OF MONTE SOMMA AND VESUVIUS. 69 
primary eruptive material, but in its escape tears away the sides of 
the vent. As the escape of vapour takes place, the point where it 
leaves the fluid magma will gradually recede from the surface, in 
proportion as the latter is ejected, unless there was a compensatory 
upflow of material from below. It is only when the gaseous materials 
leave the liquid that the former can exert their eroding power. 
These remarks give the interpretation of the prevalence of 
pumice first, then the Tertiary marls, followed by limestone, and 
last, ash and lapilli, with lava blocks, which are derived from the 
erumbling-in of the undermined crater walls and their re-ejection. 
We see by this that as the apex of the enlarging craterial hollow 
was lowered new materials made their appearance amongst the 
ejectamenta. Towards the latter end of the eruption, when the 
explosions were becoming feeble and irregular, no more planing or 
excavation of the crater took place; but the loose crumbling compo- 
nents of the walls as they fell down the sides into the vent were 
subjected to churning and violent attrition, and from time to time 
were hurled forth on the slopes of the cone. 
These are the facts we may learn from the demonstrations left 
written by the features, arrangement, and structure of the ejecta- 
menta of this great eruption. They are confirmed by the great 
similarity in the characters of deposits of later eruptions and by 
what has been witnessed by man in historic outbursts of this and 
other volcanos. In the Vallone Sanseverino, overlying the deposit 
just described, is a bed of re-sorted lapilli, rolled pumice, and pieces 
of scoria, which seem to have been disturbed by the action of water. 
The best example is to be seen in the Vallone Pietri Pomice, 
where this deposit is often seen to be eroded, and the subsequent 
materials rest unconformably on it. We have here again the same 
evidence as with earlier interruptive periods, namely, a certain 
amount of denudation going on whilst the volcano was in a state 
of repose. The epoch of inactivity was probably not of long duration, 
if we judge by the thinness of these alluvial deposits and the slight 
amount of erosion, which in many sections is not even visible. 
Puase VI., Period 2. 
This is represented by another bed of pumice and its derivatives 
with secondary materials, having its own special characters both as 
a rock-forming matter and in its minute structure, so that it can be 
distinguished from its fellows with great facility. It ranges from 
1 metre in thickness, as at Cisterna, to 4 metres or even more, as 
in the Vallone Sanseverino and in other localities. 
The whole stratum is capable of subdivision into three, a lower 
yellow loam-like decomposed ash-bed, the middle or pumiceous bed 
proper, which graduates up, though rather sharply, into another fine 
yellow ash-bed. The inferior of these subdivisions may probably 
be in part alluvial (?), preceding this eruptive phase, or more likely 
the terminal ash-bed of Paasz VI., Period 1, although it has few 
diagnostic characters. The middle division appears to be composed 
