acting with reversed data, arrive at similar results in the
aiguilles and crests. In the aiguilles, which are of such hard rock that
the fall of snow and trickling of streams do not affect them, the inner
structure is so disposed as to bring out the curvatures by the mere
fracture. In the crests and lower hills, which are of softer rock, and
largely influenced by external violence, the inner structure is
straight, and the necessary curvatures are produced by perspective, by
external modulation, and by the balancing of adverse influences of
cleavage. But, as the accuracy of an artist's eye is usually shown by
his perceiving the inner anatomy which regulates growth and form, and as
in the aiguilles, while we watch them, we are continually discovering
new curves, so in the crests, while we watch them, we are continually
discovering new straightnesses; and nothing more distinguishes good
mountain-drawing, or mountain-seeing, from careless and inefficient
mountain-drawing, than the observance of the marvellous parallelisms
which exist among the beds of the crests.

[Illustration: FIG. 62.]

Sec. 21. It indeed happens, not unfrequently, that in hills composed of
somewhat soft rock, the aqueous contours will so prevail over the
straight cleavage as to leave nothing manifest at the first glance but
sweeping lines like those of waves. Fig. 43, p. 196, is the crest of a
mountain on the north of the valley of Chamouni, known, from the rapid
decay and fall of its crags, as the Aiguille _Pourri_; and at first
there indeed seems little distinction between its contours and those of
the summit of a sea wave. Yet I think also, if it _were_ a wave, we
should immediately suppose the tide was running towards the right hand;
and if we examined the reason for this supposition, we should perceive
that along the ridge the steepest falls of crag were always on the
right-hand side; indicating a tendency in them to break rather in the
direction of the line _a b_ than any other. If we go half-way down the
Montanvert, and examine the left side of the crest somewhat more
closely, we shall find this tendency still more definitely visible, as
in Fig. 62.

Sec. 22. But what, then, has given rise to all those coiled plungings of
the crest hither and thither, yet with such strange unity of motion?

Yes. There is the cloud. How the top of the hill was first shaped so as
to let the currents of water act upon it in so varied a way we know not,
but I th