BASALT, a hard dark-coloured rock of igneous origin. It can only be considered as a variety (and a comparatively recent 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. Basalt is a rock of very extensive occurrence on the surface of the earth, and is very frequently found in the vicinity of volcanoes, both extinct and active: The greatest mass yet observed is that noticed by Colonel Sykes in the Deccan, constituting the surface of many thousand square miles of that part of India. When basalt occurs in horizontal tabular masses, and is columnar, the columns are generally perpendicular. When basalt forms the substance of a perpen dicular dyke, cutting through other rocks, and is columnar, the columns are usually horizon tal. Basaltic columns are sometimes also
curved, and of this mode of occurrence there is a beautiful example in the island of Staffa.
When basaltic columns are jointed, and ex posed to the destructive action of breakers on a coast, they often present the appearance of some great ruined work of art. Such decep tive appearances are, however, not confined to coasts, for in some countries, and especially 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 columns, if un broken, appear like a pavement composed of numerous polygonal pieces of stone fitted into each other.