446 Dr. H. J. Johnston-Lavis— On the form of Vesuvius, ete. 
writers were insufficient to explain the greater truncation of the 
Somma cone to the south. 
Prof. Pasquale Franco last year published a paper, “ Il Vesuvio ai 
tempi di Spartaco e di Strabone,” Atti dell’ Accad. Pontaniana, 
vol. xvii., in which my views are severely criticized, and a new 
hypothesis put forward to explain the lower truncation of Somma to 
the south. 
My critic commences by accusing me of having neglected mention- 
ing that Palmieri had pointed out that in pre-Plinian times the 
lower edge of Somma was to the south, and he overlooks the fact, 
that this was so evident to any one who had devoted attention to the 
subject, that it would have been prolix to mention it again. What I 
really did was, taking such for granted, to give a rational explanation 
of the cause of it. 
The next objection is raised against my supposition that the 
cone of Somma was a regular one before its truncation. There 
is no evidence whatever that it was otherwise, for the following 
reasons: Ist, It was built up by the constant and regular emission 
of lava, at any rate while a mantle of from 3800 to 400 metres 
were added to its flanks, as evidenced by the great section of 
the Atrio, conditions which result in the construction of a fairly 
symmetrical cone obliterating old irregularities, examples of which 
we have in the Vesuvian cone itself, Htna, etc. 2nd, The plan of the 
base of the whole mountain was nearly circular (it is now being 
deformed and extended by the lavas and alluvions being directed 
more to its south, east and west sides). 
Iam then blamed for choosing the 650 contour-line to find the 
original axis of Somma, and my critic proceeds to try my method on 
that and eight other contour lines, on a section 40° N. of E. to 40° S. 
of W., which was not the line I chose, always with the result that 
the Vesuvian axis is found displaced 480 metres, the minimum dis- 
placement being 300 and the maximum 620. My reason for choosing 
the 650 contour-line was that it is the highest limit of the Atrio 
crater, or the Somma edge on the south, and as a ridge obviously . 
less exposed to alteration in height than the slopes below, according 
to well-known mechanical principles of denudation, and the repose of 
material. The deposits of lava, etc., on this point cannot amount to 
many metres, whilst on the less inclined slopes below it is demon- 
strable the lava flows are much thicker, and the alluvial deposits very 
great indeed, as examination in the field will make more evident 
than speculation at home. My friend himself admits far greater 
accumulations on the southern slopes, whereas in the middle zone of 
the northern slopes of Somma there has been more removal than 
addition on the whole. This is made strikingly evident by his own 
table of calculated eccentricities for six contour-lines below the 650, 
which shows, with one exception, a continuous and progressive 
diminution in the apparent value of eccentricity, which the author 
in the third paragraph of page 11 shows a thorough appreciation of. 
But he immediately contradicts himself in the next statement that 
there is reason to believe that the denudation was much greater on 
