448 Dr. H. J. Johnston-Lavis—On the form of Vesuvius, ete. 
much under what they would be in a direction N. and §., or still 
more W. of 8., yet these are the results :— 
Contour-line. Distance of the two axes. 
675 metres. = 400 metres. 
650) ie = alow We 
600 4s SHAGOuly 5 
550) aos =" 4604.) ;3 
BOOM e580. 
A50N = 540 PS 
400 = 460 
If we have such an eccentricity indicated by observation at this 
point, there evidently must be far more in the azimuth of displace- 
ment. But there is still another geological point that makes our 
figures too low, and that is, that the post-Plinian mantle, and probably 
the Plinian also, is very thick at B. Cognoli, whereas erosion has 
been very active on the opposite side of the volcano. 
My learned friend then, on geometrical grounds, calculates the 
proper position of the vertex of Somma, and finds that, adopting the 
600 m. contour-line as the southern boundary of the Atrio crater 
edge, a point opposite and symmetrical would fall outside a vertical 
line dropped from the crest of Somma. This could hardly be 
otherwise, as I absolutely reject the 600 contour-line as being the 
edge of the Atrio crater for reasons I have already and am about to 
give. Now, strangely enough, when my critic makes use of the 
650 metre contour-line, the symmetrical point falls within the vertical 
line dropped from the crest of Somma, and the author admits that 
“the theory finds nothing absurd in its application.” This, I think, 
still further confirms me in having chosen the latter contour-line as 
being the most justifiable to represent the original edge of Somma. 
Diagrammatic sketch-section of a cone truncated by a crater formed around an axis 
eccentric to the original cone. 
We then have a description of the figure given (Le Pitture di 
Ercolano e Contorni IV. p. 843), which is undoubtedly the cone of 
Monte Somma truncated by the great crater of Atrio, the result of 
the great explosive eruptions of Phase II., around an eccentric axis. 
The author then quotes Virgil and other authors, who use words 
equivalent to the modern Italian “giogaja” or mountain-ridge, 
which no doubt was the appearance Somma had at that time as seen 
from Nola; for now, when we look at that mountain from the same 
