ETNA.
External physiognomy of Etna--Lateral cones--Their successive
obliteration--Early eruptions--Monti Rossi in 1669--Towns overflowed
by lava--Part of Catania overflowed--Mode of advance of a current of
lava--Subterranean caverns--Marine strata at base of Etna--Val del
Bove not an ancient crater--Its scenery--Form, composition, and
origin of the dikes--Linear direction of cones formed in 1811 and
1819--Lavas and breccias--Flood produced by the melting of snow by
lava--Glacier covered by a lava stream--Val del Bove how
formed--Structure and origin of the cone of Etna--Whether the
inclined sheets of lava were originally horizontal--Antiquity of
Etna--Whether signs of diluvial waves are observable on Etna.
_External physiognomy of Etna._--After Vesuvius, our most authentic
records relate to Etna, which rises near the sea in solitary grandeur to
the height of nearly eleven thousand feet.[560] The base of the cone is
almost circular, and eighty-seven English miles in circumference; but if
we include the whole district over which its lavas extend, the circuit
is probably twice that extent.
_Divided into three regions._--The cone is divided by nature into three
distinct zones, called the _fertile_, the _woody_, and the _desert_
regions. The first of these, comprising the delightful country around
the skirts of the mountain, is well cultivated, thickly inhabited, and
covered with olives, vines, corn, fruit-trees, and aromatic herbs.
Higher up, the woody region encircles the mountain--an extensive forest
six or seven miles in width, affording pasturage for numerous flocks.
The trees are of various species, the chestnut, oak, and pine being most
luxuriant; while in some tracts are groves of cork and beech. Above the
forest is the desert region, a waste of black lava and scoriae; where, on
a kind of plain, rises a cone of eruption to the height of about eleven
hundred feet, from which sulphureous vapors are continually evolved.
_Cones produced by lateral eruption._--The most grand and original
feature in the physiognomy of Etna is the multitude of minor cones which
are distributed over its flanks, and which are most abundant in the
woody region. These, although they appear but trifling irregularities
when viewed from a distance as subordinate parts of so imposing and
colossal a mountain, would, nevertheless, be deemed hills of
considerable altitude in almost any other region. Without enumerating
numerous monticules of ashes thrown out at different points, there are
about eighty of these secondary volcanoes, of considerable dimensions;
fifty-two on the west and north, and twenty-seven on the east side of
Etna. One of the largest, called Monte Minardo, near Bronte, is upwards
of 700 feet in height, and a double hill near Nicolosi, called Monti
Rossi, formed in 1669, is 450 feet high, and the base two miles in
circumference; so that it somewhat exceeds in size Monte Nuovo, before
described. Yet it ranks only as a cone of the second magnitude amongst
those produced by the lateral eruptions of Etna. On looking down from
the lower borders of the desert region, these volcanoes present us with
one of the most delightful and characteristic scenes in Europe. They
afford every variety of height and size, and are arranged in beautiful
and picturesque groups. However uniform they may appear when seen from
the sea, or the plains below, nothing can be more diversified than their
shape when we look from above into their craters, one side of which is
generally broken down. There are, indeed, few objects in nature more
picturesque than a wooded volcanic crater. The cones situated in the
higher parts of the forest zone are chiefly clothed with lofty pines;
while those at a lower elevation are adorned with chestnuts, oaks,
beech, and holm.
_Successive obliteration of these cones._--The history of the eruptions
of Etna, imperfect and interrupted as it is, affords us, nevertheless,
much insight into the manner in which the whole mountain has
successively attained its present magnitude and internal structure. The
principal cone has more than once fallen in and been reproduced. In 1444
it was 320 feet high, and fell in after the earthquakes of 1537. In the
year 1693, when a violent earthquake shook the whole of Sicily, and
killed sixty thousand persons, the cone lost so much of its height, says
Boccone, that it could not be seen from several places in Valdemone,
from which it was before visible. The greater number of eruptions happen
either from the great crater, or from lateral openings in the desert
region. When hills are thrown up in the middle zone, and project beyond
the general level, they gradually lose their height during subsequent
eruptions; for when lava runs down from the upper parts of the mountain,
and encounters any of these hills, the stream is divided, and flows
round them so as to elevate the gently sloping grounds from which they
rise. In this manner a deduction is often made at once of twenty or
thirty feet, or even more, from their height. Thus, one of the minor
cones, called Monte Peluso, was diminished in altitude by a great lava
stream which encircled it in 1444; and another current has recently
taken the same course--yet this hill still remains four or five hundred
feet high.
There is a cone called Monte Nucilla near Nicolosi, round the base of
which several successive currents have flowed, and showers of ashes have
fallen, since the time of history, till at last, during an eruption in
1536, the surrounding plain was so raised, that the top of the cone
alone was left projecting above the general level. Monte Nero, situated
above the Grotta dell' Capre, was in 1766 almost submerged by a current:
and Monte Capreolo afforded, in the year 1669, a curious example of one
of the last stages of obliteration; for a lava stream, descending on a
high ridge which had been built up by the continued superposition of
successive lavas, flowed directly into the crater, and nearly filled it.
The lava, therefore, of each new lateral cone tends to detract from the
relative height of lower cones above their base: so that the flanks of
Etna, sloping with a gentle inclination, envelop in succession a great
multitude of minor volcanoes, while new ones spring up from time to
time.
_Early eruptions of Etna._--Etna appears to have been in activity from
the earliest times of tradition; for Diodorus Siculus mentions an
eruption which caused a district to be deserted by the Sicani before the
Trojan war. Thucydides informs us, that in the sixth year of the
Peloponnesian war, or in the spring of the year 425 B. C., a lava stream
ravaged the environs of Catania, and this he says was the third eruption
which had happened in Sicily since the colonization of that island by
the Greeks.[561] The second of the three eruptions alluded to by the
historian took place in the year 475 B. C., and was that so poetically
described by Pindar, two years afterwards, in his first Pythian ode:--
[Greek: kion
D' ourania synechei
Niphoess' Aitna, panetes
Chionos oxeias tithena.]
In these and the seven verses which follow, a graphic description is
given of Etna, such as it appeared five centuries before the Christian
era, and such as it has been seen when in eruption in modern times. The
poet is only making a passing allusion to the Sicilian volcano, as the
mountain under which Typhoeus lay buried, yet by a few touches of his
master-hand every striking feature of the scene has been faithfully
portrayed. We are told of "the snowy Etna, the pillar of heaven--the
nurse of everlasting frost, in whose deep caverns lie concealed the
fountains of unapproachable fire--a stream of eddying smoke by day--a
bright and ruddy flame by night; and burning rocks rolled down with loud
uproar into the sea."
[Illustration: Fig. 46.
Minor cones on the flanks of Etna.
