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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 Pulich.e.l.lo, erupted in 1603, gave well-developed examples of the presence of olivine, and also of plagioclase. The ground ma.s.s was found to consist of felspar microliths, and grains of olivine, augite, and magnet.i.te, with some interst.i.tial gla.s.s.
Magnet.i.te 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.
t.i.taniferous 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 t.i.taniferous iron, and in the absence of this, of definite crystalline form, and of chemical a.n.a.lysis, it seems better to speak of this mineral with reserve, although t.i.tanium is very probably present, since much magnet.i.te is known to be t.i.taniferous.
The vitreous matter which occurs in these lavas is princ.i.p.ally present in the form of inclosures in the felspar, and, sometimes, the augite and olivine crystals previously described. Its occurrence in the groundma.s.s of these rocks has also been alluded to. In this interst.i.tial condition its amount is usually very small--a fact already pointed out by Zirkel.
I have unfortunately had no opportunity of examining the volcanic sands and ashes of Etna, but Zirkel's description of them seems to indicate their close mineralogical relation to lavas erupted in this district, with one exception, as pointed out by Rosenbusch, namely, that he makes no mention of the occurrence of olivine in these ejectamenta.
 "Mikroskopische Physiographie der Ma.s.sigen Gesteine. Stuttgart, 1877; p. 547.
Reference to the Figures 1 and 2 will suffice to show how close a relations.h.i.+p in mineral const.i.tution exists between these two lavas, separated in the dates of their eruption by an interval of over two thousand years.
_New Maps of Etna._--After these pages had received their final revision in type, I met with two new maps of Etna in the Paris Exhibition. The literature of our subject will obviously be incomplete without some notice of them, although this belongs properly to the first chapter rather than to the last. The one is a map in relief constructed by Captain Francesco Pistoja for the _Ist.i.tuto Topografico Militare_ of Florence. The vertical scale is 1/25,000 and the horizontal is 1/50,000.
The surface is coloured geologically: the lavas erupted during each century being differently coloured, while the course of each stream is traced. This map, although by no means free from errors, is a vast improvement on the relief map of M. Elie de Beaumont. One defect, which might be easily remedied, is due to the fact that the lavas of three consecutive centuries are coloured so much alike, that it is almost impossible to distinguish them. The minor cones are well shown, the Val del Bove fairly well, and the map is altogether a valuable addition to our knowledge of the mountain.
The other map is a _Carta Agronomica dell' Etna_, showing the surface cultivation. Different colours denote different plants, pistachio nuts, vines, olives, chestnuts, etc. It is beautifully drawn and coloured by hand, and is the work of Signor L. Ardini, of Catania.