106 THE BOOK OF NATURE STUDY 



of the material it transports is obtained by this erosion, the majority 

 is but a modified form of that delivered to it by its lateral dis- 

 integrating agencies, screes at the higher levels, soil at the lower. 

 Its main function is to scavenge out the material disinte- 

 grated and delivered to it by all agencies working all over the 

 land surface. Thus the rivers will contain, until they are com- 

 pelled to deposit it, the whole of the material denuded on the 

 surface of the entire country, and the measure of that material at 

 the mouth of the river gives the rate at which the whole surface 

 of the land is being lowered by all types of subaerial denudation. 

 This rate will, of course, express the rate at which the water course is 

 being lowered, but each lowering of the river-bed will accentuate 

 the slope of soil and scree towards the river, and quicken the 

 downward movement of such material to it. Hence the total 

 delivery of the river to the sea will measure also the general 

 lowering of the entire surface of the country by superficial de- 

 nudation. 



The rate of lowering by delivery of debris to the sea varies 

 considerably under different circumstances. The Mississippi 

 lowers its basin at the rate of i foot in 6000 years ; the Danube 

 is somewhat slower, i foot in 6840 years, while the swift-flowing 

 torrent of the Po is responsible for lowering its basin at an average 

 rate of i foot in 730 years. To this must be added matter in 

 solution, which averages the removal of i foot in 13,200 years, 

 while the work of the sea is about equal to the river average of 

 i foot in 3000 years. 



This study of denuding agents makes it clear that the land of 

 the world is being steadily destroyed at the rate of about i foot in 

 1350 years, and the products carried to the sea margins. At the 

 rates given the whole land of Europe, of an average height of 

 671 feet above sea level, would be planed down and disappear 

 in less than a million years, its debris being spread out beneath 

 the ocean. The inevitable conclusion is either that denudation 

 could not have been going on for all this time at this rate, or 

 that new land has been formed to compensate for the continuous 

 destruction. This proposition must now be discussed in the light 

 of conclusions drawn from the disposal of denuded material. 



