MEASUREMENTS IN GEOLOGY 187 



furnish valuable materials for estimates of this kind. 

 The universal degradation of the land, so notable a 

 characteristic of the earth's surface, has been regarded 

 as an extremely slow process. Though it goes on 

 without ceasing, yet from century to century it seems 

 to leave hardly any perceptible trace on the landscapes 

 of a country. Mountains and plains, hills and valleys, 

 appear to wear the same familiar aspect which is 

 indicated in the oldest pages of history. This obvious 

 slowness in one of the most important departments 

 of geological activity, doubtless contributed in large 

 measure to form and foster a vague belief in the 

 vastness of the antiquity required for the evolution 

 of the earth. 



But, as geologists eventually came to perceive, the 

 rate of degradation of the land is capable of actual 

 measurement. The amount of material worn away 

 from the surface of any drainage-basin and carried in 

 the form of mud, sand, or gravel, by the main river 

 into the sea, represents the extent to which that 

 surface has been lowered by waste in any given period 

 of time. But denudation and deposition must be 

 equivalent to each other. As much material must be 

 laid down in sedimentary accumulations as has been 

 mechanically removed, so that in measuring the annual 

 bulk of sediment borne into the sea by a river, we 

 obtain a clue not only to the rate of denudation of the 

 land, but also to the rate at which the deposition of 

 new sedimentary formations takes place. 



As might be expected, the activities involved in the 

 lowering of the surface of the land are not everywhere 

 equally energetic. They are naturally more vigorous 



