the shape of Vesuvius to have undergone an entire change, and if
these assertions appear to be confirmed by picturesque views sketched
at Naples, the cause of the error consists in the outlines of the
margin of the crater having been confounded with those of the cones of
eruption accidentally formed in the middle of the crater on its floor
or bottom which has been upheaved by vapours. Such a cone of eruption,
consisting of loosely heaped-up rapilli and scoriæ, had in the course
of the years 1816-1818 gradually risen so as to be seen above the
south-eastern margin of the crater; and the eruption of the month of
February 1822 augmented it so much, that it even became from 100 to 110
(about 107 to 117 English) feet higher than the north-western margin
of the crater (the Rocca del Palo). This remarkable cone, which it
had become customary in Naples to regard as the true summit of the
mountain, fell in, with a dreadful noise, in the last eruption, on the
night of the 22d of October (1822): so that the floor of the crater,
which had been constantly accessible since 1811, is now 750 (almost
800 English) feet lower than the northern, and 200 (213 English)
feet lower than the southern edge of the volcano. Variations in the
form and relative position of the cones of eruption,--the openings
of which ought not to be confounded, as they often are, with the
crater of the volcano itself,--give to Vesuvius at different epochs
a different appearance, which would enable a person well acquainted
with the history of the volcano, on a mere inspection of Hackert’s
paintings in the palace of Portici, to tell from the outlines of the
summit, according as the northern or the southern side of the mountain
is represented as the highest, in what year the artist had taken the
sketch from which the picture was made.
In the last eruption, in the night of the 23d to the 24th of October,
twenty-four hours after the falling in of the great cone of scoriæ
which has been mentioned, and when the small but numerous currents of
lava had already flowed off, the fiery eruption of ashes and rapilli
commenced: it continued without intermission for twelve days, but was
greatest in the first four days. During this period the detonations in
the interior of the volcano were so violent that the mere concussion of
the air, (for no earthquake movement was perceived), rent the ceilings
of the rooms in the palace of Portici. In the neighbouring villages of
Resina, Torre del Greco, Torre del Annunziata, and Bosche Tre Case,
a remarkable phenomenon was witnessed. Throughout the whole of that
part of the country the air was so filled with ashes as to cause in
the middle of the day profound darkness, lasting for several hours:
lanterns were carried in the streets, as has so often been done at
Quito during the eruptions of Pichincha. The flight of the inhabitants
had never been more general: lava currents are regarded by those who
dwell near Vesuvius with less dread than an eruption of ashes, a
phenomenon which had never been known to such a degree in modern times;
and the obscure tradition of the manner in which the destruction of
Herculaneum, Pompeii, and Stabiæ took place, filled the imaginations of
men with appalling images.
The hot aqueous vapours which rose from the crater during the eruption
and spread themselves in the atmosphere, formed, in cooling, a dense
cloud, surrounding the column of fire and ashes, which rose to a height
of between nine and ten thousand feet. So sudden a condensation of
vapour, and even, as Gay-Lussac has shewn, the formation of the cloud
itself, augmented the electric tension. Flashes of forked lightning,
issuing from the column of ashes, darted in every direction; and the
rolling thunders were distinctly heard, and distinguished from the
sounds which proceeded from the interior of the volcano. In no other
eruption had the play of the electric forces formed so striking a
feature.
On the morning of the 26th of October, a surprising rumour prevailed,
to the effect that a torrent of boiling water was gushing from the
crater, and pouring down the slope of the cone of ashes. The learned
and zealous observer of the volcano, Monticelli, soon discovered that
this erroneous rumour had arisen from an optical illusion. The supposed
torrent of water was in reality a flow of dry ashes, which, being as
loose and moveable as shifting sands, issued in large quantities from
a crevice in the upper margin of the crater. The cultivated fields
had suffered much from a long-continued drought which had preceded
the eruption; towards its close the “volcanic thunder-storm” which
has been described produced an exceedingly violent and abundant fall
of rain. This phenomenon is associated in all climates with the close
of a volcanic eruption. As during the eruption the cone of ashes is
generally enveloped in cloud, and as it is in its immediate vicinity
that the rain is most violent, torrents of mud are seen to descend
from it in all directions, which the terrified husbandman imagines to
consist of waters which have risen from the interior of the volcano
and overflowed the crater; while geologists have erroneously thought
they recognised in them either sea-water or muddy products of the
volcano, “Eruptions boueuses,” or, in the language of some old French
systematists, products of an igneo-aqueous liquefaction.
Where, as is generally the case in the Andes, the summit of the
volcano rises into the region of perpetual snow, (even attaining,
in some cases, an elevation twice as great as that of Etna), the
melting of the snows renders such inundations as have been described
far more abundant and disastrous. The phenomena in question are
meteorologically connected with the eruptions of volcanos, and are
variously modified by the height of the mountain, the dimensions of
that part of it which is always covered with snow, and the extent and
degree to which the sides of the cone of cinders become heated; but
they are not to be regarded as volcanic phenomena properly so called.
