plain, and surrounded
by steep rocks.

Now we might imagine a similar event, or a series of subsidences to have
formerly occurred on the eastern side of Etna, although such
catastrophes have not been witnessed in modern times, or only on a very
trifling scale. A narrow ravine, about a mile long, twenty feet wide,
and from twenty to thirty-six in depth, has been formed, within the
historical era, on the flanks of the volcano, near the town of
Mascalucia; and a small circular tract, called the Cisterna, near the
summit, sank down in the year 1792, to the depth of about forty feet,
and left on all sides of the chasm a vertical section of the beds,
exactly resembling those which are seen in the precipices of the Val del
Bove. At some remote periods, therefore, we might suppose more extensive
portions of the mountain to have fallen in during great earthquakes.

But we ought not to exclude entirely from our speculations another
possible agency, by which the great cavity may in part at least have
been excavated, namely, the denuding action of the sea. Whether its
waves may once have had access to the great valley before the ancient
portion of Etna was upheaved to its present elevation, is a question
which will naturally present itself to every geologist. Marine shells
have been traced to a height of 800 feet above the base of Etna, and
would doubtless be seen to ascend much higher, were not the structure of
the lower region of the mountain concealed by floods of lava. We cannot
ascertain to what extent a change in the relative level of land and sea
may have been carried in this spot, but we know that some of the
tertiary strata in Sicily of no ancient date reach a height of 3000
feet, and the marine deposits on the flanks of Etna, full of recent
species of shells, may ascend to equal or greater heights. The narrow
Valley of Calanna leading out of the Val del Bove, and that of San
Giacomo lower down, have much the appearance of ravines swept out by
aqueous action.

_Structure and origin of the cone of Etna._--Our data for framing a
correct theory of the manner in which the cone of Etna has acquired its
present dimensions and internal structure are very imperfect, because it
is on its eastern side only, in the Val del Bove above described, that
we see a deep section exposed. Even here we obtain no insight into the
interior composition of the mountain beyond a depth of between three and
four thousand feet below the base of tha