whole body into waves, apparently following the action of the
force that fractured them, like waves of sea under the wind. Truly the
cloud lies darkly upon us here!

[Illustration: FIG. 13.]

Sec. 17. And it renders these precipices more remarkable that there is in
them no principle of compensation against destructive influences. They
are not cloven back continually into new cliffs, as our chalk shores are
by the sea; otherwise, one might attribute their first existence to the
force of streams. But, on the contrary, the action of years upon them is
now always one of deterioration. The increasing heap of fallen fragments
conceals more and more of their base, and the wearing of the rain lowers
the height and softens the sternness of their brows, so that a great
part of their terror has evidently been subdued by time; and the farther
we endeavor to penetrate their history, the more mysterious are the
forms we are required to explain.

The three great representative forms of stratified mountains.

Sec. 18. Hitherto, however, for the sake of clearness, we have spoken of
hills as if they were composed of a single mass or volume of rock. It is
very seldom that they are so. Two or three layers are usually raised at
once, with certain general results on mountain form, which it is next
necessary to examine.

1. Wall above slope.

1st. Suppose a series of beds raised in the condition _a_, Fig. 13, the
lowest soft, the uppermost compact; it is evident that the lower beds
would rapidly crumble away, and the compact mass above break for want of
support, until the rocks beneath had reached a slope at which they could
securely sustain themselves, as well as the weight of wall above, thus
bringing the hill into the outline _b_.

[Illustration: FIG. 14.]

2. Slope above wall.

2d. If, on the other hand, the hill were originally raised as at _c_,
the softest beds being at the top, these would crumble into their smooth
slope without affecting the outline of the mass below, and the hill
would assume the form _d_, large masses of debris being in either of
these two cases accumulated at the foot of the slope, or of the cliff.
These first ruins might, by subsequent changes, be variously engulfed,
carried away, or covered over, so as to leave nothing visible, or at
least nothing notable, but the great cliff with its slope above or below
it. Without insisting on the evidences or probabilities of such
construction, it is su