1. Monti Rossi, near Nicolosi, formed in 1669. 2. Vampeluso?[562]]
_Eruption of 1669--Monti Rossi formed._--The great eruption which
happened in the year 1669 is the first which claims particular
attention. An earthquake had levelled to the ground all the houses in
Nicolosi, a town situated near the lower margin of the woody region,
about twenty miles from the summit of Etna, and ten from the sea at
Catania. Two gulfs then opened near that town, from whence sand and
scoriae were thrown up in such quantity, that in the course of three or
four months a double cone was formed, called Monti Rossi, about 450 feet
high. But the most extraordinary phenomenon occurred at the commencement
of the convulsion in the plain of S. Lio. A fissure six feet broad, and
of unknown depth, opened with a loud crash, and ran in a somewhat
tortuous course to within a mile of the summit of Etna. Its direction
was from north to south, and its length twelve miles. It emitted a most
vivid light. Five other parallel fissures of considerable length
afterwards opened, one after the other, and emitted smoke, and gave out
bellowing sounds which were heard at the distance of forty miles. This
case seems to present the geologist with an illustration of the manner
in which those continuous dikes of vertical porphyry were formed, which
are seen to traverse some of the older lavas of Etna; for the light
emitted from the great rent of S. Lio appears to indicate that the
fissure was filled to a certain height with incandescent lava, probably
to the height of an orifice not far distant from Monti Rossi, which at
that time opened and poured out a lava current. When the melted matter
in such a rent has cooled, it must become a solid wall or dike,
intersecting the older rocks of which the mountain is composed; similar
rents have been observed during subsequent eruptions, as in 1832, when
they ran in all directions from the centre of the volcano. It has been
justly remarked by M. Elie de Beaumont, that such star-shaped fractures
may indicate a slight upheaval of the whole of Etna. They may be the
signs of the stretching of the mass, which may thus be raised gradually
by a force from below.[563]
The lava current of 1669, before alluded to, soon reached in its course
a minor cone called Mompiliere, at the base of which it entered a
subterranean grotto, communicating with a suite of those caverns which
are so common in the lavas of Etna. Here it appears to have melted down
some of the vaulted foundations of the hill, so that the whole of that
cone became slightly depressed and traversed by numerous open fissures.
_Part of Catania destroyed._--The lava, after overflowing fourteen towns
and villages, some having a population of between three and four
thousand inhabitants, arrived at length at the walls of Catania. These
had been purposely raised to protect the city; but the burning flood
accumulated till it rose to the top of the rampart, which was sixty feet
in height, and then it fell in a fiery cascade and overwhelmed part of
the city. The wall, however, was not thrown down, but was discovered
long afterwards by excavations made in the rock by the Prince of
Biscari; so that the traveller may now see the solid lava curling over
the top of the rampart as if still in the very act of falling.
This great current performed the first thirteen miles of its course in
twenty days, or at the rate of 162 feet per hour, but required
twenty-three days for the last two miles, giving a velocity of only
twenty-two feet per hour; and we learn from Dolomieu that the stream
moved during part of its course at the rate of 1500 feet an hour, and in
others it took several days to cover a few yards.[564] When it entered
the sea it was still six hundred yards broad, and forty feet deep. It
covered some territories in the environs of Catania which had never
before been visited by the lavas of Etna. While moving on, its surface
was in general a mass of solid rock; and its mode of advancing, as is
usual with lava streams, was by the occasional fissuring of the solid
walls. A gentleman of Catania, named Pappalardo, desiring to secure the
city from the approach of the threatening torrent, went out with a party
of fifty men whom he had dressed in skins to protect them from the heat,
and armed with iron crows and hooks. They broke open one of the solid
walls which flanked the current near Belpasso, and immediately forth
issued a rivulet of melted matter which took the direction of Paterno;
but the inhabitants of that town, being alarmed for their safety, took
up arms and put a stop to farther operations.[565]
As another illustration of the solidity of the walls of an advancing
lava stream, I may mention an adventure related by Recupero, who, in
1766, had ascended a small hill formed of ancient volcanic matter, to
behold the slow and gradual approach of a fiery current, two miles and a
half broad; when suddenly two small threads of liquid matter issuing
from a crevice detached themselves from the main stream, and ran rapidly
towards the hill. He and his guide had just time to escape, when they
saw the hill, which was fifty feet in height, surrounded, and in a
quarter of an hour melted down into the burning mass, so as to flow on
with it.
But it must not be supposed that this complete fusion of rocky matter
coming in contact with lava is of universal, or even common, occurrence.
It probably happens when fresh portions of incandescent matter come
successively in contact with fusible materials. In many of the dikes
which intersect the tuffs and lavas of Etna, there is scarcely any
perceptible alteration effected by heat on the edges of the horizontal
beds, in contact with the vertical and more crystalline mass. On the
side of Mompiliere, one of the towns overflowed in the great eruption
above described, an excavation was made in 1704; and by immense labor
the workmen reached, at the depth of thirty-five feet, the gate of the
principal church, where there were three statues, held in high
veneration. One of these, together with a bell, some money, and other
articles, were extracted in a good state of preservation from beneath a
great arch formed by the lava. It seems very extraordinary that any
works of art, not encased with tuff, like those in Herculaneum, should
have escaped fusion in hollow spaces left open in this lava-current,
which was so hot at Catania eight years after it entered the town, that
it was impossible to hold the hand in some of the crevices.
_Subterranean caverns on Etna._--Mention was made of the entrance of a
lava-stream into a subterranean grotto, whereby the foundations of a
hill were partially undermined. Such underground passages are among the
most curious features on Etna, and appear to have been produced by the
hardening of the lava, during the escape of great volumes of elastic
fluids, which are often discharged for many days in succession, after
the crisis of the eruption is over. Near Nicolosi, not far from Monti
Rossi, one of these great openings may be seen, called the Fossa della
Palomba, 625 feet in circumference at its mouth, and seventy-eight deep.
After reaching the bottom of this, we enter another dark cavity, and
then others in succession, sometimes descending precipices by means of
ladders. At length the vaults terminate in a great gallery ninety feet
long, and from fifteen to fifty broad, beyond which there is still a
passage, never yet explored; so that the extent of these caverns remains
unknown.[566] The walls and roofs of these great vaults are composed of
rough and bristling scoriae, of the most fantastic forms.
_Marine strata at base of Etna._--If we skirt the fertile region at the
base of Etna on its southern and eastern sides, we behold marine strata
of clay sand, and volcanic tuff, cropping out from beneath the modern
lavas. The marine fossil shells occurring in these strata are all of
them, or nearly all, identical with species now inhabiting the
Mediterranean; and as they appear at the height of from 600 to 800 feet
above the sea near Catania, they clearly prove that there has been in
this region, as in other parts of Sicily farther to the south, an upward
movement of the ancient bed of the sea. It is fair, therefore, to infer
that the whole mountain, with the exception of those parts which are of
very modern origin, has participated in this upheaval.
If we view Etna from the south, we see the marine deposits above alluded
to, forming a low line of hills (_e_, _e_, Fig. 47), or a steep inland
slope or cliff (_f_), as in the annexed drawing taken from the limestone
platform of Primosole. It should be observed however, in reference to
this view, that the height of the volcanic cone is ten times greater
than the hills at its base (_e_, _e_), although it appears less
elevated, because the summit of the cone is ten or twelve times more
distant from the plain of Catania than is Licodia.
[Illustration: Fig. 47.
View of Etna from the summit of the limestone platform of Primosole.
_a_, Highest cone.
_b_, Montagnuola.
_c_, Monte Minardo, with smaller lateral cones above.
_d_, Town of Licodia dei Monaci.
_e_, Marine formation called creta, argillaceous and sandy beds with a
few shells, and associated volcanic rocks.
_f_, Escarpment of stratified subaqueous volcanic tuff, &c., northwest
of Catania.
_g_, Town of Catania.
_h i_, Dotted line expressing the highest boundary along which the
marine strata are occasionally seen.
_k_, Plain of Catania.
_l_, Limestone platform of Primosole of the Newer Pliocene period.
_m_, La Motta di Catania.]
The mountain is in general of a very symmetrical form, a flattened cone
broken on its eastern side, by a deep valley, called the Val del Bove,
or in the provincial dialect of the peasants, "Val di Bua," for here the
herdsman
---- "in reducta valle mugientium
Prospectat errantes greges."
Dr. Buckland was, I believe, the first English geologist who examined
this valley with attention, and I am indebted to him for having
described it to me, before I visited Sicily, as more worthy of attention
than any single spot in that island, or perhaps in Europe.
