Dr. H. J. Johnston-Lavis—On the form of Vesuvius, ete, 446 
II].—Furtuer OpsERvVATIONS ON THE Form oF VESUVIUS AND 
Monte Soma. 
By H. J. Jounston-Lavis, M.D., F.G.S., B.-és-S. 
N the year 1884 a paper of mine, entitled “The Geology of Monte 
Somma and Vesuvius,” was published (Quart. Journ. Geol. Soe. 
vol. xl. pp. 85 to 119), in which I proposed a new explanation for 
the peculiarities in form of this voleano. An endeavour was made 
to show that the truncation of Monte Somma by the series of 
explosive eruptions, especially of Phase VI., had occurred around an 
eruptive axis different from that which belonged to the period of 
vesuvian activity, by which the original Somma cone had been 
built up. Furthermore, a law was enunciated, which is applicable 
to a very large number of volcanoes, of which, even in Italy, the 
following examples may be taken in their order of perfection, viz. 
Roccamonfina, Mt. Vultura, Etna, Stromboli, Vulcano, and some of 
the Roman ones. The following are some of the words used: 
“There is very good reason to deny the concentricity of the great 
crater of the Atrio with the original cone of Monte Somma. 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 (7.e. the centre of Vesuvius), from, say, the 650 metres 
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 
between! 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. 
“Tt 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.” 
I showed that at the same time the hypotheses given by earlier 
1 This is an error of calculation : it should be between 445 and 475. 
