the same time M. Abich mentions, that when, in August, 1834, a great
subsidence took place in the platform of lava within the great crater,
so that the structure of the central cone was laid open, it was seen to
have been evidently formed, _not by upheaval_, but by the fall of
cinders and scoriae which had been thrown out during successive
eruptions.[531]

Previous to the year 79, Vesuvius appears, from the description of its
figure given by Strabo, to have been a truncated cone, having a level
and even outline as seen from a distance. That it had a crater on its
summit, we may infer from a passage in Plutarch, on which Dr. Daubeny
has judiciously commented in his treatise on volcanoes.[532] The walls
of the crater were evidently entire, except on one side, where there was
a single narrow breach. When Spartacus, in the year 72, encamped his
gladiators in this hollow, Clodius, the praetor, besieged him there,
keeping the single outlet carefully guarded, and then let down his
soldiers by scaling-ladders over the steep precipices which surrounded
the crater, at the bottom of which the insurgents were encamped. On the
side towards the sea, the walls of this original cavity, which must have
been three miles in diameter, have been destroyed, and Brieslak was the
first to announce the opinion, that this destruction happened during the
tremendous eruption which occurred in 79, when the new cone, now called
Vesuvius, was thrown up, which stands encircled on three sides by the
ruins of the ancient cone, called Monte Somma.

[Illustration: Fig. 45.

Supposed section of Vesuvius and Somma.

_a_, Monte Somma, or the remains of the ancient cone of Vesuvius.

_b_, The Pedamentina, a terrace-like projection, encircling the base of
the recent cone of Vesuvius on the south side.

_c_, Atrio del Cavallo.[533]

_d_, _e_, Crater left by eruption of 1822.

_f_, Small cone thrown up in 1828, at the bottom of the great crater.

_g_, _g_, Dikes intersecting Somma.

_h_, _h_, Dikes intersecting the recent cone of Vesuvius.]

In the annexed diagram (fig. 45) it will be seen that on the side of
Vesuvius opposite to that where a portion of the ancient cone of Somma
(_a_) still remains, is a projection (_b_) called the Pedamentina, which
some have supposed to be part of the circumference of the ancient crater
broken down towards the sea, and over the edge of which the lavas of the
modern Vesuvius have poured; the axis of the present con