e of Vesuvius
being, according to Visconti, precisely equidistant from the escarpment
of Somma and the Pedamentina.

In the same diagram I have represented the slanting beds of the cone of
Vesuvius as becoming horizontal in the Atrio del Cavallo (at _c_), where
the base of the new cone meets the precipitous escarpment of Somma; for
when the lava flows down to this point, as happened in 1822, its
descending course is arrested, and it then runs in another direction
along this small valley, circling round the base of the cone. Sand and
scoriae, also, blown by the winds, collect at the base of the cone, and
are then swept away by torrents; so that there is always here a flatish
plain, as represented. In the same manner, the small interior cone
(_f_) must be composed of sloping beds, terminating in a horizontal
plain; for, while this monticule was gradually gaining height by
successive ejections of lava and scoriae, in 1828, it was always
surrounded by a flat pool of semi-fluid lava, into which scoriae and sand
were thrown.

In the steep simicircular escarpment of Somma, which faces the modern
Vesuvius, we see a great number of sheets of lava inclined at an angle
of about 26 degrees. They alternate with scoriae, and are intersected
by numerous dikes, which, like the sheets of lava, are composed chiefly
of augite, with crystals of leucite, but the rock in the dikes is more
compact, having cooled and consolidated under greater pressure. Some of
the dikes cut through and shift others, so that they have evidently been
formed during successive eruptions. While the higher region of Somma is
made up of these igneous products, there appear on its flanks, for some
depth from the surface, as seen in a ravine called the "Fossa Grande,"
beds of white pumiceous tuff, resembling the tuff which, at Pausilippo,
and other places, near Naples, contain shells of living Mediterranean
species. It is supposed by Pilla, Von Buch, and others, that the
tufaceous beds, which rise in Somma to more than half the height of that
mountain, are, in like manner, of submarine origin, because a few
sea-shells have been found in them, here and there, together with
serpulae of recent species attached to included blocks of
limestone.[534]

It is contended, therefore, that as these strata were once accumulated
beneath the sea, they may have been subjected as they rose to such an
upward movement as may have given rise to a conical hill; and this
hypothesis, it