266 



a progressive motion of the finer particles of earth 

 from the mountains, towards the valleys." 



" Admitting this to be true, and also that a propor- 

 tion of it is swept away by the torrents : the longer 

 the rivers continue to run, the less quantity of earth 

 they must carry away with them ; and therefore the 

 increase of the deltas and other alluvions of capital 

 rivers, must have been more rapid in the earlier pe- 

 riods of the world? s age, than now."* 



This, however, is evidently a mistaken opinion; 

 for we may confidently assert, that the annual exten- 

 sion of deltas or alluvial formations, is in an increas- 

 ed ratio, proportionate to the improvement and culti- 

 vation of the districts in their vicinities, and par- 

 ticularly so in that of most maritime cities, situated at, 

 or near, the mouths of rivers ; nevertheless, it does 

 not depend, but in part, on the increased quantity of 

 alluvion brought down by the currents of such rivers. 



I have remarked that the formation of deltas in 

 general, depends on three causes : The first is that 

 of the alluvion of rivers, so called, being that which 

 is held suspended and brought down by their currents. 



This I shall proceed first to consider. 



The alluvion of all rivers, in the earlier periods of 

 time, and even at the present, when they run through a 

 country, overgrown with forest trees and covered with 

 leaves, or a thick sward, that extends to the utmost 

 extremity of their auxiliary branches, must necessarily 



* Rennell's Map of Hindoostan, page 257. 



