§ 3. How far this apparent unity is the result of elevatory force in mountain, and how far of the sculptural force of water upon the mountain, is the question we have mainly to deal with in the present chapter.
But first look back to Fig. 7, of Plate 8, Vol. III., there given as the typical representation of the ruling forces of growth in a leaf. Take away the extreme portion of the curve on the left, and any segment of the leaf remaining, terminated by one of its ribs, as a or b, [Fig. 44], will be equally a typical contour of a common crested mountain. If the reader will merely turn Plate 8 so as to look at the figure upright, with its stalk downwards, he will see that it is also the base of the honeysuckle ornament of the Greeks. I may anticipate what we shall have to note with respect to vegetation so far as to tell him that it is also the base of form in all timber trees.
§ 4. There seems something, therefore, in this contour which makes its production one of the principal aims of Nature in all her compositions. The cause of this appears to be, that as the cinqfoil is the simplest expression of proportion, this is the simplest expression of opposition, in unequal curved lines. If we take any lines, a x and e g, [Fig. 45], both of varied curvature (not segments of circles), and one shorter than the other, and join them together so as to form one line, as b x, x g, we shall have one of the common lines of beauty; if we join them at an angle, as c x, x y, we shall have the common crest, which is in fact merely a jointed line of beauty. If we join them as at a, [Fig. 46], they form a line at once monotonous and cramped, and the jointed condition of this same line, b, is hardly less so. It is easily proved, therefore, that the junction of lines c x, x y, is the simplest and most graceful mode of opposition; and easily observed that in branches of trees, wings of birds, and other more or less regular organizations, such groups of line are continually made to govern the contours. But it is not so easily seen why or how this form should be impressed upon irregular heaps of mountain.
§ 5. If a bed of coherent rock be raised, in the manner described in Chap. XIII., so as to form a broken precipice with its edge, and a long slope with its surface, as at a, [Fig. 47] (and in this way nearly all hills are raised), the top of the precipice has usually a tendency to crumble down, and, in process of time, to form a heap of advanced ruins at its foot. On the other side, the back or slope of the hill does not crumble down, but is gradually worn away by the streams; and as these are always more considerable, both in velocity and weight, at the bottom of the slope than the top, the ground is faster worn away at the bottom, and the straight slope is cut to a curve of continually increasing steepness. [Fig. 47] b represents the contour to which the hill a would thus be brought in process of time; the dotted line indicating its original form. The result, it will be seen, is a crest.[67]