LAND WATERS STREAMS 



139 



7,000,000 years to bring the continent to sea-level. But this rate 

 of degradation could not continue to the end, for as the continent 

 became lower, streams would become sluggish and erosion less 

 rapid. Long before the continent reached base-level, the rate of 

 degradation, so far -as dependent on mechanical erosion, would 

 become so slow that the time necessary to bring the continent to 

 sea-level would be indefinitely prolonged. Furthermore, it is 

 quite possible that the land is suffering, or is liable to suffer, uplift, 

 relative or absolute. If the rate of rise were equal to the rate of 

 degradation, the average height of the continent would of course 

 not be affected. 



The following table 1 gives the percentage of material carried 

 in suspension by various rivers : 



The composition of rain-water falling near London, as deter- 

 mined by analysis, was as follows: 2 



Organic carbon 99 part in 1,000,000 of water 



Organic nitrogen 22 " " " " " 



Ammonia 50 " " 



Nitrogen as nitrates and nitrites 07 "" 



Chlorine 6.30 parts in " " " 



Total solids 39.50 " " " " " 



1 Babb. Science, Vol. XXI, p. 343. 1893. 



2 Quoted by Mason. Water-supply, p. 204. 



