ore its formation is shown by the
lightly-shaded portion above B.]

(_b._) _Val del Bove._--The most wonderful feature of Mount Etna is the
celebrated Val del Bove (Valle del Bue), of which S. von Waltershausen
has furnished a very beautiful plate[6]--a vast amphitheatre hewn out of
the eastern flank of the mountain, just below the snow-mantled platform.
It is a physical feature somewhat after the fashion of Monte Somma in
Vesuvius, but exceeds it in magnitude as Etna exceeds Vesuvius. The Val
del Bove is about five miles in diameter, bounded throughout
three-fourths of its circumference by precipitous walls of ashes,
scoriae, and lava, traversed by innumerable dykes, and rising inwards to
a height of between 3000 and 4000 feet. Towards the east the cliffs
gradually fall to a height of about 500 feet, and at this side the vast
chasm opens out upon the slope of the mountain. At the head of the Val
del Bove rises the platform, surmounted by the great cone and crater. It
will thus be seen that by means of this hollow we have access almost to
the very heart of the mountain.

What is very remarkable about the structure of this valley is that the
beds exhibit "the _qua-qua_ versal dip"--in other words, they dip away
on all sides from the centre--which has led to the conclusion that in
the centre is a focus of eruption which had become closed up
antecedently to the formation of the valley itself. Lyell has explained
this point very clearly by showing that this focus had ceased to eject
matter at some distant period, and that the existing crater at the
summit of the mountain had poured out its lavas over those of the
extinct orifice. This was prior to the formation of the Val del Bove
itself; and the question remains for consideration how this vast natural
amphitheatre came to be hollowed out; for its structure shows
unquestionably that it owes its form to some process of excavation.

In the first place, it is certainly not the work of running water, as in
the case of the canyons of Colorado; the porous matter of which the
mountain is formed is quite incapable of originating and supporting a
stream of sufficient volume to excavate and carry away such enormous
masses of matter within the period required for the purpose. We must
therefore have recourse to some other agency. Numerous illustrations are
to be found of the explosive action of volcanoes in blowing off either
the summits of mountains, or portions of their sides. For