Diabase

Diabase, a dark green or black crystalline rock, occurring mainly in the earlier geological formations, and consisting of a plagioclase felspar, generally oligoclase, and augite, with or without quartz, olivine, apatite, and mica. There is usually more or less magnetite or ilmenite, and the augite has generally been to a considerable extent transformed into a viridite, or green decomposition-product, believed to be epichlorite. Diabase is accordingly considered to be an altered condition of some basaltic lava. It contains from 48 to 50 per cent. of silica.