should find not simply a deep empty cavity, but an
irregular opening, where many rents converged; and these rents would be
now seen breaking through the walls of the crater, widening as they
approach the centre. (See Fig. 44, _a_, _b_.)[510] Not a single fissure
of this kind is observable in the interior of Monte Nuovo, where the
walls of the crater are continuous and entire; nor are there any dikes
implying that rents had existed, which were afterwards filled with lava
or other matter.

[Illustration: Fig. 44.]

It has moreover been often urged by Von Buch, De Beaumont, and others,
who ascribe the conical form of volcanoes chiefly to upheaval from
below, that in such mountains there are a great number of deep rents and
ravines, which diverge on all sides like the spokes of a wheel, from
near the central axis to the circumference or base of the cone, as in
the case of Palma, Cantal, and Teneriffe. Yet the entire absence of such
divergent fissures or ravines, in such cases as Monte Nuovo, Somma, or
Etna, is passed by unnoticed, and appears to have raised in their minds
no objection to their favorite theory.

It is, indeed, admitted by M. Dufranoy that there are some facts which
it is very difficult to reconcile with his own view of Porzio's record.
Thus, for example, there are certain Roman monuments at the base of
Monte Nuovo, and on the borders of Lake Avernus, such as the temples of
Apollo (before mentioned) and Pluto, which do not seem to have suffered
in the least degree by the supposed upheaval. "The walls which still
exist have preserved their vertical position, and the vaults are in the
same state as other monuments on the shores of the Bay of Baiae. The long
gallery which led to the Sibyl's Cave, on the other side of Lake
Avernus, has in like manner escaped injury, the roof of the gallery
remaining perfectly horizontal, the only change being that the soil of
the chamber in which the Sibyl gave out her oracles is now covered by a
few inches of water, which merely indicates a slight alteration in the
level of Lake Avernus."[511] On the supposition, then, that pre-existing
beds of pumiceous tuff were upraised in 1538, so as to form Monte Nuovo,
it is acknowledged that the perfectly undisturbed state of the
contiguous soil on which these ancient monuments stand, is very
different from what might have been expected.

Mr. Darwin, in his "Volcanic Islands," has described several crateriform
hills in the Galapagos A