202 
ARE SOILS ENRICHED, IMPOVERISHED, [Part III. 
plains compared with the unduly large size of 
the channels of their upper parts, or of torrents. 
Lyell tells us that, from the sea to the upper 
end of the alluvial plain of the Mississippi (800 
miles, including 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 ad¬ 
vance of the delta, and that, as the delta con¬ 
tinues to advance, the rise will increase. In this 
light, seas as well as rivers may be considered as 
filling valleys, as well as excavating them. 
The steeper the slope and the more rapid the 
stream, the straighter. It is in the alluvial part 
that the river winds, and shifts its course. But 
u erosion of banks” and “ acting on lines” must 
not be claimed from these changes; for, besides 
that the river fills up the old course which it has 
left, in the change it only takes what it had be¬ 
fore deposited, and had brought from a distance. 
The Adriatic is filling up from its tributary 
valleys, and 100,000 years may see it an alluvial 
plain, the Po running through it, and falling 
into the Mediterranean. 
In this case, would the alluvial plain be at a 
dead level, or would it slope ? I think it would 
slope from the head of the Adriatic to the Me¬ 
diterranean. If so, as the level of the sea would 
