VOLCANIC DISTRICT OF NAPLES.
History of the volcanic eruptions in the district round
Naples--Early convulsions in the island of Ischia--Numerous cones
thrown up there--Lake Avernus--The Solfatara--Renewal of the
eruptions of Vesuvius, A.D. 79--Pliny's description of the
phenomena--His silence respecting the destruction of Herculaneum and
Pompeii--Subsequent history of Vesuvius--Lava discharged in Ischia
in 1302--Pause in the eruptions of Vesuvius--Monte Nuovo thrown
up--Uniformity of the volcanic operations of Vesuvius and Phlegraean
Fields in ancient and modern times.
I shall next give a sketch of the history of some of the volcanic vents
dispersed throughout the great regions before described, and consider
the composition and arrangement of their lavas and ejected matter. The
only volcanic region known to the ancients was that of the
Mediterranean; and even of this they have transmitted to us very
imperfect records relating to the eruptions of the three principal
districts, namely, that round Naples, that of Sicily and its isles, and
that of the Grecian Archipelago. By far the most connected series of
records throughout a long period relates to the first of these
provinces; and these cannot be too attentively considered, as much
historical information is indispensable in order to enable us to obtain
a clear view of the connection and alternate mode of action of the
different vents in a single volcanic group.
_Early convulsions in the Island of Ischia._---The Neapolitan volcanoes
extend from Vesuvius, through the Phlegraean Fields, to Procida and
Ischia, in a somewhat linear arrangement, ranging from the northeast to
the southwest, as will be seen in the annexed map of the volcanic
district of Naples (fig. 40). Within the space above limited, the
volcanic force is sometimes developed in single eruptions from a
considerable number of irregularly scattered points; but a great part of
its action has been confined to one principal and habitual vent,
Vesuvius or Somma. Before the Christian era, from the remotest periods
of which we have any tradition, this principal vent was in a state of
inactivity. But terrific convulsions then took place from time to time
in Ischia (Pithecusa), and seem to have extended to the neighboring isle
of Procida (Prochyta); for Strabo[490] mentions a story of Procida
having been torn asunder from Ischia; and Pliny[491] derives its name
from its having been poured forth by an eruption from Ischia.
The present circumference of Ischia along the water's edge is eighteen
miles, its length from west to east about five, and its breadth from
north to south three miles. Several Greek colonies which settled there
before the Christian era were compelled to abandon it in consequence of
the violence of the eruptions. First the Erythraeans, and afterwards the
Chalcidians, are mentioned as having been driven out by earthquakes and
igneous exhalations. A colony was afterwards established by Hiero, king
of Syracuse, about 380 years before the Christian era; but when they had
built a fortress, they were compelled by an eruption to fly, and never
again returned. Strabo tells us that Timaeus recorded a tradition, that,
a little before his time, Epomeus, the principal mountain in the centre
of the island, vomited fire during great earthquakes; that the land
between it and the coast had ejected much fiery matter, which flowed
into the sea, and that the sea receded for the distance of three stadia,
and then returning, overflowed the island. This eruption is supposed by
some to have been that which formed the crater of Monte Corvo on one of
the higher flanks of Epomeo, above Foria, the lava-current of which may
still be traced, by aid of the scoriae on its surface, from the crater to
the sea.
[Illustration: Fig. 40.
VOLCANIC DISTRICT OF NAPLES.
A. Astroni. B. Monte Barbaro. M. Monte Nuovo. S. The Solfatara.]
To one of the subsequent eruptions in the lower parts of the isle, which
caused the expulsion of the first Greek colony, Monte Rotaro has been
attributed, and it bears every mark of recent origin. The cone, which I
examined in 1828, is remarkably perfect, and has a crater on its summit
precisely resembling that of Monte Nuovo near Naples; but the hill is
larger, and resembles some of the more considerable cones of single
eruption near Clermont in Auvergne, and, like some of them, it has given
vent to a lava-stream at its base, instead of its summit. A small ravine
swept out by a torrent exposes the structure of the cone, which is
composed of innumerable inclined and slightly undulating layers of
pumice, scoriae, white lapilli, and enormous angular blocks of trachyte.
