Basalt, the hardest, most compact, and heaviest of the trap rocks, frequently columnar in structure, the columns or prisms having three, five, or more sides, regular and jointed. Some of the columns of the isle of Skye are 400 feet long, while in other localities they do not exceed an inch in length. The diameters of the prisms range from nine feet to an inch across the face. The columnar structure is most noticeable when the rock is viewed at a distance, as at the Palisades on the Hudson. Remarkable examples of basalt have been found on the N. W. shore of Lake Superior, at the Giant's Causeway, Ireland, and Fingal's cave, Scotland, and on the island of St. Helena. Basalt belongs to the augitic series of the igneous rocks resembling dolerite, and consists of labra-dorite, augite, and chrysolite in grains looking like green glass. Its specific gravity varies from 2.9 to 3.2. Owing to its hardness, basalt has been much used for pavements and for macadamizing roads. When melted and cooled rapidly it is converted into a kind of obsidian (volcanic glass), and can be cast into ornamental blocks and mouldings.
Artificial building stone was at one time made of it in England.