
’ " ih Ne ae 
444 . DYNAMICAL GEOLOGY. - [Boox III, _ 
a 
the fraction of a foot of rock (reckoning the specific gravity of the — 
silt at 1:9 and that of rock at 2°5) which each river must remove ~ 
from the general surface of its drainage basin in one year, 
Fiaction of foot of 












Name of River. | “UCC miles, | sediment in cuble fect, ||_aran of teste 
lowered in one year. | 
Mississippi . . 1,147,000 7,459,267,200 aio 
Ganges (Upper) . 143,000 6,368,077,440 ahs 
Hoang Ho . . 700,000 17,520,000,000(?) sigs 
PRBS eon Be 8 25,000 600,381,800 Ta55 
Wanube ~.'’. 234,000 1,253,738,600 aes 
Oye aime ae ala 30,000 1,510,137,000 535 

At the present rate of erosion, the rivers named in this table 
remove one foot of rock from the general surface of their basins 
in the following ratio:—The Mississippi removes one foot in 6000 
years; the Ganges above Ghazipir does the same in 823 years; ? 
the Hoang Ho in 1464 years; the Rhone in 1528 years; the Danube 
in 6846 years; the Po in 729 years. If these rates should continue, 
the Mississippi basin will be lowered 10 feet in 60,000 years, 100 
feet in 600,000 years, 1000 feet in 6,000,000. Assuming Humboldt’s 
estimate of the mean height of the North American continent, 748 
feet,’ we fiud that at the Mississippi’s rate of denudation, this con- 
tinent would be worn away in about four and a half million years. 
The Ganges works still more rapidly. It removes one foot of rock 
in 823 years, and if Humboldt’s estimate of the average height of 
the Asiatic continent be accepted, viz., 1132 Knglish feet, that mass 
of laud, worn down at the rate at which the Ganges destroys it, 
would be reduced to the sea-level in little more than 930,000 years. 
Still more remarkable is the extent to which the River Po denndes 
its area of drainage. Even though measurements had not been 
made of the ratio of sediment contained in its water, we should be 
prepared to find that proportion a remarkably large one if we look 
at the enormous clianges which, within historic times, have been 
made by the alluvial accumulations of this river (p. 890). If the 
Po removes one foot of rock from its drainage basin in 729 years, it 
will lower that basin 10 feet in 7290 years, 100 feet in 72,900 
years. Ifthe whole of Europe (taken at a mean height of 671 feet) 
were denuded at the same rate, it would be levelled in rather less 
than half a million of years. 
It is not pretended that these results are strictly accurate. On 
‘ 
1 In my original paper the arca of drvinage of the Ganges was given as 432,480 
square miles. But the area from which the annual discharge of silt was there givin 
was only that part of the Gangetic basin above Ghazipfir, which Dr. Haughton estimates 
at 143,000 square miles (Proe. Roy. Dublin Soc, 1879, No. xxxix.). Hene>, as he has 
pointed out, the rate of erosion is really much greater than I had made it. I have 
yecaleulated the rate from the altered data, and the result is as given above, 
2 Ante, p. du. 
