
- Boox VII.] INFLUENCE OF DENUDATION. 921 
3. Terrestrial Features due to Denudation.—The general 
- results of denudation have been discussed in Book ILI. Part IT. sect. 11. 
Every portion of the land, as soon as it rises above the sea-level, is 
attacked by denuding agents. Hence the older a terrestrial surface 
the more may it be expected to show the results of the operation of 
these agents. We have already seen how comparatively rapid are the 
_ processes of subaerial waste (p. 444). It is accordingly evident that 
the present contours of the land cannot be expected toreveal any trace 
whatever of the early terrestrial surfaces of the globe. The most 
recent mountain chains and volcanoes may, indeed, retain more or 
less markedly their original superficial outlines; but these must 
be more and and more effaced in proportion to their geological 
antiquity. 
The fundamental law in the erosion of the terrestrial surfaces 
is that harder rocks resist decay more, while softer rocks resist it less. 
The former consequently are left projecting, while the latter are 
worn down. The terms“ hard” and “soft” are used here in the sense 
of being less easily and more easily abraded, though every rock suffers 
in some measure. If, therefore, a perfectly level surface, composed 
of rocks exceedingly unequal in power of resistance, were to be raised 
above the sea, and to be exposed to the action of weathering, it 
would eventually be carved into a system of ridges and valleys. The 
eminences would be determined by the position of the harder rocks, 
the depressions by the site of the softer. very region of Mesozoic 
or Paleozoic rocks affords ample illustration of this result. The hills 
and prominent ridges are found to be where they are, not because 
they have been specially upheaved, but because they are composed 
of more durable materials, or because by the disposition of the 
original drainage lines they have been less eroded than the valleys. 
In this marvellous process of land-sculpture we have to consider 
on the one hand the agents and combinations of agents which are 
at work, and on the other the varying powers of resistance arising 
from declivity, composition, and structure of the materials on which 
these agents act. The forces or conditions required in denudation— 
- air, aridity, rain, springs, frost, rivers, glaciers, the sea, plant and 
animal life—have been described in Book IIT. Part II. Every country 
and climate must obviously have its own combination of erosive 
activities. The decay of the surface in Egypt or Arizona arises 
from a different group of forces from that which can be seen in the 
west of Europe or in New England. 
In tracing the sculpture of the land we are soon led to perceive 
the powerful influence of the angle of slope of the ground upon 
the rate of erosion. ‘This rate decreases as the angle lessens, till on 
level plains it reaches its minimum. Other things being equal a 
steep mountain ridge will be more deeply eroded than one of the 
same elevation which rises gradually out of the plains. Hence 
the declivity of the ground at its first elevation into land must 
have had an important bearing upon the subsequent erosion of the 
