OF MONTE SOMMA AND VESUVIUS. 79 
holds a very doubtful place there. It occurs as blocks, very splintery, 
which attain a considerable size, and consist of a great abundance 
of sanidine crystals, large and well formed, scattered through a light 
grey or bufft-coloured matrix, the uniformity of which is broken by 
a number of small garnets and prisms of amphibole. There seems 
to be no clear distinction between this rock and the pumice amongst 
which it is found, and of which it seems to be a compact variety. 
When examined microscopically it proves to be very free from ex- 
traneous impurities, and is often slightly vesicular. The minerals 
that go to form its principal components are exceedingly perfect and 
uninjured. The most abundant are amphibole, garnet (melanite), 
and sanidine. The latter, in many of its crystals, presents a great 
number of amphibole microliths arranged along the planes of growth 
as in that of the pumice, which seems to confirm the common origin 
of the two. The analysis of this rock by Vauquelin quoted by Breislak 
shows it to be identical with pumice. Although some difference ap- 
parently exists between the two, it is necessary to repeat the analysis 
with careful choice of the specimens, and with the modern methods of 
chemical research. If this rock should really turn out to be of the 
same origin as the pumice, the amphibole would be the only repre- 
sentative of the bisilicate group to which it belongs, the presence of 
the pyroxene being simply accidental except that of second consoli- 
dation. Such evidence would point to this rock being the same 
magma that formed the pumice, but cooled slowly under pressure. 
We may follow this rock through many most delicate gradations 
into a syenite, so called. Nepheline, guarinite, titanite, zircon, all 
make their appearance in the inseparable varieties of it, which, in 
its highest crystalline states, seems to be composed of a friable ag- 
gregate of sanidine, black hornblende (syntagmite), and garnet (me- 
lanite), the other minerals being subsidiary. Through such masses, 
and often forming the greater part of the detached blocks, are to be 
found veins rich in black, green, or brown mica, which may support 
beautiful crystals of red garnet or idocrase, and these again sustain 
dodecahedra of sodalite or hexagonal prisms of nepheline. 
By.an equally imperceptible gradation this micaceous rock may be 
mixed with pyroxene, anorthite, humite, and olivine, each of which 
may preponderate, so that a block may, for example, be composed 
of nothing but an aggregate of pyroxene crystals. Passing into 
these there is a most extraordinary series of varieties of altered lime- 
stones which have developed within them, to a greater or less extent 
—pyrrhotite, periclase, lapis lazuli, magnetite, spinel, wollastonite, 
pyroxene, meionite, anorthite, nepheline, leucite, &. Next come a 
series of metamorphosed dolomitic limestones, which may range 
from the purest white, porcellanous in structure and fracture, through 
the saccharoidal stage, until each grain may reach the size of a wal- 
nut or more. ‘The last degree consists of stratified limestones, in 
which the lines of deposition may be more or less distinctly exhibited 
by difference of structure or colour. Between the lamine I have 
occasionally found the carbonized remnants of fuci or other vegetable 
remains. On the one hand we may have this limestone in such a 
