OF MONTE SOMMA AND VESUVIUS. 83 
the mass friable. The explosions, as they became feebler, would no 
longer hurl the pumice and lapilli beyond the crater-rim, and on falling, 
they would be reduced to dust by the violent attrition to which they 
would be subjected. The continual abrasion of each particle would 
result in dust which, from its lightness, would be all that was carried 
to any height by the ascending column of vapour, and therefore the 
only material that could fall on the outer slopes of the cone. It is 
possible that the pisolitic grains may be due to the falling of the 
pulverized water derived from the inflow from the sides of the crater 
below drainage-level and the condensed vapour mixing with this ash 
either as it fell or afterwards, so as to convert it into a mud or 
“ moya.” 
From these deposits immediately supporting the products of the 
Plinian eruption, which is the first true historic outburst of the 
mountain, we may conclude that this is the last prehistoric one, 
although there is a kind of legend that may possibly apply to this. 
Vitruvius * says that, according to history, Vesuvius once burst and 
covered the environs with its fires, and Diodorus Siculus yf reports 
that Hercules saw Vesuvius, which at that time vomited fire like 
tna. 
So far as we have gone in the study of the histology of the 
pumiceous products of Monte Somma, the absence of leucite, either 
as erystals or microliths, as a component of these vesicular primary 
or essential materials of paroxysmal eruptive origin, has been very 
remarkable. Henceforward in all the pumice of similar and ana- 
logous deposits leucite plays a very important part as one of the 
constituents. 
Puassz VII., Period 1. 
The eruption of a.v. 79, or the Plinian eruption. 
The letters of Pliny the younger to Tacitus, although written half 
a century after the event, by an unscientific observer, have yet all 
the appearance of an impartial and correct account of the youthful 
impressions of an educated Roman. It would be useless to repeat 
the two letters m their entirety, since they are so well known, and 
therefore only those passages are quoted that bear directly on our 
subject. It will be convenient to follow the same order as in dis- 
cussing earlier eruptions, namely, examination “en. gros” and in 
minute structure, the manner of deposition, and then the comparison 
of the results with what Pliny records. 
Reposing directly on the streets of Pompeii we have about four 
metres of pumice mixed with many leucilitic lapilli and that 
same series of ejected metamorphic and other foreign rocks that 
we see in Puasz VI., Period 4. Like that deposit also, the eruptive 
materials are capable of subdivision into three groups similar to those 
of the preceding ejectamenta. 
The lowest of these is a white light pumice containing a number 
7 hip. 1. chap. vi- 
+ Lib. v. cap. xxi., or ‘ Histoire naturelle des voleans,’ &e., par C. N. Ordinaire, 
ci-devant chanoine de Riom, Paris, 1802. 
Ga 
