o vegetation so far as to tell him that
it is also the base of form in all timber trees.

[Illustration: FIG. 46.]

Sec. 4. There seems something, therefore, in this contour which makes its
production one of the principal aims of Nature in all her compositions.
The cause of this appears to be, that as the cinqfoil is the simplest
expression of proportion, this is the simplest expression of opposition,
in unequal curved lines. If we take any lines, _a x_ and _e g_, Fig. 45,
both of varied curvature (not segments of circles), and one shorter than
the other, and join them together so as to form one line, as _b x_, _x
g_, we shall have one of the common lines of beauty; if we join them at
an angle, as _c x_, _x y_, we shall have the common crest, which is in
fact merely a jointed line of beauty. If we join them as at _a_, Fig.
46, they form a line at once monotonous and cramped, and the jointed
condition of this same line, _b_, is hardly less so. It is easily
proved, therefore, that the junction of lines _c x_, _x y_, is the
simplest and most graceful mode of opposition; and easily observed that
in branches of trees, wings of birds, and other more or less regular
organizations, such groups of line are continually made to govern the
contours. But it is not so easily seen why or how this form should be
impressed upon irregular heaps of mountain.

[Illustration: FIG. 47.]

Sec. 5. If a bed of coherent rock be raised, in the manner described in
Chap. XIII., so as to form a broken precipice with its edge, and a long
slope with its surface, as at _a_, Fig. 47 (and in this way nearly all
hills are raised), the top of the precipice has usually a tendency to
crumble down, and, in process of time, to form a heap of advanced ruins
at its foot. On the other side, the back or slope of the hill does not
crumble down, but is gradually worn away by the streams; and as these
are always more considerable, both in velocity and weight, at the bottom
of the slope than the top, the ground is faster worn away at the bottom,
and the straight slope is cut to a curve of continually increasing
steepness. Fig. 47 _b_ represents the contour to which the hill _a_
would thus be brought in process of time; the dotted line indicating its
original form. The result, it will be seen, is a crest.[67]

[Illustration: FIG. 48.]

Sec. 6. But crests of this uniform substance and continuous outline occur
only among hills composed of the softest coherent rocks