Sec. 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. 2. Lines of Projection. Produced by fragments bounding or carried forward from the bases of hills. Sec. 26. The fragments carried down by the torrents from the 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 debris itself, after it has fallen, undergoes farther modification by surface streamlets. But in a general way debris 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. [Illustration: FIG. 103.] 3. Lines of Escape. 3. Lines of Escape. Produced by the lateral dissemination of the fragments.