276 DISCUSSIONS IN COSMOLOGY. 



Rough estimates have been made of the sediment 

 carried down by some eight or ten European rivers ; 

 and although those estimates cannot be depended upon 

 as being anything like accurate, still they show that 

 it is extremely probable that the European continent 

 is being denuded at about the same rate as the 

 American. 



I think we may assume, without the risk of any 

 great error, that the average rate of sub-aerial 

 denudation during past geological ages did not differ 

 much from the present. The rate at which a country 

 is lowered by sub-aerial denudation is determined (as 

 has been shown, ' Climate and Time,' p. 334) not so 

 much by the character of its rocks as by the sedi- 

 ment-carrying power of its river systems. And 

 this again depends mainly upon the amount of rain- 

 fall, the slope of the ground, and the character of the 

 soil and vegetation covering the surface of the country. 

 And in respect of these we have no reason to believe 

 that the present is materially different from the past. 

 No doubt the average rain-fall during some past 

 epochs might have been greater than at present, but 

 there is just as little reason to doubt that during other 

 epochs it might have been less than now. We may, 

 therefore, conclude that about one foot of rock removed 

 from the general surface of the country in 6000 years 

 may be regarded as not very far from the average rate 

 of denudation during past ages. 



But some of the cases we have given of great denu- 

 dation refer to comparatively small areas, and others 

 to beds which form anticlinal axes, and which, as is 

 well known, denude more rapidly than either synclinal 

 or horizontal beds. We shall therefore, to prevent 

 the possibility of over-estimating the length of time 

 necessary to effect the required amount of denudation, 



