RATE OF SUB-MR1AL DENUDATION. 167 



mean level of Europe at 600 feet, and supposing it everywhere worn 

 clown uniformly at this rate, little more than three and a half millions 

 of years would be required to wear it level with the sea. But de- 

 nudation is far more rapid in valleys and mountain regions ; so that 

 while T V of an inch may be removed in seventy-five years from plains 

 and table-lands, an equal amount of denuded matter is carried away 

 from valleys in eight years and a half, so that a valley 1000 feet 

 deep might be excavated in 1,200,000 years. 1 



1 Geikie, Geographical Evolution. Proc. Roy. Gepg. Society, July 1879. 



