38 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
We may arrive approximately at a conception of the enormous 
space and depth this great crater had, by comparing it with the one 
produced by the eruption of 1822, a space calculated at one kilo- 
metre in diameter, and variously at from 309 to 700 metres in depth. 
It was drilled through lava, scorie, ash, &c., identical in composition 
with those of the ancient Somma. 
Comparing these measurements with the diameter of the crater 
in A.D. 79, at, say, the 650-metre contour-line, the altitude of its 
lower edge, which was three kilometres, we may estimate the depth. 
Such a diameter calculated with the median depth of the 1822 crater 
as 500 metres, would show that of 79 to have been between 800 and 
900 metres below sea-level. We therefore see that all the ejecta- 
menta derived from the eruptions since the commencement of the 
Christian era are not represented by the mass of the Vesuvian cone, 
but also by another cone, inverted, whose base is now the plain in 
the Atrio, and whose apex must be more than 1500 metres below 
its base. 
If the Plinian eruption had formed the greater part of the present 
Vesuvian cone, it must, besides the materials that cover Pompeii and 
the mountain slopes in that direction, have ejected sufficient also to 
form a cone twice the size of that of Vesuvius, to fill up the great 
crater, and upon the base of this, another at least half the size of 
that now visible. 
When we look at the question in this light we can hardly suppose 
that any thing like a cone of eruption was visible above the edge of 
the crater after this Plinian eruption. 
We shall return to this question more than once as confirmatory 
evidence is brought out by other facts. 
If we speculate for a moment on the results of such a hollow as 
we have shown this great crater to have been, we may possibly find 
that some difficult facts are cleared up, and an explanation given of 
that much disputed question, the eruption of mud and water. 
Taking a volcano, such as Vesuvius in its present active state, 
let us endeavour to examine the conditions which must exist 
amongst the components of the base of the mountain below sea- 
level. 
From artesian wells sunk in the neighbourhood, and from the 
surrounding geology, it may be learned that this volcano reposes on 
a considerable thickness of highly porous strata, through which 
water can percolate with great ease and rapidity. Only a thickness 
of six or seven kilometres of such materials separates the fluid 
magma occupying the chimney, from the open sea. The question 
therefore arises, what are the consequences of the necessary contact 
of the molten rock with the water disseminated in the porous walls 
of the volcanic chimney? This is assuming conditions not differing 
from those lately put forward by Prof. Prestwich. 
The natural tendency of the water in immediate contact with the 
heated magma would be to flash into steam, which is prevented by 
the pressure of the superincumbent materials. In such a way we 
in all probability have a gradual intervention of water at various 
