BASALT, in mineralogy, occurs mas sive, in blunt and rolled pieces, and some times vesicular: its common colour is greyish black, of various degrees of in tensity ; from this it passes into ash-grey ; sometimes to brown, and even in some varieties to raven black. It is dull in ternally, and the fracture is commonly coarse-grained and uneven. It occurs almost always in distinct concretions, which are usually columnar, and from a few inches to several fathoms, and even to upwards of 100 feet, in length. They are sometimes straight, sometimes bent, and either paraltelor diverging.:lanoun tains these concretions are collectedinto larger groupes, of which many together form a hill or a mountain. Sometimes the columns are articulated, and the joints have convex and concave faces. The specific gravity is by Bergman put at 3.0 : by Brisson at 2.86: and by Kir wan at 2.98. Before the blowpipe it ea sily melts without addition into an opaque black glass. By analysis the constituent parts have been brought out differently by different chemists, but according to Klaproth they are as follow : Silica 44 50 Alumina . . . . 16.75 Oxide of iron . . 20.00 Lime 9 50 Magnesia . . . . 2.25 Oxide of Manganese 0.12 Soda 2 60 Water 2 00 97,62 It is found in vast mountainous beds, in most parts of the world, and almost al ways accompanies coal. The island of Staffa, on the western coast of Scotland, is entirely composed of basaltic pillars : the Giants' Causeway, on the coast of Antrim, in Ireland, is a huge pavement of straight pillars, running to an unknown distance in the sea : the promontory of Fairhead, a little further to the north, ex hibits a continued range, about a mile long, of columns 250 feet in height, and from 10 to 20 in diameter, being the larg est yet known.
Basalt is employed as a building stone and touchstone ; as a flux for certain ores of iron ; in glass manufactures ; in mak ing the common green glass. The vesi cular varieties are employed for mill stones. Owing to its great hardness, the
ancients, who were acquainted with its indestructibility, executed several fine works in it ; many of which are preserved in great perfection to this day. The origin and formation of basalt are much controverted. Bergman introduced the theory of its aqueous formation ; and to this Jameson inclines, from observing that the strata which Are in contact with basalt generally exhibit appearances incompa• tible with the action of fire. Since the time of Bergman, the two theories have nearly equally divided the mineralogical world. The Swedes, Germans, and nerians in Britain, maintain the aqueous theory : they have shewn basalt resting upon and alternating with strata of ac• knowledged aqueous origin ; they have discovered shells and- vegetable remains imbedded in its substance ; they have found its cavities filled by officious nodules containing water ; they have melted ba salt in their furnaces, and have found it to produce glass; they have moreover shewn that the lava of Vesuvius and Etna differs in many important particulars from basalt ; and they have pointed out the prismatic structure in many substances, which are not supposed to have under gone the action of fire The French, the Italians, and Dr. Hut ton and his disciples in this country, main tain the igneous origin of basalt ; in de fence of their system they have shewn the prismatic structure of some undoubt ed Italian lavas : they have shewn beds of coal charred by the contact of dykes of basalt, and the forcible disrupt,i,on, incur vation, and induration of argillaceous strata, when pierced through by means of this substance. Sir James Hall has proved that basalt, after it has undergone the vitreous fusion, may be made to as sume a perfect stony appearance : and Mr. Watt has demonstrated, by experi ment, that basalt may, by the medium of fire, acquire those peculiarities of struc ture that cannot readily be explained by the aqueous theory.