208 
ISLAND LIFE. 
[part I. 
The Mississippi 
removes 
one foot in 6,000 
„ Ganges 
99 
„ 2,358 
,, Hoang Ho 
99 
„ 1,464 
„ Rhone 
„ 
„ 1,528 
,, Danube 
99 
„ 6,846 
„ Po 
99 
„ 729 
,, Nith 
99 
„ 4,723 
Here we see an intelligible relation between the character of 
the river basin and the amount of denudation. The Mississippi 
has a large portion of its basin in an arid country, and its sources 
are either in forest-clad plateaux or in mountains free from 
glaciers and with a scanty rainfall. The Danube flows through 
Eastern Europe where the rainfall is considerably less than in 
the west, while comparatively few of its tributaries rise among 
the loftiest Alps. The proportionate amounts of denudation 
being then what we might expect, and as all are probably 
under rather than over the truth, we may safely take the aver- 
age of them all as representing an amount of denudation which, 
if not true for the whole land surface of the globe, will certainly 
be so for a very considerable proportion of it. This average is 
almost exactly one foot in three thousand years . 1 The mean 
altitude of the several continents has been estimated to be as 
1 It has usually been the practice to take the amount of denudation in 
the Mississippi valley, or one foot in six thousand years, as a measure of the 
rate of denudation in Europe, from an idea apparently of being on the 
“safe side,” and of not over-estimating the rate of change. But this 
appears to me a most unphilosophical mode of proceeding and unworthy 
of scientific inquiry. What should we think of astronomers if they always 
took the lowest estimates of planetary or stellar distances, instead of the 
mean results of observation, “in order to be on the safe side !” ? As if 
error in one direction were any worse than error in another. Yet this is 
what geologists do systematically. Whenever any calculations are made 
involving the antiquity of man, it is those that give the lowest results that 
are always taken, for no reason apparently except that there was, for so long 
a time, a prejudice, both popular and scientific, against the great antiquity 
of man ; and now that a means has been found of measuring the rate of 
denudation, they take the slowest rate instead of the mean rate, apparently 
only because there is now a scientific prejudice in favour of extremely slow 
geological change. I take the mean of the whole ; and as this is almost 
exactly the same as the mean of the three great European rivers — the 
Rhone, Danube, and Po — I cannot believe that this will not be nearer the 
truth for Europe than taking one North American river as the standard. 