These last have evidently been thrown out by violent explosions, like
those which in 1822 launched from Vesuvius a mass of augitic lava, of
many tons' weight, to the distance of three miles, which fell in the
garden of Prince Ottajano. The cone of Rotaro is covered with the
arbutus, and other beautiful evergreens. Such is the strength of the
virgin soil, that the shrubs have become almost arborescent; and the
growth of some of the smaller wild plants has been so vigorous, that
botanists have scarcely been able to recognize the species.
The eruption which dislodged the Syracusan colony is supposed to have
given rise to that mighty current which forms the promontory of Zaro and
Caruso. The surface of these lavas is still very arid and bristling, and
is covered with black scoriae; so that it is not without great labor that
human industry has redeemed some small spots, and converted them into
vineyards. Upon the produce of these vineyards the population of the
island is almost entirely supported. It amounted when I was there, in
1828, to about twenty-five thousand, and was on the increase.
[Illustration: Fig 41.
Part of Ischia seen from the West.
_a._ Monte Epomeo or San Niccola.
_b._ Monte Vico.
_c._ Another of the minor cones with a crater.[492]]
From the date of the great eruption last alluded to, down to our own
time, Ischia has enjoyed tranquillity, with the exception of one
emission of lava hereafter to be described, which, although it
occasioned much local damage, does not appear to have devastated the
whole country, in the manner of more ancient explosions. There are, upon
the whole, on different parts of Epomeo, or scattered through the lower
tracts of Ischia, twelve considerable volcanic cones which have been
thrown up since the island was raised above the surface of the deep; and
many streams of lava may have flowed, like that of "Arso" in 1302,
without cones having been produced; so that this island may, for ages
before the period of the remotest traditions, have served as a
safety-valve to the whole Terra di Lavoro, while the fires of Vesuvius
were dormant.
_Lake Avernus._--It seems also clear that Avernus, a circular lake near
Puzzuoli, about half a mile in diameter, which is now a salubrious and
cheerful spot, once exhaled mephitic vapors, such as are often emitted
by craters after eruptions. There is no reason for discrediting the
account of Lucretius, that birds could not fly over it without being
stifled, although they may now frequent it uninjured.[493] There must
have been a time when this crater was in action; and for many centuries
afterwards it may have deserved the appellation of "atri jauna Ditis,"
emitting, perhaps, gases as destructive of animal life as those
suffocating vapors given out by Lake Quilotoa, in Quito, in 1797, by
which whole herds of cattle on its shores were killed,[494] or as those
deleterious emanations which annihilated all the cattle in the island of
Lancerote, one of the Canaries, in 1730.[495] Bory St. Vincent mentions,
that in the same isle birds fell lifeless to the ground; and Sir William
Hamilton informs us that he picked up dead birds on Vesuvius during an
eruption.
_Solfatara._--The Solfatara, near Puzzuoli, which may be considered as a
nearly extinguished crater, appears, by the accounts of Strabo and
others, to have been before the Christian era in very much the same
state as at present, giving vent continually to aqueous vapor, together
with sulphureous and muriatic acid gases, like those evolved by
Vesuvius.
_Ancient history of Vesuvius._--Such, then, were the points where the
subterranean fires obtained vent, from the earliest period to which
tradition reaches back, down to the first century of the Christian era;
but we then arrive at a crisis in the volcanic action of this
district--one of the most interesting events witnessed by man during the
brief period throughout which he has observed the physical changes on
the earth's surface. From the first colonization of Southern Italy by
the Greeks, Vesuvius afforded no other indications of its volcanic
character than such as the naturalist might infer, from the analogy of
its structure to other volcanoes. These were recognized by Strabo, but
Pliny did not include the mountain in his list of active vents. The
ancient cone was of a very regular form, terminating not as at present
in two peaks, but with a summit which presented, when seen from a
distance, the even outline of an abruptly truncated cone. On the summit,
as we learn from Plutarch, there was a crater with steep cliffs, and
having its interior overgrown with wild vines, and with a sterile plain
at the bottom. On the exterior, the flanks of the mountain were clothed
with fertile fields richly cultivated, and at its base were the populous
cities of Herculaneum and Pompeii. But the scene of repose was at length
doomed to cease, and the volcanic fire was recalled to the main channel,
which at some former unknown period had given passage to repeated
streams of melted lava, sand, and scoriae.
