PLEISTOCENE RIVER-DEPOSITS. 



129 



one foot of rock from the general surface of their basins in the 



The Mississippi, therefore, is lowering the surface of the great 

 area it drains at the rate of one foot in 6000 years, which would 

 give 100 feet in 600,000 years. At the rate at which the Po 

 works 100 feet would be removed in 72,900 years. Of course it 

 will be understood that the whole surface of a country does not 

 suffer denudation to an equal degree. Some districts, owing to 

 a variety of circumstances, such as differences in the composi- 

 tion and geological structure of the rocks, inequalities of rain- 

 fall, variations in the configuration of the ground, and so forth, 

 are lowered at a more rapid rate than others, the chief amount 

 of waste going on along the course of the valleys. According 

 to the calculations, it appears that the mean annual quantity of 

 detritus carried to the sea may with some probability be regarded 

 as equal to the yearly loss of -g^o °f a f°°t °f roc ^ : fr° m the 

 general surface of the land, the larger proportion of the loss 

 being sustained by the valleys and sloping surfaces. To appor- 

 tion this loss between the different parts of a land -surface 

 can of course only be done in a rough-and-ready manner. For 

 the sake of illustration we may assume, with my brother, that 

 the erosion of the surface is nine times greater over the valleys 

 than over the plains and tablelands ; — while the more level parts 

 of the country have been lowered one foot, the valleys have 

 lost nine feet. " Apportioning this loss over the surface in the 

 ratio just given, we find that it amounts to -f of a foot from the 

 more level grounds in 6000 years, and five feet from the valleys 

 in the same space of time. Then if -| of a foot be removed from 

 the level grounds in 6000 years, one foot will be removed in 



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