[L. basaltes.] A rock of igneous origin, occurring chiefly in the ancient trap series of rocks. Basaltic rocks are composed of feldspar, augite, and magnetic iron, and other minerals, such as olivine. In color they are blue, brown, and black. They occur both as lava-flows and as sheets or dikes intruded between other strata, and are sometimes found in columns, which may be perpendicular or inclined, and of regular or irregular structure, as at the Giants' Causeway in Ireland.