Basalt

Basalt, a dark-coloured lava, finely crystalline. compact or sometimes porphyritic in texture, composed essentially of a plagioclase felspar (labradorite or anorthite) and augite. Olivine, magnetite, apatite, and other minerals commonly occur in it as accessories, and there are varieties characterised by the more or less complete replacement of the felspar by nepheline or by leucite. The specific gravity of the rock ranging from 2.6 to 3.1, it belongs to the basic class of igneous rocks. It occurs in sheets, dykes and veins, and commonly exhibits columnar jointing produced by contraction during cooling. The columns are perpendicular to the surface of cooling. and may be three, four, six or eight-sided and of great length. There are also sometimes cross-joints parallel to the surface, and percolating water acting along all these joints produces spheroidal weathering. The surface of a basalt-flow is commonly covered with a thin glassy layer known as tachylite. Basalt is a hard, tough rock, suitable for road-metal. It surface weathers to a rust-brown. The "toadstone" of Derbyshire is an amygdaloidal basalt, and the columns of the Giant's Causeway in Antrim, and of Fingal's Cave, Staffa, are composed of an olivine-basalt. When coarse-grained, a basalt is termed dolerite.