_Renewal of its eruptions._--The first symptom of the revival of the
energies of this volcano was the occurrence of an earthquake in the year
63 after Christ, which did considerable injury to the cities in its
vicinity. From that time to the year 79 slight shocks were frequent; and
in the month of August of that year they became more numerous and
violent, till they ended at length in an eruption. The elder Pliny, who
commanded the Roman fleet, was then stationed at Misenum; and in his
anxiety to obtain a near view of the phenomena, he lost his life, being
suffocated by sulphureous vapors. His nephew, the younger Pliny,
remained at Misenum, and has given us, in his Letters, a lively
description of the awful scene. A dense column of vapor was first seen
rising vertically from Vesuvius, and then spreading itself out
laterally, so that its upper portion resembled the head, and its lower
the trunk of the pine, which characterizes the Italian landscape. This
black cloud was pierced occasionally by flashes of fire, as vivid as
lightning, succeeded by darkness more profound than night. Ashes fell
even upon the ships at Misenum, and caused a shoal in one part of the
sea--the ground rocked, and the sea receded from the shores, so that
many marine animals were seen on the dry sand. The appearances above
described agree perfectly with those witnessed in more recent eruptions,
especially those of Monte Nuovo, in 1538, and of Vesuvius in 1822.
The younger Pliny, although giving a circumstantial detail of so many
physical facts, and describing the eruption and earthquake, and the
shower of ashes which fell at Stabiae, makes no allusion to the sudden
overwhelming of two large and populous cities, Herculaneum and Pompeii.
In explanation of this omission, it has been suggested that his chief
object was simply to give Tacitus a full account of the particulars of
his uncle's death. It is worthy, however, of remark, that had the buried
cities never been discovered, the accounts transmitted to us of their
tragical end might well have been discredited by the majority, so vague
and general are the narratives, or so long subsequent to the event.
Tacitus, the friend and contemporary of Pliny, when adverting in general
terms to the convulsions, says merely that "cities were consumed or
buried."[496]
Suetonius, although he alludes to the eruption incidentally, is silent
as to the cities. They are mentioned by Martial, in an epigram, as
immersed in cinders; but the first historian who alludes to them by name
is Dion Cassius,[497] who flourished about a century and a half after
Pliny. He appears to have derived his information from the traditions of
the inhabitants, and to have recorded, without discrimination, all the
facts and fables which he could collect. He tells us, "that during the
eruption a multitude of men of superhuman stature, resembling giants,
appeared, sometimes on the mountain, and sometimes in the environs--that
stones and smoke were thrown out, the sun was hidden, and then the
giants seemed to rise again, while the sounds of trumpets were heard,
&c., &c.; and finally," he relates, "two entire cities, Herculaneum and
Pompeii, were buried under showers of ashes, while all the people were
sitting in the theatre." That many of these circumstances were invented,
would have been obvious, even without the aid of Pliny's letters; and
the examination of Herculaneum and Pompeii enables us to prove, that
none of the people were destroyed in the theatres, and indeed that there
were very few of the inhabitants who did not escape from both cities.
Yet some lives were lost, and there was ample foundation for the tale in
its most essential particulars.
It does not appear that in the year 79 any lava flowed from Vesuvius;
the ejected substances, perhaps, consisted entirely of lapilli, sand,
and fragments of older lava, as when Monte Nuovo was thrown up in 1538.
