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most abundantly nearest to the river, on its immediate banks. 

 The tendency of the banks to be raised in this way is greater in 

 the climate of the Mississippi than in that of the Nile, because of 

 the abundant vegetation of the former, which during inundations 

 acts in some degree as a filter, detaining the mud and letting the 

 water pass through. 



The part of a river valley where there is more deposition 

 than denudation may be called the alluvial part. In connexion 

 with the fact that in the alluvial part of a valley, the banks of 

 the river are generally higher than the rest of the country, is 

 the fact that in such regions the stream often divides or bifur- 

 cates, without uniting again. The district lying between the 

 arms of the river below its first or highest bifurcation, constitutes 

 the delta. The delta of a river is of course included in that part 

 of its valley where there is more deposition than denudation. 



The writer mentioned having once stood on the Roth- 

 horn mountain above the lake of Brientz in Switzerland, 

 and seen as in a map the little delta formed by the Aar as it flows 

 into the lake. It showed him the way in which deltas appear 

 to be formed, which he believes to be as follows : — 



It is well known that the quantity of silt which running 

 water is able to carry in suspension, depends on the velocity of 

 the current ; and consequently when the velocity of a stream 

 is checked by its flowing into the comparatively still water of a 

 lake or sea, a great part of its silt is deposited. When the sea 

 is not too deep, and the silt not removed by currents, an alluvial 

 island is thus formed in front of the mouth of the river. By 

 this island the stream of the river is divided into two, forming 

 two mouths ; in front of each of these mouths another island 

 is formed, producing further divisions ; and so on indefinitely. 



It frequently happens that these islands are separated, not 

 merely by river channels, but by extensive shallow lakes. Such 

 lakes are characteristic of deltas ; of this kind are the Sea of 

 Haarlem in Holland, now drained and converted into land, and 

 Lake Pontchartrain near New Orleans. But Lake Mareotis, 

 and the other lakes of the Egyptian delta, appear to be formed 

 in quite a different way, by the action of wind and surf throwing 



