200 C. Lloyd Morgan — Geological Time. 



continually, washed down seawards ; but that as fast as soil is carried 

 away, fresh soil is formed by the action of rain and the weather. 

 Only in steep and mountainous regions is the flow of water so rapid 

 as to carry away the material abraded from the rocks, directly, 

 without any formation of soil. Now all the material washed from 

 the surface of the land is carried into the rivers. To ascertain, 

 therefore, the rate of subaerial denudation in any area, we have only 

 to find the amount of sediment transported by the rivers which 

 drain that area. If we knew, for example, the exact amount of 

 suspended matter which is carried into the sea by all the rivers of 

 England, we could readily calculate the rate at which that country 

 is yielding to subaerial denudation. At present, however, we have 

 not the requisite data for making this calculation. The matter 

 is much simplified if we take the cases of individual rivers, and 

 calculate the rate at which they are lowering the average level of the 

 area of country which they drain. This has been done for a few 

 rivers, and the following table, from Professor Geikie's paper on this 

 subject, shows the number of years which it would take, at the 

 present rate of denudation, for each river named to remove one foot 

 of solid rock (the average sp. gr. of river-silt being taken at 1*9 and 

 that of rock at 2-5). 



Mississippi one foot in 6000 years. 



Ganges „ „ 2358 



Hoango Ho 



Rhone 



Danube 



Po 



Nith 



Taking the mean height of the American Continent at 1496 feet, 

 at the present Mississippi rate of denudation that continent would 

 be worn away in about nine million years. Asia, at the rate at 

 which the Ganges destroys it, would disappear in five million years ; 

 and if the whole of Europe were denuded at the same rate as the 

 basin of the Po, it would be levelled in rather less than a million 

 of years. 



But these calculations take no account of the material removed 

 in solution by Chemical denudation. If this be taken into con- 

 sideration, the figures above quoted will have to be considerably 

 modified. Mr. T. Mellard Eeade, in his Presidential Address to 

 the Liverpool Geological Society, 1877, calculated that the general 

 area of England and Wales was lowered by chemical denudation 

 at the rate of "0077 of a foot in a century, or one foot in 12,978 

 years. He also states that the matter in suspension in the waters 

 of the Danube are in amount three times those in solution ; but 

 the solids in solution come down constantly ; the mud is pushed 

 along in times of flood. Certain experiments of mine on Thames 

 water (taken from the river near the Waterworks, Surbiton) tend 

 to show, that the matter in solution, even in time of flood, was 

 seven times that in suspension, while in summer time this ratio 

 was exceeded, and was more nearly twenty times. In these ex- 

 periments, however, no account was taken of the solid matter 





1464 , 





1528 , 





6846 , 





729 , 





4723 , 



