central groups are often themselves composed
of beds lying in a precisely opposite direction; so that if we analyze
carefully the structure of the dark mass in the centre of Fig. 25, we
shall find it arranged in lines which slope downwards to the centre; the
flanks of it being of slaty crystalline rock, and the summit of compact
crystallines, as at _a_, Fig. 26.

[Illustration: FIG. 26.]

In speaking of the sculpture of the central peaks in the last chapter, I
made no reference to the _nature_ of the rocks in the banks on which
they stood. The diagram at _a_, Fig. 27, as representative of the
original condition, and _b_, of the resultant condition will, compared
with Fig. 24, p. 170, more completely illustrate the change.[58]

[Illustration: FIG. 27.]

[Illustration: FIG. 28.]

Sec. 4. By what secondary laws this structure may ultimately be discovered
to have been produced is of no consequence to us at present; all that it
is needful for us to note is the beneficence which appointed it for the
mountains destined to assume the boldest forms. For into whatever
outline they may be sculptured by violence or time, it is evident at a
glance that their stability and security must always be the greatest
possible under the given circumstances. Suppose, for instance, that the
peak is in such a form as _a_ in Fig. 26, then, however steep the slope
may be on either side, there is still no chance of one piece of rock
sliding off another; but if the same outline were given to beds disposed
as at _b_, the unsupported masses might slide off those beneath them at
any moment, unless prevented by the inequalities of the surfaces.
Farther, in the minor divisions of the outline, the tendency of the peak
at _a_ will be always to assume contours like those at _a_ in Fig. 28,
which are, of course, perfectly safe; but the tendency of the beds at
_b_ in Fig. 27 will be to break into contours such as at _b_ here, which
are all perilous, not only in the chance of each several portion giving
way, but in the manner in which they would _deliver_, from one to the
other, the fragments which fell. A stone detached from any portion of
the peak at _a_ would be caught and stopped on the ledge beneath it; but
a fragment loosened from _b_ would not stay till it reached the valley
by a series of accelerating bounds.

Sec. 5. While, however, the secure and noble form represented at _a_ in
Figs. 26 and 28 is for the most part ordained to be that of the hi