Basalt

[Fr. basalte; Lat. basaltes; Ethiop. basal, iron.] ( Geol. and Min.) A close-grained, hard, black, or dark-brown rock, of igneous origin, occurring both in the trap and the volcanic series; sp. grav. 287 to 3. It is one of the dolorites or augitic lavas, and consists e entially of augite and felspar, the former being in exce . It often contains crystals of the olive-green mineral olivine, grains of magnetic iron, and other bodies. Ma es of basalt are frequently found divided into columns or prisms, with three, five, or more sides. That this columnar structure is the result of contraction on consolidation, is shown by the prisms usually Fig. 298. -THE ISLE OF STAFF A. being at right angles to the greatest extension of the ma , that is to say, being vertical in an horizontal bed, and horizontal in a vertical dyke, -proving that the fi uring commenced at the cooling surfaces, and struck thence directly toward the centre of the ma . Sometimes it is found that the two sets of prisms thus originating at each surface did not exactly fit when they met in the centre. At other times, however, the prisms proceed uninterruptedly from side to side, the two sets either having coalesced, or one surface having cooled before the other, and given rise to divisions that were carried right acro the ma . In addition to these prismatic joints, other irregular joints, more or le nearly at right angles to the prisms, also occur; and in very regular columnar basalt the columns are articulated, or separated, at regular or irregular intervals, into short blocks, by divisions, which are sometimes quite flat. and sometimes curved into concave and convex surfaces, forming an approach to the ball-and socket joint. The origin of this structure is explained by the celebrated observations of Gregory Watt. If a ma of basalt be melted in a furnace and allowed to cool again, the following results are observed: - If a small part be removed and allowed to cool quickly, a kind of slag-like gla is obtained, not differing in appearance from obsidian; if it cool in larger ma , and more slowly, it returns to its original stony state. During the cooling, small globules make their appearance, which increase by the succe ive formation of external concentric coats, like those of an onion; and the simultaneous obliteration of the previously formed internal coats, so that, ultimately, a number of solid balls are formed. As these balls continue to increase in size, their external coats at length touch, and then they mutually compre each other. Now in a layer of equal-sized balls, each ball is touched by exactly six others, and if all be squeezed together by an equal force acting in every direction, each ball will nece arily be converted into a regular hexagon. The same resuit will also follow from an equal expansive force acting from the centre of each ball, or from the tendency to indefinite enlargement in their concentric coats. Thus, each spheroidal ma , under favorable circumstances, will a ume the form of a short hexagonal pillar. If there are many lavers of balls. each ball resting directly and centrically on the one below t, a long column of Basaltic, a. [Fr. basalt i que .] Pertaining to basalt; formed of, or containing, basalt. Basalt i form, a . Columnar; in the form of basalt. Basal'tine, n . ( Min . ) A column of basalt. Ba's an, n. See BASIL.