The first era at which we have authentic accounts of the flowing of a
stream of lava, is the year 1036, which is the seventh eruption from the
revival of the fires of the volcano. A few years afterwards, in 1049,
another eruption is mentioned, and another in 1138 (or 1139), after
which a great pause ensued of 168 years. During this long interval of
repose, two minor vents opened at distant points. First, it is on
tradition that an eruption took place from the Solfatara, in the year
1198, during the reign of Frederick II., Emperor of Germany; and
although no circumstantial detail of the event has reached us from those
dark ages, we may receive the fact without hesitation.[498] Nothing
more, however, can be attributed to this eruption, as Mr. Scrope
observes, than the discharge of a light and scoriform trachytic lava, of
recent aspect, resting upon the strata of loose tuff which covers the
principal mass of trachyte.[499]
_Volcanic eruption in Ischia_, 1302.--The other occurrence is well
authenticated--the eruption, in the year 1302, of a lava-stream from a
new vent on the southeast end of the Island of Ischia. During part of
1301, earthquakes had succeeded one another with fearful rapidity; and
they terminated at last with the discharge of a lava-stream from a point
named the Campo del Arso, not far from the town of Ischia. This lava ran
quite down to the sea--a distance of about two miles; in color it varies
from iron-gray to reddish black, and is remarkable for the glassy
felspar which it contains. Its surface is almost as sterile, after a
period of five centuries, as if it had cooled down yesterday. A few
scantlings of wild thyme, and two or three other dwarfish plants, alone
appear in the interstices of the scoriae, while the Vesuvian lava of 1767
is already covered with a luxuriant vegetation. Pontanus, whose
country-house was burnt and overwhelmed, describes the dreadful scene as
having lasted two months.[500] Many houses were swallowed up, and a
partial emigration of the inhabitants followed. This eruption produced
no cone, but only a slight depression, hardly deserving the name of a
crater, where heaps of black and red scoriae lie scattered around. Until
this eruption, Ischia is generally believed to have enjoyed an interval
of rest for about seventeen centuries; but Julius Obsequens,[501] who
flourished A. D. 214, refers to some volcanic convulsions in the year
662 after the building of Rome (91 B. C.) As Pliny, who lived a century
before Obsequens, does not enumerate this among other volcanic
eruptions, the statement of the latter author is supposed to have been
erroneous; but it would be more consistent, for reasons before stated,
to disregard the silence of Pliny, and to conclude, that some kind of
subterranean commotion, probably of no great violence, happened at the
period alluded to.
_History of Vesuvius after_ 1138.--To return to Vesuvius:--the next
eruption occurred in 1306; between which era and 1631 there was only one
other (in 1500), and that a slight one. It has been remarked, that
throughout this period Etna was in a state of such unusual activity, as
to lend countenance to the idea that the great Sicilian volcano may
sometimes serve as a channel of discharge to elastic fluids and lava
that would otherwise rise to the vents in Campania.
_Formation of Monte Nuovo_, 1538.--The great pause was also marked by a
memorable event in the Phlegraean Fields--the sudden formation of a new
mountain in 1538, of which we have received authentic accounts from
contemporary writers.
[Illustration: Fig. 42.
Monte Nuovo, formed in the Bay of Baiae, Sept. 29th, 1538.
1. Cone of Monte Nuovo.
2. Brim of crater of ditto.
3. Thermal spring, called Baths of Nero, or Stufe di Tritoli.
]
The height of this mountain, called ever since Monte Nuovo, has been
determined by the Italian mineralogist Pini, to be 440 English feet
above the level of the bay; its base is about eight thousand feet, or
more than a mile and a half in circumference. According to Pini, the
depth of the crater is 421 English feet from the summit of the hill, so
that its bottom is only nineteen feet above the level of the sea. The
cone is declared, by the best authorities, to stand partly on the site
of the Lucrine Lake (4, fig. 43),[502] which was nothing more than the
crater of a pre-existent volcano, and was almost entirely filled during
the explosion of 1538. Nothing now remains but a shallow pool, separated
from the sea by an elevated beach, raised artificially.
[Illustration: Fig. 43.
The Phlegraean Fields.
1. Monte Nuovo.
2. Monte Barbaro.
3. Lake Avernus.
4. Lucrine Lake.
5. The Solfatara.
6. Puzzuoli.
7. Bay of Baiae.
]
Sir William Hamilton has given us two original letters describing this
eruption. The first, by Falconi, dated 1538, contains the following
passages.[503] "It is now two years since there have been frequent
earthquakes at Puzzuoli, Naples, and the neighboring parts. On the day
and in the night before the eruption (of Monte Nuovo), above twenty
shocks, great and small, were felt. The eruption began on the 29th of
September, 1538. It was on a Sunday, about one o'clock in the night,
when flames of fire were seen between the hot baths and Tripergola. In a
short time the fire increased to such a degree, that it burst open the
earth in this place, and threw up so great a quantity of ashes and
pumice-stones, mixed with water, as covered the whole country. The next
morning (after the formation of Monte Nuovo) the poor inhabitants of
Puzzuoli quitted their habitations in terror, covered with the muddy and
black shower which continued the whole day in that country--flying from
death, but with death painted in their countenances. Some with their
children in their arms, some with sacks full of their goods; others
leading an ass, loaded with their frightened family, towards Naples;
others carrying quantities of birds, of various sorts, that had fallen
dead at the beginning of the eruption; others, again, with fish which
they had found, and which were to be met with in plenty on the shore,
the sea having left them dry for a considerable time. I accompanied
Signor Moramaldo to behold the wonderful effects of the eruption. The
sea had retired on the side of Baiae, abandoning a considerable tract,
and the shore appeared almost entirely dry, from the quantity of ashes
and broken pumice-stones thrown up by the eruption. I saw two springs in
the newly discovered ruins; one before the house that was the queen's,
of hot and salt water," &c.
