pee i ee 
446 _. DYNAMICAL GEOLOGY. __ [Boox Ik 
more detailed observation is needed before any estimate of this 
kind can be based upon accurate and reliable data. But it illustrates 
a method of vividly bringing before the mind the reality and extent 
of the denudation now in progress. 
2. Subaerial denudation—the unequal erosion of land. 
It is obvious that the earthy matter annually removed from 
the surface of the land does not come equally from the whole 
surface. ‘The determination of its total quantity furnishes no 
aid in apportioning the loss, or in ascertaining how much each part — 
of the surface has contributed to the total amount of sediment. 
On plains, watersheds, and more or less level ground, the pro- 
portion of loss may be small, while on slopes and in valleys it may 
be great, and it may not be easy to fix the true ratios in these cases. 
But it must be borne in mind that estimates and measurements of the 
sum-total of denudation are not thereby affected. If we allow too 
little for the loss from the surface of the tablelands, we increase the 
proportion of the loss sustained by the sides and bottoms of the valleys, 
and vice versa. ? 
While these proportions vary indefinitely with the form of the 
surface, rainfall, &c., the balance of loss must always be, on the whole, 
on the side of the sloping surfaces. In order to show the full import 
of this part of the subject, certain ratios may here be assumed which 
are probably understatements rather than exaggerations. Let us 
take the proportion between the extent of the plains and tablelands 
of a country, and the area of its valleys, to be as nine to one; in other 
words, that of the whole surface of the country, nine-tenths consists 
of broad undulating plains, or other comparatively level ground, and 
one-tenth of steeper slopes. Let it be further assumed that the 
eresion of the surface is nine times greater over the latter than over ~ 
the former area, so that while the more level parts of the country 
have been lowered one foot, the valleys have lost nine feet. If, fol- 
lowing the measurements and calculations already given, we admit 
that the mean annual quantity of detritus carried to the sea may, 
with some probability, be regarded as equal to the yearly loss of .355 
of a foot of rock from the general surface of the country, then, appor- 
tioning this loss over the surface in the ratio just given, we find that 
it amounts to > of a foot from the more level grounds in 6000 years, 
and 5 feet from the valleys in the same space of time. Now, if 4 of 
a foot be removed from tle level grounds in 6000 years, 1 foot will 
be removed in 10,800 years; and if 5 feet be worn out of the 
valleys in 6000 years, 1 foot will be worn out in 1200 years. This is 
equal to a loss of only +4, of an inch from the tableland in 75 years, 
while the same amount is excavated from the valleys in 84 years. 
It may seem at first sight that such a loss as only a single line 
from the surface of the open country during more than the lapse of 
a long human life is almost too trifling to be taken into account, as 

