BASALT, in petrology, one of the oldest rock names, sup posed to be derived from an Ethiopian word basal, signifying a stone which yields iron; according to Pliny, the first basalts were obtained in Ethiopia. In current usage the term includes a large variety of types of igneous rock belonging to the basic subdivision, dark in colour, weathering to brown, and compara tively rich in magnesia and iron. Some basalts are in large measure glassy (tachylytes), and many are very fine grained and compact; but it is more usual for them to exhibit porphyritic structure, showing larger crystals of olivine, augite, or felspar in a finely crystalline groundmass. Olivine and augite are the commonest porphyritic minerals in basalts, the former green or yellowish (and weathering to green or brown serpentine), the latter pitch-black. Porphyritic plagioclase felspars, however. are also very common, and may be one or two inches in length, though usually not exceeding a quarter of an inch. Basaltic lavas are frequently spongy or pumiceous, especially near their sur faces; and the steam cavities become filled with secondary min erals such as calcite, chlorite, and zeolites.
Till recent years it was widely believed by continental geolo gists that the pre-Tertiary basalts differed so fundamentally from their Tertiary and recent representatives that they were entitled to be regarded as a distinct class. For the older rocks the names anamesite, diabase porphyrite, diabasmandelstein, or melaphyre were used, and are still favoured by many writers, to indicate varieties and states of more or less altered basalts and dolerites, though no longer held to differ in any essential respects from the better preserved basalts. Still older is the term trap, which is derived from a Swedish word meaning "a stair," for in many places superposed sheets of basalt weather with well-marked step-like or terraced features.
In the early years of the 1 gth century a great controversy convulsed the geological world as to the origin of the older basalts or "fioetz-traps." Werner, the Saxon mineralogist, and his school held them to be of aqueous origin, the chemical pre cipitates deposited in primeval seas, but Hutton and a number of French geologists maintained that they were really volcanic rocks emitted by craters now extinct (see GEOLOGY : Historical) .
Minerals of the f elspathoicl group occur in a large number of basaltic rocks belonging to the alkaline group; nepheline, anal cite, and leucite are the most common, but haiiyne is occasion ally present. If nepheline entirely replaces felspar, the rock is known as nepheline-basalt ; if the replacement is only partial the term nepheline-basanite is used. Similarly there are analcite and leucite-basalts and leucite-basanites. The nepheline is in small six-sided prisms, and usually cannot be detected with the unaided eye. Even with the help of the microscope nepheline basalts are not always easy to determine, as the crystals may be exceedingly small and imperfect, and they readily decompose into analcite and zeolites. Most nepheline-basalts are fine grained, very dark coloured rocks, and belong to the Tertiary period. They are fairly common in some parts of Germany and occur also in Tripoli, Asia Minor, Montana, Cape Verde Islands, etc. Leucite basalts contain small rounded crystals of leucite in place of plagio clase felspar. Rocks of this group are well known in the Eifel, and other volcanic districts in Germany, also in Bohemia, Italy, Montana, Java, Celebes, etc.
The distribution of basalts is world-wide; and in some places they occur in immense masses, and cover great areas. In Wash ington, Oregon, and Idaho many thousands of square miles are occupied by basaltic lava-flows. In the Sandwich Islands and Iceland they are the prevalent lavas; and the well-known columnar jointed basalts of Skye, Staffa, and Antrim (Giant's Causeway) form a southward extension of the Icelandic volcanic province, with which they are connected by the similar rocks of the Faeroe Islands. In the Deccan in India and in western Vic toria great basaltic lava fields are known; and Etna and Vesuvius emit basaltic rocks. Liquid of essentially basaltic composition is now commonly regarded as the parent magma from which the greater number of igneous rock types is derived. (See PETROLOGY.)