Chapter 12

GEOLOGY AND MINERALOGY OF THE MOUNTAIN.

Elie de Beaumont's classification of rocks of Etna.--Hoffman's geological map.--Lyell's researches.--The period of earliest eruption.--The Val del Bove.--Two craters of eruption.--Antiquity of Etna.--The lavas of Etna.--Labradorite.--Augite.--Olivine.-- Analcime.--Titaniferous iron.--Mr. Rutley's examination of Etna lavas under the microscope.

The opinion of geologists is divided as to the manner in which a volcano is first formed. Some hold that the volcanic forces have upraised the rocks from beneath, and at last finding vent have scattered the lighter portions of such rocks into the air, and have poured out lava through the rent masses, thus forming a _crater of elevation_. Others maintain that the volcanic products are ejected from an aperture or fissure already existing in rocks previously formed, and that the accumulation of these products around the vent forms the mass of the volcano and the _crater of eruption_. Lyell favours the latter view; Von Buch, Dufrénoy, and Elie de Beaumont the former.

According to M. Elie de Beaumont, Etna is an irregular crater of elevation. The original deposits were nearly horizontal, and lavas were poured through fissures in these, and accumulated at first in layers; afterwards the whole mass was upheaved and a crater formed.[20] The upheaving force does not appear to have acted at one point, but along a line traversing the Val del Bove. The latter he refers to a subsidence of a portion of the mountain. He divides the rocks of Etna into six orders: 1. The lowest basis of the mountain would appear to consist of granite, because masses of that rock have from time to time been ejected. 2. Calcareous and arenaceous rocks, of which the mountains surrounding Etna are composed, and which appear capped with lava near Bronte and elsewhere. 3. Basaltic rocks, which are met with near Motta S. Anastasia, Paterno, Licodia, and Aderno, and in the Isole de'Ciclopi. 4. Rolled pebbles, which form a range of slightly rising ground between the first slopes of Etna on the southern side and the plain of Catania. (Lyell speaks of this rising ground as consisting of "argillaceous and sandy beds with marine shells, nearly all of living Mediterranean species, and with associated and contemporaneous volcanic rocks.") 5. Ancient lavas forming the escarpments around the Val del Bove; and 6th, Modern lavas. He considers that the fissures which abound on Etna are shifts or faults produced by dislocation, and that the minor cones are points along such fissures from which ashes and lava have been ejected. He admits the existence of two cones. The geological map of Etna prepared by M. Elie de Beaumont to accompany his memoir can scarcely be regarded as a great addition to our knowledge of the mountain. For although in the main points it is correct, so many details have been omitted that the map must be considered to have now been quite superseded by those of Von Waltershausen and Friedrich Hoffmann.

[20] "Récherches sur la structure et sur l'origine du Mont Etna." 1836.

The most convenient geological map of the mountain is without doubt that of Hoffmann, given in the _Vulkanen Atlas_ of Dr. Von Leonhard; and here reproduced. Von Waltershausen's geological map has been the foundation of all others which have subsequently appeared. It is a marvel of accurate work, and patient industry. The form however is inconvenient, as it nowhere appears as a whole, but in separate portions, which are scattered through the folio sheets of the very expensive _Atlas des Aetna_. It is accurate, and at the same time very clear and intelligible. By reference to the map it will be seen that from Capo di Schiso westward, to near Paterno, Etna is surrounded by sandstone hills; at the south we have recent clays, and, at intervals, chalk. A large triangular space having the two angles at its base, respectively near Maletto and Aderno, and its apex at the great crater, is covered with new lava; while around Nicolosi there is volcanic sand. At the Isole de'Ciclopi, Motta S. Anastasia, and a few other places, basalt is seen; on each side of the Val del Bove, dolerite; and near Misterbianco and Piedemonte, small deposits of clay slate. The great mass of the surface of the mountain, not specially mentioned above, is volcanic tuff.