Vast cavities also often exist on the slope or at the foot of volcanos
which, communicating through many channels with the mountain torrents,
form large subterranean lakes or reservoirs of water. When earthquake
shocks, which, in the Andes, usually precede all igneous eruptions,
convulse the entire mass of the volcano, these subterranean reservoirs
are opened, and there issue from them water, fishes, and tufaceous mud.
This is the singular phenomenon which brings to light an otherwise
unknown fish, the Pimelodes Cyclopum, called by the inhabitants of the
highlands of Quito “Preñadilla,” and which I described soon after my
return. When, on the night of the 19th of June, 1698, the summit of a
mountain situated to the north of Chimborazo, the Carguairazo, above
19000 English feet high, fell in, the country for nearly thirty English
geographical square miles round was covered with mud and fishes; and
seven years earlier a putrid fever, in the town of Ibarra, was ascribed
to a similar eruption of fish from the volcano of Imbaburu.
I recall these facts, because they throw some light on the difference
between the eruption of dry ashes and miry inundations of tufa and
trass, carrying with them wood, charcoal, and shells. The quantity
of ashes emitted by Vesuvius in the recent eruption, like every
thing connected with volcanos and other great natural phenomena of a
character to excite terror, has been exceedingly exaggerated in the
public papers; and two Neapolitan chemists, Vicenzo Pepe and Giuseppe
di Nobili, notwithstanding the statements of Monticelli and Covelli to
the contrary, even describe the ashes as containing silver and gold.
According to the results of my researches and inquiries, the thickness
of the bed of ashes formed by the twelve days’ shower was but little
above three feet, towards Bosche Tre Case, on the slope of the cone
where rapilli were mingled with them; and in the plain, from 15-1/2 to
19 inches at the utmost. Such measurements ought not to be taken in
places where the ashes have been heaped up by the action of wind, like
drifted snow or sand, or have accumulated from being carried thither by
water. The times are passed for seeking only the marvellous in volcanic
phenomena, in the manner of the ancients among whom Ctesias made the
ashes of Etna to be conveyed as far as the Indian peninsula. There are
in Mexico veins of gold and silver in trachytic porphyry; but in the
ashes of Vesuvius which I brought back with me, and which an excellent
chemist, Heinrich Rose, has examined at my request, no traces of either
gold or silver have been discovered.
Although the above mentioned results, which are quite in accordance
with the exact observations of Monticelli, differ much from the
accounts which have been current during the short interval which has
elapsed, it is nevertheless true that the eruption of ashes from
Vesuvius from the 24th to the 28th of last October (1822) is the most
memorable of any of which we possess an authentic account, since
that which occasioned the death of the elder Pliny. The quantity of
ashes is, perhaps, three times as great as has ever been seen to fall
since volcanic phenomena have been attentively observed in Italy. A
stratum of ashes, from 16 to 19 inches thick, appears at first sight
insignificant compared with the mass which we find covering Pompeii;
but, not to speak of the increase which that mass has probably received
by the effects of heavy rains and other causes during the centuries
which have since elapsed, and without renewing the animated debate
respecting the causes of the destruction of the Campanian towns,
and which, on the other side of the Alps, has been carried on with
a considerable degree of scepticism, it should here be recalled to
recollection that the eruptions of a volcano, at widely separated
epochs, do not well admit of comparison, as respects their intensity.
All inferences derived from analogy are inadequate where quantitative
relations are concerned; as the quantity of lava and ashes, the
height of the column of smoke, and the loudness or intensity of the
detonations.
From the geographical description of Strabo, and from an opinion given
by Vitruvius respecting the volcanic origin of pumice, we perceive
that, up to the year of the death of Vespasian, _i. e._ previous to the
eruption which overwhelmed Pompeii, Vesuvius had more the appearance of
an extinct volcano than of a Solfatara. When, after long repose, the
subterranean forces suddenly opened for themselves new channels, and
again broke through the beds of primitive and trachytic rocks, effects
must have been produced for which subsequent ones do not furnish a
standard. From the well-known letter in which the younger Pliny informs
Tacitus of his uncle’s death, it may be clearly seen that the renewal
of volcanic outbursts, or what might be called the revival of the
slumbering volcano, began with an eruption of ashes. The same thing
was observed at Jorullo when, in September 1759, the new volcano,
breaking through beds of syenite and trachyte, rose suddenly in the
plain. The country-people took flight on finding their huts strewed
with ashes which had been emitted from the everywhere opening ground.
In the ordinary periodical manifestations of volcanic activity, on the
contrary, the shower of ashes marks the termination of each particular
eruption. There is a passage in the letter of the younger Pliny which
shews clearly that, at a very early stage of the eruption, the dry
ashes which had fallen had reached a thickness of four or five feet,
without accumulation from drift or other extraneous cause. He writes,
in the course of his narrative, “the court which had to be crossed to
reach the room in which Pliny was taking his noon-day repose was so
filled with ashes and pumice, that, if he had longer delayed coming
forth, he would have found the passage stopped.” In an enclosed space
like a court, the action of wind in drifting the ashes can scarcely
have been very considerable.
