208 



ISLAND LIFE. 



[part I. 



The Mississippi removes one foot in 6,000 years. 



Ganges 

 Hoang Ho 

 Rhone 

 Danube 

 Po 

 Nith 



2,358 

 1,464 

 1,528 

 6,846 

 729 

 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.^ The mean 

 altitude of the several continents has been estimated to be as 



^ 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. 



