ALLUVIUM. 1 



the increased fall and tlie cutting thereby entailed, produce features very- 

 similar to those now presented by the older alluvium, but in a far 

 less striking manner than would result from the partial depression of 

 the delta proper ; and such a movement would not at aU account for 

 other features of the area under notice, such as the existence of the Silhet 

 jheels : The delta of tjie older alluvium, although its outer margin need 

 not perhaps have been more advanced than that of the present delta, 

 must have been much more formed ; and simple denudation, due to the 

 cause we are now supposing, could not have reduced any part of its 

 surface to a level so far below that of the surface of fluviatile deposition 

 (calculated from the supposed outer margin), as is that of the Silhet jheels. 

 The argument, indeed, cannot proceed so far ; for, if the surface of the 

 older alluvixmi is not now, as I have just conjectured, more elevated than 

 the limiting plane of deposition due to the actual seaward base-line, an 

 upheaval which brought it rapidly to that level could not cause the 

 extensive denudation we are seeking to account for.* We are thus com- 

 pelled to adopt the supposition of the depression of a large portion of the 

 old delta ; and it must have been a very extensive operation to produce 

 the features now presented by the older alluvium — effects reaching to the 

 upper limits of the alluvial plains, as evinced by the permanent valleys 

 (khaxiars) through those older deposits in which the great rivers flow for 

 the upper part of their course, reversing there the conditions of delta- 

 forming rivers.f The position of the Madhopur area in the middle of 

 the delta is very important as giving some clue to the limits of the area 

 of depression. The conclusion thus arrived at as to the depression of 



* The assertions running througli these sentences depend upon certain propositions 

 regarding the growth of fluviatile deposits illustrated by Mr. Fergusson, and which are 

 in some degree or other of mathematical certainty. 



t This feature of the upper plains, the formation of permanent doais, might perhaps 

 to some extent he produced by the gradual erosion of the steep torrential gorges of the great 

 rivers within the mountain area, entailing the formation of corresponding channels through 

 previously deposited alluviuin in the plains, 



( 157 ) 