Among the more important and recent additions to our knowledge of the geology of Etna may be mentioned Lyell's paper on the subject, communicated to the Royal Society in 1858, the matter of which is incorporated in a lengthy chapter on Etna in the "Principles of Geology." Lyell visited the mountain in 1828, 1857, and 1858, and he then collected together a great number of personal observations; he also made use of the maps and plans of Von Waltershausen, and he has analysed the views of Elie de Beaumont and other writers. He alludes at the outset to the numerous minor cones of Etna produced by lateral eruption, and points out the fact that they are gradually obliterated by the lava descending from the upper part of the mountain, which flows around them and heightens the ground on which they stand. In this way the crater of Monte Nocilla is now level with the plain, and the crater of Monte Capreolo was nearly filled by a lava stream in 1669. Thus without doubt beneath the sloping sides of Etna a multitude of obliterated monticules exist.

The strata which surround Mount Etna on the south are of Newer Pliocene date, and contain shells which are nearly all of species still living in the Mediterranean. Out of sixty-five species collected by Lyell in 1828, sixty-one were found to belong to species still common in the Mediterranean. These strata are about the age of the Norwich crag; and the oldest eruptions of Etna must have taken place during the glacial period, but before the period of greatest cold in Northern Europe.

Before visiting Etna, Lyell had been told by Dr. Buckland that in his opinion the Val del Bove was the most interesting part of Etna, accordingly he specially and minutely examined that part of the mountain. This vast valley is situated on the eastern flanks of the mountain, and it commences near the base of the cone, stretching for nearly five miles downwards. It is a large oval basin formed in the side of the mountain, and surrounded by vast precipices, some of which at the head of the valley are between three and four thousand feet in height. The surface is covered with lava of various dates, and several minor cones, notably those of 1852, are within its boundaries. The abrupt precipices reveal the presence of a large number of vertical dikes, radiating from a point within the valley, some of them, according to Von Waltershausen, being of ancient greenstone. Other dikes of more modern doleritic lava radiate from the present crater. From the slope of the beds in the Val del Bove, Lyell and Von Waltershausen have independently inferred that there was once a second great centre of eruption in the Val del Bove between the Sierra Giannicola, and Zoccolaro (_vide_ the Figure on p. 117). The axis of eruption passing through this point Lyell calls the _Axis of Trifoglietto_; while he distinguishes the present centre of eruption as the _Axis of Mongibello_. These centres probably existed simultaneously, but were unequal as regards eruptive violence; the crater of Mongibello was the more active of the two, and eventually overwhelmed the crater of Trifoglietto with its products, by which means the whole mountain became a fairly symmetrical cone, having the crater of Mongibello at its apex (_vide_ the Figures on pp. 119 and 121). Subsequently the Val del Bove was formed, probably by some paroxysmal explosion, caused by pent-up gases escaping from fissures. Possibly also subsidence may have occurred.

We must then in the first place think of Etna as a submarine volcano of the Newer Pliocene age; when it reached the surface it increased rapidly in bulk by pouring out scoriæ and lava from its two centres of eruption--the centre of Mongibello, and the centre of Trifoglietto,--general upheaval of the surrounding district followed, and ultimately the crater of Trifoglietto was obliterated by the discharges from the crater of Mongibello. Afterwards the Val del Bove was blown out by sudden eruptive force from beneath, and the mountain assumed its present aspect. Then the historical eruptions commenced, and of these we have given an account in the preceding chapter.

