54 GEOLOGICAL NOTES ON ASSAM. 



high ground^ one might conjecture that it was purely a suhaeriul forma- 

 tion^ hy the "wash of rain/-' to use Colonel Greenwood^s expression^ 

 But its appearance in the open plains implies some other possible mode of 

 production. We do not yet know enough of the many conditions o^ 

 river valley deposits to assert that it cannot have been formed by that 

 process. And as for the fact of its isolation as a raised area surrounded 

 by ground subject to inundation, such is a natural result of the process 

 advocated by Mr. Ferguson, and without the aid of local or of continental 

 upheaval. Were the case of the Madoopore jungle area an isolated 

 one, it might be most reasonable to explain it as a special upheaval ; but 

 it would be inadmissible to make such a supposition for every similar 

 case that might be cited. In the secular changes of a river during the 

 process of raising its valley, it may cut away previously formed land^ 

 leaving portions of it for an indefinite time projecting high over the 

 newly formed Jchadur (local river valley) ; or, the extensive destruction 

 of a delta, whether by settlement, or by subsidence, or by a permanent 

 change in the conditions of the receiving water basin, or even by some 

 rare occurrence of oceanic violence, might so disturb the status of the 

 river that it would have for a long period to prey upon previously formed 

 deposits throughout the greater part of its valley ; leaving portions of 

 them as we now find the older alluvium in the valley of the Ganges. A 

 very mild amount of such action would account for what I have described 

 in Upper Assam ; an erosion of a few feet in the river bed below Goalparra 

 would have allowed of some permanent dry land appearing in the eastern 

 districts. About Goalparra itself it seemed to me as if the hills alone were 

 secure from flooding. The difiference in kind between the old alluvium in 

 Assam and the silt deposits of the actual river courses is even more striking 

 than I have seen it elsewhere, although the origin of the former is most 

 unmistakably not marine, but fluviatile (under the extensive meaning 

 that must now be given to this word) . The other deposits difler greatly 

 on the opposite sides of the valley, as do also the sediments of the 

 ( 4.iO ) 



