Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/260

166 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 27, plate set at right angles to the course of the dyke. These plates appear to he of different material from the dyke itself; and where this affects the needle they do not partake of that quality. In aspect these selvages resemble greenstone, although so closely connected with the basalt. They may, no doubt, represent a band of trap, fused by the dyke, or more probably a part of the dyke itself, which, under electrical action, has assumed this structure.

The general direction of the dykes is more or less nearly parallel to the bands of volcanic vents, main or subordinate, near to which also they are most numerous. Hence, although this rule has, as was to be expected, many exceptions, the greater number of the dykes near to the central axis of the Concan range north and south, and those near the subordinate band in the Malseje Valley range more nearly E.N.E. and W.S.W. In both, as in the adjacent districts, occur a few dykes taking other directions; but out of thirty-nine dykes, thirteen range north and south, and sixteen between N.E. and E. and S.W. and W. Three only lie S.E. and N.W.

The plane of each dyke is usually vertical; and the exceptions are few and local. Also they range in straight or nearly straight lines. They now and then swell out or contract, and in one or two instances have been observed to include a rider, though commonly their faces are parallel. They are very seldom indeed connected with any vertical displacement of the rocks; and the few exceptions to this are of very limited extent. I have not observed any dyke to be connected with a considerable fault.

Though the basalt of the dykes is peculiarly hard, and undergoes very little disintegration from the weather, its fissile character leaves it with little power of resisting eroding forces; and, in consequence, although sometimes a dyke stands up, out of the trap ridge already described, as a rugged black wall, more commonly its position is marked by a depressed furrow along the top of the ridge, the sides of which are thickly strewed with its fragments.

The most precipitous passes across the Ghauts are those produced by the removal of the dykes. In such cases the pass is a vast fissure, the walls of which are perpendicular and parallel, sometimes 300 or 400 feet high, and not above 10 or 20 feet apart. These walls are almost as hard as the removed basalt, and, not being fissile, are much less readily disturbed. Here the lower or unremoved part of the dyke forms the rough steps up which such passes are ascended. The "Nisnee cha Dara," or "pass of stone," near Jooneer is a very fine example of such a pass.

Owing to this same hardening of the trap, the course of a dyke across a river is often marked by a bund or dam, which the people turn to advantage for the irrigation of their lands. There are several such near Moorbar; and the navigation of the Callian river is seriously impeded by such an obstruction a few miles below that town.

These dyke ridges form rather a leading feature in the topography of the Concan. Some resemble raised causeways, and are so used during the rains: others have all the regularity of an artificial