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OF MONTE SOMMA AND VESUVIUS,. ol 
enclosed crystals. ‘The latter break out long before the section is 
nearly thin enough. 
By ordinary light and a low power the section is seen to bea 
meshwork of rock material enclosing a multitude of air-cells, which, 
if unopened, present their peculiar refractive properties. They 
assume every imaginable form, the commonest being spherical, 
ovoid, pear-shaped, fusiform, multilocular, tabular, or irregular. 
Scattered about in the matrix are a few small dark green crystals 
of pyroxene, unfortunately rarely well formed. Sanidine in limpid 
well-terminated crystals, often twinned on the Carlsbad type, occur, 
besides a very few with rounded borders. Some examples enclose 
smaller crystals that extinguish at a different angle. The sanidine 
is usually surrounded by a coat of dark brown dusty matter, pro- 
bably magnetite (?), just as we see clear crystals of ice separate out 
from mud or muddy water. ‘The pyroxene has few cleavage-planes, 
is very faintly, if at all, polychroic, and has a very large angle of 
extinction, from which we may safely conclude as to its nature. 
There are a few crystals, however, that present more the characters 
of amphibole. One very dark bluish green, rather polychroic crystal 
gave 18° as the angle of extinction. 
The pyroxenic minerals are often surrounded by a wreath of 
sanidine. Magnetite occurs in well-defined crystals and irregular 
masses, though none microlithic. Biotite is to be seen in a few 
cases as large crystals of first consolidation, very polychroic, varying 
from greenish brown to pinkish black tints. 
Under a high power the ground mass is seen to be composed of a 
very light straw-coloured vitreous base, with remarkably few im- 
purities. The only microliths visible are a few odd ones here and 
there. They are usually minute rods, too small for exact determina- 
tion; but by analogy one would judge them to be pyroxene. 
The order of crystallization would appear to be :— 
I, Amphibole ?, magnetite, and biotite. 
II. Pyroxene and sanidine. 
Ii. Vitreous matrix and odd microliths. 
There are to be found, amongst the uppermost or last fragments 
ejected, pumice of a somewhat darker tint. These pieces prove to 
be very rich in small beautifully well-formed hexagonal plates of 
biotite * of sepia tint. In the same specimens are a considerable 
but variable number of crystallites or small microliths, rod-like in 
form, probably of pyroxene (?). There are also minute crystals, 
hexagonal in outline, of a blood-red colour, which appear to belong 
to the cubic system, for they seem to be extinguished without excep- 
tion between crossed nicols. They might be mistaken at first for 
small garnets (melanite), a mineral not uncommon in other pumice 
of similar origin. Their homology in position, arrangement, and 
other characters, point them out as magnetite partly altered; their 
* “Mica is generally of first consolidation ; it may, however, have the micro- 
lithic stamp.” Messrs. F. Fouqué and M. Lévy, ‘Mém. pour servir 4 l’explica- 
tion détaillée de la carte géologique de la France’ (Paris, 1879), p. 340. 
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