Part III.] OR POISONED BY VEGETABLE GROWTH? 
201 
of the river always at a dead level. Now a river 
flowing for 200 miles, or even fifty miles, or one 
mile at a dead level, is very ill-calculated to dis¬ 
charge the floods poured into it in the rainy sea¬ 
sons from inclined channels. The consequence 
is ponding back, overflow, and deposit from the 
overflow: but the banks, catching the first and 
heaviest deposit, grow more quickly than the 
plain, and the last yard protruded at the level of 
the sea, by preventing the spreading of the 
water, tends to force it over the yard behind it, 
and to raise that yard above the level of the sea; 
and so every yard raises the yard behind it, in 
succession, to the highest point of the alluvial 
plain ; though in any part, and particularly in 
the higher part, the growth of the plain may 
(from excess of the longitudinal over the lateral 
deposit) keep pace with the growth of the banks, 
—and possibly banks thus built may tend to as¬ 
sume that slope or gradient lengthwise which, 
when flooded, would allow the water to escape 
sideways at the same instant, and at the same 
depth, everywhere. 
And this tendency to a gentle simultaneous 
overflow of the whole banks prevents their ero¬ 
sion and the enlargement of the channel in the 
flood season, and, I think, is the reason of the 
small size of the channels of rivers in alluvial 
