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ON MONTE SOMMA AND VESUVIUS. 97 
nicols, neither is it very rich in inclusions, which, when present, 
consist more of glass-cavities than microliths. ‘The mineral is easily 
recognized by the beautiful neatness with which we may see all the 
crystalline edges by focussing up and down, aided by the purity of 
the glassy base in which the leucite is imbedded. 
A very few well-formed sanidine crystals occur, especially upon 
and in the neighbourhood of the pyroxene. 
There are two distinct varieties of microliths. The first are neat, 
sharp-cut, but rather small rods, which seem to be principally, if 
not all, pyroxene. ‘Six carefully checked measurements gave an 
average angle of extinction of 373°. It is possible that of the smaller 
microliths some may be felspar. 
Scattered about, but more especially collected into clouds, are 
minute crystals of a mineral identical with what we have already 
described as probably magnetite. The more perfect ones show under 
a high power a six-sided outline, almost without exception ; now 
such could hardly be derived from any other form than a rhombic 
dodecahedron ; for hexagonal plates, as biotite, or prisms, as nephe- 
line or other minerals of the same system, give sections very variable 
in form, according to the plane of the slice in relation to the crystal. 
So far as can be judged from their small size, they are uniaxial; the 
question of choice therefore lies between garnet, titaniferous iron, 
and magnetite. Their great abundance, their peculiar gregarious 
arrangement, and their occupying the exact relative distribution 
that magnetite does in similar rocks, certainly points to that mineral, 
though the dodecahedron is rare, and the red light transmitted 
resembles somewhat that of melanite. We have already shown a 
similar mineral to exist in earlier ejectamenta in which there is 
little doubt of its constitution. It might be confused with hematite, 
but that mineral usually lines the cavities of a scoria, and is not, as 
in the present case, enveloped in a glassy base, from which it has 
crystallized. Besides, this mineral forms more irregular crystals, 
both in form and size, and the colour is much redder. Sodalite 
is another mineral not uncommon as a product of sublimation (?), 
lining cavities in Vesuvian lava, but is rare in the rock mass, and 
then is of very light colour, if not transparent. This dubious 
mineral is, as we have already seen, not an uncommon component 
of pumice &c. 
Although the perfect crystalline condition we have described is 
not uncommon, yet the mineral is usually to be seen in the form of 
six-rayed stars, or in the shape of those spiked balls used in ancient 
warfare, and with which the representations of Gog and Magog are 
armed. This seems to be due to imperfect development, similar to 
the hollow crystals of halite, bismuth, &c. In this condition each 
individual is nearly opaque, and we can trace the mineral into 
granular masses that are quite so. 
The whole of these minerals are enveloped in a very pure glass 
of dark sepia-colour, which is very abundant. The spongy nature 
of the mass is due to the great abundance of large vesicular cavi- 
ties: whereas the numerous minute ones characteristic of pumice, 
Q.J.G.8. No. 157. . H 