The most obvious method of obtaining some idea as to the age of Etna, is to ascertain the thickness of matter added during the historical period to the sides of the mountain, and to compare this with the thickness of the beds of ancient lava and scoriæ exposed at the abrupt precipices of the Val del Bove. There is reason for believing, however, that none of the ancient lavas equalled in volume the lava streams of 1809 and 1852, and the question is much complicated by other considerations. Lyell compares the growth of a volcano to that of an exogenous tree, which increases both in bulk and height by the external application of ligneous matter. Branches which shoot out from the trunk, first pierce the bark and proceed outwards, but if they die or are broken off they become inclosed in the body of the tree, forming knots in the wood. Similarly the volcano consists of a series of conical masses placed one above the other, while the minor cones, corresponding to the branches of the tree, first project, and then become buried again, as successive layers of lava flow around them. But volcanic action is very intermittent, the layers of lava and scoriæ do not accumulate evenly and regularly like the layers of a tree. A violent paroxysmal outbreak may be succeeded by centuries of quiescence, or by a number of ordinary eruptions; or, again, several paroxysmal outbreaks may occur in succession. Moreover, each conical envelope of the mountain is made up of a number of distinct currents of lava, and showers of scoriæ. "Yet we cannot fail to form the most exalted conception of the antiquity of this mountain, when we consider that its base is about 90 miles in circumference; so that it would require ninety flows of lava, each a mile in breadth at their termination, to raise the present foot of the volcano as much as the average height of one lava current." If all the minor cones now visible on Etna could be removed, with all the lava and scoriæ which have ever proceeded from them, the mountain would appear scarcely perceptibly smaller. Other cones would reveal themselves beneath those now existing. Since the time when, in the Newer Pliocene period, the foundations of Etna were laid in the sea, it is quite impossible even to hint at the number of hundreds of thousands of years which have elapsed.

We collected specimens of lava from various points around and upon the mountain. They presented a wonderful similarity of structure, and a mineralogist to whom they were shown remarked that they might almost all have come from the same crater, at the same time. A specimen of the lava of 1535 found near Borello, was ground by a lapidary until it was sufficiently transparent to be examined under the microscope by polarised light. It was found to contain good crystals of augite and olivine, well striated labradorite, and titaniferous iron ore.

Elie de Beaumont affirms that the lavas of Etna consist of labradorite, pyroxene (augite), peridot (olivine), and titaniferous iron. Rose was the first to prove that the lavas of Etna do not contain ordinary felspar (or potash felspar), but labradorite (or lime felspar.) (_Annales des Mines_, 3 serie, t. viii., p. 3.) Elie de Beaumont detached a quantity of white crystals from the interior of a lava found between Giarre and Aci Reale; these were analysed by M. Auguste Laurent with the following results in 100 parts:--

Silica 47·9 Alumina 34·0 Oxide of Iron 2·4 Soda (Na{2}O) 5·1 Potash (K{2}O) ·9 Lime 9·5 Magnesia ·2 ----- 100·0

Von Waltershausen gives the following as the composition of two specimens of Labradorite from Etna:--

I. II.

Silica 53·56 55·83 Alumina 25·82 25·31 Sesquioxide of Iron 3·41 3·64 Magnesia ·52 ·74 Lime 11·69 10·49 Soda 4·09 3·52 Potash ·54 ·83 Water ·95 -- ------ ------ 100·58 100·36

Specimens of Augite from Etna have been examined by Von Waltershausen and Rammelsberg, with the following results:--

_Greenish_ _From_ _From_ _Black._ _Black._ _Mascali._ _Monti Rossi._

Silica 47·63 51·70 49·69 47·38 Alumina 6·74 4·38 5·22 5·52 Protoxide of Iron 11·39 4·24 10·75 7·89 " Manganese ·21 -- -- ·10 Magnesia 12·90 21·11 14·74 15·29 Lime 20·87 18·02 18·44 19·10 Sesquioxide of Iron -- -- -- 3·85 Water ·28 ·49 ·51 ·43 ------ ----- ----- ----- 100·02 99·94 99·35 99·56

Olivine is generally met with in the lavas of Etna. It has an olive, or bottle-glass green colour, and is disseminated through the lavas in the form of small crystalline grains, sometimes of some magnitude. Specific gravity 3·334. A specimen from Etna gave the following results on analysis:--

Silica 41·01 Protoxide of Iron 10·06 Magnesia 47·27 Alumina ·64 Oxide of Nickel ·20 Water 1·04 ------ 100·22

The titaniferous iron of Etna is found disseminated through the mass of the lavas, and is plainly distinguished when a thin section is examined under the microscope. It is sometimes met with in masses. A specimen from Etna, analysed by Von Waltershausen, was found to contain:--

Titanic Acid 11·14 Sesquioxide of Iron 58·86 Protoxide of Iron 30·00 ------ 100·00

