IX.] THE WORK OF RAIN AND RIVERS. 149 



present rate of denudation, it would require about 5^ 

 million years to reduce the British Isles to a flat plane at 

 the level of the sea. It must be remembered, however, 

 that such calculations are beset with grave difficulties, and 

 that the results can only be put forth as rough approxima- 

 tions. Nevertheless, they are not without their value in 

 enabling us to form a conception of the great wearing down 

 of land which must be effected by rain and rivers. 



