ure 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.

Sec. 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