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Dacite

1911 Encyclopedia Britannica

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(from 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 microcrystalline, 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 augiteand 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. F.)