[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 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 been alluded to. In this interstitial condition its amount is usually very small--a fact already pointed out by Zirkel.