CHAP. X 



THE EARTH'S AGE 



215 



the matter so deposited does not come down to the sea. 

 After a careful examination of all the best records, 

 Sir A. Geikie arrives at the following results, as to the 

 quantity of matter removed by seven rivers from their 

 basins, estimated by the number of years required to lower 

 the whole surface an average of one foot : 



The Mississippi removes one foot in 6,000 years. 



Ganges " 2,358 ,, 



,, Hoang Ho ,, ,, 1,464 



,, Rhone ,, ,, 1,528 



,, Danube ,, „ 6,846 ,, 



„ Po „ „ 729 ,, 



,, Nith 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 

 average of them all as representing an amount of denuda- 

 tion which, if not true for the whole land surface of the 

 globe, will certainly be so for a very considerable propor- 

 tion of it. This average is almost exactly one foot in 

 three thousand years.^ The mean altitude of the several 



•'• 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 



