sal in its influence on the
aggregate of all. It is that the lines by which rocks are terminated,
are always steeper and more inclined to the vertical as we approach the
summit of the mountain. Thousands of cases are to be found in every
group, of rocks and lines horizontal at the top of the mountain and
vertical at the bottom; but they are still the exceptions, and the
average out of a given number of lines in any rock formation whatsoever,
will be found increasing in perpendicularity as they rise. Consequently
the great skeleton lines of rock outline are always concave; that is to
say, all distant ranges of rocky mountain approximate more or less to a
series of concave curves, meeting in peaks, like a range of posts with
chains hanging between. I do not say that convex forms will not
perpetually occur, but that the tendency of the majority will always be
to assume the form of sweeping, curved valleys, with angular peaks; not
of rounded convex summits, with angular valleys. This structure is
admirably exemplified in the second vignette in Rogers's Italy, and in
Piacenza.

Sec. 11. The gentle convexity caused by aqueous erosion.

But although this is the primary form of all hills, and that which will
always cut against the sky in every distant range, there are two great
influences whose tendency is directly the reverse, and which modify, to
a great degree, both the evidences of stratification and this external
form. These are aqueous erosion and disintegration. The latter only is
to be taken into consideration when we have to do with minor features of
crag; but the former is a force in constant action--of the very utmost
importance--a force to which one-half of the great outlines of all
mountains is entirely owing, and which has much influence upon every one
of their details.

Now the tendency of aqueous action over a large elevated surface is
_always_ to make that surface symmetrically and evenly convex and
dome-like, sloping gradually more and more as it descends, until it
reaches an inclination of about 40 deg., at which slope it will descend
perfectly straight to the valley; for at that slope the soil washed from
above will accumulate upon the hill-side, as it cannot lie in steeper
beds. This influence, then, is exercised more or less on all mountains,
with greater or less effect in proportion as the rock is harder or
softer, more or less liable to decomposition, more or less recent in
date of elevation, and mo