lines of rest, because, whenever
a mountain side is composed of soft stone which splits and decomposes
fast, it has a tendency to choke itself up with the ruins, and gradually
to get abraded or ground down towards the debris slope; so that vast
masses of the sides of Alpine valleys are formed by ascents of nearly
uniform inclination, partly loose, partly of jagged rocks, which break,
but do not materially alter the general line of ground. In such cases
the fragments usually have accumulated without disturbance at the foot
of the slope, and the pine forests fasten the soil and prevent it from
being carried down in large masses. But numerous instances occur in
which the mountain is consumed away gradually by its own torrents, not
having strength enough to form clefts or precipices, but falling on each
side of the ravines into even banks, which slide down from above as they
are wasted below.

Sec. 17. By all these various expedients, Nature secures, in the midst of
her mountain curvatures, vast series of perfectly straight lines
opposing and relieving them; lines, however, which artists have almost
universally agreed to alter or ignore, partly disliking them
intrinsically, on account of their formality, and partly because the
mind instantly associates them with the idea of mountain decay. Turner,
however, saw that this very decay having its use and nobleness, the
contours which were significative of it ought no more to be omitted
than, in the portrait of an aged man, the furrows on his hand or brow;
besides, he liked the lines themselves, for their contrast with the
mountain wildness, just as he liked the straightness of sunbeams
penetrating the soft waywardness of clouds. He introduced them
constantly into his noblest compositions; but in order to the full
understanding of their employment in the instance I am about to give,
one or two more points yet need to be noticed.

Sec. 18. Generally speaking, the curved lines of convex, _fall_ belong to
mountains of hard rock, over whose surfaces the fragments _bound_ to the
valley, and which are worn by wrath of avalanches and wildness of
torrents, like that of the Cascade des Pelerins, described in the note
above. Generally speaking, the straight lines of _rest_ belong to softer
mountains, or softer surfaces and places of mountains, which, exposed to
no violent wearing from external force, nevertheless keep slipping and
mouldering down spontaneously or receiving gradual access