[Illustration: PLATE III
VIEW LOOKING UP THE VAL DEL BOVE, ETNA]
The Val del Bove commences near the summit of Etna, and descending
into the woody region, is farther continued on one side by a second and
narrower valley, called the Val di Calanna. Below this another, named
the Val di St. Giacomo, begins,--a long narrow ravine, which is
prolonged to the neighborhood of Zaffarana (_e_, fig. 48), on the
confines of the fertile region. These natural incisions into the side of
the volcano are of such depth that they expose to view a great part of
the structure of the entire mass, which, in the Val del Bove, is laid
open to the depth of from 3000 to above 4000 feet from the summit of
Etna. The geologist thus enjoys an opportunity of ascertaining how far
the internal conformation of the cone corresponds with what he might
have anticipated as the result of that mode of increase which has been
witnessed during the historical era.
[Illustration: Fig. 48.
Great valley on the east side of Etna.
_a_, Highest cone.
_b_, Montagnuola.
_c_, Head of Val del Bove.
_d_, _d_, Serre del Solfizio.
_e_, Village of Zaffarana on the lower border of the woody region.
_f_, One of the lateral cones.
_g_, Monti Rossi.]
_Description of Plate III._--The accompanying view (Pl. III.) is part of
a panoramic sketch which I made in November, 1828, and may assist the
reader in comprehending some topographical details to be alluded to in
the sequel, although it can convey no idea of the picturesque grandeur
of the scene.
The great lava-currents of 1819 and 1811 are seen pouring down from the
higher parts of the valley, overrunning the forests of the great plain,
and rising up in the foreground on the left with a rugged surface, on
which many hillocks and depressions appear, such as often characterize a
lava-current immediately after its consolidation.
The small cone, No. 7, was formed in 1811, and was still smoking when I
saw it in 1828. The other small volcano to the left, from which vapor is
issuing, was, I believe, one of those formed in 1819.
The following are the names of some of the other points indicated in the
sketch:--
1, Montagnuola.
2, Torre del Filosofo.
3, Highest cone.
4, Lepra.
5, Finocchio.
6, Capra.
7, Cone of 1811.
8, Cima del Asino.
9, Musara.
10, Zocolaro.
11, Rocca di Calanna.
DESCRIPTION OF PLATE IV.--The second view (Pl. IV.) represents the same
valley as seen from above, or looking directly down the Val del Bove,
from the summit of the principal crater formed in 1819.[567] I am
unable to point out the precise spot which this crater would occupy in
the view represented in Plate III.; but I conceive that it would appear
in the face of the great precipice, near which the smoke issuing from
the cone No. 7 is made to terminate. There are many ledges of rock on
the face of that precipice where eruptions have occurred.
The circular form of the Val del Bove is well shown in this view. (Pl.
IV.) To the right and left are the lofty precipices which form the
southern and northern sides of the great valley, and which are
intersected by dikes projecting in the manner afterwards to be
described. In the distance appears the "fertile region" of Etna,
extending like a great plain along the sea-coast.
The spots particularly referred to in the plate are the following:--
_a_, Cape Spartivento, in Italy, of which the outline is seen
in the distance.
_b_, The promontory of Taormino, on the Sicilian coast.
_c_, The river Alcantra.
_d_, The small village of Riposto.
_f_, The town of Aci Reale.
_g_, Cyclopian islands, or "Faraglioni," in the Bay of Trezza.
_h_, The great harbor of Syracuse.
_k_, The Lake of Lentini.
_i_, The city of Catania, near which is marked the course of
the lava which flowed from the Monti Rossi in 1669,
and destroyed part of the city.
_l_, To the left of the view is the crater of 1811, which is
also shown at No. 7 in Plate III.
_m_, Rock of Musara, also seen at No. 9 in Plate III.
_e_, Valley of Calanna.
The Val del Bove is of truly magnificent dimensions, a vast amphitheatre
four or five miles in diameter, surrounded by nearly vertical
precipices, varying from 1000 to above 3000 feet in height, the loftiest
being at the upper end, and the height gradually diminishing on both
sides. The feature which first strikes the geologist as distinguishing
the boundary cliffs of this valley, is the prodigious multitude of
verticle dikes which are seen in all directions traversing the volcanic
beds. The circular form of this great chasm, and the occurrence of these
countless dikes, amounting perhaps to several thousands in number, so
forcibly recalled to my mind the phenomena of the Atrio del Cavallo, on
Vesuvius, that I at first imagined that I had entered a vast crater, on
a scale as far exceeding that of Somma, as Etna surpasses Vesuvius in
magnitude.
But I was soon undeceived when I had attentively explored the different
sides of the great amphitheatre, in order to satisfy myself whether the
semicircular wall of the Val del Bove had ever formed the boundary of a
crater, and whether the beds had the same quaqua-versal dip which is so
beautifully exhibited in the escarpment of Somma. Had the supposed
analogy between Somma and the Val del Bove held true, the tufts and
lavas at the head of the valley would have dipped to the west, those on
the north side towards the north, and those on the southern side to the
south. But such I did not find to be the inclination of the beds; they
all dip towards the sea, or nearly east, as in the valleys of St.
Giacomo and Calanna below.
[Illustration: PLATE IV
VIEW OF THE VAL DEL BOVE, ETNA, AS SEEN FROM ABOVE, OR FROM THE CRATER
OF 1819]
_Scenery of the Val del Bove._--Let the reader picture to himself a
large amphitheatre, five miles in diameter, and surrounded on three
sides by precipices from 2000 to 3000 feet in height. If he has beheld
that most picturesque scene in the chain of the Pyrenees, the celebrated
"cirque of Gavarnie," he may form some conception of the magnificent
circle of precipitous rocks which inclose, on three sides, the great
plain of the Val del Bove. This plain has been deluged by repeated
streams of lava; and although it appears almost level, when viewed from
a distance, it is, in fact, more uneven than the surface of the most
tempestuous sea. Besides the minor irregularities of the lava, the
valley is in one part interrupted by a ridge of rocks, two of which,
Musara and Capra, are very prominent. It can hardly be said that they
----"like giants stand
To sentinel enchanted land;"
for although, like the Trosachs, in the Highlands of Scotland, they are
of gigantic dimensions, and appear almost isolated, as seen from many
points, yet the stern and severe grandeur of the scenery which they
adorn is not such as would be selected by a poet for a vale of
enchantment. The character of the scene would accord far better with
Milton's picture of the infernal world; and if we imagine ourselves to
behold in motion, in the darkness of the night, one of those fiery
currents which have so often traversed the great valley, we may well
recall
----"yon dreary plain, forlorn and wild,
The seat of desolation, void of light,
Save what the glimmering of these livid flames
Casts pale and dreadful."
The face of the precipices already mentioned is broken in the most
picturesque manner by the vertical walls of lava which traverse them.
These masses visually stand out in relief, are exceedingly diversified
in form, and of immense altitude. In the autumn, their black outline may
often be seen relieved by clouds of fleecy vapor which settle behind
them, and do not disperse until mid-day, continuing to fill the valley
while the sun is shining on every other part of Sicily, and on the
higher regions of Etna.
As soon as the vapors begin to rise, the changes of scene are varied in
the highest degree, different rocks being unveiled and hidden by turns,
and the summit of Etna often breaking through the clouds for a moment
with its dazzling snows, and being then as suddenly withdrawn from the
view.
An unusual silence prevails; for there are no torrents dashing from the
rocks, nor any movement of running water in this valley such as may
almost invariably be heard in mountainous regions. Every drop of water
that falls from the heavens, or flows from the melting ice and snow, is
instantly absorbed by the porous lava; and such is the dearth of
springs, that the herdsman is compelled to supply his flocks, during
the hot season, from stores of snow laid up in hollows of the mountain
during winter.
The strips of green herbage and forest land, which have here and there
escaped the burning lavas, serve, by contrast, to heighten the
desolation of the scene. When I visited the valley, nine years after the
eruption of 1819, I saw hundreds of trees, or rather the white skeletons
of trees, on the borders of the black lava, the trunks and branches
being all leafless, and deprived of their bark by the scorching heat
emitted from the melted rock; an image recalling those beautiful
lines:--
----"As when heaven's fire
Hath scath'd the forest oaks, or mountain pines,
With singed top their stately growth, though bare,
Stands on the blasted heath."