The basalts of the Isole de'Ciclopi enclose beautiful transparent crystals of Analcime, the _zeolite dure_ of Dolomieu. The word is derived from ~analkis~ weak, in allusion to the weak electric power which the mineral acquires when heated or rubbed. Dana prefers the term _analcite_. Specimens from the Cyclops Islands have been analysed by Von Waltershausen and Rammelsberg, with the following results:--

I. II. III.

Silica 53·72 55·22 54·34 Alumina 24·03 23·14 23·61 Lime 1·23 ·25 ·21 Soda (Na{2}O) 7·92 12·19 12·95 Potash (K{2}O) 4·46 1·52 ·66 Water 8·50 7·68 8·11 Magnesia ·05 -- -- Sesquioxide of Iron -- -- ·12 ----- ------ ------ 99·91 100·00 100·00

The minerals of Etna are not nearly as numerous as those of Vesuvius. It has been remarked that no area of equal size on the face of the globe furnishes so many different species of minerals as Vesuvius and its immediate neighbourhood. Out of the 380 species of simple minerals enumerated by Hauy, no less than 82 had been found on and around Vesuvius, as long ago as 1828, and many have been since found.

Of other common products of Etna, there are sulphur in various forms, sulphurous acid gas, ammonia salts, hydrochloric acid gas, and steam. A curious white mass, which we found near the summit, proved to be the result of the decomposition of lava by hot acid vapours. In the different lavas, the crystals of labradorite, and of olivine, vary in size considerably. Magnetic oxide of iron is very visible in thin slices of the lavas when placed under the microscope; and iron appears to be a constant constituent in nearly all the products of the mountain.

Within the last few months Prof. Silvestri has detected a mineral oil in the cavities of a prehistoric doleritic lava found near Paterno.[21] The lava is in close contiguity to the clay deposits of a mud volcano, and when examined under the microscope is seen to consist mainly of augite, together with olivine and transparent crystals of labradorite. It contains numerous cavities coated with arragonite, and filled with a mineral oil which constitutes about one per cent of the whole weight of the lava. It was taken from the lava at a temperature of 24° C., (75·2° F.), and solidified at 17° C. (62·6° F.) to a yellowish green mass, which on analysis gave the following percentage composition:--

Liquid hydrocarbons boiling at 79° C. = 17·97 Hydrocarbons solidifying below 0° C., boiling } between 280° and 400° C. } = 31·95 Paraffine melting between 52° and 57° C. = 42·79 Asphalt containing 12 per cent of ash = 2·90 Sulphur = 4·32 ------ 99·93

[21] "Atti Accademia Gioenia," serie iii., vol. xii.

Prof. Silvestri has recently made some interesting determinations of the specific gravity and chemical composition of the different products of Etna. They are given in full in his work entitled, "_I Fenomeni Vulcanici presentati dall'Etna, nel 1863, 1864, 1865, 1866_," which was published in Catania in 1867. The following table gives the specific gravity of various ancient and modern forms of lava, ashes, etc. of Etna:--

_Sp. Gr._

Ashes ejected in 1865 2·644 Sand " " " 2·715 Scoriæ " " " 2·633 Compact lava " " 2·771 Scoriæ ejected in 1669 2·622 Compact lava " " 2·697 Lapilli ejected in 1444 2·420 Compact lava ejected in prehistoric times 2·854

A very decided change in the specific gravity was found to take place after fusion. This can only be accounted for on the supposition that a chemical change is effected during the fusion:--

_Sp. Gr._ _Sp. Gr._ _before fusion._ _after fusion._

Pyroxene of Etna 3·453 2·148 Felspar " " 2·925 1·361 Olivine " " 3·410 2·290 Lava of 1865 2·771 1·972 Ancient basaltic lava from } 2·854 2·000 the Scogli de'Ciclopi } Ancient basaltic lava from } 2·795 1·947 Aci Reale }

It will be seen from the following analyses that the sand, ashes, scoriæ, and compact lava have virtually the same composition--indeed they consist of the same substance in different states of aggregation.

