356 GEIKIE 



represents, of course, the extent to which the general level 

 of the surface of the river's drainage basin is annually 

 lowered. According to such measurements and computa- 

 tions as have been already made, it appears that some- 

 where about nfa v of a foot is every year removed from 

 the surface of its drainage basin by a large river. This 

 seems a small fraction, yet by the power of mere addition 

 it soon mounts up to a large total. Taking the mean level 

 of Europe to be 600 feet, its surface, if everywhere worn 

 away at what seems to be the present mean normal rate, 

 would be entirely reduced to the sea-level in little more than 

 three and a half millions of years. 



But of course the waste is not uniform over the whole 

 surface. It is greatest on the slopes and valleys, least on 

 the more level grounds. A few years ago, in making some 

 estimates of the ratios between the rates of waste on these 

 areas, I assumed that the tracts of more rapid erosion 

 occupy only one-ninth of tihe whole surface affected, and 

 that in these the rate of destruction is nine times greater 

 than on the more level spaces. Taking these proportions, 

 and granting that ^iforr of a foot is the actual ascertained 

 amount of loss from the whole surface, we learn by a 

 simple arithmetical process that TS of an inch is carried 

 away from the plains and tablelands in seventy-five years, 

 while the same amount is worn out of the valleys in eight 

 and a half years. One foot must be removed from the 

 former in 10,800 years, and from the latter in 1200 years. 

 Hence, at the present rate of erosion, a valley 1000 feet 

 deep may be excavated in 1,200,000 years by no means a 

 very long period in the conception of most geologists. 



I do not offer these figures as more than tentative results. 

 They are based, however, not on mere guesses, but on data 

 which, though they may be corrected by subsequent inquiry, 

 are the best at present available, and are probably not far 

 from the truth. They are of value in enabling us more 

 vividly to realise how the prodigious waste of the land, 

 proved by the existence of such enormous masses of sedi- 

 mentary rock, went quietly on age after age, until results 

 were achieved which seem at first scarcely possible to so 

 slow and gentle an agency. 



