of Escape.

3. Lines of Escape. Produced by the lateral dissemination of the
fragments.

Sec. 27. But this form is much modified by the special direction of the
descending force as it escapes from confinement. For a stream coming
down a ravine is kept by the steep sides of its channel in concentrated
force: but it no sooner reaches the bottom, and escapes from its ravine,
than it spreads in all directions, or at least tries to choose a new
channel at every flood. Let _a b c_, Fig. 104, be three ridges of
mountain. The two torrents coming down the ravine between them meet, at
_d_ and _e_, with the heaps of ground formerly thrown down by their own
agency. These heaps being more or less in the form of cones, the torrent
has a tendency to divide upon their apex, like water poured on the top
of a sugar-loaf, and branch into the radiating channels _e x_, _e y_,
&c. The stronger it is, the more it is disposed to rush straightforward,
or with little curvature, as in the line _e x_, with the impetus it has
received in coming down the ravine; the weaker it is, the more readily
it will lean to one side or the other, and fall away in the lines of
escape, _e y_, or _e h_; but of course at times of highest flood it
fills all its possible channels, and invents a few new ones, of which
afterwards the straightest will be kept by the main stream, and the
lateral curves occupied by smaller branches; the whole system
corresponding precisely to the action of the ribs of the young leaf, as
shown in Plate +8+ of Vol. III., especially in Fig. 6,--the main
torrent, like the main rib, making the largest fortune, i. e. raising
the highest heap of gravel and dust.

[Illustration: FIG. 104.]

Sec. 28. It may easily be imagined that when the operation takes place on a
large scale, the mass of earth thus deposited in a gentle slope at the
mountain's foot becomes available for agricultural purposes, and that
then it is of the greatest importance to prevent the stream from
branching into various channels at its will, and pouring fresh sand over
the cultivated fields. Accordingly, at the mouth of every large ravine
in the Alps, where the peasants know how to live and how to work, the
stream is artificially embanked, and compelled as far as possible to
follow the central line down the cone. Hence, when the traveller passes
along any great valley,--as that of the Rhone or Arve,--into which
minor torrents are poured by lateral ravines, he wil