Geol. and Min. [a. F. trachyte (Haüy); f. Gr. τρᾱχύς rough, or perh. τρᾱχύτης roughness.] A group of volcanic rocks, having a characteristically rough or gritty surface. The name was given by Haüy to certain volcanic rocks from Auvergne, and at first used in a wide sense; now confined to rocks consisting mainly of sanidine (or glassy orthoclase) felspar, as distinguished from oligoclase- and quartz-trachytes, and intermediate forms: see TRACHY- b.
1821. R. Jameson, Man. Mineral., 427. Rocks of extinct and ancient volcanoes…. 1. Trachyte. This rock which is of the nature of felspar, is generally porphyritic, the imbedded crystals being most frequently of the glassy kind.
1830. Lyell, Princ. Geol., I. 386. These isles are formed of brown trachyte … full of crystals of glassy felspar.
1854. Murchison, Siluria, xviii. 425. These were, in ancient times, penetrated by granites, porphyries, trachytes, and other eruptive matters.
1876. Page, Adv. Text-bk. Geol., v. 105. The trachytes are rough-grained subcrystalline varieties of felspathic Lava.
1911. Encycl. Brit., XXVII. 116/2. Trachyte … was long used in a much wider sense … in fact it included quartz-trachytes (now known as liparites and rhyolites) and oligoclase-trachytes, more properly assigned to Andesites.
b. attrib., as trachyte rock, porphyry; trachyte tuff, a tuff having the composition and structure of trachyte.
1872. C. King, Mountain. Sierra Nev., ix. 188. Rounded domes of trachyte rock.
1877. Tylor, in Nature, 5 July, 191/1. In a still larger chulpa [i.e., Peruvian burial-tower] there are hewn trachyte blocks as large as twelve feet long [etc.].
1825. Geikie, Text-bk. Geol., II. II. vii. (ed. 2), 166. Thus we have felsite-tuffs, trachyte-tuffs, basalt-tuffs, pumice-tuffs, porphyrite-tuffs, etc.
