ess pervades
their ground mass as soon as the Nicols are crossed, and this general
darkness does not appear to be dissipated during the horizontal
revolution of the sections themselves. The translucent minerals in these
sections are all doubly refracting, and as I have not been able to
detect the presence of hauyne, noseau, sodalite, analcime, or any other
cubic mineral in them, the natural inference is that the obscurity
between crossed Nicols is due to amorphous matter. I have only been able
to ascertain the presence of glass distinctly in a microscopic section
of the lava of Salto di Pulichello. In the other sections which I have
examined there appears to be a small quantity of interstitial glass, but
it is so finely disseminated between the microliths of felspar and
granules of olivine, augite and magnetite, which constitute the
ground-mass of these rocks, that it is most difficult to determine the
single refraction of such minute specks during revolution between
crossed Nicols, and I therefore merely express a belief, which, in some
instances, I cannot demonstrate with any certainty.

[Illustration: Sections of Etna Lavas seen under the Microscope]

Plagioclastic felspars are unquestionably the dominant constituents of
these lavas. Lyell, in his "Principles of Geology," (9th Edition, p.
411), states that the felspar is Labradorite. He does not, however, give
the grounds for this conclusion, and, as microscopic examination alone
merely indicates the crystalline system and not the species of felspar,
it is unsafe to speculate upon this point in the absence of chemical
investigation. In some of these lavas Sanidine is also present, but it
is always subordinate to the plagioclase, and does not, as a rule,
appear to play a part sufficiently prominent to entitle the rock to the
appellation Trachy-dolerite.

Augite and olivine are generally present in the Etna lavas, especially
the latter mineral.

Magnetite appears to occur in all of them. Titaniferous iron may also be
represented, but I have failed to detect any well-defined crystals, or
any traces of the characteristic white decomposition product which would
justify me in citing the presence of this mineral, although it is stated
by Lyell to occur in these rocks.

The constituent minerals of the Etna lavas now to be described, namely,
those of B.C. 396 and A.D. 1535, 1603 and 1689, are:--

Plagioclase, augite, olivine, magnetite, and, in some cases,
sanidine--po