in a very
fluid state, it moves with great velocity. As it cools the sides and
surface begin to harden, its velocity decreases, and in the course of a
few days it only moves a few yards in an hour. The internal portions,
however, part slowly with their heat, and months after the eruption,
clouds of steam arise from the black and externally cold lava beds after
rain, which, having penetrated through the cracks, has found its way to
the heated mass within.

Of the seventy-eight eruptions described above, it will be noticed that
not more than nineteen have been of extreme violence, while the majority
have been of a slight and comparatively harmless character.

[Illustration: Geological Map of Etna]

CHAPTER VI.

GEOLOGY AND MINERALOGY OF THE MOUNTAIN.

Elie de Beaumont's classification of rocks of Etna.--Hoffman's
geological map.--Lyell's researches.--The period of earliest
eruption.--The Val del Bove.--Two craters of eruption.--Antiquity of
Etna.--The lavas of Etna.--Labradorite.--Augite.--Olivine.--
Analcime.--Titaniferous iron.--Mr. Rutley's examination of Etna
lavas under the microscope.

The opinion of geologists is divided as to the manner in which a volcano
is first formed. Some hold that the volcanic forces have upraised the
rocks from beneath, and at last finding vent have scattered the lighter
portions of such rocks into the air, and have poured out lava through
the rent masses, thus forming a _crater of elevation_. Others maintain
that the volcanic products are ejected from an aperture or fissure
already existing in rocks previously formed, and that the accumulation
of these products around the vent forms the mass of the volcano and the
_crater of eruption_. Lyell favours the latter view; Von Buch, Dufrenoy,
and Elie de Beaumont the former.

According to M. Elie de Beaumont, Etna is an irregular crater of
elevation. The original deposits were nearly horizontal, and lavas were
poured through fissures in these, and accumulated at first in layers;
afterwards the whole mass was upheaved and a crater formed.[20] The
upheaving force does not appear to have acted at one point, but along a
line traversing the Val del Bove. The latter he refers to a subsidence
of a portion of the mountain. He divides the rocks of Etna into six
orders: 1. The lowest basis of the mountain would appear to consist of
granite, because masses of that rock have from time to time been
ejected. 2. Calcare