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, Dufrenoy,
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] "Recherches 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.
[Illustration: Map of the Val del Bove, to illustrate the theory of a
double axis of eruption. (_Lyell_).]
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.
[Illustration: Ideal section of Mount Etna]
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.
[Illustration: Profile of Etna]
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 scoriae 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 scoriae 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 scoriae 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 scoriae. "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
scoriae 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 deg. C., (75.2 deg.
F.), and solidified at 17 deg. C. (62.6 deg. F.) to a yellowish green mass,
which on analysis gave the following percentage composition:--
Liquid hydrocarbons boiling at 79 deg. C. = 17.97
Hydrocarbons solidifying below 0 deg. C., boiling }
between 280 deg. and 400 deg. C. } = 31.95
Paraffine melting between 52 deg. and 57 deg. 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
Scoriae " " " 2.633
Compact lava " " 2.771
Scoriae 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,
scoriae, and compact lava have virtually the same composition--indeed
they consist of the same substance in different states of aggregation.
_Ashes._ _Sand._ _Scoriae._ _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.
[Illustration: Sections of Etna Lavas seen under the Microscope]
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 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
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
AEschylus, 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
" " Scoriae, 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 Phlegraei, 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
Scoriae, 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'.