_Ashes._ _Sand._ _Scoriæ._ _Compact lava._

Silica 50·00 49·80 50·00 49·95 Alumina 19·08 18·20 19·00 18·75 Protoxide of iron 12·16 12·42 11·70 11·21 Protoxide of Manganese ·40 ·45 ·50 ·49 Lime 9·98 11·00 10·28 11·10 Magnesia 4·12 4·00 4·20 4·05 Potash ·60 ·49 ·69 ·70 Soda 3·72 3·60 3·40 3·71 Water ·36 ·29 ·33 ·23 Phosphoric acid } Titanic acid } traces traces traces traces Vanadic acid } Sesquioxide of iron } ------ ------ ------ ------ 100·42 100·25 100·10 100·19

With these we may compare the composition of the lava which issued from Monti Rossi in 1669, and was analysed by Lowe, and of an ancient lava of Etna ejected during an unknown eruption, and analysed by Hesser.

_Ancient lava._ _Lava of 1669._

Silica 49·63 48·83 Alumina 22·47 16·15 Protoxide of Iron 10·80 16·32 Protoxide of Manganese ·63 ·54 Lime 9·05 9·31 Magnesia 2·68 4·58 Soda 3·07 3·45 Potash ·98 ·77 ----- ----- 99·31 99·95

The sublimations from the fumaroles are chiefly chloride of ammonium, perchloride of iron, and sulphur. An analysis of the gases of the fumaroles of 1865 gave the following results:--

Carbonic acid 50·5 Hydrosulphuric acid 11·9 Oxygen 7·1 Nitrogen 30·5 ----- 100·0

An account of microscopic analysis of some of the lavas of Etna, for which I am indebted to Mr. Frank Rutley, will be found appended to this chapter. He considers that they are Plagioclase-basalts, and occasionally Olivine-basalts; and that they consist of Plagioclase, Augite, Olivine, Magnetite, Titaniferous iron, and a residuum of glass.

Near the summit of the great crater I found a mass of perfectly white, vesicular, and very friable substance, somewhat pumiceous in appearance. It proved to be a decomposed lava, and was found elsewhere on the sides of the crater. Mr. Rutley examined a section of it, and reports: "Under the microscope a tolerably thin section shows the outlines of felspar crystals, lying in a hazy milk-white semi-opaque granular matrix. The felspar crystals are lighter and more translucent than the matrix, but are of much the same character, having a granulated or flocculent appearance, somewhat like that of the decomposed felspars in diabase. There are numerous roundish cavities in the section which may once have contained olivine, or some other mineral, or they may be merely vesicles."

A qualitative analysis of this substance, made by Mr. H. M. Elder, has proved that it contains a large quantity of Silica (about 70 per cent.), and smaller proportions of Alumina, Iron, Magnesium, Calcium, and Potash; together with very small amounts of Sulphuric Acid and a trace of Ammonia. Lithium is absent, and Sodium is only present in very minute quantity. Water is present to the extent of nearly 20 per cent.

During the eruption of Etna in 1869 Von Waltershausen noticed on some of the lava blocks which were still hot and smoking, silver-coloured particles, which rapidly underwent change. An insufficient quantity for analysis was collected, but during the eruption of 1874, Silvestri found a quantity of the substance and analysed it. (_Poggendorff's Annalen_, CLVII. 165, 1876.) It possesses a specific gravity of 3·147, and shows a metallic lustre similar to that of steel. On analysis it was found to consist of:--

Iron 90·86 Nitrogen 9·14 ------ 100·00

which corresponds with the formula Fe{5}N{2},--a formula assigned by Fremy to Nitride of iron. It has been named _Siderazote_. This new mineral species appears to be formed by the action of hydrochloric acid, and of ammonia on red-hot lava containing a large percentage of iron. It was formed artificially by exposing fragments of lava alternately to the action of hydrochloric acid and ammonia in a red-hot tube. At a high temperature Siderazote undergoes decomposition, nitrogen being evolved. In contact with steam at a red heat it forms magnetite and ammonia.

