Basalt

Basalt, dense rock formed by the solidification of lava. It underlies ocean floors and is the basis of most oceanic islands. Frequently dark in color, its main constituents are labradorite feldspar and pyroxene. Basalt strata usually consist of hexagonal columns produced by crystallization of the molten lava during a slow cooling process, as seen in the Palisades along the Hudson River, the Devil's Postpile National Monument, Calif., and the Giant's Causeway in Ireland.