28 A. Tylor on Changes of the Sea-Level effected by 
physical conditions is sufficient to raise the ocean level 3 or 4 
inches in 10,000 years, provided no subsidence or elevation dis- 
turbed the result. 
To this subject [now proceed. Sir Charles Lyell’s published 
statements of the quantity of mud annually carried down by the 
Mississippi and Ganges appear to have been made with so much 
care, that they may be a better guide to the general rate of remo-- 
val of soil by rivers than information obtained from a greater 
number of smaller rivers, which of course are more likely to be 
influenced by local circumstances. Eleven hundred thousand 
square miles of land are drained by the Mississippi,* which annu- 
ally discharges a quantity of water equal in volume to 4 inches of 
rain, or about one-tenth of the total rain-fall over this entire sur- 
face, which forms one-fifth part of North America.+ From the 
mean of a great number of observations, the average quantity of 
alluvium suspended in the water appears to be 1 part in 3000. 
Consequently, as the water annually drawn off would cover an 
area of eleven hundred thousand square miles to the depth of four 
inches, the quantity of mud removed in the water (as measured 
at or near the mouth of the river) would.cover the same exten- 
sive surface to the depth of ,,),;dth part of four inches, or to the 
depth of ;,';;dth part of a foot. Or, in other words, the Missis- 
Sippi at its present rate would occupy 9000 years in carrying 
away detritus before the mean surface level of one-fifth part of 
North America would be reduced one foot. 
‘The Ganges discharges into the Indian Ocean a supply of wa- 
ter equal to about six inches of rain on 400,000 square miles, ora 
much greater volume of water than the Mississippi pours into the 
Gulf of Mexico, taking into consideration the difference in size 
of the countries they drain. PTs 
which it is derived to the depth of +,'-+ of a foot ; that is to say, 
2737 
: propose to estimate (aS — 
before mentioned) that only half the land contributes detritus iD - 
* See art. Meiers, Penny Cyclopedia, vol. xxv, p. 277. 
+ The total rain-fall of the United States is 39 inches between 24}° and 45° N. 

