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Ch. XIX.] 



AGE OF EXISTING DELTAS. 



485 



a partial union at first takes place by the confluence of some 



one or more 



arms 



it is 



mam 



common 



that a complete intermixture of their joint waters and sedi- 

 ment takes place. 



The 



union, therefore, of the Po and 



Adige, and of the Ganges and Brahmapootra, is still incom- 

 plete. If we reflect on the geographical extent of surface 

 drained by rivers such as now enter the Bay of Bengal, and 

 then consider how complete the blending together of the 

 greater part of their transported matter has already become, 

 and throughout how vast a delta it is spread by numerous 

 arms, we no longer feel so much surprise at the area occu- 

 pied by some ancient formations of homogeneous mineral 

 composition. But our surprise will be still farther lessened, 

 whan we afterwards enauire (Ch. XXIL") into the action of 



tides and currents in disseminating sediment. 



Age of existing deltas. — If we could take for granted, that 

 the relative level of land and sea had remained stationary 

 ever since all the existing deltas began to be formed — could 

 we assume that their growth commenced at one and the 

 same instant when the present continents acquired their 

 actual shape — we might understand the language of geolo- 

 gists who speak of ' the epoch of existing continents. 5 They 

 endeavour to calculate the age of deltas from this imaginary 

 fixed period ; and they calculate the gain of new land upon 

 the sea, at the mouths of rivers, as having begun everywhere 

 simultaneously. But the 



more 



become 



movements of the land and contiguous bed of the sea have 

 exerted, and continue to exert, an influence on the physical 

 geography of many hydrographical basins, on a scale com- 

 parable in magnitude or importance to the amount of fluvia- 

 tile deposition effected in an equal lapse of time. In the 

 basin of the Mississippi, for example, proofs both of descend- 



w movements to a vertical amount of several 

 hundred feet, can be shown to have taken place since the 

 existing species of land and freshwater shells lived in that 



region.* 



Lyell's Second Visit to the United States, vol. ii. chap. 34. 





