very zonal; sanidine occurs also in some dacites, and when abundant gives rise to rocks which form transitions to the rhyolites . The biotite is brown; the hornblende brown or greenish brown; the augite usually green . The ground-mass of these rocks is often micro-crystalline, with a web of minute felspars mixed with interstitial grains of quartz; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline . In the hand specimen many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white felspars, and black crystals of biotite and hornblende . Other dacites, especially augite- and enstatite-dacites, are darker coloured . The rocks of this group occur in Hungary, Almeria (Spain), Argyllshire and other parts of Scotland, New Zealand, the Andes, Martinique, Nevada and other districts of western North America, Greece, &c . They are mostly associated with andesites and trachytes, and form lava flows, dikes, and in some cases massive intrusions in the centres of old volcanoes . Among continental petrographers the older dacites (Carboniferous, &c.) are often known as " porphyrites." (J: S .