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Basalt. A well-known igneous rock occuring in the ancient trap and the recent volcanic series of rocks, but most abundantly in the former. It is a fine-grained, heavy crystalline rock, consisting of felspar, augite, and magnetic iron, and sometimes contains a little olivine. Basalt is amorphous, columnar, tabular, or globular. The columnar form is straight or curved, perpendicular or inclined, sometimes nearly horizontal; the diameter of the columns from three to eighteen inches, sometimes with transverse semispherical joints, in which the convex part of one is inserted in the concavity of another; and the height from five feet to one hundred fifty. The forms of the columns generally pentagonal, hexagonal, or octagonal. When decomposed it is found also in round masses, either spherical or compressed and lenticular. These rounded masses are sometimes composed of concentric layers, with a nucleus, and sometimes of prisms radiating from a center. Fingal's Cave, in the island of Staffa, furnishes a remarkable instance of basaltic columns. The pillars of the Giant's Causeway, Ireland, composed of this stone, and exposed to the roughest sea for ages, have their angles as perfect as those at a distance from the waves. Basalt often assumes curious and fantastic forms, as for example, those masses popularly known as "Sampson's Ribs" at Arthur's Seat, Edinburgh, and "Lot" and "Lot's Wife" near the southern coast of St. Helena.

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