of the Calcutta basin. 455 



will now consider whether the deposits took place during 

 the interval alluded to, as they do at present. 



A submarine basin, or a lake filled up by the uniform dis- 

 charge of sediment from a great river, will exhibit in its 

 lower parts a deposit of water-worn fragments, more or 

 less coarse, according to the velocity of the current, and the 

 distance the transported matter has been carried forward. 

 As the cavity of the basin becomes obliterated, the velocity 

 of the currents entering it will also proportionally diminish, 

 and the sediment from being coarse gravel, will gradually 

 pass into fine sand, and this last into clay and mud. So far 

 from the section afforded by Lieut. Smith of the boring 

 operations in Fort William presenting this uniform gradation, 

 we find various beds of fine clay and sand alternating with 

 beds of gravel, several of these different beds being nearly 

 100 feet in thickness. 



Thus proving considerable variation in the velocity of the 

 currents from which they were derived, and the lapse of 

 a considerable period of time in some cases from the termina- 

 tion of one change to the introduction of another. 



The coarse conglomerate from 392 feet down to 480, 

 where the work terminated, corresponds with that which is 

 rolled forward, as Mr. Smith has remarked, by mountain 

 streams. The large branch of the Bramaputra which fails 

 into Assam from the Eastward, is the greatest and most 

 rapid torrent we have seen in India, and yet its power of 

 transporting gravel is lost at some distance above Suddyah, 

 or within about 30 miles of the Mishmee mountains, from 

 which the river falls. In the lesser, but equally rapid 

 streams in Assam we find the distance to which gravel is 

 transported to vary from five to twenty miles, and on the 

 surface of these coarse gravels we often see occasional frag- 

 ments of bones of, various animals, pieces of coal, and trunks 

 of trees, the latter often changed to lignite, especially when 

 buried in the hot sands, which during the dry season are 

 exposed to a parching sun. 