[Illustration: Fig. 49.
Dikes at the base of the Serre del Solfizio, Etna.]
_Form, composition, and origin of the dikes._--But without indulging the
imagination any longer in descriptions of scenery, I may observe that
the dikes before mentioned form unquestionably the most interesting
geological phenomenon in the Val del Bove. Some of these are composed of
trachyte, others of compact blue basalt with olivine. They vary in
breadth from two to twenty feet and upwards, and usually project from
the face of the cliffs, as represented in the annexed drawing (fig. 49).
They consist of harder materials than the strata which they traverse,
and therefore waste away less rapidly under the influence of that
repeated congelation and thawing to which the rocks in this zone of
Etna are exposed. The dikes are for the most part vertical, but
sometimes they run in a tortuous course through the tuffs and breccias,
as represented in fig. 50. In the escarpment of Somma, where similar
walls of lava cut through alternating beds of sand and scoriae, a coating
of coal-black rock, approaching in its nature and appearance to
pitchstone, is seen at the contact of the dike with the intersected
beds. I did not observe such parting layers at the junction of the
Etnean dikes which I examined, but they may perhaps be discoverable.
[Illustration: Fig. 50.
Tortuous veins of lava at Punto di Giumento, Etna.]
The geographical position of these dikes is most interesting, as they
are very numerous near the head of the Val del Bove, where the cones of
1811 and 1819 were thrown up, as also in that zone of the mountain where
lateral eruptions are frequent; whereas in the valley of Calanna, which
is below that parallel, and in a region where lateral eruptions are
extremely rare, scarcely any dikes are seen, and none whatever still
lower in the valley of St. Giacomo. This is precisely what we might have
expected, if we consider the vertical fissures now filled with rock to
have been the feeders of lateral cones, or, in other words, the channels
which gave passage to the lava-currents and scoriae that have issued from
vents in the forest zone. In other parts of Etna there may be numerous
dikes at as low a level as the Valley of Calanna, because the line of
lateral eruptions is not everywhere at the same height above the sea;
but in the section above alluded to, there appeared to me an obvious
connection between the frequency of dikes and of lateral eruptions.
Some fissures may have been filled from above, but I did not see any
which, by terminating downwards, gave proof of such an origin. Almost
all the isolated masses in the Val del Bove, such as Capra, Musara, and
others, are traversed by dikes, and may, perhaps, have partly owed their
preservation to that circumstance, if at least the action of occasional
floods has been one of the destroying causes in the Val del Bove; for
there is nothing which affords so much protection to a mass of strata
against the undermining action of running water as a perpendicular dike
of hard rock.
In the accompanying drawing (fig. 51), the flowing of the lavas of 1811
and 1819, between the rocks Finochio, Capra, and Musara, is represented.
The height of the two last-mentioned isolated masses has been much
diminished by the elevation of their base, caused by these currents.
They may, perhaps, be the remnants of lateral cones which existed before
the Val del Bove was formed, and may hereafter be once more buried by
the lavas that are now accumulating in the valley.
[Illustration: Fig. 51.
View of the rocks Finochio, Capra, and Musara, Val del Bove.]
From no point of view are the dikes more conspicuous than from the
summit of the highest cone of Etna; a view of some of them is given in
the annexed drawing. (Fig. 52.)
_Eruption of 1811._--I have alluded to the streams of lava which were
poured forth in 1811 and 1819. Gemmellaro, who witnessed these
eruptions, informs us that the great crater in 1811 first testified by
its loud detonations that a column of lava had ascended to near the
summit of the mountain. A violent shock was then felt, and a stream
broke out from the side of the cone, at no great distance from its apex.
Shortly after this had ceased to flow, a second stream burst forth at
another opening, considerably below the first; then a third still lower,
and so on till seven different issues had been thus successively formed,
all lying upon the same straight line. It has been supposed that this
line was a perpendicular rent in the internal framework of the mountain,
which rent was probably not produced at one shock, but prolonged
successively downwards, by the lateral pressure and intense heat of the
internal column of lava, as it subsided by gradual discharge through
each vent.[568]
_Eruption of 1819._--In 1819 three large mouths or caverns opened very
near those which were formed in the eruptions of 1811, from which
flames, red-hot cinders, and sand were thrown up with loud explosions. A
few minutes afterwards another mouth opened below, from which flames
and smoke issued; and finally a fifth, lower still, whence a torrent of
lava flowed, which spread itself with great velocity over the deep and
broad valley called "Val del Bove." This stream flowed two miles in the
first twenty-four hours, and nearly as far in the succeeding day and
night. The three original mouths at length united into one large crater,
and sent forth lava, as did the inferior apertures, so that an enormous
torrent poured down the "Val del Bove." When it arrived at a vast and
almost perpendicular precipice, at the head of the Valley of Calanna, it
poured over in a cascade, and, being hardened in its descent, made an
inconceivable crash as it was dashed against the bottom. So immense was
the column of dust raised by the abrasion of the tufaceous hill over
which the hardened mass descended, that the Catanians were in great
alarm, supposing a new eruption to have burst out in the woody region,
exceeding in violence that near the summit of Etna.
[Illustration: Fig. 52.
View from the summit of Etna into the Val del Bove.[569]]
_Mode of advance of the lava._--Of the cones thrown up during this
eruption, not more than two are of sufficient magnitude to be numbered
among those eighty which were before described as adorning the flanks of
Etna. The surface of the lava which deluged the "Val del Bove," consists
of rocky and _angular blocks_, tossed together in the utmost disorder.
Nothing can be more rugged, or more unlike the smooth and even
superficies, which those who are unacquainted with volcanic countries
may have pictured to themselves, in a mass of matter which had
consolidated from a liquid state. Mr. Scrope observed this current in
the year 1819, slowly advancing down a considerable slope, at the rate
of about a yard an hour, nine months after its emission. The lower
stratum being arrested by the resistance of the ground, the upper or
central part gradually protruded itself, and, being unsupported, fell
down. This in its turn was covered by a mass of more liquid lava, which
swelled over it from above. The current had all the appearance of a huge
heap of rough and large cinders rolling over and over upon itself by the
effect of an extremely slow propulsion from behind. The contraction of
the crust as it solidified, and the friction of the scoriform cakes
against one another, produced a crackling sound. Within the crevices a
dull red heat might be seen by night, and vapor issuing in considerable
quantity was visible by day.[570]
It was stated that when the lava of 1819 arrived at the head of the
Valley of Calanna, after flowing down the Val del Bove, it descended in
a cascade. This stream, in fact, like many previous currents of lava
which have flowed down successively from the higher regions of Etna, was
turned by a great promontory projecting from the southern side of the
Val del Bove. This promontory consists of the hills called Zocolaro and
Calanna, and of a ridge of inferior height which connects them. (See
fig. 53.)
[Illustration: Fig. 53.
A, Zocolaro.
B, Monte di Calanna.
C, Plain at the head of the Valley of Calanna.
_a_, Lava of 1819 descending the precipice and flowing through
the valley.
_b_, Lavas of 1811 and 1819 flowing round the hill of Calanna.]
It happened in 1811 and 1819 that the flows of lava overtopped the ridge
intervening between the hills of Zocolaro and Calanna, so that they fell
in a cascade over a lofty precipice, and began to fill up the valley of
Calanna (_a_, fig. 53). Other portions of the same lava-current (_b_)
flowed round the promontory, and they exhibit one of the peculiar
characteristics of such streams, namely that of becoming solid
externally, even while yet in motion. Instead of thinning out gradually
at their edges, their sides may often be compared to two rocky walls
which are sometimes inclined at an angle of between thirty and forty
degrees. When such streams are turned from their course by a projecting
rock, they move right onwards in a new direction; and in the Valley of
Calanna a considerable space has thus been left between the steep sides
of the lavas _b b_, so deflected, and the precipitous escarpment of
Zocolaro, A, which bounds the plain C.