_The Mineral Constitution and Microscopic Characters of some of the Lavas of Etna._

By Frank Rutley, F.R.G.S., of H.M. GEOLOGICAL SURVEY.

A cursory examination of the series of specimens collected by Mr. Rodwell, seemed to show that all the lavas of Etna, irrespective of their differences in age, exhibit a remarkable similarity in mineralogical constitution. Occasionally, however, there appears to have been a little difference in their respective viscidity at the time of the eruption, the crystals in some of them lying in all directions, while in others there appears to be a more or less definite arrangement of the felspar crystals, as seen in the lava of A.D. 1689.

Although the specimens which I have examined microscopically do not appear to differ in the nature of their constituents, yet in some of them certain minerals fluctuate in quantity, some containing a comparatively large amount of olivine and well-developed crystals of augite, while, in others, these minerals, although one or other is always present, are but poorly represented by minute and sparsely-disseminated grains. It seems probable that all the Etna lavas contain traces of a vitreous residuum, since, when sections are examined under the microscope, a more or less general darkness 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.

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--possibly 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 lamellæ 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 lamellæ. 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 lamellæ.

[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.

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,[24] namely, that he makes no mention of the occurrence of olivine in these ejectamenta.

[24] "Mikroskopische Physiographie der Massigen Gesteine. Stuttgart, 1877; p. 547.

Reference to the Figures 1 and 2 will suffice to show how close a relationship in mineral constitution 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 _Istituto 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.

INDEX.

Abich, 22 Acesines, River, 81 Aci Reale, 72, 73 Acis, River, 73 Acque Grande, 73 Aderno, 65 Adranum, City of, 66 Æschylus, 4 Aetna, Town of, 64 Ajo, Fountain of, 85 Alcantara, Valley of, 69 Amphinomus, 80 Analcime, Analysis of, 128 Analcite, 127 Analysis of Augite, 126 " " Labradorite, 125, 126 " " Olivine, 127 " " Titaniferous Iron, 127 " " Analcime, 128 " " Mineral Oil, 129 " " Volcanic Ashes, 131 " " Lava, 131, 132 " " Sand, 131 " " Scoriæ, 131 " Microscopic, of Lavas, 135-141 " " Siderazote, 133, 134 " " Sublimations, 132 " " white friable substance, 133 Anapias, 80 Arethusa, Fountain of, 85 Ascent, Fatigue of, 60 Ashes, Volcanic, Analysis of, 131 " " Specific Gravity of, 130 Atlas des Aetna, 23 Augite, Analysis of, 126 Axis of Mongibello, 119, 122 " " Trifoglietto, 119, 122

Baltzer, 24 Basalt, Columnar, 75 Base, Circumference of, 30 Belpasso, 63 Bembo, Cardinal, 10 Biancavilla, 65 Boccarelle del Fuoco, 36 Bocche eruttive, 110 " del Fuoco, 36 Borelli, 90 Bosco di Bronte, 99 Botanical Regions, 40-42 Boundaries of Etna, 29 Brilliancy of Stars, 48, 49 Bronte, 67, 68 Brydone, 15

Campi Phlegræi, 14 Campus Piorum, 80 Cardinal Bembo, 10 Carpinetto, Forest of, 39 Carrera, 12 Casa del Bosco, 48 " Inglesi, 51, 52 Castagno di Cento Cavalli, 39 " della Galea, 39 Catania, 76-78 " Destruction of, 94-96 Caverns of Etna, 37 Cavern of Thalia, 37 Centenario, 108 Chestnuts, 39 Circumference of base, 30 Cisterna, 101 Coltivata, Regione, 37, 38 Cones, Minor, 34-36 Crater of Elevation, 114 " of Eruption, 114 " The Great, 56-58 Craters, Minor, 34-36 Cultivated Region, 38 Cyclops, Rocks of, 75

