260 OSCILLATION OF LEVEL. [CHAP. XXXIV. 



overflow the adjoining plains, raising their level by 

 repeated layers of fluviatile matter or silt, frequently 

 containing the shells of land and amphibious mol- 

 lusks. 



If, even now, the Mississippi, when flooded, dams 

 up the mouths of its great tributaries, and transforms 

 them for months into temporary lakes, it must have 

 produced the same effect to a far greater extent if at 

 any time the general fall of the country towards the 

 sea was less rapid. 



In narrow valleys bounded by ancient rocks 500 

 or 600 feet high, such as that of the Ohio, the 

 alluvial formation could never acquire great breadth. 

 Its thickness would depend entirely on the length of 

 time throughout which the subsidence was prolonged. 

 But nearer the sea, where the continent falls with a 

 gentle slope towards the Gulf, the encroachment of 

 the freshwater deposits (No. 2. fig. 11. p. 262.), 

 of the great river on the tertiary strata (No. 3.), 

 constituting the original bluffs on its eastern and 

 western boundaries, might be very great. 



If we then suppose the downward movement to 

 cease, and to be at length converted into an ascend 

 ing one, the rate of upheaval being greatest in the 

 more inland country, the fall of every river, and con 

 sequently its velocity, would begin immediately to 

 augment. Their power of carrying earthy matter 

 seaward, and of scouring out and deepening their 

 channels, would be greater and greater, till at length, 

 after a lapse of many thousand years, each of them 

 would have eroded a deep channel or valley through 

 the fluviatile formation previously accumulated. The 



