Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/221

ROGERS — WESTERN INDIA. 121 further eastward, in the position of the Greater and Lesser Runns, to the west and east of Cutch, and again in that of the country between the Lesser Runn and the Gulf of Cambay. In this last-mentioned tract of country, coinciding to a great extent with the black-soil belt, there can be clearly traced a natural depression in the surface of the country for some twenty miles from the head of the Gulf, terminating in a shallow lake of brackish water called the Null. This lake, in the rainy season, is probably twenty miles long and three or four broad, and finds an outlet for its superfluous waters at that season through the Bhogava, which enters the Gulf at its north-west corner. Shells of the genus Cerithium, an estuarine form, are found lying loose in the black soil many miles from this point; and the records of the old Revenue Survey of Goozerat state that there were formerly found in the Null large round stones with holes through them, which had evidently served as anchors for boats of some size. In all this we have strong presumptive evidence of a connexion between the Gulf of Cambay and the Runns at a comparatively modern period; and there is historical and well-known proof of the alteration of the level of the larger of these salt flats as the consequence of an earthquake in a.d. 1819. May we not, then, imagine a time before the great trap effusions of Western India had been poured out, when the granite rocks on the west and north-east of the Gulf were wave-beaten islets surrounded by the Indian Ocean, which then probably covered the whole region up to the Cretaceous formations of Central India, at the mouth or one of the mouths of the Indus or some corresponding great river? In a report lately made by Mr. Blanford, of the Geological Survey of India, the age of the great trap formation of the Deccan and Malwa, of which that lying on the east of the Gulf of Cambay forms a part, has been determined to be between the Cretaceous and the Lower Eocene. This conclusion has been come to mainly from an examination of the Nummulitic formation between the Taptee and Nerbudda rivers. Mr. Blanford has also shown, from an examination of these traps and the intertrappean formations of Bombay and other places, that there have been not one but many successive eruptions and accumulations, probably extending over long periods. May it not, then, have been that previously to any of these the Indus had, if not its sole, at least its most easterly mouth in the Gulf of Cambay? that on the upheaval of the trap the tract to the west and south-west of the primary and metamorphic rocks to the north-east of the gulf was raised, and has since risen gradually so as to throw the course of the river more and more to the westward, drying up successively its channels of communication with the Gulf of Cambay and the Lesser and Greater Runns until it attained its present course and exit into the Indian Ocean near Kurrachee? If there is truth in the theory that rivers running towards the equator from the north and south have a tendency, in consequence of the earth's diurnal motion, to wear away their western banks, this may have assisted the westward motion of the river. Before the final upheaval of trap on the north-east of