; and near Misterbianco and
Piedemonte, small deposits of clay slate. The great mass of the surface
of the mountain, not specially mentioned above, is volcanic tuff.

[Illustration: Map of the Val del Bove, to illustrate the theory of a
double axis of eruption. (_Lyell_).]

Among the more important and recent additions to our knowledge of the
geology of Etna may be mentioned Lyell's paper on the subject,
communicated to the Royal Society in 1858, the matter of which is
incorporated in a lengthy chapter on Etna in the "Principles of
Geology." Lyell visited the mountain in 1828, 1857, and 1858, and he
then collected together a great number of personal observations; he also
made use of the maps and plans of Von Waltershausen, and he has analysed
the views of Elie de Beaumont and other writers. He alludes at the
outset to the numerous minor cones of Etna produced by lateral eruption,
and points out the fact that they are gradually obliterated by the lava
descending from the upper part of the mountain, which flows around them
and heightens the ground on which they stand. In this way the crater of
Monte Nocilla is now level with the plain, and the crater of Monte
Capreolo was nearly filled by a lava stream in 1669. Thus without
doubt beneath the sloping sides of Etna a multitude of obliterated
monticules exist.

[Illustration: Ideal section of Mount Etna]

The strata which surround Mount Etna on the south are of Newer Pliocene
date, and contain shells which are nearly all of species still living in
the Mediterranean. Out of sixty-five species collected by Lyell in 1828,
sixty-one were found to belong to species still common in the
Mediterranean. These strata are about the age of the Norwich crag; and
the oldest eruptions of Etna must have taken place during the glacial
period, but before the period of greatest cold in Northern Europe.

Before visiting Etna, Lyell had been told by Dr. Buckland that in his
opinion the Val del Bove was the most interesting part of Etna,
accordingly he specially and minutely examined that part of the
mountain. This vast valley is situated on the eastern flanks of the
mountain, and it commences near the base of the cone, stretching for
nearly five miles downwards. It is a large oval basin formed in the side
of the mountain, and surrounded by vast precipices, some of which at the
head of the valley are between three and four thousand feet in height.
The surface is covered with lava of various