| Fig. 102. |
§ 24. It follows, also, that not only the whole of the nearer curves, will be steeper, but, if seen from below, the steepest parts of them will be the more important. Supposing each, instead of a curve, divided into a sloping line and a precipitous one, the perspective of the precipice, raising its top continually, will give the whole cone the shape of a or b in [Fig. 102], in which, observe, the precipice is of more importance, and the slope of less, precisely in proportion to the nearness of the mass.
§ 25. [Fig. 102], therefore, will be the general type of the form of a convex retiring hill symmetrically constructed. The precipitous part of it may vary in height or in slope according to original conformation; but the heights being supposed equal along the whole flank, the contours will be as in that figure; the various rise and fall of real height altering the perspective appearance accordingly, as we shall see presently, after examining the other three kinds of line.
2. Lines of Projection.
§ 26. The fragments carried down by the torrents from the 2. Lines of Projection. Produced by fragments bounding or carried forward from the bases of hills. flanks of the hill are of course deposited at the base of it. But they are deposited in various ways, of which it is most difficult to analyze the laws; for they are thrown down under the influence partly of flowing water, partly of their own gravity, partly of projectile force caused by their fall from the higher summits of the hill; while the débris itself, after it has fallen, undergoes farther modification by surface streamlets. But in a general way débris descending from the hill side, a b, [Fig. 103], will arrange itself in a form approximating to the concave line d c, the larger masses remaining undisturbed at the bottom, while the smaller are gradually carried farther and farther by surface streams.
| Fig. 103. |
3. Lines of Escape.
§ 27. But this form is much modified by the special direction 3. Lines of Escape. Produced by the lateral dissemination of the fragments. of the descending force as it escapes from confinement. For a stream coming down a ravine is kept by the steep sides of its channel in concentrated force: but it no sooner reaches the bottom, and escapes from its ravine, than it spreads in all directions, or at least tries to choose a new channel at every flood. Let a b c, [Fig. 104], be three ridges of mountain. The two torrents coming down the ravine between them meet, at d and e, with the heaps of ground formerly thrown down by their own agency. These heaps being more or less in the form of cones, the torrent has a tendency to divide upon their apex, like water poured on the top of a sugar-loaf, and branch into the radiating channels e x, e y, &c. The stronger it is, the more it is disposed to rush straightforward, or with little curvature, as in the line e x, with the impetus it has received in coming down the ravine; the weaker it is, the more readily it will lean to one side or the other, and fall away in the lines of escape, e y, or e h; but of course at times of highest flood it fills all its possible channels, and invents a few new ones, of which afterwards the straightest will be kept by the main stream, and the lateral curves occupied by smaller branches; the whole system corresponding precisely to the action of the ribs of the young leaf, as shown in Plate 8 of Vol. III., especially in Fig. 6,—the main torrent, like the main rib, making the largest fortune, i. e. raising the highest heap of gravel and dust.
| Fig. 104. |