OF MONTE SOMMA AND VESUVIUS. 93 
the bottom of the crater to the suitable conditions for the deposition 
of that group of hydrous silicates, before they were ejected. 
Remarks.—From various authorities we learn that there were 
eruptions during the years a.p. 245, 305, and 321, of a feeble type. 
These are probably represented by the pumiceous breccia; but from 
the absence of any unconformability or variation of structure be- 
tween one part of the deposit and another, it is quite impossible to 
know whether this may represent one or all of these eruptive phases. 
At any rate we may suppose that they added their ejectamenta 
principally to swell the cone of eruption forming at the bottom of 
the great crater. 
PHasst VIL., Period 4. 
General Description.—The next in order in the section of the 
Canale di Arena is a bed about a metre in thickness. It consists 
principally of masses of black pumiceous scoria, loose fragments of 
large leucite crystals, a few leucilitic lapilli, and foreign metamorphic 
and other rocks, overlain by a metre of coarse breccia. 
Around the foot of the mountain are very extensive deposits of 
this eruption. For example, all the valloni or lagni between the 
Fossa Faraone and the Lagno di Trocchia usually show in their sections 
thick accumulations of these scoriaceous materials, mixed with much 
sand and dust of the same origin. In the Lagno di Pollena we ob- 
serve that these materials have completely filled the old valley that 
now lies to the south of the present one, where their thickness is 
10 metres or more. 
Patches occur in this section where the surface at least has changed 
colour to a brown, owing to the higher oxidation and hydration of the 
magnetite. The materials are also very subject to consolidation 
en masse by the infiltration of calcareous matter ; their consequent 
resistance to denudation plays some part in modifying the super- 
ficial features of that portion of the mountain where they are found. 
The south and west slopes of Monte Somma, although not showing 
this deposit in sections, which are nearly absent, nevertheless where 
the mountain is left uncovered by lava-streams, present many !oose 
fragments belonging to it collected together in the garden stone- 
heaps or employed for wall-building. 
This scoria may be detected all over the mountain, but seems not 
to be abundant west of a line drawn from north-west to south-east. 
The scoria, which still retains a slight pumiceous facies about it, 
is a heavy and fairly compact rock, but very ragged, and containing 
many irregular cavities, which may be partly occupied by a leucite 
erystal or fragment of some rock caught up in the plastic mass. 
The surface presents an irregular corded and denser coat. 
The size is very variable, but pieces some pounds in weight are 
exceedingly common, and still larger ones are not rare. I lately broke 
up a block that must have weighed 60 or 70 kilogrammes, and 
which yielded over a hundred fine crystals of leucite more than one 
centimetre in diameter. One splendid crystal, measuring three 