_Lavas and breccias._--In regard to the volcanic masses which are
intersected by dikes in the Val del Bove, they consist in great part of
graystone lavas, of an intermediate character between basalt and
trachyte, and partly of porphyritic lava resembling trachyte, but to
which that name cannot, according to Von Buch and G. Rose, be in
strictness applied, because the felspar belongs to the variety called
Labradorite. There is great similarity in the composition of the ancient
and modern lavas of Etna, both consisting of felspar, augite, olivine,
and titaniferous iron. The alternating breccias are made up of scoriae,
sand, and angular blocks of lava. Many of these fragments may have been
thrown out by volcanic explosions, which, falling on the hardened
surface of moving lava-currents, may have been carried to a considerable
distance. It may also happen that when lava advances very slowly, in the
manner of the flow of 1819, the angular masses resulting from the
frequent breaking of the mass as it rolls over upon itself, may produce
these breccias. It is at least certain that the upper portion of the
lava-currents of 1811 and 1819 now consist of angular masses to the
depth of many yards. D'Aubuisson has compared the surface of one of the
ancient lavas of Auvergne to that of a river suddenly frozen over by the
stoppage of immense fragments of drift-ice, a description perfectly
applicable to these modern Etnean flows. The thickness of the separate
beds of conglomerate or breccia which are seen in the same vertical
section, is often extremely different, varying from 3 to nearly 50 feet,
as I observed in the hill of Calanna.
_Flood produced by the melting of snow by lava._--It is possible that
some of the breccias or conglomerates may be referred to aqueous causes,
as great floods occasionally sweep down the flanks of Etna, when
eruptions take place in winter, and when the snows are melted by lava.
It is true that running water in general exerts no power on Etna, the
rain which falls being immediately imbibed by the porous lavas; so that,
vast as is the extent of the mountain, it feeds only a few small
rivulets, and these, even, are dry throughout the greater portion of the
year. The enormous rounded boulders, therefore, of felspar-porphyry and
basalt, a line of which can be traced from the sea, from near Giardini,
by Mascali, and Zafarana, to the "Val del Bove," would offer a
perplexing problem to the geologist, if history had not preserved the
memorials of a tremendous flood which happened in this district in the
year 1755. It appears that two streams of lava flowed in that year, on
the 2d of March, from the highest crater; they were immediately
precipitated upon an enormous mass of snow which then covered the whole
mountain, and was extremely deep near the summit. The sudden melting of
this frozen mass, by a fiery torrent three miles in length, produced a
frightful inundation, which devastated the sides of the mountain for
eight miles in length, and afterwards covered the lower flanks of Etna,
where they were less steep, together with the plains near the sea, with
great deposits of sand, scoriae, and blocks of lava.
Many absurd stories circulated in Sicily respecting this event; such as
that the water was boiling, and that it was vomited from the highest
crater; that it was as salt as the sea, and full of marine shells; but
these were mere inventions, to which Recupero, although he relates them
as tales of the mountaineers, seems to have attached rather too much
importance.
Floods of considerable violence have also been produced on Etna by the
fall of heavy rains, aided, probably, by the melting of snow. By this
cause alone, in 1761, sixty of the inhabitants of Acicatena were killed,
and many of their houses swept away.[571]
_Glacier covered by a lava-stream._--A remarkable discovery was made on
Etna in 1828 of a great mass of ice, preserved for many years, perhaps
for centuries, from melting, by the singular accident of a current of
red-hot lava having flowed over it. The following are the facts in
attestation of a phenomenon which must at first sight appear of so
paradoxical a character. The extraordinary heat experienced in the South
of Europe, during the summer and autumn of 1828, caused the supplies of
snow and ice which had been preserved in the spring of that year, for
the use of Catania and the adjoining parts of Sicily and the island of
Malta, to fail entirely. Great distress was consequently felt for want
of a commodity regarded in those countries as one of the necessaries of
life rather than an article of luxury, and the abundance of which
contributes in some of the larger cities to the salubrity of the water
and the general health of the community. The magistrates of Catania
applied to Signor M. Gemmellaro, in the hope that his local knowledge of
Etna might enable him to point out some crevice or natural grotto on the
mountain, where drift-snow was still preserved. Nor were they
disappointed; for he had long suspected that a small mass of perennial
ice at the foot of the highest cone was part of a large and continuous
glacier covered by a lava-current. Having procured a large body of
workmen, he quarried into this ice, and proved the superposition of the
lava for several hundred yards, so as completely to satisfy himself that
nothing but the subsequent flowing of the lava over the ice could
account for the position of the glacier. Unfortunately for the
geologist, the ice was so extremely hard, and the excavation so
expensive, that there is no probability of the operations being renewed.
On the first of December, 1828, I visited this spot, which is on the
southeast side of the cone, and not far above the Casa Inglese; but the
fresh snow had already nearly filled up the new opening, so that it had
only the appearance of the mouth of a grotto. I do not, however,
question the accuracy of the conclusion of Signer Gemmellaro, who, being
well acquainted with all the appearances of drift-snow in the fissures
and cavities of Etna, had recognized, even before the late excavations,
the peculiarity of the position of the ice in this locality. We may
suppose that, at the commencement of the eruption, a deep mass of
drift-snow had been covered by volcanic sand showered down upon it
before the descent of the lava. A dense stratum of this fine dust mixed
with scoriae is well known to be an extremely bad conductor of heat; and
the shepherds in the higher regions of Etna are accustomed to provide
water for their flocks during summer, by strewing a layer of volcanic
sand a few inches thick over the snow, which effectually prevents the
heat of the sun from penetrating.
Suppose the mass of snow to have been preserved from liquefaction until
the lower part of the lava had consolidated, we may then readily
conceive that a glacier thus protected, at the height of ten thousand
feet above the level of the sea, would endure as long as the snows of
Mont Blanc, unless melted by volcanic heat from below. When I visited
the great crater in the beginning of winter (December 1st, 1828), I
found the crevices in the interior incrusted with thick ice, and in some
cases hot vapors were actually streaming out between masses of ice and
the rugged and steep walls of the crater.[572]
After the discovery of Signor Gemmellaro, it would not be surprising to
find in the cones of the Icelandic volcanoes, which are covered for the
most part with perpetual snow, repeated alternations of lava-streams and
glaciers. We have, indeed, Lieutenant Kendall's authority for the fact
that Deception Island, in New South Shetland, lat. 62 degrees. 55
minutes S., is principally composed of alternate layers of volcanic
ashes and ice.[573]
_Origin of the Val del Bove._--It is recorded, as will be stated in the
history of earthquakes (ch. 29), that in the year 1772 a great
subsidence took place on Papandayang, the largest volcano in the island
of Java; an extent of ground _fifteen miles in length_, _and six in
breadth_, covered by no less than forty villages, was engulphed, and the
cone lost 4000 feet of its height. In like manner the summit of
Carguairazo, one of the loftiest of the Andes of Quito, fell in on the
19th July, 1698; and another mountain of still greater altitude in the
same chain, called Capac Urcu, a short time before the conquest of
America by the Spaniards.
It will also be seen in the next chapter that, so late as the year
1822, during a violent earthquake and volcanic eruption in Java, one
side of the mountain called Galongoon, which was covered by a dense
forest, became an enormous gulf in the form of a semicircle. The new
cavity was about midway between the summit and the plain, and surrounded
by steep rocks.
