PT. nr. OR POISONED BY VEGETABLE GROAVTH ? 177 



plains compared with the unduly hirge size of the 

 channels of their npper parts, or of torrents. 



Lyell tells us that from the sea to the upper end of 

 the alluvial plain of the Mississippi (800 miles, includ- 

 ing the delta) there is a rise of three inches in a mile, 

 amounting to 200 feet * I imagine that this rise is the 

 effect of the advance of the delta, and that, as the delta 



* This was thus published in the second edition of this bof^k 

 in 1853. From Ellett's Mississippi, pubhshed in the same year, 

 and which will be alluded to in the sequel, I find that the slope 

 of the surface of the river is about three inches in the mile, but 

 the slope of the land about eight ; and I think that an approach 

 to these two slopes may be universal in alluvial plains. The 

 banks of an alluvial river are built by deposit from the overflow 

 of its waters. Therefore the height of the banks above low 

 water at any part is the difference between the high water and 

 low water. This difference at the sea is nothing, therefore the 

 banks die off to nothing at the sea ; and I think that there must 

 be a fixed rule for the height of alluvial banks according to the 

 distance from the sea ; in other words, that there must be a fixed 

 certain rule for the increase of the rise of high water necessary to 

 overflow alluvial banks directly as their increased distance from 

 the sea. I have seen somewhere that at Mendes it required a 

 rise of seven cubits to overflow the banks of the ISTile, at Memphis 

 fourteen, at Syene twenty-eight. The rule, however, would apply 

 only where the banks and bed are completely alluvial. The 

 deposits on the unalluvial valley must tend to die off to nothing 

 at the upper end of the alluvial valley, unless the slope of the 

 unalluvial valley is suddenly terminated by a ridge of rock. I 

 only talk of general tendencies, and these will be perpetually in- 

 fluenced by accidental causes, both general and local : as the 

 perpetual variation, general and local, of the quantity and sud- 

 denness of rain, or the mode in which flood- water escapes. For 

 instance, in some places it may be obliged to return over the 

 same ground to the river ; or, in deltas, it may escape by lateral 

 forward channels to the sea ; in alluvial plains, by longitudinal 

 channels parallel to the main channel and entering the river 