So far Falconi: the other account is by Pietro Giacomo di Toledo, which
begins thus:--"It is now two years since this province of Campagna has
been afflicted with earthquakes, the country about Puzzuoli much more so
than any other parts; but the 27th and the 28th of the month of
September last, the earthquakes did not cease day or night in the town
of Puzzuoli: that plain which lies between Lake Avernus, the Monte
Barbaro, and the sea, was _raised a little_, and many cracks were made
in it, from some of which issued water; at the same time the sea,
immediately joining the plain, _dried up about two hundred paces_, so
that the fish were left on the sand a prey to the inhabitants of
Puzzuoli. At last, on the 29th of the same month, about two o'clock in
the night, the earth opened near the lake, and discovered a horrid
mouth, from which were vomited furiously smoke, fire, stones, and mud,
composed of ashes, making at the time of its opening a noise like the
loudest thunder. The stones which followed were by the flames converted
to pumice, and some of these were _larger than an ox_. The stones went
about as high as a cross-bow can carry, and then fell down, sometimes on
the edge, and sometimes into the mouth itself. The mud was of the color
of ashes, and at first very liquid, then by degrees less so, and in such
quantities, that in less than twelve hours, with the help of the
above-mentioned stones, a mountain was raised of 1000 paces in height.
Not only Puzzuoli and the neighboring country was full of this mud, but
the city of Naples also; so that many of its palaces were defaced by it.
Now this eruption lasted two nights and two days without intermission,
though, it is true, not always with the same force; the third day the
eruption ceased, and I went up with many people to the top of the new
hill, and saw down into its mouth, which was a round cavity about a
quarter of a mile in circumference, in the middle of which, the stones
which had fallen were boiling up, just as a caldron of water boils on
the fire. The fourth day it began to throw up again, and the seventh
much more, but still with less violence than the first night. At this
time many persons who were on the hill were knocked down by the stones
and killed, or smothered with the smoke. In the day the smoke still
continues, and you often see fire in the midst of it in the
nighttime."[504]
It will be seen that both these accounts, written immediately after the
birth of Monte Nuovo, agree in stating that the sea retired; and one
mentions that its bottom was upraised; but they attribute the origin of
the new hill exclusively to the jets of mud, showers of scoriae, and
large fragments of rock, cast out from a central orifice, for several
days and nights. Baron Von Buch, however, in his excellent work on the
Canary Islands, and volcanic phenomena in general, has declared his
opinion that the cone and crater of Monte Nuovo were formed, not in the
manner above described, but by the upheaval of solid beds of white tuff,
which were previously horizontal, but which were pushed up in 1538, so
as to dip away in all directions from the centre, with the same
inclination as the sloping surface of the cone itself. "It is an error,"
he says, "to imagine that this hill was formed by eruption, or by the
ejection of pumice, scoriae, and other incoherent matter; for the solid
beds of upraised tuff are visible all round the crater, and it is merely
the superficial covering of the cone which is made up of ejected
scoriae."[505]
In confirmation of this view, M. Dufranoy has cited a passage from the
works of Porzio, a celebrated physician of that period, to prove that in
1538 the ground where Monte Nuovo stands was pushed up in the form of a
great bubble or blister, which on bursting, gave origin to the present
deep crater. Porzio, says, "that after two days and nights of violent
earthquakes, the sea retired for nearly 200 yards; so that the
inhabitants could collect great numbers of fish on this part of the
shore, and see some springs of fresh water which rose up there. At
length, on the third day of the calends of October (September 29), they
saw a large tract of ground intervening between the foot of Monte
Barbaro, and part of the sea, near the Lake Avernus, rise, and suddenly
assume the form of an incipient hill; and at two o'clock at night, this
heap of earth, opening as it were its mouth, vomited, with a loud noise,
flames, pumice-stones, and ashes."[506]
So late as the year 1846 a fourth manuscript (written immediately after
the eruption) was discovered and published in Germany. It was written in
1538 by Francesco del Nero,[507] who mentions the drying up of the bed
of the sea near Puzzuoli, which enabled the inhabitants of the town to
carry off loads of fish. About eight o'clock in the morning of the 29th
September, the earth sunk down about 14 feet in that place where the
volcanic orifice now appears, and there issued forth a small stream of
water, at first cold, and afterwards tepid. At noon, on the same day,
the earth began to swell up in the same spot where it had sunk down 14
feet, so as to form a hill. About this time fire issued forth, and gave
rise to the great gulf, "with such a force, noise, and shining light,
that I, who was standing in my garden, was seized with terror. Forty
minutes afterwards, although unwell, I got upon a neighboring height,
from which I saw all that took place, and by my troth it was a splendid
fire, that threw up for a long time much earth and many stones, which
fell back again all round the gulf, in a semicircle of from one to three
bow-shots in diameter, and, filling up part of the sea, formed a hill
nearly of the height of Monte Morello. Masses of earth and stones, as
large as an ox, were shot up from the fiery gulf into the air, to a
height which I estimate at a mile and a half. When they descended, some
were dry, others in a soft muddy state." He concludes by alluding again
to the sinking of the ground, and the elevation of it which followed,
and says that to him it was inconceivable how such a mass of stones and
ashes could have been poured forth from the gulf. He also refers to the
account which Porzio was to draw up for the Viceroy.