Now we might imagine a similar event, or a series of subsidences to have
formerly occurred on the eastern side of Etna, although such
catastrophes have not been witnessed in modern times, or only on a very
trifling scale. A narrow ravine, about a mile long, twenty feet wide,
and from twenty to thirty-six in depth, has been formed, within the
historical era, on the flanks of the volcano, near the town of
Mascalucia; and a small circular tract, called the Cisterna, near the
summit, sank down in the year 1792, to the depth of about forty feet,
and left on all sides of the chasm a vertical section of the beds,
exactly resembling those which are seen in the precipices of the Val del
Bove. At some remote periods, therefore, we might suppose more extensive
portions of the mountain to have fallen in during great earthquakes.
But we ought not to exclude entirely from our speculations another
possible agency, by which the great cavity may in part at least have
been excavated, namely, the denuding action of the sea. Whether its
waves may once have had access to the great valley before the ancient
portion of Etna was upheaved to its present elevation, is a question
which will naturally present itself to every geologist. Marine shells
have been traced to a height of 800 feet above the base of Etna, and
would doubtless be seen to ascend much higher, were not the structure of
the lower region of the mountain concealed by floods of lava. We cannot
ascertain to what extent a change in the relative level of land and sea
may have been carried in this spot, but we know that some of the
tertiary strata in Sicily of no ancient date reach a height of 3000
feet, and the marine deposits on the flanks of Etna, full of recent
species of shells, may ascend to equal or greater heights. The narrow
Valley of Calanna leading out of the Val del Bove, and that of San
Giacomo lower down, have much the appearance of ravines swept out by
aqueous action.
_Structure and origin of the cone of Etna._--Our data for framing a
correct theory of the manner in which the cone of Etna has acquired its
present dimensions and internal structure are very imperfect, because it
is on its eastern side only, in the Val del Bove above described, that
we see a deep section exposed. Even here we obtain no insight into the
interior composition of the mountain beyond a depth of between three and
four thousand feet below the base of that highest cone, which has been
several times destroyed and renewed. The precipices seen at the head of
the Val del Bove, in the escarpment called the Serre del Solfizio,
exhibit merely the same series of alternating lavas and breccias, which,
descending with a general dip towards the sea, form the boundary cliffs
of all other parts of the Val del Bove. If then we estimate the height
of Etna at about 11,000 feet, we may say that we know from actual
observation less than one-half of its component materials, assuming it
to extend downwards to the level of the sea; namely, first, the highest
cone, which is about 1000 feet above its base; and, secondly, the
alternations of lava, tuff, and volcanic breccia, which constitute the
rocks between the Cisterna, near the base of the upper cone, and the
foot of the precipices at the head of the Val del Bove. At the lowest
point to which the vertical section extends, there are no signs of any
approach to a termination of the purely volcanic mass, which may perhaps
penetrate many thousand feet farther downwards. There is, indeed, a rock
called Rocca Gianicola, near the foot of the great escarpment, which
consists of a large mass between 150 and 200 feet wide, not divided into
beds, and almost resembling granite in its structure, although agreeing
very closely in mineral composition with the lavas of Etna in
general.[574] This mass may doubtless be taken as a representative of
those crystalline or plutonic formations which would be met with in
abundance if we could descend to greater depths in the direction of the
central axis of the mountain. For a great body of geological evidence
leads us to conclude, that rocks of this class result from the
consolidation, under great pressure, of melted matter, which has risen
up and filled rents and chasms, such, for example, as may communicate
with the principal and minor vents of eruption in a volcano like Etna.
But, if we speculate on the nature of the formation which the lava may
have pierced in its way upwards, we may fairly presume that a portion of
these consist of marine tertiary rocks, like those of the neighboring
Val di Noto, or those which skirt the borders of the Etnean cone, on its
southern and eastern sides. Etna may, in fact, have been at first an
insular volcano, raising its summit but slightly above the level of the
sea; but we have no grounds for concluding that any of the beds exposed
in the deep section of the Val del Bove have formed a part of such a
marine accumulation. On the contrary, all the usual signs of subaqueous
origin are wanting; and even if we believe the foundations of the
mountain to have been laid in the sea, we could not expect this portion
to be made visible in sections which only proceed downwards from the
summit through one-half the thickness of the mountain, especially as the
highest points attained by the tertiary strata in other parts of Sicily
very rarely exceed 3000 feet above the sea.
On the eastern and southern base of Etna, a marine deposit, already
alluded to, is traced up to the height of 800 or 1000 feet, before it
becomes concealed beneath that covering of modern lavas which is
continually extending its limits during successive eruptions, and
prevents us from ascertaining how much higher the marine strata may
ascend. As the imbedded shells belong almost entirely to species now
inhabiting the Mediterranean, it is evident that there has been here an
upheaval of the region at the base of Etna at a very modern period. It
is fair, therefore, to infer that the volcanic nucleus of the mountain,
partly perhaps of submarine, and partly of subaerial origin,
participated in this movement, and was carried up bodily. Now, in
proportion as a cone gains height by such a movement, combined with the
cumulative effects of eruptions, throwing out matter successively from
one or more central vents, the hydrostatic pressure of the columns of
lava augments with their increasing height, until the time arrives when
the flanks of the cone can no longer resist the increased pressure; and
from that period they give way more readily, lateral outbursts becoming
more frequent. Hence, independently of any local expansion of the
fractured volcanic mass, those general causes by which the modern
tertiary strata of a great part of Sicily have been raised to the height
of several thousand feet above their original level, would tend
naturally to render the discharge of lava and scoriae from the summit of
Etna less copious, and the lateral discharge greater.
If, then, a conical or dome-shaped mass of volcanic materials was
accumulated to the height of 4000, or perhaps 7000 feet, before the
upward movement began, or, what is much more probable, during the
continuance of the upward movement, that ancient mass would not be
buried under the products of newer eruptions, because these last would
then be poured out chiefly at a lower level.
Since I visited Etna in 1828, M. de Beaumont has published a most
valuable memoir on the structure and origin of that mountain, which he
examined in 1834;[575] and an excellent description of it has also
appeared in the posthumous work of Hoffmann.[576]
In M. de Beaumont's essay, in which he has explained his views with
uncommon perspicuity and talent, he maintains that all the alternating
stony and fragmentary beds, more than 3000 feet thick, which are exposed
in the Val del Bove, were formed originally on a surface so nearly flat
that the slope never exceeded three degrees. From this horizontal
position they were at length heaved up suddenly (d'un seul coup) into a
great mountain, to which no important additions have since been made.
Prior to this upthrow, a platform is supposed to have existed above the
level of the sea, in which various fissures opened; and from these
melted matter was poured forth again and again, which spread itself
around in thin sheets of uniform thickness. From the same rents issued
showers of scoriae and fragmentary matter, which were spread out so as to
form equally uniform and horizontal beds, intervening between the sheets
of lava. But although, by the continued repetition of these operations,
a vast pile of volcanic matter, 4000 feet or more in thickness, was
built up precisely in that region where Etna now rises, and to which
nothing similar was produced elsewhere in Sicily, still we are told that
Etna was not yet a mountain. No hypothetical diagram has been given to
help us to conceive how this great mass of materials of supramarine
origin could have been disposed of in horizontal beds, so as not to
constitute an eminence towering far above the rest of Sicily; but it is
assumed that a powerful force from below at length burst suddenly
through the horizontal formation, uplifted it to a considerable height,
and caused the beds to be, in many places, highly inclined. This
elevatory force was not all expended on a single central point as Von
Buch has imagined in the case of Palma, Teneriffe, or Somma, but rather
followed for a short distance a linear direction.[577]
Among other objections that may be advanced against the theory above
proposed, I may mention, first, that the increasing number of dikes as
we approach the head of the Val del Bove, or the middle of Etna, and the
great thickness of lava, scoriae, and conglomerates in that region, imply
that the great centre of eruption was always where it now is, or nearly
at the same point, and there must, therefore, have been a tendency, from
the beginning, to a conical or dome-shaped arrangement in the ejected
materials. Secondly, were we to admit a great number of separate points
of eruption, scattered over a plain or platform, there must have been a
great number of cones thrown up over these different vents; and these
hills, some of which would probably be as lofty as those now seen on the
flanks of Etna, or from 300 to 750 feet in height, would break the
continuity of the sheets of lava, while they would become gradually
enveloped by them. The ejected materials, moreover, would slope at a
high angle on the sides of these cones, and where they fell on the
surrounding plain, would form strata thicker near the base of each cone
than at a distance.
