Basalt

Dark-colored, compact, crystalline rock of igneous origin, consisting e entially of labradorite and augite, though small quantities of chrysolite or other minerals are often present as acce ories. It occurs as the typical basic eruptive rock in many parts of the earth, and frequently takes the form of regular, jointed columns, as at the "Giants' Causeway," on n. IMG:content-0438.png:[graphic][merged small] coast of Ireland, where it forms a series of ma ive steps projecting into the sea, and at Fingal's Cave in Staff a, near the opposite coast of Scotland. Both of these are relics of a once continuous sheet of basalt poured out upon the top of the chalk. The Palisades of the Hudson are another example of the same formation. By decomposition basalt produces a variety of alteration products, known by special names. Dolerite is a coarse grained rock of the same mineralogical composition. Compare Trachyte.