Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/262

168 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 27, The dykes are observed to traverse and to pass at tangents to particular vents, and, although most numerous about such points, not to radiate from them. Nor do such vents present any traces of a flux of regular basalt. From this it seems reasonable to infer that the dykes are due not only to a later period, but to a period after the cessation of the regular eruptive action, and possibly therefore that the capping beds may be the overflow of the dykes.

Salsette, indeed, seems to be an exception, and to contain evidences of still later eruptive action, though local only.

The dykes themselves seem to admit of division into at least two periods, since dykes of different grain frequently intersect each other. Near Moorbar these intersections are most numerous; but at such points the rock is commonly very loose, covered with jungle, and full of poisonous snakes; they are therefore difficult of examination. I think, however, that in many cases the dykes lying north-east and south-west traverse and often very slightly dislocate those lying more nearly north and south; whence I infer these to be the later, and the cross band of the Malseje vents to have ejected its dykes since those connected with the general outline of the Ghauts. It is probable, however, that a minute examination of specimens from all these dykes would throw more light upon this matter.

The dykes, as has been stated, are neither vesicular nor amygdaloidal, although they pass through rocks most of which are both one and the other. But though the basalt contains no vesicles, it is fissured in all directions, and must have been fissured, more or less, very soon after it was poured out, as it began to cool. All the contiguous rocks contain kernels of zeolite, and now and then of chalcedony and calcareous spar. None of these minerals are found in the basalt, although its cracks must have lain open to receive them. I do not know what is the prevailing theory upon the deposition of these included minerals; but these appearances seem to point to their segregation from the trap, and to be opposed to any theory of infiltration.

Among the dykes alluded to as not basaltic may be mentioned one on Trombay Island, which branches off from a basalt dyke, but is quite void of the mineral, and contains laumonite.

In most cases the great and numerous fissures, so common on the Ghauts, mark the passage and removal of a dyke. This is not always the case; and hence there seems ground to suspect that the fissures may have been formed, not by the forcible injection of the basalt, but independently of and before its appearance.

Discussion.

Mr. David Forbes did not see any proof of the dykes being of different ages. In modern eruptions, lasting over some years, the lava first erupted sometimes became fissured, and the fissures filled at a later period of the eruption.