GEOLOGICAL PRINCIPLES. 117 



solid matter carried down by the rivers to the sea. 

 This has been measured with considerable accuracy for 

 several important rivers; and by comparing the quantity 

 of matter, both in suspension and solution, with the area 

 of the river basin, we know exactly the average amount 

 of lowering of the whole surface per annum. It has 

 thus been calculated that 



The Mississippi removes one foot of the surface of 



its basin in 6000 years. 



' Ganges " " " 2358 " 



' HoangHo " " " 1464 " 



1 Rh6ne " " " 1528 " 



' Danube " 6846 " 



Po " " 729 " 



Nith " " " " 4723 " 



The average of these rivers gives us one foot as the 

 lowering of the land by sub-aerial denudation in 3000 

 years, or a thousand feet in three million years; but as 

 Europe has a mean altitude of less than a thousand feet, 

 it follows that, at the present rate of denudation, the 

 whole of Europe would be reduced to nearly the sea- 

 level in about three million years. Before this method 

 of measuring the rate of the lowering of continents was 

 hit upon by Mr. Alfred Tylor in 1853, no one imagined 

 that it was anything like so rapid; and, as a million years 

 is certainly a short period as compared with the whole 

 geological record, it is clear that elevation must, on the 

 whole, have always kept pace with the two lowering 

 agencies sinking and denudation. Again, as in every 

 continent the areas occupied by plains and lowlands, 

 where denudation is comparatively slow, are large as 

 compared with the mountain areas, where all the denud- 

 ing agencies are most powerful, it is probable that most 