Decomposed Lava, Analysis of, 133 Desert Region, 39, 40

Effects of Refraction, 59, 60 Elevation, Crater of, 114 Elie de Beaumont, 21 Elie de Beaumont's Classification, 115, 116 Empedokles, 10 Errors in Maps, 24 Eruption, Crater of, 114 Eruptions, General Character of, 112 " Number of, 113 Etna, the Home of Early Myths, 9 " a Submarine Volcano, 122 Enceladus, 4

Fatigue of Ascent, 60 Fazzello, 11 Ferrara, 15, 16 Filoteo, 11 Flood of 1755, 98 Forest of Carpinetto, 39 Fratelli Pii, Cone of, 111 Fumaroles, Sublimations of, 132 Fuoco, Regione del, 37

Gemellaro, Giuseppe, 18 " Mario, 19 " Carlo, 19, 20 General aspect of Etna, 34 _Genista Etnensis_, 45 Geological Maps of Etna, 117-18 Gibel Uttamat, 3 Gladstone's Account of Eruption, 58 Gravity, Specific, of Ejectamenta, 130 Great Crater, 57, 58 Grotto of Polyphemus, 75 " delle Palombe, 37 Growth of a Volcano, 123 Gurrita, Lake, 68

Hamilton, 14 Height of Etna, 26, 27 Hephaistos, Forge of, 4 Himilco, 81 Hoffmann, 22 Homer, 4 Houel, 14 Hybla Major, 63

Increase of Population, 33 Inessa, City of, 64 Inglesi, Casa, 51, 52 Inns, Sicilian, 65, 67, 69 Iron, Nitride of, 134 " Perchloride of, 132 Iron, Titaniferous, 127 Isola d'Aci, 75

Katana, 76, 77 Kircher, 12

Labradorite, Analysis of, 125, 126 La Cisterna, 101 Latitude of Crater, 2 La Scaletta, 75 Lava, Analysis of, 131, 132 " Specific Gravity of, 130 " Decomposed, Analysis of, 133 Lavas, Microscopic Analysis of, 135-141 Lavas of Etna, 125-141 Linguaglossa, 70 Longitude of Crater, 2 Lucilius Junior, 7-9 Lucretius, 6 Lyell, 23 Lyell's Researches, 118-122

Magnetite, 139 Maletto, 68 Maps, Geological, of Etna, 117, 118 " of Etna, 13, 16, 21-24 Mascali, 71 Microscopic Analysis of Lavas, 135-141 Milo, 108 Mineral Oil in Lava, 129 Minor Cones, 34-36 Mongibello, 3 " Axis of, 122 Monte Calanna, 107 " Capreolo, 118 " di Mojo, 36,81 " Minardo, 34 " Ste. Sofia, 36 " Fusara, 37 " Grigio, 111 " Lepre, 85 " Nocilla, 118 " Rosso, 99 " Spagnuolo, 69 " Ste. Sofia, 95 Monti Rossi, 35, 91 Mules, 47

Name of Etna, 3 Natural Boundaries of Etna, 30 Naxos, 81 Newer Pliocene Strata, 120 Nicolosi, 46 Nitride of Iron, 134

Observatory on Etna, 50 Oil, Mineral, in Lava, 129 Olivine, 139 " Analysis of, 127

Palombe, Grotto delle, 37 Paterno, 63 Pennisi, Baron, 71, 72 Piano del Lago, 51 " di S. Leo, 90 Piedimontana, Regione, 37, 38 Pii Fratres, 80, 81 Pindar, 4 Polarized Light, applied to Analysis of Lavas, 136-139 Polyphemus, Grotto of, 75 Population, 31-33 Position of Etna, 2 Presl's _Flora Sicula_, 41, 42

Radius of Vision, 28 Randazzo, 68, 69 Recupero, 17 Recupero's Account of Eruption, 99 Refraction, Effects of, 59, 60 Regions of Etna, 37-42 Regione del Fuoco, 37 River, Acis, 73 Road around Etna, 29 Rocca di Musarra, 98 Rocks of Cyclops, 75 Rutley's, Mr. Frank, Analysis of Lavas, 135-141

