ssibly titaniferous iron--and in some, if not in all, a
slight residuum of glass. These lavas must therefore be regarded as
plagioclase-basalts, or occasionally as olivine-basalts. The plagioclase
crystals vary greatly in size, some being mere microliths while others
are over the eighth of an inch in length. They show the characteristic
twin lamellation by polarized light, but the lamellae are often very
irregular as regards their boundaries. The sections of the crystals
themselves are also frequently bounded by irregular outlines, but they
often show internally delicate zonal markings, as indicated in Fig.
1,[22] which correspond with the outlines of perfectly developed
crystals. The inclosures in the larger plagioclastic felspars consist
for the most part either of brownish glass, containing fine dark
granular matter--probably magnetite, which often renders them
opaque,--or of matter similar to that which constitutes the groundmass
of the surrounding rock. These stone and glass cavities are very
numerous and most irregular in outline, as shown in Figs. 1 and 2. They
appear, however, to be elongated generally in the direction of the
planes of composition of the twin lamellae. Zirkel has noted the
plentiful occurrence of these glass inclosures in the felspar crystals
and fragments of crystals which partly constitute the volcanic sands of
Etna, in which he has also detected the presence of numerous isolated
particles of brownish glass.[23] The felspar microliths, which
constitute so large a proportion of the ground-mass in the Etna lavas,
are in most instances probably triclinic. Monoclinic felspar does,
however, occur in some of these rocks; but the difficulty of
ascertaining the precise character of microliths renders it unsafe to
speculate on the amount of sanidine which may be present. Some crystals,
such as that shown in the centre of Fig. 2, appear at first sight to be
sanidine, twinned on the Carlsbad type, but closer inspection often
demonstrates the presence of other and very delicate twin lamellae.

[22] _The figures in this plate are magnified 35 diameters._ Fig. 1.
Lava of B.C. 396. The upper half of the drawing is occupied by a crystal
of plagioclastic felspar showing twin lamellation and faint zonal
markings, and with numerous irregular dark-brown inclosures of glass,
probably containing magnetite dust and matter similar to that of the
groundmass of the rock which consists of felspar microliths, granules of