Basalt. Basalt-rocks are of igneous origin, and are composed essentially of plagioclase felspar, augite, olivine, and generally magnetite or titaniferous iron. They show all varieties of texture, from smooth-compact up to coarsely crystalline, and vary in colour from pale blue up to dark-grayish blue, brownish, and black. The vitreous varieties are Tachylite and Hyalomelane; compact or crypto-crystalline varieties are termed Basalt; fine-grained kinds are called Anamesite; while Dolerite is the name given to the more coarsely crystalline kinds. Seen under the microscope, the compact and fine-grained basalt-rocks often show a certain proportion of glassy or devitrified matter lying between the various crystalline minerals of which the rock is chiefly composed. Basalt-rocks are often vesicular and amygdaloidal. Sometimes they occur as lava-flows, at other times they appear as intrusive sheets, dikes, and masses. They are of common occurrence in Britain, more especially in Scotland. As examples of lava-form basalt-rocks may be cited those of Mull, Staffa, and other islands of the Inner Hebrides. Similar lava-form basalt-rocks are well developed in Antrim. Intrusive basalt-rocks are abundant in Central Scotland—Salisbury Craigs, Edinburgh Castle Rock, Dalmaihoy Craigs, Abbey Craig, &c. are examples.
The older basalt-rocks have frequently undergone some changes, owing to the chemical action of percolating water. Such altered rocks have often a dull greenish colour, the greenish tinge being due to the conversion of the augite and olivine into green serpentinous and chloritic products. Such more or less altered varieties of basalt-rock sometimes acquire special names—the finer-grained kinds being called melaphyre, and the coarser-grained ones diabase. When basalt-rocks have been intruded amongst coals or black shales, they often become gray, white, or yellow in colour, and assume a dull earthy appearance. This is the so-called white-trap.
There are volcanic masses of Tertiary age which occur in such regions as the Thüringer Wald, Erzgebirge, the Eifel, Italy, &c. which closely resemble basalt-rocks. In these rocks, however, the minerals nepheline and leucite play the part of plagioclase felspar. The rocks, therefore, are known as nepheline-basalt and leucite-basalt, to distinguish them from ordinary basalt, or, as it is sometimes called, plagioclase-basalt. The latter ranges in age from Lower Carboniferous times at least, up to the present; the former are not as yet known from any older stage than the Tertiary.
Basalt-rocks, especially the compact varieties, often assume a columnar structure. This structure, however, is not confined to basalt-rocks. The columns vary in the number of their angles from 3 to 12, but they have most commonly from 5 to 7 sides. In some cases they are more or less perfectly hexagonal. They are generally divided transversely by joints at tolerably equal distances, and in the case of the more perfectly columnar rocks, these joints often show at each articulation a cup and ball socket. The columns are always arranged at right angles to the planes of cooling, so that in the case of an approximately horizontal bed or sheet the columns are vertical, while in the case of a vertical dike, they are horizontal. Various explanations of this remarkable structure have been advocated, none of which can be said to be perfectly satisfactory. The general belief, however, is that they are the result of contraction upon cooling. Two of the best known and finest examples of columnar structure in basalt-rocks are Fingal's Cave in the island of Staffa (q.v.), on the west coast of Scotland, and the Giants' Causeway (q.v.), on the north coast of Ireland.
Basalt-rocks, owing to their toughness and hardness, and to the fact that their mineral ingredients yield unequally, are much employed for causeway-stones and 'road-metal.'