mountain gradually piled up.

_Third Stage._--The vent under the Val del Bove ceased to extrude more
matter, and became extinct. Meanwhile the second vent continued active,
and, piling up more and more matter round the central crater, surmounted
the former vent, and covered its _ejecta_ with newer sheets of lava,
ashes, and lapilli, while numerous smaller vents, scattered all over the
sides of the mountain, gave rise to smaller cones and craters.

_Fourth Stage._--This stage is signalised by the formation of the Val
del Bove through some grand explosion, or series of explosions, by which
this vast chasm was opened in the side of the mountain, as already
explained.

_Fifth Stage._--This represents the present condition of the mountain,
whose height above the sea is due, not only to accumulation of volcanic
materials round the central cone, but to elevation of the whole island,
as evinced by numerous raised beaches of gravel and sand, containing
shells and other forms of marine species now living in the waters of the
Mediterranean.[7] Since then the condition and form of the mountain has
remained very much the same, varied only by the results of occasional
eruptions.

(_d._) _Dissimilarity in the Constitution of the Lavas of Etna and
Vesuvius._--Before leaving the subject we have been considering, it is
necessary that I should mention one remarkable fact connected with the
origin of the lavas of Etna and Vesuvius respectively; I refer to their
essential differences in mineral composition. It might at first sight
have been supposed that the lavas of these two volcanic
mountains--situated at such a short distance from each other, and
evidently along the same line of fracture in the crust--would be of the
same general composition; but such is not the case. In the lava of
Vesuvius leucite is an essential, and perhaps the most abundant mineral.
It is called by Zirkel _Sanidin-Leucitgestein_. (See Plate IV.) But in
that of Etna this mineral is (as far as I am aware) altogether absent.
We have fortunately abundant means of comparison, as the lavas of these
two mountains have been submitted to close examination by petrologists.
In the case of the Vesuvian lavas, an elaborate series of chemical
analyses and microscopical observations have been made by the Rev.
Professor Haughton, of Dublin University, and the author,[8] from
specimens collected by Professor Guiscardi from the lava-flows extending
from 1631 to 1868, in every on