ESTUARINE. 257 



matter of terrestrial uprise than of sedimentary accumula- 

 tion. As with the estuarine plains of our own islands, so 

 to a great extent with those of other regions : they owe 

 their accumulation wholly to silt and sediment, "but their 

 conversion into dry land, partly to silting, and partly to 

 terrestrial upheavals. 



Whatever their mode of accretion, the composition of 

 these estuarine formations is much the same in every 

 region : mud -silts, clays, sands, gravels, drift-wood, shell- 

 beds, peat and swamp earths the whole being usually 

 surmounted by loamy, vegetable soils of extraordinary fer- 

 tility. Their imbedded remains are partly terrestrial, partly 

 fresh-water, and partly marine ; and these, of course, will 

 differ according to the latitudes in which the estuary occurs, 

 and the regions through which its affluent rivers flow. 

 Thus the Mississippi will sweep down the terrestrial and 

 fresh- water spoils of temperate North America, the Amazon 

 those of tropical South America, the Niger those of Equa- 

 torial Africa, and the Ganges and Irawaddy those of sub- 

 tropical Asia. Every estuary, in fact, is characterised by 

 its own fossil flora and fauna, and these of varying anti- 

 quity, from the spoils swept down by the latest land-flood 

 or deposited by yesterday's tide, back to the confines of the 

 glacial epoch, if in the higher latitudes, and it may be to 

 the tertiary itself, if occurring in intertropical regions. In 

 the estuarine silts of our own islands, for example, we pass 

 through every gradation of antiquity, from the plants and 

 animals now flourishing around us, back through those 

 which, like the bear, wild-boar, wolf, and beaver, have 

 long since been ^extirpated, and from these backwards still 

 to the seals, whales, and boreal shells that inhabited our 

 firths and estuaries in times immediately post-glacial. The 

 reader may readily trace this gradation in the estuarine 

 deposits of the Clyde, Forth, Tay, or any other of our 



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