BASALT, a hard dark-coloured rock of igneous origin. The chemical composition in variable, as appears from different analyses, two of which, by Beudant and Phillips, are as follows :— True Basalt has been regarded as composed of Augite, Felspar, and oxide of iron ; but this definition is far too limited for either theoreti cal or practical purposes, unless the constituent minerals be considered of variable chemical compositions, as appears to be the case. Since Augite and Hornblende may, from the researches of Rose, be regarded as the same mineral, it follows that a very fine-grained Greenstone, containing a considerable percentage of oxide of iron, can, even under this definition, be considered a true Basalt. There can indeed be little doubt that the same igneous rock has been termed Greenstone when the grains of Felspar and Hornblende were sufficiently distinct, which, when exceedingly fine-grained, has been named Basalt. Basalt can only be considered as one variety of that mass of melted rock which has been ejected at various periods from beneath the crust of the globe, and to which various names have been assigned, according to the characters which circumstances have impressed upon different portions of it.
Like others of the same class, Basalt occasionally passes into many rocks which have been in a state of fusion beneath the surface of the earth, and subsequently ejected. Dr. Hibbert notices a passage of Basalt into Granite in the Shetland Islands. (Brewster's 'Edinburgh Journal of Science,' toL i. p. 107.) When however we view the mass of igneous rocks generally, it appears that Basalts are the products of comparatively late geological epochs. We may therefore infer that during the earlier states of our planet, conditions were not favourable to their production, or at least to their propulsion to the surface ; though probably some varieties of Hornblende Rock, particularly when impregnated with much oxide of iron, do not differ materially from Basalt in their chemical contents. The mode of occurrence of these rocks and of Basalts is however very different.
Basalt is a rock of very extensive occurrence on the surface of the earth, and is very frequently detected in the vicinity of volcanoes, both extinct and active. The greatest mass of Basalt yet observed is
that noticed by Colonel Sykes in the Deccan, constituting the surface of many thousand square miles of that part of India. This immense mass of Basalt is either massive, prismatic, or globular, occurs in hori zontal beds, and is traversed by dykes [DYKE) of Basalt, which some times cross each other. There is no trace of any crater in this basaltic region ; and indeed this is the case with numerous other districts of Basalt, whence it has been inferred that such tabular masses have not been ejected from a conical vent similar to those of volcanoes, but that the Basalt of which they are formed rose through'eracks and fissures while in a highly liquid state, spreading out in sheets of melted matter over the adjacent rocks.
As Basalt is frequently columnar, it is a rock which has excited much popular attention, and travellers have been some times induced to describe rocks as basaltic merely be cause they were columnar, which however is a character that this rock possesses in common with many'others of igneous origin. When Basalt occurs in horizontal tabular masses, and is columnar, the columns are generally perpen dicular, as in the annexed figure. When Basalt forms the substance of a perpendi cular dyke, cutting through other rocks, and is columnar, the columns are usually hori zontal, in the manner repre sented beneath, a being the basaltic dyke, and bb the rocks through which the dyke passes. Basaltic columns are some times also curved, and of this mode of occurrence there is a beautiful example in the island of Staffa.
'Mien basaltic columns are jointed, and exposed to the destructive action of breakers on a coast, they often present the appearance of some great ruined work of art. Such deceptive appear ances are however not confined to coasts, for in some countries, and es pecially in India, masses of Basalt rise suddenly from the plains, and the broken columns, shooting upwards, may readily at a distance be mistaken for buildings. When viewed from above, the heads of a number of basaltic inns, if unbroken, appear like a pavement composed of numerous polygonal pieces of stone fitted into each other, as in the following figure.