A group of plutonic rocks intermediate in composition between acidic and basic, characteristically composed of dark-colored amphibole (esp. hornblende), acid plagioclase (oligoclase, andesine), pyroxene, and sometimes a small amount of quartz; also, any rock in that group; the approximate intrusive equivalent of andesite. Diorite grades into monzonite with an increase in the alkali feldspar content. Etymol: Greek diorizein, to distinguish, in reference to the fact that the characteristic mineral, hornblende, is usually identifiable megascopically. Compare: dolerite; gabbro.