What then are the facts, it will be asked, to account for which this
hypothesis of original horizontality, followed by a single and sudden
effort of upheaval, which gave to the beds their present slope, has been
invented? M. de Beaumont observes, that in the boundary precipices of
the Val del Bove, sheets of lava and intercalated beds of cinders, mixed
with pulverulent and fragmentary matter evidently cast out during
eruptions, are sometimes inclined at steep angles, varying from 15
degrees. to 27 degrees. It is impossible, he says, that the lavas could
have flowed originally on planes so steeply inclined, for streams which
descend a slope even of 10 degrees form narrow stripes, and never
acquire such a compact texture. Their thickness, moreover, always
inconsiderable, varies with every variation of steepness, in the
declivity down which they flow; whereas, in several parts of the Val del
Bove, the sheets of lava are continuous for great distances, in spite of
their steep inclination, and are often compact, and perfectly parallel
one to the other, even where there are more than 100 beds of
interpolated fragmentary matter.
The intersecting dikes also terminate upwards in many instances, at
different elevations, and blend (or, as M. de Beaumont terms it,
articulate) with sheets of lava, which they meet at right angles. It is
therefore assumed that such dikes were the feeders of the streams of
lava with which they unite, and they are supposed to prove that the
platform, on the surface of which the melted matter was poured out, was
at first so flat, that the fluid mass spread freely and equally in every
direction, and not towards one point only of the compass, as would
happen if it had descended the sloping sides of a cone. This argument is
ingeniously and plainly put in the following terms:--"Had the melted
matter poured down an inclined plane, after issuing from a rent, the
sheet of lava would, after consolidation, have formed an elbow with the
dike, like the upper bar of the letter F, instead of extending itself on
both sides like that of a T."[578] It is also contended that a series of
sheets of lava, formed on a conical or dome-shaped mountain, would have
been more numerous at points farthest from the central axis, since every
dike which had been the source of a lava-stream, must have poured its
contents downwards, and never upwards.
[Illustration: Fig. 54.
Dikes as they would now appear had they been originally perpendicular.]
In reference to the facts here stated, I may mention that the dikes
which I saw in the Val del Bove were either vertical, or made almost all
of them a near approach to the perpendicular, which could not have been
the case had they been the feeders of horizontal beds of lava, and had
they consequently joined them originally at right angles, for then the
dikes, as at _a_, _b_, _c_, fig. 54, ought subsequently to have acquired
a considerable slope, like the beds which they intersect. I may also
urge another objection to the views above set forth, namely, that had
the dikes been linear vents, or orifices of eruption, we must suppose
the inter-stratified scoriae and lapilli, as well as the lavas, to have
come out of them, and in that case the irregular heaping up of
fragmentary matter around the vents would, as before hinted, have
disturbed that uniform thickness and parallelism of the beds which M. de
Beaumont describes.
If, however, some of the sheets of lava join the dikes in such a manner,
as to imply that they were in a melted state simultaneously with the
contents of the fissures,--a point not easily ascertained, where the
precipices are for the most part inaccessible,--the fact may admit of a
different interpretation from that proposed by the French geologists.
Rents like those before alluded to (p. 399), which opened in the plain
of S. Lio in 1669, filled below with incandescent lava, may have lain in
the way of currents of melted matter descending from higher openings. In
that case, the matter of the current would have flowed into the fissure
and mixed with the lava at its bottom. Numerous open rents of this kind
are described by Mr. Dana as having been caused, during a late eruption,
in one of the volcanic domes of the Sandwich Islands. They remained open
at various heights on the slopes of the great cone, running in different
directions, and demonstrate the possibility of future junctions of
slightly inclined lava-streams with perpendicular walls of lava.
To me, therefore, it appears far more easy to explain the uniform
thickness and parallelism of so many lavas and beds of fragmentary
matter seen in the Val del Bove, by supposing them to have issued
successively out of one or more higher vents near the summit of a great
dome, than to imagine them to have proceeded from lateral dikes or rents
opening in a level plain. In the Sandwich Islands, we have examples of
volcanic domes 15,000 feet high, produced by successive outpourings from
vents at or near the summit. One of these, Mount Loa, has a slope in all
directions of 6 degrees 30 minutes; another, Mount Kea, a mean
inclination of 7 degrees 46 minutes. That their lavas may occasionally
consolidate on slopes of 25 degrees, and even more, and still preserve
considerable solidity of texture, has been already stated; see above, p.
383.
We know not how large a quantity of modern lava may have been poured
into the bottom of the Val del Bove, yet we perceive that eruptions
breaking forth near the centre of Etna have already made some progress
in filling up this great hollow. Even within the memory of persons now
living, the rocks of Musara and Capra have, as before stated, lost much
of their height and picturesque grandeur by the piling up of recent
lavas round their base (see fig. 51, p. 408), and the great chasm has
intercepted many streams which would otherwise have deluged the fertile
region below, as has happened on the side of Catania. The volcanic
forces are now laboring, therefore, to repair the breach which
subsidence has caused on one side of the great cone; and unless their
energy should decline, or a new sinking take place, they may in time
efface this inequality. In that event, the restored portion will always
be unconformable to the more ancient part, yet it will consist, like it,
of alternating beds of lava, scoriae;, and conglomerates, which, with all
their irregularities, will have a general slope from the centre and
summit of Etna towards the sea.
I shall conclude, then, by remarking that I conceive the general
inclination of the alternating stony and fragmentary beds of the Val del
Bove, from the axis of Etna towards its circumference or base, and the
greater thickness of the volcanic pile as we approach the central parts
of the mountain, to be due to the preponderance of eruptions from that
centre. These gave rise, from the first, to a dome-shaped mass, which
has ever since been increasing in height and area, being fractured again
and again by the expansive force of vapors, and the several parts made
to cohere together more firmly after the solidification of the lava with
which every open fissure and chasm has been filled. At the same time the
cone may have gained a portion of its height by the elevatory effect of
such dislocating movements, and the sheets of lava may have acquired in
some places a greater, in others a less, inclination than that which at
first belonged to them.
[Illustration: Fig. 55.
Non-volcanic protuberance and valley of elevation.]
But had the mountain been due solely, or even principally, to upheaval,
its structure would have resembled that which geologists have so often
recognized in dome-shaped hills, or certain elevated regions, which all
consider as having been thrust up by a force from below. In this case
there is often an elliptical cavity at the summit, due partly to the
fracture of the upraised rocks, but still more to aqueous denudation, as
they rose out of the sea. The central cavity, or valley, exposes to view
the subjacent formation _c_, fig. 55, and the incumbent mass dips away
on all sides from the axis, but has no tendency to thin out near the
base of the dome, or at _x_, _x_; whereas at this point the volcanic
mass terminates (see fig. 56) and allows the fundamental rock _c_ to
appear at the surface. In the last diagram, the more ordinary case is
represented of a great hollow or crater at the summit of the volcanic
cone; but instead of this, we have seen that in the case of Etna there
is a deep lateral depression, called the Val del Bove, the upper part of
which approaches near to the central axis, and the origin of which we
have attributed to subsidence.
[Illustration: Fig. 56.
Volcanic mountain and crater.]
