The ideas expressed in this paper are the outcome of a study of the Punjab oil-belt. The principal hypothesis proposed has been advanced simultaneously by Dr. H. G. E. Pilgrim. F.G.S., in a paper still in manuscript, to explain the formation of the Siwalik boulder-conglomerates. That paper will, I hope, soon be published, but I have taken the liberty of following up an interesting point to which Dr. Pilgrim has drawn attention: namely, the frequent V-shaped course of the north-bank tributaries of the Ganges where they leave the belt of Siwalik deposits. Briefly, the hypotheses that I desire to bring forward may be summarized as follows:— (i) That in Eocene times a gulf extended from Sind northwards as far as Afghanistan, and thence curved eastwards and south-eastwards through Kohat and the Punjab to the neighbourhood of Naini Tal. (ii) That this gulf gave place to a great river, the head-waters of which consisted of the portion of the Brahmaputra flowing through Assam. This river flowed westwards and north-westwards along the foot of the Himalaya us far as the North-West Punjab, where it turned southwards along a line not very different from that of the modern Indus, and emptied itself into the Arabian Sea. In other words, the Assam Brahmaputra was once the head- waters of the Indus. (iii) That two separate rivers or two branches of the same river, debouching into the Bay of Bengal, cut back and beheaded this old Indus, the