Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/261

1869.] CLARK INDIAN BASALT DYKES. 167 earthwork; and many traverse the country for miles together, varying from one to two or three hundred feet in height, but always narrow. Near Moorbar a number of these ridges intersect each other, and form some very singular triangular enclosures.

The dykes are observed not only near to the parallel of Bombay, but they are seen in great numbers both north and south of that line, and still present the same characters. A sufficient number of dykes have been traced up from the Concan to the crests of the Ghauts to make it clear that elevations of 3000 and 4000 feet produce no perceptible difference in their structure or density.

The dykes are less numerous and of a smaller size as they recede from the volcanic district. East of the Ghauts, therefore, they are not numerous. Perhaps half a dozen have been noted between the Ghauts and Poona, and none between Poona and Shoolapoor. North of Jooneer, or rather of the Hurreechunder range, they are said to be more frequent, and to be found of large dimensions 50 or 60 miles from the sea. If this be so in these as well as in the more eastern examples in the metamorphic districts, it seems probable that such dykes are not to be traced to the Concan vents, but to sources more to the eastward—sources independent above, though, no doubt, identical below.

It may be mentioned that where fissures in a lava stream have been filled up by the overpouring of a later stream, the included matter, probably from its contact with cooled surfaces, assumes a denser structure than either its parent or including rock, it is, as founders say, "chilled;" and when the parent and superincumbent mass is swept away, what is left much resembles a regular dyke. Many such occur in the Deccan, but they are usually on a small scale, zig-zag, and may be readily traced to an end.

There seems every reason to believe that basalt dykes belong to a very late, perhaps nearly the latest, period of general volcanic action in the vast cape of India. The regular basalt beds which they resemble have not been observed lower than near to, if not at the very summit of, the series upon the Ghauts, and at the actual summit in Bombay—that is to say, on the two remaining margins of the volcanic region. The Bombay basalt, indeed, seems precisely to resemble, mineralogically, that composing the dykes, and to differ only in the local accident of a more perfect rhomboidal and columnar structure, due, no doubt, to its mass. The Ghaut sheet-basalt, well seen on the great tableland upon and east of Beema Sunker, is rather prismatic than columnar, and it contains the honey-coloured mineral and affects the needle. In the possession of these two qualities it differs from the basalt of the adjacent dykes; but I know not if this be fatal to their general identity, or at any rate to their being regarded as of one general period. The nodular basalt common as a capping-bed on the hills near Poona and the Beema is certainly cleft by the dykes. Upon Beema Sunker, and, I believe, elsewhere at great altitudes, beds of laterite, or of an amygdaloid much resembling it, are found above the basalt-bed. I know not whether these beds are reached or cleft by the dykes.