On comparing these four accounts, recorded by eye-witnesses, there
appears to be no real discrepancy between them. It seems clear that the
ground first sunk down 14 feet on the site of the future volcano, and
after having subsided it was again propelled upwards by the lava mingled
with steam and gases, which were about to burst forth. Jets of red-hot
lava, fragments of fractured rock, and occasionally mud composed of a
mixture of pumice, tuff, and sea-water, were hurled into the air. Some
of the blocks of stone were very large, leading us to infer that the
ground which sank and rose again was much shattered and torn to pieces
by the elastic vapors. The whole hill was not formed at once, but by an
intermittent action extending over a week or more. It seems that the
chasm opened between Tripergola and the baths in its suburbs, and that
the ejected materials fell and buried that small town. A considerable
part, however, of the hill was formed in less than twenty-four hours,
and in the same manner as on a smaller scale the mud cones of the air
volcanoes are produced, with a cavity in the middle. There is no
difficulty in conceiving that the pumiceous mud, if so thrown out, may
have set into a kind of stone on drying, just as some cements, composed
of volcanic ashes, are known to consolidate with facility.
I am informed that Baron Von Buch discovered some marine shells of
existing species, such as occur fossil in the tuff of the neighborhood,
in beds exposed low down in the walls of the crater of Monte Nuovo.
These may have been ejected in the mud mixed with sea-water which was
cast out of the boiling gulf; or, as Signor Arcangelo Scacchi has
suggested,[508] they may have been derived from the older tuff, which
contains marine shells of recent species. The same observer remarks that
Porzio's account upon the whole corroborates the doctrine of the cone
having been formed by eruption, in proof of which he cites the
following passage:--"But what was truly astonishing, a hill of
pumice-stones and ashes was heaped up round the gulf to the height of a
mile in a single night."[509] Signor Scacchi also adds that the ancient
temple of Apollo, now at the foot of Monte Nuovo, and the walls of which
still retain their perfect perpendicularity, could not possibly have
maintained that position had the cone of Monte Nuovo really been the
result of upheaval.
Tripergola was much frequented as a watering-place, and contained a
hospital for those who resorted there for the benefit of the thermal
springs; and it appears that there were no fewer than three inns in the
principal street. Had Porzio stated that any of these buildings, or the
ruins of them, were seen by himself or others raised up above the plain,
a short time before the first eruption, so as to stand on the summit or
slope of a newly-raised hillock, we might have been compelled, by so
circumstantial a narrative, to adopt M. Dufranoy's interpretation.
But in the absence of such evidence, we must appeal to the crater
itself, where we behold a section of the whole mountain, without being
able to detect any original nucleus of upheaved rock distinct from the
rest; on the contrary, the whole mass is similar throughout in
composition, and the cone very symmetrical in form; nor are there any
clefts, such as might be looked for, as the effect of the sudden upthrow
of stony masses. M. C. Prevost has well remarked, that if beds of solid
and non-elastic materials had yielded to a violent pressure directed
from below upward, we should find not simply a deep empty cavity, but an
irregular opening, where many rents converged; and these rents would be
now seen breaking through the walls of the crater, widening as they
approach the centre. (See Fig. 44, _a_, _b_.)[510] Not a single fissure
of this kind is observable in the interior of Monte Nuovo, where the
walls of the crater are continuous and entire; nor are there any dikes
implying that rents had existed, which were afterwards filled with lava
or other matter.
