EXCURSIONS OF AN EVOLUTIONIST 



formed this enormous amount of work in some- 

 thing less than a million of years." l 



Now all the fossil-bearing rocks on the globe 

 have been formed from the sediment brought 

 down by rivers to the sea, and this sediment has 

 been worn off from the hills and valleys and 

 plains of ancient continents. In recent years it 

 has been attempted to calculate the amounts of 

 sediment worn off by various great rivers from 

 the surface of the regions drained by them ; and 

 the results are very interesting and instructive. 

 The Mississippi, for example, draining a country 

 with scanty rainfall, and having its sources in the 

 Alleghanies and the Rocky Mountains, where 

 there are no glaciers, performs its work of denu- 

 dation slowly. The Mississippi wears off from 

 the whole immense area drained by it about one 

 foot in 6000 years. While the Po, on the other 

 hand, having its sources in the glaciers of the 

 Alps, works with great rapidity, and lowers the 

 area drained by it at the rate of one foot in 729 

 years. The mean rate of denudation over the 

 globe seems to be not far from one foot in 3000 

 years. Now at this rate, and from the action of 

 rivers alone, it would take only two million years 

 to wear the whole existing continent of Europe, 

 with all its huge mountain masses, down to the 

 sea-level, while North America, in similar wise, 

 would be washed away in less than three millions. 



1 Croll, Climate and Time, p. 327. 

 12 



