1837.] 



the Assam Tea Plant, 



42; 



pass within, the'mountains on the frontier of China. It forms the na- 

 tural boundary between the Abor, and the Mishmee tribes. 



The third branch of the Bramaputra, is that which retains the name 

 of the great river, from its falling straight into the axis of the main 

 trunk, from the opposite, or eastern extremity of Assam. It enters the 

 valley by a series of cascades, it is said, rather than by a deep defile : 

 and indeed this peculiarity is distinguished at a distance of thirty miles, 

 by the accumulations of rolled stones which have been propelled for- 

 ward, causing a succession of rapids which gradually increase in num- 

 ber and difficulty. This river, after its entrance into the valley, re- 

 ceives the Digaroo and Kondul rivers from the Mishmee mountains on 

 the north, and the Noa Dihing and Tenga from the south-east. For 

 reasons to be afterwards assigned, it will be well to keep the direction 

 of the two last named rivers in view. 



The fourth great branch of the Bramaputra, called Suban-Shieree, 

 takes its origin, it is supposed, in Thibet; and enters Assam from the 

 north, below the junction of the other great branches, and at right 

 angles with the great fluviatile trunk. From its southerly direction, 

 and the peculiarity of its source in the snows of an elevated chain, 

 from which it descends transversely, the inundations of this river take 

 possession of Assam, previous to those of the other rivers. The silt 

 which its floods convey into the valley (where they spread and lose 

 their impetuosity), is consequently deposited so as to impede the course 

 of the Bramaputra. The great stream is thus caused to diverge from 

 its direct course, and from time to time to force its way, by the bursting 

 of new channels. Between two of these at present, an island, sixty- 

 miles long and ten broad, is formed at the confluence of the Suban- 

 Shieree, chiefly by its silt. 



It has now been shewn that Upper Assam is an alluvial basin, form- 

 ed by the confluence of various great rivers, which flowing from oppo- 

 site directions, meet in its centre. I have been unable to determine 

 Whether the original surface underwent any subsidence of its former 

 levels, or whether the elevation of the rocks in Lower Assam was the 

 only cause of the production of the basin. The total absence of any- 

 rocky protrusions through the alluvium, and the general levels rela- 

 tively, between Upper and Lower Assam, as well as Bengal, being only- 

 such as to afford an ordinary current of about two miles or three an 

 hour, indicate a depression of the older rocks in Upper Assam. More 

 extensive observations, and experiments in boring, can alone deter- 

 mine this point. 



The rivers as they fall into the valley, precipitate such portion of 



