plutonist to fix
upon the point at which fracture begins to play a material part.

In ascending one of the larger valleys, we enter it where it is wide
and where the eminences are gentle on either side. The flanking
mountains become higher and more abrupt as we ascend, and at length we
reach a place where the depth of the valley is a maximum. Continuing
our walk upwards, we find ourselves flanked by gentler slopes, and
finally emerge from the valley and reach the summit of an open col, or
depression in the chain of mountains. This is the common character of
the large valleys. Crossing the col, we descend along the opposite
slope of the chain, and through the same series of appearances in the
reverse order. If the valleys on both sides of the col were produced
by fissures, what prevents the fissure from prolonging itself across
the col? The case here cited is representative; and I am not
acquainted with a single instance in the Alps where the chain has been
cracked in the manner indicated. The cols are simply depressions; in
many of which the unfissured rock can be traced from side to side.

The typical instance just sketched follows as a natural consequence
from the theory of erosion. Before either ice or water can exert
great power as an erosive agent, it must collect in sufficient mass.
On the higher slopes and plateaus--in the region of cols--the power is
not fully developed; but lower down tributaries unite, erosion is
carried on with increased vigour, and the excavation gradually reaches
a maximum. Lower still the elevations diminish and the slopes become
more gentle; the cutting power gradually relaxes, until finally the
eroding agent quits the mountains altogether, and the grand effects
which it produced in the earlier portions of its course entirely
disappear.

I have hitherto confined myself to the consideration of the broad
question of the erosion theory as compared with the fracture theory;
and all that I have been able to observe and think with reference to
the subject leads me to adopt the former. Under the term erosion I
include the action of water, of ice, and of the atmosphere, including
frost and rain. Water and ice, however, are the principal agents, and
which of these two has produced the greatest effect it is perhaps
impossible to say. Two years ago I wrote a brief note 'On the
Conformation of the Alps,' [Footnote: Phil. Mag. vol. xxiv. p. 169]
in which I ascribed the paramount influen