St. Agatha, Veil of, 83 " Maria di Licodia, 64 S. Niccola, Monastery of, 92 S. Simone, 102 Salto della Guimenta, 106 Sand, Volcanic, Analysis of, 131 " Specific Gravity of, 130 Scogli di Ciclopi, 75 Scoriæ, Volcanic, Analysis of, 131 " Specific Gravity of, 130 Serra delle Concazze, 59 " del Solfizio, 59 " di Giannicola, 107 Serrapizzuta, 89 Sicilian Inns, 65, 67, 69 Siderazote, 133, 134 Silvestri, Researches of, 25 Simeto, Valley of, 67 Smyth, 18 Smyth's Observations, 2 Snow of Etna, 59 Specific Gravity, Alteration of, 130 " " of Ejectamenta, 130 Strabo, 6 Stars, Brilliancy of, 48, 49 Stato Maggiore, Map of, 24 Sulphur in Sublimations, 132 Sublimations from Fumaroles, 132 Summit of Cone, 53, 54 Sunrise seen from Summit, 54, 55

Tacchini, 50 Taormina, 71 Tertiary Sandstone of Bronte, 67 Thucydides, 5 Titaniferous Iron, 127 Torre del Filosofo, 10, 59 Towns on Etna, 31 Trezza, 75 Trifoglietto, Axis of, 121 Trunks of Large Chestnuts, 39

Val del Bove, 34, 71, 98, 100, 119-121 " Serbo, 103 Valley of Alcantara, 69 Virgil, 5 Vision, Radius of, 28 Volcano, Growth of, 123 Von Waltershausen, 23, 120 Vulkanen Atlas, 116

Winchelsea, Lord, 12, 94, 95 Woody Region, 38

_Zeolite dure_, 127 Zones of Temperature, 38

* * * * *

Transcribers Notes:

OE ligatures have been written as two letters. The chemical formulae in this text have subscripts written in curly brackets. For example, water appears as H{2}O.

Passages in italics indicated by _underscores_. Passages in Greek indicated by ~tildes~.

Printer's errors corrected and other transcription points as follows:

Page Correction --------------------------------------------------------------------- - Some arcane spellings retained where verified as correct for the time of publication or quote, such as 'musquitoes', 'plaintain', 'mettals', 'felspar', etc. - Italian quotations sometimes include phrases such as "sull' Etna" or "dell' Asia". There should be no space after the apostrophes but spaces are often present. This may have been done to better space the text during justification. Such errors have not been corrected. - The document is inconsistent in the use of commas where quoting numbers of over 1,000; sometimes they are used and sometimes they are not used. These inconsistencies remain. ix Added '.' at end of 'Silvestri' in text block to maintain style of other entries. x Added '.' at end of 'Eruptions' in text block to maintain style of other entries. x Added '.' after '1408' in text block to maintain style of other entries. xi Added '.' after 'microscope' in text block to maintain style of other entries. 4 Corrected 'ength' to 'length'. 12 Corrected 't'jncedie' to 't'incedie'. 't'jncedie' and 'treblement' both printed with tildes over the first occurrence of e in each word. Tildes omitted in this version. 17 Corrected 'Guiseppe' to 'Giuseppe'. 27 Added end quotes after final word on page. 29 Inserted full-stop after '(S.W.)', to end sentence. 33 Corrected 'f' to 'of'. 42 'of-course' corrected to 'of course'. 44 Corrected 'unusally' to 'unusually'. 59 Corrected 'subsequenly' to 'subsequently'. 64 Corrected 'athough' to 'although'. 79 Added '.' after '812' in text block to maintain style of other entries. 80 Anapias and Amphinomus printed with diacritic marks omitted in this text (breves over first 'a' in 'anapias' and 'i' in 'Amphinomus'). 82 Numbered list runs 1, 2, 3, 4, 5, 5, 7, 8... Second occurrence of 5 amended to 6. 85 'In the 1329' corrected to 'In the year 1329'. 127 'the time analcite' corrected to 'the term analcite'. 144 Corrected 'Guiseppe' to 'Giuseppe'. 145 Corrected 'Miscroscope' to 'Microscope'. 152 Corrected 'magnitite' to 'magnetite'.