Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/220

120 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. miles to the south and south-east of the former near its mouth, and on the west bank of the latter to about halfway between Ahmedabad and the head of the Gulf, the soil is all light-coloured, containing particles mostly derived from the primary and metamorphic rocks to the north-east. The black soil lies in one extensive Bed to the west of the last-mentioned limit of the light-coloured soil, widening out on both sides towards the mouth of the Saburmuttee, and down the west coast of the Gulf, but becoming lighter after it reaches the granitic region. This westerly belt stretches beyond the head of the Gulf as far at least as the Lesser Runn, and probably impinges on the Great Runn itself. The alluvial soil to the north of Baroda, between the Myhee and the trap range running to the north-east from Powaghur, beyond the Champaneer beds mentioned by Mr. Blanford, is of a general character between those of the light-coloured and black varieties; and the deltas between the mouths of the Myhee, the Nerbudda, the Taptee, and a point about fifty miles to the south of Surat, where the trap touches the present coast-line, are again all black.

We have now to consider whence these enormous alluvial deposits can have been derived. With regard to those on the east coast of the Gulf of Cambay, there can be no hesitation in ascribing them to the gradual action of the two large rivers Nerbudda and Taptee, and the numerous smaller streams which flow from the east, either from the western range of Ghauts or passing through gorges in that range from further east. There is no doubt that the washings of the trap-rocks which compose that range enter largely into the composition of the black soil which, as already stated, occupies almost the whole of the east coast of the Gulf; for much of the trap is amygdaloidal, and zeolitic minerals derived from it are found in abundance in the soil. But this cannot be said of the light-coloured soil on the north-east of the Gulf, which must be chiefly derived from the primary and metamorphic rocks in that direction mentioned above — or of the black belt running from the head of the Gulf in a north-westerly direction, in which there are no rock formations at all to account for the colour of the soil. It is as well to notice here that the fact of this belt being just of as dark a colour as that lying on the east coast apparently militates against the theory that this colour is derived from the presence of iron in the trap-rocks from which it is believed to be formed. The Myhee, the Saburmuttee, and other streams which now fall into the Gulf at its northern extremity are not sufficiently large to account for the deposit, especially for the belt of black soil to the north-west, extending as far as the Lesser Runn. Many considerations point to the existence in former ages of some large river flowing down from the north, and falling into the Indian Ocean somewhere in the position of the present Gulf of Cambay; and it is not improbable that that river may have been the Indus. It may have been that the original course of the Indus from the Punjaub was in a more south-easterly direction than that of the present day; or it may from the first have had several mouths, some in the position of its existing delta, others