Andesite.

Andesite. a group of volcanic rocks, gray, reddish, or dark brown in colour. The ground-mass of these rocks is usually composed of felspar-microliths, scattered through which are abundant crystals of plagioclase felspar. Hornblende and augite, one or both, are generally present, together with magnetite, which is often very abundant. Andesite occurs chiefly in Tertiary and more recent strata, and is found in Hungary, Transylvania, Siebengebirge, Santorin, Iceland, the Andes, the western territories of United States, &c.