598 THE POPULAR SCIENCE MONTHLY. 



a rule, our conceptions of the rate of this degradation are exceedingly- 

 vague. Yet they may easily be made more definite by a consideration 

 of present changes on the surface of the land. Every river carries 

 yearly to the sea an immense amount of sand and mud. But this 

 amount is capable of measurement. It represents, of course, the ex- 

 tent to which the general level of the surface of the river's drainage- 

 basin is annually lowered. According to such measurements and com- 

 putations as have been already made, it appears that somewhere about 

 g-^g-o 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 of the 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 the 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 6o * QO 

 of a foot is the actual ascertained amount of loss from the whole sur- 

 face, we ascertain by a simple arithmetical process that T V of an inch 

 is carried away from the plains and table-lands 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 1,200 years. Hence we learn that at the present rate 

 of erosion a valley 1,000 feet deep may be excavated in 1,200,000 years 

 by no means a very long period in the conceptions 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 realize how the prodigious waste of the 

 land, proved by the existence of such enormous masses of sedimentary 

 rock, went quietly on age after age, until results were achieved which 

 seem at first scarcely possible to so slow and gentle an agency. 



It is during this quiet process of decay and removal that all the dis- 

 tinctive minor features of the land are wrought out. When first ele- 

 vated from the sea, the land doubtless presents on the whole a feature- 

 less surface. It may be likened to a block of marble raised out of the 

 quarry rough and rude in outline, massive in solidity and strength, 

 but giving no indication of the grace into which it will grow under the 

 hand of the sculptor. What art effects upon the marble block, Nature 

 accomplishes upon the surface of the land. Her tools are many and 



