1863.] FERGUSSON DELTA OF THE GANGES. 327 



Fig. 2. — Diagram illustrating the Secular Elevation of Deltas. 



Patna. 



50 miles. 



200 miles. 



20, and during the last 13 or 14, or less than one-third of what was 

 due to the accumulation during the first period. But this is far from 

 representing the whole truth ; first, in consequence of the greater 

 quantity of silt brought down by the river when its slope was 12 

 inches, which it must have been during the first period, instead of 

 less than 6 inches as it is now. And, secondly, the area or breadth 

 of the delta is necessarily less near its apex than at its base, and, 

 consequently, the area over which the silt had to be deposited much 

 less than at present. 



From these and other minor causes I feel convinced that if we 

 assumed the deposit to be 50 feet, or one-half, during the first 

 period, it would be rather under than over the mark ; and, say, 25 

 feet during the second, 12 or 13 during the third, and 5 or 6 during 

 the last of the four periods indicated in the diagram. From the 

 conformation of the ground I should be inclined to assign a higher 

 rate of progression for the rise of the land of Egypt in the neigh- 

 bourhood of Memphis. But it is difficult to speculate on the phe- 

 nomena of that river while we remain so ignorant of the physical 

 condition of the country from which the inundation proceeds. But 

 even more uncertainty has been superinduced, as pointed out above; 

 by artificial obstructions. 



All this is assuming that the silt is distributed quite evenly over 

 the whole delta. This, however, is practically very far from being 

 the case. The mode in which deltas are raised is by a river, flowing 

 through some low part of the country, gradually embanking itself, 

 and then raising its bed till the body of its water is higher than some 

 neighbouring region ; it then falls into this, and, going through the 

 same process, it fills that depression, and then goes on to the next. 

 After a long cycle of years it comes back again to the country it first 

 left, which probably has not risen 1 foot since, while the neighbour- 

 ing country may have been raised 30 or 40. 



From these data it will be perceived how fallacious any conclusions 

 must be which are drawn from borings into the strata of deltas, and 

 calculations formed from local superficial deposits. I myself have 

 seen the bricks which formed the foundation of a house I had built 

 carried away, and strewed along the bottom of a river at a depth of 



z2 



