Dr. H. J. Johnston-Lavis—On the form of Vesuvius, ete. 449 
locality, Somma appears as a beautiful symmetrical cone, truncated 
by a horizontal serrated ridge (well explained by the word “giogaja”’). 
The quotations from the other well-known authors agree quite as 
well with the geological evidence I have brought forward, as with 
the figure under consideration, or as these agree together, the one 
confirming the others. It should be remarked, however, that none 
of these authors were scientists, or gave any detailed description or 
references to the mountain. 
We then have to deal with another geometrical argument, for the 
author points out that the plan of the Atrio crater is circular (the 
interruptions in its limits are so many and so irregular that circular 
is somewhat a far-fetched expression to describe its form), whereas 
in truncating a regular cone sensibly an ellipse would result. This 
is perfectly true when we deal with a cone whose slopes are straight, 
but those of a volcano are curved with a concavity towards the axis. 
In truncating such a cone, we should not obtain an ellipse, but an 
ovoid. In truth, in our present consideration, geometrical arguments 
are of secondary consideration, in comparison to the physical facts 
concerned. In fact, it is hardly just to assume in one argument that 
the cone of the old volcano was very far from symmetrical, and to 
suit another argument to consider it as not only symmetrical, but 
homogeneous, which system of employing different data is pursued 
throughout the whole of my critic’s memoir. The central core of a 
voleano is far more dense and compact than the peripheral parts,— 
first, because it has been exposed to greater superincumbent pressure, 
and secondly to the intense heat of the neighbouring chimney, which 
has cemented the fragmentary constituents together into one solid 
compact mass, such as we see constituting the bottom of the great 
Atrio section of Monte Somma, whereas towards the periphery, we 
have simply thin beds of lava, loose scorize, ash, etc. It is just this 
central resistent nucleus which is the probable cause of the eccen- 
tricity of explosive eruptions, after long inactivity taking place at 
some distance from the old axis. When the explosive eruption does 
take place, it will excavate and erode the part opposite the nucleus 
with much greater ease. The consequence of this is, that the lower 
crater edge will be removed further away from the new eruptive axis, 
so that in plan, it may even appear more distant from that axis, than 
the upper edge of the crater. This extra destruction of the peripheral 
part will be greatest near the lowest edge or point most distant from 
the nuclear part of the volcano, and will diminish, as we approach 
that part, so that the tendency always is to form a circular and 
neither an elliptical nor an ovoid-shaped crater. But this is counter- 
balanced on the northern edge, by subsequent denudation, and 
crumbling in of the Somma crest. Which of these two influences 
have been greatest in modifying the original outlines of the mountain, 
it would be rash to speculate upon. Above all other proofs, however, 
of the modern eruptive axis being nearer the §. or 8.W. slope of the 
mountain is that all eccentric lava eruptions below the 650 contour- 
line have taken place within the southern semicircle since those of 
Phase IV. 
DECADE III.—VOL. V.—NO. X, 29 
