, and seldom
attain any elevation such as to make them important or impressive. The
notable crests are composed of the hard coherents or slaty crystallines,
and then the contour of the crests depends mainly on the question
whether in the original mass of it, the beds lie as at _a_ or as at _b_,
Fig. 48. If they lie as at _a_, then the resultant crest will have the
general appearance seen at _c_; the edges of the beds getting separated
and serrated by the weather. If the beds lie as at _b_, the resultant
crest will be of such a contour as that at _d_.

The crests of the contour _d_ are formed usually by the harder coherent
rocks, and are notable chiefly for their bold precipices in front, and
regular slopes, or sweeping curves, at the back. We shall examine them
under the special head of _precipices_. But the crests of the form at c
belong usually to the slaty crystallines, and are those properly called
crests, their edges looking, especially when covered with pines, like
separated plumes. These it is our chief business to examine in the
present chapter.

[Illustration: FIG. 49.]

Sec. 7. In order to obtain this kind of crest, we first require to have our
mountain beds thrown up in the form _a_, Fig. 48. This is not easily
done on a large scale, except among the slaty crystallines forming the
flanks of the great chains, as in Fig. 29, p. 176. In that figure it
will be seen that the beds forming each side of the chain of Mont Blanc
are thrown into the required steepness, and therefore, whenever they
are broken towards the central mountain, they naturally form the front
of a crest, while the torrents and glaciers falling over their longer
slopes, carve them into rounded banks towards the valley.

Sec. 8. But the beauty of a crest or bird's wing consists, in nature, not
merely in its curved terminal outline, but in the radiation of the
plumes, so that while each assumes a different curve, every curve shall
show a certain harmony of direction with all the others.

We shall have to enter into the examination of this subject at greater
length in the 17th chapter; meanwhile, it is sufficient to observe the
law in a single example, such as Fig. 49, which is a wing of one of the
angels in Durer's woodcut of the Fall of Lucifer.[68] At first sight,
the plumes seem disposed with much irregularity, but there is a sense of
power and motion in the whole which the reader would find was at once
lost by a careless copyist; for it de