BASALT' (Lat. basaltes, an African word). A volcanic rock of basic composition, character ized by a porphyritic texture, and having generally one or more of the feldspathoid min erals, lime-soda feldspar. nephelite, or leucite, associated with pyroxene or hornblende, and magnetite or ilmenite; frequently also with oli vine. According as one or the other of these minerals predominates. the chemical composition varies between wide limits, so that an average for basalt would have little significance. Ba salts are distinguished as olivine basalts, nephe line basalts, leucite basalts. or as basalts proper, when containing as essential constituents only lime-soda feldspar and pyroxene or hornblende. The terms (laterite, anamesite, and basalt were used to distinguish basalts of coarse, medium, and fine grains, respectively but since the in troduction of the petrographical microscope has made accurate rock-study possible, these terms have fallen into disuse, and others, describing the intimate texture of the rocks as revealed by microscopic study, have taken their place. In the preliminary study in the field. however. they have still some value for cartographical pur poses.
Basalts are in all cases the products of con solidation of molten magmas, and generally of lavas which were poured out at the surface of the earth. In contrast with rhyolites and trach ytes, which in their genesis they resemble, ba salts, when molten, flow as comparatively thin fluids, which travel rapidly down a slope, and on solidifying ultimately build up mountains which have comparatively gentle slopes. Thus the
Hawaiian Islands, which are built up of basalt from the sea-bottom, have, in contrast with the slopes of the American Cordilleran volcanoes, which are generally rhyolite or andesite, ex ceedingly gentle slopes. Masses of basalt, when solidified at the surface, have, quite generally near the original upper surfaces, a separation by cracks into layers parallel to the surface; where as at greater depths there is produced a series of columns, generally hexagonal in section, and with their columnar axes perpendicular to the original surface. The upper surfaces of a basalt outflow being curved, the columns generally radi ate from some point near the bottom of the mass. As the upper layers of rock have been re moved by erosion in all save the most recent extrusions of lava, the platy parting is less commonly observed than is the columnar. The Giant's Causeway, in the north of Ireland, is the best-known example of columnar joints in ba salt. In common with most other lavas, basalt exhibits all varieties of cellular and scoriaceous texture, depending upon the quantity of steam the lava had absorbed and upon the opportuni ties afforded this steam to expand when it reached the surface. ( For illustration, see GIANT'S CAUSEWAY.) Very similar rocks to basalt are diabase, melaphyre, and augite por phyritc.