256 RECENT FORMATIONS. 



fresh-water organisms, and thus they are regarded as estu- 

 arine or fluvio- marine, in contradistinction to those of 

 strictly fluviatile or lacustrine origin. The low-lying deltas 

 of the Mississippi, the plain of Lower Egypt, the jungle- 

 swamps of the Niger, the sunderbunds or mud-islands of 

 the Ganges and Irawaddy, and the alluvial plain of China, 

 are familiar examples on a great scale of these estuarine or 

 deltic deposits; "but as with these, so with almost every 

 other river that discharges its waters into the ocean. The 

 magnitude of estuarine formations is one of the most 

 notable features in the geology of the current epoch, and 

 this magnitude is increased by a twofold process which the 

 reader would do well to consider. 



In the first place, the delta that makes its appearance as 

 dry land may form but a small portion of the sedimentary- 

 matter borne down by a river, the greater portion being 

 carried forward and projected, as it were, over the bed of 

 the ocean. An estuarine formation is thus partly sub- 

 aerial and partly submarine, and it necessarily requires a 

 long and gradual process of silting to convert the sub- 

 marine into sub -aerial. But during the oscillations or 

 crust-motions to which the earth is subjected, it frequently 

 happens that a whole island or portion of a continent is 

 gradually upraised, and thus the submarine portion of an 

 estuary may be upheaved into dry land, and this altogether 

 independent of the slow and ordinary process of silting. 

 An uprise of fifty feet would convert a large portion of 

 the Yellow Sea into a lower terrace of the great Chinese 

 plain ; and by a similar uprise thousands of square miles 

 of Mississippi swamp would assume the character of fertile 

 prairie-ground. To such upheavals much of the " carses " 

 of Scotland and " levels " of England are no doubt due ; 

 for though wholly composed of river and marine silts, their 

 final conversion into dry alluvial plains has been more a 



