40) H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
into the hard mass we now see covering that town and the neigh- 
bourhood. 
Exactly similar circumstances attended the eruption of Monte 
Nuovo *, in which the crater bottom is now only a little above sea- 
level. It is possible that part of the first violence of the outburst 
of a voleano may be due to the meeting of the incandescent rock 
and highly aquiferous strata near the surface, as the former gradually 
rises in the vent. Such conditions are somewhat analogous to the 
violent explosion that occurs when lava flows into a well of water. 
In the same manner the latter part of a paroxysmal eruption might 
vary as the rising incandescent matter battled with the unlimited 
inpouring water from the walls of the chimney. 
There is no doubt that in the latter part of a paroxysmal eruption 
the volcanic forces can no longer hoist the ejectamenta beyond the 
crater-rim, and therefore they would fall in again and go towards 
filling up the hollow. It must be remembered, however, that it is a 
generally recognized fact that paroxysmal eruptions do not spread 
their decrescence over a long period of moderate activity suitable 
to the formation of a cone of eruption, in which category Vesuvius 
must be placed in its relation to Somma. 
In the accounts of all the great known paroxysmal outbursts, the 
resulting crater has rarely possessed a cone of eruption occupying its 
bottom. Even admitting such to have been the case in the present 
instance, the subsequent eruptions, as we shall see, must have done 
much to destroy any eruptive cone. 
Another question that presents itself to one’s mind is, What was 
the original form and height of Monte Somma ? 
From the remarkable similarity in the lavas and their fragmentary 
derivatives, and their arrangement in the oldest sections of the moun- 
tain, there is every reason to suppose that, like those of Vesuvius, 
they were ejected under similar conditions and produced quite as 
regular a cone as Vesuvius, having the same inclination of its flanks. 
This was interthreaded by radial sheets of lava, which, cooling as 
dykes, strengthened the great mass, just as was seen in the section 
of the 1822 crater by Mallet. 
An idea of this prehistoric cone may be obtained by the following 
method. 
Construct upon a base which shall be any one contour-line, a cone 
whose sides are inclined at the same angle of rest as that assumed 
by the products of Vesuvius. The height of the mountain will be 
equal to such a cone, plus the height of the contour-line used for 
the base. 
Take a correct outline of the whole mountain and continue 
curves from the peak of Somma and the Pedimentina parallel to the 
corresponding slope and irregularities of Vesuvius. 
* “Jn a short time the fire increased to such a degree that it burst open the 
earth at this place, and threw up so great a quantity of ashes, pumice mixed with 
water, as covered the whole country; and in Naples a shower of these ashes 
and water fell great part of the night.’—Marco Antonio della Falconi, ‘ Dell’ 
Incendio di Pozzuoli,’ 1538. 
