36 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
cone to be concentric within the old crater, and consequently to have 
the same eruptive axis. This method is not so easily applicable if 
used on a line east and west, since there are many irregularities 
produced by lava and ash of modern date; but as far as possible the 
evidence is confirmatory. 
There is very good reason to deny the concentricity of the great 
crater of the Atrio with the original cone of MonteSomma. If within 
a cone we scoop out an inverted conical hollow around an axis 
eccentric, but parallel, to that of the solid, we shall have the included 
space bounded by an annular ridge, not horizontal, but sloping down 
in the direction of the axis of excavation, and inclined proportionally 
more, the further the axis of excavation is removed from that of 
the solid. 
It is just with such a condition of things that we have to deal in 
the present instance. If we measure the distance of the modern 
eruptive axis (2. e. the centre of Vesuvius), from, say, the 650 metre 
contour-line on the northern slope of Somma, and compare it with 
that of the same line on the-south, we shall find that this axis is be— 
tween 850 and 950 metres to the south of the centre of the contour- 
lines of the ancient Somma. In a regular cone, such as we suppose 
this ancient volcano to have been, the centre of the contour-line 
would be the eruptive axis of the mountain. 
From these facts we must conclude that although the eruptions 
that excavated the great crater of the Atrio, and subsequently piled 
up the cone of Vesuvius, occurred from the same axis, this was nearly 
a kilometre to the south of the ancient one, of the primary cone 
of Somma. 
It seems also that the modern axis is slightly displaced to the 
west of south; but from the obscured features of the ground the 
exact amount cannot be accurately measured, although it was hardly 
south-west, as thought by Prof. Phillips. 
This change in the position of the eruptive axis appears to be the 
most rational explanation of the great difference in height of nearly 
500 metres, between the southern and northern ridge of Monte 
Somma. 
Breislak * endeavoured to explain this lower truncation to the 
south, by supposing that the vast mass that would fall back into 
the crater after the first explosion might be such as to cause the 
following one to search the weakest point, which would be to the 
south. 
If the igneous forces were able to elevate the great mass of ejected 
materials in. the first instance, they would hardly be directed 
obliquely by that portion in a fragmentary condition which would 
fall back again. ‘That any amount of blocking, even by a solid com- 
pact mass, would be capable of causing 500 metres of a mountain’s 
side to be carried away, whilst the eruptive force could not again 
lift that filling up the aperture, hardly seems probable. But such a 
plug could not be one solid coherent mass, from the fragmentary 
nature of its components, and it would therefore allow the gas or 
* Voyages physiques et lithologiques dans la Campanie,’ 1801, p. 182. 
