stony and fragmentary beds of the Val del
Bove, from the axis of Etna towards its circumference or base, and the
greater thickness of the volcanic pile as we approach the central parts
of the mountain, to be due to the preponderance of eruptions from that
centre. These gave rise, from the first, to a dome-shaped mass, which
has ever since been increasing in height and area, being fractured again
and again by the expansive force of vapors, and the several parts made
to cohere together more firmly after the solidification of the lava with
which every open fissure and chasm has been filled. At the same time the
cone may have gained a portion of its height by the elevatory effect of
such dislocating movements, and the sheets of lava may have acquired in
some places a greater, in others a less, inclination than that which at
first belonged to them.

[Illustration: Fig. 55.

Non-volcanic protuberance and valley of elevation.]

But had the mountain been due solely, or even principally, to upheaval,
its structure would have resembled that which geologists have so often
recognized in dome-shaped hills, or certain elevated regions, which all
consider as having been thrust up by a force from below. In this case
there is often an elliptical cavity at the summit, due partly to the
fracture of the upraised rocks, but still more to aqueous denudation, as
they rose out of the sea. The central cavity, or valley, exposes to view
the subjacent formation _c_, fig. 55, and the incumbent mass dips away
on all sides from the axis, but has no tendency to thin out near the
base of the dome, or at _x_, _x_; whereas at this point the volcanic
mass terminates (see fig. 56) and allows the fundamental rock _c_ to
appear at the surface. In the last diagram, the more ordinary case is
represented of a great hollow or crater at the summit of the volcanic
cone; but instead of this, we have seen that in the case of Etna there
is a deep lateral depression, called the Val del Bove, the upper part of
which approaches near to the central axis, and the origin of which we
have attributed to subsidence.

[Illustration: Fig. 56.

Volcanic mountain and crater.]

_Antiquity of the cone of Etna._--It was before remarked that confined
notions in regard to the quantity of past time have tended, more than
any other prepossessions, to retard the progress of sound theoretical
views in geology;[579] the inadequacy of our conceptions of the earth's
antiquity having