88 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
We may easily explain the similarity in general features and 
arrangement of these two distinct deposits, by the conditions under 
which they were formed and ejected being almost analogous. 
We have in the section of the Canale di Arena, overlying the 
pisolitic bed of Puasz VI., Period 4, the already mentioned deposits 
of brown pumice, moderately compact, resembling the upper sub- 
division of that covering Pompeii even to the minutest details of 
microscopic structure. It may seem curious that this is the only 
northern part of Monte Somma where this pumice has yet been met 
with, and that both the lower subdivision of the pumice and the 
pisolitic ash-beds are wanting. The two absent subdivisions were 
the lightest and most capable of transport by the wind, which we 
know from Pliny’s account to have blown somewhere to southwards. 
We must also remember that the crater had higher walls to the 
north, so as to prevent to some extent the directing of the ejecta- 
menta on that side. Also the wind reaching the eruptive column 
at a higher point on the north would impress itself more power- 
fully on the uprushing materials, there moving at a proportionally 
diminished speed, so as to direct them towards the south or least 
resistent side. 
That a powerful wind did blow in the direction above indicated 
is confirmed by the great abundance of pumice, for the most part 
similar in all details, which is to be found on Monte St. Angelo be- 
hind Castellamare. Almost at the summit of this mass of Apennine 
limestone are extensive deposits of pumice with a few leucilitic 
lapilli and erratic fragments. 
Prof. A. Scacchi* says @ propos of this :—‘‘ But the lapilli ejected 
in the great eruption of a.p. 79, which entombed Pompeii, and those 
which, in the direction of Vesuvius and Pompeii, form thick strata 
on the mountains of Castellamare and Sorrento, are composed in great 
part of pumice.” 
That the phenomena of this paroxysmal eruption exhibited ex- 
traordinary violence and force, history but too well recounts; yet we 
are most powerfully impressed with this fact when we encounter large 
quantities of its products carried a distance of twenty-two kilometres, 
and there deposited on a mountain which must then have been more 
than 300 or 400 metres higher than the then existing crater-edge. 
Nevertheless the ejectamenta of this eruption are comparatively of 
small volume compared with those of that which preceded it, and it 
is doubtful whether the already existing crater had more than its 
bottom cleared out and its walls shaved by the explosions. 
Strabo has been often quoted to prove that the summit of the 
mountain before the Plinian eruption was a plain, which could not 
possibly have given the peculiar natural protection that Spartacus 
and his followers sought and found. In fact the words of the great 
geographer, ‘“‘Imminet hisce locis Vesuvius mons, agris cinctus 
optimis, dempto vertice qui magna sui parte planus,” may well be 
understood as a craterial hollow enclosing a large plain and having 
* «Lezioni di geologia,’ Napoli, 1843, p. 169, and ‘Notizie geologiche dei 
vulcani della Campania.’ (These are the same.) i 
