332 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [-^P r « 1> 



of the rocks of which these hills are composed, or because the violence 

 of the monsoon has long ago denuded them of every moveable par- 

 ticle, these rivers bring down little or no silt even in the height of 

 the monsoon, and are quite clear in the cold weather. 



The Brahmapootra, on the other hand, depends for its floods 

 partly on the melting of the snow, partly on the far more moderate 

 and later rains of the valley of Assam, probably hardly exceeding 

 100 or 120 inches ; having also a longer course to run, it arrived later 

 at the scene of the struggle, and found the country already occupied by 

 the waters of the Megna to such an extent as to be able to dam back 

 its waters for the first month of the rains, and to force it to deposit its 

 silt in its own bed. It could not, of course, have done this with any 

 effect until the large river had reached the higher lands beyond the 

 jheels to the southward, and with its silt had bridged across the whole 

 width of the jheel-country, and, by this process, had embanked 

 itself along* the whole extent, so as to make it difficult for it to 

 change its course. So long as the Brahmapootra was only forming an 

 inland delta in the depressed country, the Megna had no hold upon 

 it ; but when it came to flow in what was practically an aqueduct, 

 along the top of an embankment of its own making, it was rendered 

 powerless, and the struggle was soon over. Had it not been for the 

 upheaved tract already alluded to, it would, of course, have sidled 

 away westward, and so have avoided the contest with the Sylhet 

 rivers ; but that being impossible, we find it retracing its steps nearly 

 seventy miles northwards, and finding a new channel for itself above 

 Dewangunge in the bed of the Jennai. 



3. Eastern Gap in the Seaward Face of the Bella. — Before leaving 

 this branch of the subject, it may be well to allude to another 

 geographical fact, which I believe to have been in a great measure 

 the result of this diversion of the Brahmapootra into the Sylhet 

 Jheels. It is the great gap or gulf that exists to the eastward of 

 the Gangetic half of the delta. 



From the Hoogly to the Horringotta the seaward face of the 

 Sunderbuns is tolerably level and fixed ; at all events, it has under- 

 gone no sensible change within any period to which our knowledge 

 extends ; and, so far as can be ascertained, it shows no tendency to 

 go forward. In that portion of the delta, however, allotted to the 

 Brahmapootra a great deal of work has yet to be done ; everything 

 there is so new, and in such a constant state of change, that, even 

 in that climate, vegetation has not been able to settle upon the 

 islands, and these are continually moving and changing their places. 

 A great deal has been done to fill up the gap since the Brahma- 

 pootra last changed its course, in the beginning of the present cen- 

 tury ; but we want a new survey to be quite sure to what extent 

 this has gone. If I am correct in my view, that the gap is mainly 

 the result of the straining of the waters of the Brahmapootra 

 through the Sylhet Jheels, and their consequently reaching the Bay 

 of Bengal deprived of all their silt, it follows that the process of 

 filling up will now be comparatively rapid, and that the eastern face 

 of the delta will assume before long the same fixed character which 