[Illustration: Fig. 44.]
It has moreover been often urged by Von Buch, De Beaumont, and others,
who ascribe the conical form of volcanoes chiefly to upheaval from
below, that in such mountains there are a great number of deep rents and
ravines, which diverge on all sides like the spokes of a wheel, from
near the central axis to the circumference or base of the cone, as in
the case of Palma, Cantal, and Teneriffe. Yet the entire absence of such
divergent fissures or ravines, in such cases as Monte Nuovo, Somma, or
Etna, is passed by unnoticed, and appears to have raised in their minds
no objection to their favorite theory.
It is, indeed, admitted by M. Dufranoy that there are some facts which
it is very difficult to reconcile with his own view of Porzio's record.
Thus, for example, there are certain Roman monuments at the base of
Monte Nuovo, and on the borders of Lake Avernus, such as the temples of
Apollo (before mentioned) and Pluto, which do not seem to have suffered
in the least degree by the supposed upheaval. "The walls which still
exist have preserved their vertical position, and the vaults are in the
same state as other monuments on the shores of the Bay of Baiae. The long
gallery which led to the Sibyl's Cave, on the other side of Lake
Avernus, has in like manner escaped injury, the roof of the gallery
remaining perfectly horizontal, the only change being that the soil of
the chamber in which the Sibyl gave out her oracles is now covered by a
few inches of water, which merely indicates a slight alteration in the
level of Lake Avernus."[511] On the supposition, then, that pre-existing
beds of pumiceous tuff were upraised in 1538, so as to form Monte Nuovo,
it is acknowledged that the perfectly undisturbed state of the
contiguous soil on which these ancient monuments stand, is very
different from what might have been expected.
Mr. Darwin, in his "Volcanic Islands," has described several crateriform
hills in the Galapagos Archipelago as composed of tuff which has
evidently flowed like mud, and yet on consolidating has preserved an
inclination of twenty and even thirty degrees. The tuff does not fold in
continuous sheets round the hills as would have happened if they had
been formed by the upheaval of horizontal layers. The author describes
the composition of the tuff as very similar to that of Monte Nuovo, and
the high angles at which the beds slope, both those which have flowed
and those which have fallen in the form of ashes, entirely removes the
difficulty supposed by M. Dufranoy to exist in regard to the slope of
Monte Nuovo, where it exceeds an angle of 18 degrees. to 20
degrees.[512] Mr. Dana, also, in his account of the Sandwich
Islands,[513] shows that in the "cinder cones" of that region, the
strata have an original inclination of between 35 degrees and 40
degrees, while in the "tufa cones" formed near the sea, the beds slope
at about an angle of 30 degrees. The same naturalist also observed in
the Samoan or Navigator Islands in Polynesia, that fragments of fresh
coral had been thrown up together with volcanic matter to the height of
200 feet above the level of the sea in cones of tufa.[514]
I shall again revert to the doctrine of the origin of volcanic cones by
upheaval, when speaking of Vesuvius, Etna, and Santorin, and shall now
merely add, that, in 1538, the whole coast, from Monte Nuovo to beyond
Puzzuoli, was upraised to the height of many feet above the bed of the
Mediterranean, and has since retained the greater part of the elevation
then acquired. The proofs of these remarkable changes of level will be
considered at length when the phenomena of the temple of Serapis are
described.[515]
_Volcanoes of the Phlegraean Fields._--Immediately adjoining Monte Nuovo
is the larger volcanic cone of Monte Barbaro (2, fig. 43, p. 367), the
"Gaurus inanis" of Juvenal--an appellation given to it probably from its
deep circular crater, which is about a mile in diameter. Large as is
this cone, it was probably produced by a single eruption; and it does
not, perhaps, exceed in magnitude some of the largest of those formed in
Ischia, within the historical era. It is composed chiefly of indurated
tufa like Monte Nuovo, stratified conformably to its conical surface.
