§ 10. It often happens, however, that Nature does not choose to keep the ridge broad at the lower extremity, so as to diminish its steepness. But when this is not so, and the base is narrowed so that the slope of side shall be nearly equal everywhere, she almost always obtains her varied curvature of the plume in another way, by merely turning the crest a little round as it descends. I will not confuse the reader by examining the complicated results of such turning on the inclined lines of the strata; but he can understand, in a moment, its effect on another series of lines, those caused by rivulets of water down the sides of the crest. These lines are, of course, always, in general tendency, perpendicular. Let _a_, Fig. 53, be a circular funnel, painted inside with a pattern of vertical lines meeting at the bottom. Suppose these lines to represent the ravines traced by the water. Cut off a portion of the lip of the funnel, as at _b_, to represent the crest side. Cut the edge so as to slope down towards you, and add a slope on the other side. Then give each inner line the concave sweep, and you have your ridge _c_, of the required form, with radiant curvature.
§ 11. A greater space of such a crest is always seen on its concave than on its convex side (the outside of the funnel); of this other perspective I shall have to speak hereafter; meantime, we had better continue the examination of the proper crest, the _c_ of Fig. 48, in some special instance.
The form is obtained usually in the greatest perfection among the high ridges near the central chain, where the beds of the slaty crystallines are steep and hard. Perhaps the most interesting example I can choose for close examination will be that of a mountain in Chamouni, called the Aiguille Bouchard, now familiar to the eye of every traveller, being the ridge which rises, exactly opposite the Montanvert, beyond the Mer de Glace. The structure of this crest is best seen from near the foot of the Montanvert, on the road to the source of the Arveiron, whence the top of it, _a_, presents itself under the outline given rudely in the opposite plate (+33+), in which it will be seen that, while the main energy of the mountain mass tosses itself against the central chain of Mont Blanc (which is on the right hand), it is met by a group of counter-crests, like the recoil of a broken wave cast against it from the other side; and yet, as the recoiling water has a sympathy with the under swell of the very wave against which it clashes, the whole mass writhes together in strange unity of mountain passion; so that it is almost impossible to persuade oneself, after long looking at it, that the crests have not indeed been once fused and tossed into the air by a tempest which had mastery over them, as the winds have over ocean.