1852,
and the question is much complicated by other considerations. Lyell
compares the growth of a volcano to that of an exogenous tree, which
increases both in bulk and height by the external application of
ligneous matter. Branches which shoot out from the trunk, first pierce
the bark and proceed outwards, but if they die or are broken off they
become inclosed in the body of the tree, forming knots in the wood.
Similarly the volcano consists of a series of conical masses placed one
above the other, while the minor cones, corresponding to the branches of
the tree, first project, and then become buried again, as successive
layers of lava flow around them. But volcanic action is very
intermittent, the layers of lava and scoriae do not accumulate evenly and
regularly like the layers of a tree. A violent paroxysmal outbreak may
be succeeded by centuries of quiescence, or by a number of ordinary
eruptions; or, again, several paroxysmal outbreaks may occur in
succession. Moreover, each conical envelope of the mountain is made up
of a number of distinct currents of lava, and showers of scoriae. "Yet we
cannot fail to form the most exalted conception of the antiquity of this
mountain, when we consider that its base is about 90 miles in
circumference; so that it would require ninety flows of lava, each a
mile in breadth at their termination, to raise the present foot of the
volcano as much as the average height of one lava current." If all the
minor cones now visible on Etna could be removed, with all the lava and
scoriae which have ever proceeded from them, the mountain would appear
scarcely perceptibly smaller. Other cones would reveal themselves
beneath those now existing. Since the time when, in the Newer Pliocene
period, the foundations of Etna were laid in the sea, it is quite
impossible even to hint at the number of hundreds of thousands of years
which have elapsed.

We collected specimens of lava from various points around and upon the
mountain. They presented a wonderful similarity of structure, and a
mineralogist to whom they were shown remarked that they might almost
all have come from the same crater, at the same time. A specimen of the
lava of 1535 found near Borello, was ground by a lapidary until it was
sufficiently transparent to be examined under the microscope by
polarised light. It was found to contain good crystals of augite and
olivine, well striated labradorite, and titaniferous iron ore.

Elie de Beaumo