350 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 1, 



redeemed from jungle and swamps, during the last century, there 

 seems nothing to contradict this theory of the very modern origin of 

 the present configuration of the valley of the Ganges. 



It is not, of course, meant to be asserted that the valley of the 

 Ganges was filled up, geologically speaking, within that period, but 

 only that it became fit for man's occupation within the limits of the 

 historical period, as hundreds of square miles of the Delta have 

 become since BennelTs survey was made. 



The greater part of the valley of Assam still remains — what the 

 plains of Bengal may be conceived to have been 2000 or 3000 years 

 ago — uninhabitable swamps, with occasional spots where cities have 

 existed or do now stand. But if the principles enunciated above 

 are to be depended upon, the recent changes in the course of the 

 Brahmapootra ought rapidly to affect the level of the land in that 

 valley ; and it cannot possibly require a thousand, or half that 

 number of years, before the swamps opposite Goalparah and Goa- 

 huttee become as dry and as habitable as the plains of Purneah in 

 Tirhoot. 



§ IV. Increase of the Delta seaward. 



1. Silt held in suspension in Ganges' Water. — It will have been 

 observed that, in the previous part of this paper, I have said 

 nothing about the quantity of silt contained in the water of the 

 rivers I have been describing, nor attempted to calculate its in- 

 fluence either in extending the delta seaward, or in raising it 

 upwards. I have refrained from alluding to this simply because 

 I know of no data on which any reliance can be placed. 



To base any calculation on this agent, the experiments ought 

 to be continued for, at the very least, one whole year, on some one 

 at least of the larger rivers. But this has not yet been done ; and, 

 even if it were done for the Ganges, it must be nearly useless unless 

 we had the same knowledge as regards the Brahmapootra, which I 

 believe to be an infinitely more important stream in this respect than 

 the Ganges itself. And we ought also to know what is brought down 

 by the Mahanuddee, and the group of Natore streams debouching 

 through the Oorasagur. 



The latter rivers run through so low a country that they probably 

 deposit most of their silt en route ; but the Mahanuddee and its tri- 

 butaries are swift, and strongly embanked. The Sylhet rivers may 

 probably be disregarded ; they never possess much silt, and what 

 little they have they deposit at home, so they contribute little or 

 nothing to the delta. Supposing, however, all this were ascertained 

 for every river just as it enters the delta, another very important 

 question arises — How much is deposited on the plains of the delta, 

 and how much carried to sea ? During the cold weather, when the 

 rivers are low, almost all their silt will be earned to sea ; but then 

 the quantity of water is small, and that little comparatively clear. 

 At the height of the inundation, when the river is overflowing its 

 banks, at least one-half is deposited inland. As the rivers fall, the 

 greater part will again be carried away ; but as the force of the 



