BASALT', strictly a variety of trap rock (q.v.), although some writers use the words as synonymous. It is composed of the same materials as greenstone (q.v.), and other varieties of trap, viz., hornblende and felspar, with a small quantity of iron; but these exist in a state of finer division than in greenstone, showing that the crystalline action has been stopped at its commencement by the more rapid cooling of the mass. To this is owing its sharp conchoidal fracture and its hardness. As the hardness is frequently accompanied with tenacity, it. makes B. a valuable material in the making of roads. It is of a more uniform dark-gray color, approaching to black, than the other varieties of trap.
A rock of a similar appearance and structure occurs as a variety of lava, in volcanic districts. This lava B. differs from the older trap B. in the form which the silicates of magnesia and lime assume when crystallizing. In the newer rocks, they appear as augite; in the older, as hornblende. These two minerals can scarcely be distinguished by their chemical composition, the different formulas given by mineralogists being the result of the presence, in the specimen analyzed, of accidental ingredients or impurities. The slightly differing crystallographic angle has been accounted for by the supposed more speedy cooling of the volcanic rocks. Rose, indeed, has shown that the hornblende of melted greenstoue, in retooling, crystallizes as augite; and we have observed that the same change has taken place in specimens of recrystallized B., obtained from works which existed lately at Birmingham for converting this rock into an opaque glass for various economic uses.
The remarkable columnar structure which B. frequently assumes, is its most striking characteristic. The columns vary in the number of their angles from three to twelve; but they have most commonly from five to seven sides. They are frequently divided transversely by joints at nearly equal distances. The direction of the columns is always at right angles to the greatest extension of the mass, so that when B. occurs as a bed, either overlying, or interstratified with the regular strata, the columns are perpendic ular, while they are horizontal when the B. exists as a dike.
The columnar structure was at first believed to be owing to a modification of the crystalline force. Such a supposition was favored by the external form of the columns; but the total absence of internal structure showed that the explanation must be sought elsewhere. In 1804, Mr Gregory Watt propounded a theory of the origin of the struc ture, ascribing it to the pressure of numerous spheres on each other, during the process of cooling. such spheres being produced in planes of refrigeration or absorption. They increase by the successive formation of external concentric coats, until their growth is prevented by the contact of neighboring spheres; and as in a layer of equal-sized spheres, each is pressed on by six others, the result is that each sphere will be squeezed into a regular hexagon. Watt published this theory as the result of his celebrated observations on the cooling of a mass of molten basalt, in which he noticed the production of numer ous spheroids, having a radiate structure. Many greenstones, in weathering, present such a structure, giving often to the rock the appearance as if it were composed of a mass of cannon-halls, and Watt's experiments satisfactorily explain this phenomenon; but they will not go further. Anxious, however, that they should throw some light on
the structure of basaltic columns, lie manages it by the following remarkable assump tion: " In a stratum composed of an indefinite number in superficial extent, but only one in height, of impenetrable spheroids, with nearly equidistant centers, if their periph eries should come in contact in the same plane, it seems obvious that their mutual action would form them into hexagons; and if these were resisted below, and there was no opposing cause above them, it seems equally clear that they would extend their dimensions upwards, and thus form hexagonal prisms, whose length might be indefi nitely greater than their diameters. The further the extremities of the radii were removed from the center, the greater would be their approach to parallelism; and the structure would be finally propagated by nearly parallel fibers, still keeping within the limits of the hexagonal prism with which their incipient formation commenced; and the prisms might thus shoot to an indefinite length into the undisturbed central mass of the fluid, till their structure was deranged by the superior influence of a counteracting cause." Unfortunately, such dreams too often meet with more acceptance than the drier deductions from observed facts; which must, however, in the end, form the only basis of all geologic science. But there is no occasion here to urge even the most imaz inative to resort to hypothesis, for the formation of columns in other substances than B. is quite familiar, and their producing causes evident. In starch, columns having the external prismatic appearance, and the internal earthy structure, are produced simply from the escape of vapor, and consequent shrinking of parts. We have seen singularly regular joints produced in the argillaceous ironstone at Wardie, near Edinburgh, on its exposure on the beach, the contractions forming the columns evidently resulting from the escape of the moisture retained by the bed while it was covered by other strata. The same occurs in beds of flue clay that have been recently exposed. But the most • regular series of columns that hare been noticed by us, were produced on tricks which formed the bottom of a public oven. The long-continued and powerful heat to which they had been subjected, though it had not caused fusion, had so affected them as to produce a beautiful series of regular hexagonal prisms. The columns bud u diameter of nearly half an inch. Their direction was at right angles to the oven floor. The earthy structure of the brick remained. The columns, in short, were in every respect, except the material of which they were formed, true basaltic columns. It is surely better to look for an explanation of this structure in causes similar to those which have produced the examples adduced, than to had it in such groundless assumptions as are at the foun dation of the generally received theory of Watt. The columnar structure of B. scents to have been produced subsequently to the cooling of the mass, by changes is the solid rock, probably from the escape of sonic volatile matter.
The two best known and most beautiful examples of columnar B. are Fingals Cave, in the island of Stalin, on the west coast of Scotland, and the Giants' Causeway, on the n. coast of Ireland.