0a Charles Davison—Suberial Denudation. 
Hoang Ho 1464, and the Po 729, years. The average of these figures 
is 3378, and ‘ this,” says Dr. Croll, in his recent interesting work 
on “ Stellar Evolution” (p. 41), “gives a mean of 3378 years to 
remove one foot, or a little over one-half the time taken by the 
Mississippi.” As the mean rate of subaérial denudation is an 
important element in physical geology, it may be worth while to 
point out a slight mistake in this determination of its value, and also 
to show how a more correct estimate may, with our present data, 
be obtained. 
In the first place, let us, as in the above method, consider these 
river-basins of equal area for the purpose of ascertaining the mean 
rate of denudation. Then, in one year, a layer sexs of a foot thick 
is, On an average, worn away from the surface of the Danube basin, 
a layer sooo of a foot thick from that of the Mississippi, and so on. 
The mean thickness of the layers is the average of the seven fractions 
suis, sovo, etc., 1.€. riz of a foot. This, and not the average of 
the denominators of these fractions, gives the mean rate of denudation 
on the above understanding: so that a layer one foot thick would 
be worn away from the general surface of the average river-basin 
in 1912, instead of in 3378, years. Or, if we take the area of the 
Upper Ganges basin at Dr. Haughton’s estimate! of 143,000, in 
place of 432,480, square miles, the mean rate of denudation would 
be one foot in 1572, instead of in 3159, years. 
But, in considering the river- Thain as of equal size, undue 
importance is being given to such comparatively small rivers as 
the Rhone, the Nith, and the Po. In determining the mean rate 
of denudation, the area of each river-basin must evidently be taken 
into account, and this may be done as follows. The total volume 
of sediment brought down by these seven rivers every year is 
34,712,610,040 cubic feet. If, as Dr. Geikie estimates, 25 cubic feet 
of sediment are derived from 19 cubic feet of rock,” this is equivalent 
to the removal of 26,380,817,398 cubic feet of rock; which may be 
regarded as taken from the surface of one river-basin equal in area 
to the sum of the areas of the above seven basins, 7.e. to 2,279,400 
square miles, or to 65,546,024,960,000 square feet. From these 
figures we find that the mean rate of subaérial denudation is equal 
to one foot every 2409 years, i.e. about two-fifths of the rate of 
denudation in the Mississippi basin. 
Dr. Croll points out (p. 48) that the Rhone, the Ganges and the 
Po are charged with mud from the Alps or Himalayas; and, for this 
and other reasons, he makes use in his work of the rate of denuda- 
tion in the Mississippi valley only. But while the rates of denuda- 
tion in the above cases are among the most rapid, these rivers have 
at the same time (with the exception of the Nith, which hardly 
affects the average) the smallest basins; while the Danube, the 
1 A, Geikie, Text Book of Geology, p. 444, footnote. This value is used in the 
estimate that follows. 
* Reckoning the specific gravity of the silt at 1:9 and that of rock at 2°5. This 
tends to diminish the estimate of the mean rate of denudation, for the sediment 
deposited at the mouth of a river is partly derived from the erosion of earthy beds. 
