
- Parr Il. Suor. ii, §7.] SUBAERIAL DENUDATION. 445 
the other band they are not mere guesses. The amount of water 
flowing into the sea, and the annual discharge of sediment, have 
been in each case measured with greater or less precision. The areas 
of drainage may perhaps require to be increased or lessened, But 
_ though some change may be made upon the ultimate results just 
given, it is hardly possible to consider them attentively without being 
forced to ask whether those enormous periods which geologists have 
been in the habit of demanding for the accomplishment of geological 
phenomena, and more especially for the very phenomena of denuda- 
tion, are not in reality far too vast. Ifthe Mississippi is carrying on 
- the process of denudation so rapidly that at the same rate the whole 
of North America might be levelled in four and a half millions of 
years, surely it is most unphilosophical to demand unlimited ages 
for similar but often much less extensive denudations in the geo- 
logical past. Moreover, that rate of erosion appears on the whole 
to be rather below the average in point of rapidity. The Po, for 
instance, works more than eight times as fast. But as the physics 
of the Mississippi have been more carefully studied than those of 
perhaps any other river, and as that river drains so extensive a 
region, embracing so many varieties of climate, rock and soil, we shall 
probably not exaggerate the result if we assume the Mississippi ratios 
as an average. It is of course obvious that as the level of the land 
is lowered the rate of subaerial denudation decreases, so that on the 
supposition that no subterranean movements took place to aid or 
retard the denudation, the last stages in the demolition of a continent 
must be enormously slower than during earlier periods. 
There is another point of view from which a geologist may 
advantageously contemplate the active denudation of a country. 
He may estimate the annual rainfall and the proportion of water 
which returns to the sea. If he can obtain a probable average ratio 
for the earthy substances contained in the river water which enters 
the sea, he will be able to estimate the mean amount of loss sustained 
by the whole country. Thus, taking the average rainfall of the 
British Islands at 36 inches annually, and the superficial area over 
which this rain is discharged at 120,000 square miles, then it will be 
found that the total quantity of rain received in one year by the 
British Isles is equal to about 68 cubic miles of water. If the 
proportion of rainfall returned to the sea by streams be taken at a 
third, there are 23 cubic miles; if at a fourth, there are 17 cubic 
miles of fresh water sent off the surface of the British Islands into 
the sea in one year. Assuming, in the next place, that the average 
ratio of mechanical impurities is only 3559 by volume of the water, 
the proportion of the rainfall returned to the sea being 4, then it 
will follow that ¢55 of a foot of rock is removed from the general 
surface of Britain every year. One foot will be planed away in 8800 
years. Ifthe mean height of the British Islands be taken at 650 
feet, then, if the ratio now assumed were to continue, these islands 
might be levelled in about five and a half millions of years. Much 
