olivine, and augite crystals, grains of magnetite, and apparently a
little interstitial glass. A crystal of augite is shown near the bottom
of the drawing.

Fig. 2. Lava of A.D. 1689. On the right hand side part of a plagioclase
crystal with inclosures similar to that in the preceding figure. In the
centre a small crystal of plagioclase. Groundmass similar to that of
Fig. 1, but showing a somewhat definite arrangement of the small felspar
crystals, indicative of fluxion.

[23] "Mikroskopische Beschaffenheit der Mineralien und Gesteine."
Leipzig, 1873; p. 480.

The augite in these lavas sometimes occurs in well-formed crystals of a
green or brown colour, and often shows the characteristic cleavage very
well, especially in the augite crystals of the lava of the Boccarelle
del Fuoco, erupted in 1535. A small crystal of green augite is
represented at the bottom of Fig. 1. Augite, however, appears to be more
plentiful in the rocks in the form of small roundish grains.

Olivine is of very common occurrence in the Etna lavas, mostly in round
or irregularly shaped grains, but also in crystals which usually exhibit
rounded angles.

A specimen of lava from Salto di Pulichello, erupted in 1603, gave
well-developed examples of the presence of olivine, and also of
plagioclase. The ground mass was found to consist of felspar microliths,
and grains of olivine, augite, and magnetite, with some interstitial
glass.

Magnetite is present in all of the lavas here described. It occurs both
in octahedral crystals and in the form of irregular grains and fine
dust. To the presence of this substance much of the opacity of thin
sections of the Etna lavas is due.

Titaniferous iron may also be present. One small crystal in the lava of
1535 appeared to show a somewhat characteristic form, but although much
of the black opaque matter has undergone decomposition, I have failed to
detect any of the white or greyish alteration product which
characterises titaniferous iron, and in the absence of this, of definite
crystalline form, and of chemical analysis, it seems better to speak of
this mineral with reserve, although titanium is very probably present,
since much magnetite is known to be titaniferous.

The vitreous matter which occurs in these lavas is principally present
in the form of inclosures in the felspar, and, sometimes, the augite and
olivine crystals previously described. Its occurrence in the groundmass
of these rocks has also be