This hill was once very celebrated for its wines, and is still covered
with vineyards; but when the vine is not in leaf it has a sterile
appearance, and, late in the year, when seen from the beautiful Bay of
Baiae, it often contrasts so strongly in verdure with Monte Nuovo, which
is always clothed with arbutus, myrtle, and other wild evergreens, that
a stranger might well imagine the cone of older date to be that thrown
up in the sixteenth century.[516]
There is nothing, indeed, so calculated to instruct the geologist as the
striking manner in which the recent volcanic hills of Ischia, and that
now under consideration, blend with the surrounding landscape. Nothing
seems wanting or redundant; every part of the picture is in such perfect
harmony with the rest, that the whole has the appearance of having been
called into existence by a single effort of creative power. Yet what
other result could we have anticipated if nature has ever been governed
by the same laws? Each new mountain thrown up--each new tract of land
raised or depressed by earthquakes--should be in perfect accordance with
those previously formed, if the entire configuration of the surface has
been due to a long series of similar disturbances. Were it true that the
greater part of the dry land originated simultaneously in its present
state, at some era of paroxysmal convulsion, and that additions were
afterwards made slowly and successively during a period of comparative
repose; then, indeed, there might be reason to expect a strong line of
demarcation between the signs of the ancient and modern changes. But the
very continuity of the plan, and the perfect identity of the causes, are
to many a source of deception; since by producing a unity of effect,
they lead them to exaggerate the energy of the agents which operated in
the earlier ages. In the absence of all historical information, they are
as unable to separate the dates of the origin of different portions of
our continents, as the stranger is to determine, by their physical
features alone, the distinct ages of Monte Nuovo, Monte Barbara,
Astroni, and the Solfatara.
The vast scale and violence of the volcanic operations in Campania, in
the olden time, has been a theme of declamation, and has been contrasted
with the comparative state of quiescence of this delightful region in
the modern era. Instead of inferring, from analogy, that the ancient
Vesuvius was always at rest when the craters of the Phlegraean Fields
were burning--that each cone rose in succession,--and that many years,
and often centuries, of repose intervened between different
eruptions,--geologists seem to have generally conjectured that the whole
group sprung up from the ground at once, like the soldiers of Cadmus
when he sowed the dragon's teeth. As well might they endeavor to
persuade us that on these Phlegraean Fields, as the poets feigned, the
giants warred with Jove, ere yet the puny race of mortals were in being.
_Modern eruptions of Vesuvius._--For nearly a century after the birth of
Monte Nuovo, Vesuvius continued in a state of tranquillity. There had
been no violent eruption for 492 years; and it appears that the crater
was then exactly in the condition of the present extinct volcano of
Astroni, near Naples. Bracini, who visited Vesuvius not long before the
eruption of 1631, gives the following interesting description of the
interior:--"The crater was five miles in circumference, and about a
thousand paces deep: its sides were covered with brushwood, and at the
bottom there was a plain on which cattle grazed. In the woody parts wild
boars frequently harbored. In one part of the plain, covered with ashes,
were three small pools, one filled with hot and bitter water, another
salter than the sea, and a third hot, but tasteless."[517] But at length
these forests and grassy plains were consumed, being suddenly blown into
the air, and their ashes scattered to the winds. In December, 1631,
seven streams of lava poured at once from the crater, and overflowed
several villages, on the flanks and at the foot of the mountain. Resina,
partly built over the ancient site of Herculaneum, was consumed by the
fiery torrent. Great floods of mud were as destructive as the lava
itself,--no uncommon occurrence during these catastrophes; for such is
the violence of rains produced by the evolutions of aqueous vapor, that
torrents of water descend the cone, and becoming charged with impalpable
volcanic dust, and rolling along loose ashes, acquire sufficient
consistency to deserve their ordinary appellation of "aqueous lavas."
A brief period of repose ensued, which lasted only until the year 1666,
from which time to the present there has been a constant series of
eruptions, with rarely an interval of rest exceeding ten years. During
these three centuries, no irregular volcanic agency has convulsed other
points in this district. Brieslak remarked, that such irregular
convulsions had occurred in the Bay of Naples in every second century;
as, for example, the eruption of the Solfatara, in the twelfth; of the
lava of Arso, in Ischia, in the fourteenth; and of Monte Nuovo in the
sixteenth; but the eighteenth has formed an exception to this rule, and
this seems accounted for by the unprecedented number of eruptions of
Vesuvius during that period; whereas, when the new vents opened, there
had always been, as we have seen, a long intermittence of activity in
the principal volcano.