* 
OF MONTE SOMMA AND VESUVIUS. 91 
pressure was most favourable to its development, if we may judge 
trom subsequent eruptions. 
Puase VII., Period 2. 
General Description.—Reposing upon the pumice that we have 
considered as Plinian, we may see in the section of the Canale di 
Arena about a metre of yellowish-grey pumiceous lapilli. The pri- 
mary material consists of, ordinarily, a yellowish or brownish-grey 
pumice, graduating from amoderately spongy to a very compact tex- 
ture. A considerable number of crystals of biotite, amphibole, sani- 
dine, and leucite are scattered throughout the mass. The last of 
these minerals occurs in irregular and impure crystals, ranging from 
a centimetre indiameter downwards, which, in the more compact frag- 
ments, are very abundant. The pumice graduates into a porphyritic 
rock exactly similar to that described amongst the ejected blocks 
of the pre-Plinian eruption (PHasm VI., Period 4), except that in 
this case the porphyritically enclosed sanidine is, to a great extent, 
replaced by crystals of leucite. This rock seems to bear the same 
relation to the pumice of this period as does its earlier analogue to 
the corresponding pumice. 
The primary material is exceedingly rich in enclosed fragments 
of secondary and tertiary erratics. 
Microscopic Kxamination.—Amphibole, pyroxene, sanidine, garnet, 
and biotite, all occur in well-formed crystals, having all the cha- 
racters of the same minerals in earlier ejectamenta. Their relative 
abundance is in the order in which they are placed above, the 
amphibole (?)* being the most common. 
The leucite occurs as very pellucid crystals, except where traversed 
by long slender microlithic rods and a few cavities. Occasionally 
enclosed within the leucite is a well-formed crystal of amphibole. 
The large leucites are the only ones that exhibit the optical proper- 
ties of that mineral in polarized light, but they do so to perfection. 
Sometimes one meets with patches of an intimate interlacement 
of amphibole, garnet, and magnetite. 
Besides the large and well-developed light-green crystals of 
pyroxene there are a considerable number of large microliths, or 
perhaps more properly, small crystals of that mineral. They are 
lignt-green rod-like prisms, fairly well terminated. The leucite 
microliths are somewhat larger than those of the Plinian eruption, 
and are in such profusion that they are as closely packed together as 
they can be, only the interspaces being filled by a greenish or brownish 
mass. ‘This interstitial substance is composed of minute microliths, 
or belonites and globulites ; the former are of doubtful character, 
but, by analogy, we may refer the globulites to magnetite, though 
their minuteness prevents one getting a clear sight of any individuals, 
even under the highest powers. 
There would seem to be a small and variable quantity of vitreous 
base, according to the extent of formed material that in any speci- 
* Tam not quite sure about the determination of this mineral. 
