§ 6. But crests of this uniform substance and continuous outline occur only among hills composed of the softest coherent rocks, 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.
| Fig. 49. |
§ 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.
§ 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 depends on the fact that if we take the principal curves at any points of the wing, and continue them in the lines which they are pursuing at the moment they terminate, these continued lines will all meet in a single point, C. It is this law which gives unity to the wing.
| Fig. 50. |
All groups of curves set beside each other depend for their beauty upon the observance of this law;[69] and if, therefore, the mountain crests are to be perfectly beautiful, Nature must contrive to get this element of radiant curvature into them in one way or another. Nor does it, at first sight, appear easy for her to get, I do not say radiant curves, but curves at all: for in the aiguilles, she actually bent their beds; but in these slaty crystallines it seems not always convenient to her to bend the beds; and when they are to remain straight, she must obtain the curvature in some other way.
| Fig. 51. |