_Antiquity of the cone of Etna._--It was before remarked that confined
notions in regard to the quantity of past time have tended, more than
any other prepossessions, to retard the progress of sound theoretical
views in geology;[579] the inadequacy of our conceptions of the earth's
antiquity having cramped the freedom of our speculations in this
science, very much in the same way as a belief in the existence of a
vaulted firmament once retarded the progress of astronomy. It was not
until Descartes assumed the indefinite extent of the celestial spaces,
and removed the supposed boundaries of the universe, that just opinions
began to be entertained of the relative distances of the heavenly
bodies; and until we habituate ourselves to contemplate the possibility
of an indefinite lapse of ages having been comprised within each of the
modern periods of the earth's history, we shall be in danger of forming
most erroneous and partial views in geology.
If history had bequeathed to us a faithful record of the eruptions of
Etna, and a hundred other of the principal active volcanoes of the
globe, during the last three thousand years,--if we had an exact account
of the volume of lava and matter ejected during that period, and the
times of their production,--we might, perhaps, be able to form a correct
estimate of the average rate of the growth of a volcanic cone. For we
might obtain a mean result from the comparison of the eruptions of so
great a number of vents, however irregular might be the development of
the igneous action in any one of them, if contemplated singly during a
brief period.
It would be necessary to balance protracted periods of inaction against
the occasional outburst of paroxysmal explosions. Sometimes we should
have evidence of a repose of seventeen centuries, like that which was
interposed in Ischia, between the end of the fourth century B.C., and
the beginning of the fourteenth century of our era.[580] Occasionally a
tremendous eruption, like that of Jorullo, would be recorded, giving
rise, at once, to a considerable mountain.
If we desire to approximate to the age of a cone such as Etna, we ought
first to obtain some data in regard to the thickness of matter which has
been added during the historical era, and then endeavor to estimate the
time required for the accumulation of such alternating lavas and beds of
sand and scoriae as are superimposed upon each other in the Val del Bove;
afterwards we should try to deduce, from observations on other
volcanoes, the more or less rapid increase of burning mountains in all
the different stages of their growth.
There is a considerable analogy between the mode of increase of a
volcanic cone and that of trees of _exogenous_ growth. These trees
augment, both in height and diameter, by the successive application
externally of cone upon cone of new ligneous matter; so that if we make
a transverse section near the base of the trunk, we intersect a much
greater number of layers than nearer to the summit. When branches
occasionally shoot out from the trunk, they first pierce the bark, and
then, after growing to a certain size, if they chance to be broken off,
they may become inclosed in the body of the tree, as it augments in
size, forming knots in the wood, which are themselves composed of layers
of ligneous matter, cone within cone.
In like manner, a volcanic mountain, as we have seen, consists of a
succession of conical masses enveloping others, while lateral cones,
having a similar internal structure, often project, in the first
instance, like branches from the surface of the main cone, and then
becoming buried again, are hidden like the knots of a tree.
We can ascertain the age of an oak or pine by counting the number of
concentric rings of annual growth seen in a transverse section near the
base, so that we may know the date at which the seedling began to
vegetate. The Baobab-tree of Senegal (_Adansonia digitata_) is supposed
to exceed almost any other in longevity. Adanson inferred that one which
he measured, and found to be thirty feet in diameter, had attained the
age of 5150 years. Having made an incision to a certain depth, he first
counted three hundred rings of annual growth, and observed what
thickness the tree had gained in that period. The average rate of growth
of younger trees, of the same species, was then ascertained, and the
calculation made according to a supposed mean rate of increase. De
Candolle considers it not improbable that the celebrated Taxodium of
Chapultepec, in Mexico (_Cupressus disticha_, Linn.), which is 117 feet
in circumference, may be still more aged.[581]
It is, however, impossible, until more data are collected respecting the
average intensity of the volcanic action, to make any thing like an
approximation to the age of a cone like Etna; because, in this case, the
successive envelopes of lava and scoriae are not continuous, like the
layers of wood in a tree, and afford us no definite measure of time.
Each conical envelope is made up of a great number of distinct
lava-currents and showers of sand and scoriae, differing in quantity, and
which may have been accumulated in unequal periods of time. Yet we
cannot fail to form the most exalted conception of the antiquity of this
mountain, when we consider that its base is about ninety 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.
There are no records within the historical era which lead to the opinion
that the altitude of Etna has materially varied within the last two
thousand years. Of the eighty most conspicuous minor cones which adorn
its flanks, only one of the largest, Monti Rossi, has been produced
within the times of authentic history. Even this hill, thrown up in the
year 1669, although 450 feet in height, only ranks as a cone of second
magnitude. Monte Minardo, near Bronte, rises, even now, to the height of
750 feet, although its base has been elevated by more modern lavas and
ejections. The dimensions of these larger cones appear to bear testimony
to _paroxysms_ of volcanic activity, after which we may conclude, from
analogy, that the fires of Etna remained dormant for many years--since
nearly a century of rest has sometimes followed a violent eruption in
the historical era. It must also be remembered, that of the small number
of eruptions which occur in a century, one only is estimated to issue
from the summit of Etna for every two that proceed from the sides. Nor
do all the lateral eruptions give rise to such cones as would be
reckoned amongst the smallest of the eighty hills above enumerated; some
of them produce merely insignificant monticules, which are soon
afterwards buried by showers of ashes.
How many years then must we not suppose to have been expended in the
formation of the eighty cones? It is difficult to imagine that a fourth
part of them have originated during the last thirty centuries. But if we
conjecture the whole of them to have been formed in twelve thousand
years, how inconsiderable an era would this portion of time constitute
in the history of the volcano! If we could strip off from Etna all the
lateral monticules now visible, together with the lavas and scoriae that
have been poured out from them, and from the highest crater, during the
period of their growth, the diminution of the entire mass would be
extremely slight: Etna might lose, perhaps, several miles in diameter at
its base, and some hundreds of feet in elevation; but it would still be
the loftiest of Sicilian mountains, studded with other cones, which
would be recalled, as it were, into existence by the removal of the
rocks under which they are now buried.
There seems nothing in the deep sections of the Val del Bove to indicate
that the lava-currents of remote periods were greater in volume than
those of modern times; and there are abundant proofs that the countless
beds of solid rock and scoriae were accumulated, as now, in succession.
On the grounds, therefore, already explained, we must infer that a mass
so many thousand feet in thickness must have required an immense series
of ages anterior to our historical periods for its growth; yet the whole
must be regarded as the product of a modern portion of the tertiary
epoch. Such, at least, is the conclusion that seems to follow from
geological data, which show that the oldest parts of the mountain, if
not of posterior date to the marine strata around its base, were at
least of coeval origin.
Some geologists contend, that the sudden elevation of large continents
from beneath the waters of the sea have again and again produced waves
which have swept over vast regions of the earth.[582] But it is clear
that no devastating wave has passed over the forest zone of Etna since
any of the lateral cones before mentioned were thrown up; for none of
these heaps of loose sand and scoriae could have resisted for a moment
the denuding action of a violent flood. To some, perhaps, it may appear
that hills of such incoherent materials cannot be of very great
antiquity, because the mere action of the atmosphere must, in the course
of several thousand years, have obliterated their original forms. But
there is no weight in this objection; for the older hills are covered
with trees and herbage, which protect them from waste; and, in regard to
the newer ones, such is the porosity of their component materials, that
the rain which falls upon them is instantly absorbed; and for the same
reason that the rivers on Etna have a subterranean course, there are
none descending the sides of the minor cones.
No sensible alteration has been observed in the form of these cones
since the earliest periods of which there are memorials; and there seems
no reason for anticipating that in the course of the next ten thousand
or twenty thousand years they will undergo any great alteration in their
appearance, unless they should be shattered by earthquakes or covered by
volcanic ejections.
In other parts of Europe, as in Auvergne and Velay, in France, similar
loose cones of scoriae, probably of as high antiquity as the whole mass
of Etna, stand uninjured at inferior elevations above the level of the
sea.