I have interrupted my general comparative view of volcanos by a notice
of particular observations made on Vesuvius, partly on account of the
great interest excited by the recent eruption, and partly on account
of those recollections of the catastrophes of Pompeii and Herculaneum,
which are almost involuntarily recalled to our minds by the occurrence
of any considerable shower of ashes. I have recorded in a note the
measurements of height made by myself and others on Vesuvius and in its
vicinity.
We have hitherto been considering the structure and mode of action
of those volcanos which have a permanent communication with the
interior of the Earth by craters. The summits of such volcanos
consist of masses of trachyte and lava upheaved by elastic forces and
traversed by veins. The permanency of their action gives us reason
to infer great complexity of structure. They have, so to speak, an
individual character which remains unaltered for long periods of time.
Neighbouring mountains often present the greatest differences in their
products: leucitic and feldspathic lavas, obsidian with pumice, and
masses of basalt containing olivine. They belong to the most recent
terrestrial phænomena, breaking through almost all the sedimentary
strata, and their products and lava currents are of later origin
than our valleys. Their life, if I may permit myself to employ this
figurative mode of expression, depends on the manner and permanence
of their communications with the interior of the Earth. They often
continue for centuries in a state of repose, are then suddenly
rekindled, and end by becoming Solfataras, emitting aqueous vapours,
gases, and acids; sometimes, however, as in the case of the Peak of
Teneriffe, we find that their summit has already become a laboratory of
regenerated sulphur; while from the sides of the mountain there still
issue large torrents of lava, basaltic in the lower part, but towards
the upper part, where the pressure is less,[39] presenting the form of
obsidian with pumice.
Distinct from these volcanos provided with permanent craters, there
is another class of volcanic phenomena more rarely observed, but
particularly instructive to the geologist, as they recall the ancient
world or the earliest geological revolutions of our planet. Trachytic
mountains open suddenly, emit lava and ashes, and close again, perhaps
never to reopen. Thus it was with the gigantic mountain of Antisana in
the chain of the Andes, and with the Monte Epomeo in Ischia in 1302.
Sometimes such an outbreak has even taken place in plains: as in the
high plateau of Quito, in Iceland at a distance from Mount Hecla,
and in Eubœa in the Lelantine Fields. Many of the upheaved islands
belong to this class of transitory phænomena. In all these cases the
communication with the interior of the earth is not permanent, and
the action ceases as soon as the cleft or fissure forming a temporary
channel closes again. Veins or dykes of basalt, dolerite, and
porphyry, which in different parts of the earth traverse almost all
formations, and masses of syenite, augitic porphyry, and amygdaloid,
which characterise the recent transition and oldest sedimentary rocks,
have probably been formed in a similar manner. In the youth of our
planet, the substances of the interior being still fluid, penetrated
through the everywhere fissured crust of the globe, sometimes becoming
solidified in the form of rocky veins or dykes of granular texture, and
sometimes spreading out in broad sheets, and resembling superimposed
strata. The volcanic products or rocks transmitted to us from the
earlier ages of our planet have not flowed in narrow bands like the
lavas of the isolated conical volcanos of the present time. The
mixtures of augite, titaniferous iron, feldspar, and hornblende, may
have been the same at different epochs, sometimes approximating more to
basalt and sometimes to trachyte; and, (as we learn from the important
researches of Mitscherlich, and the analogy of artificial igneous
products) chemical substances may have united in definite proportions
in a crystalline form: in all cases we recognise that substances
similar in composition have arrived at the surface of the earth by
very different ways; either simply upheaved, or penetrating through
temporary fissures; and that breaking through the older rocks, (_i.
e._ the earlier oxydized crust of the globe), they have finally issued
as lava currents from conical mountains having a permanent crater. To
confound together phenomena so different is to throw the geological
study of volcanos and volcanic action back into the obscurity from
which, by the aid of numerous comparative observations and researches,
it has gradually began to emerge.
The question has often been propounded: What is it that burns in
volcanos,--What produces the heat which melts and fuses together earths
and metals? Modern chemical science has essayed to answer, that what
burns are the earths, the metals, the alkalies themselves; viz. the
metalloids of those substances. The solid and already-oxydised crust
of the globe separates the surrounding atmosphere, with the oxygen
which it contains, from the inflammable unoxydised substances in the
interior of our planet: when those metalloids come in contact with
the oxygen of the atmosphere there arises disengagement of heat.
The great and celebrated chemist who propounded this explanation of
volcanic phenomena soon himself relinquished it. Observations made
in mines and caverns in all climates, and which in concert with M.
Arago I have collected in a separate memoir, shew that, even at what
may be considered a very small depth, the temperature of the Earth is
much above the mean temperature of the atmosphere at the same place.
A fact so remarkable, and so generally confirmed, connects itself
with that which we learn from volcanic phenomena. The depth at which
the globe may be regarded as a molten mass has been calculated. The
primitive cause of this subterranean heat is, as in all planets,
the process of formation itself, the separation of the spherically
condensing mass from a cosmical gaseous fluid, and the cooling of the
terrestrial strata at different depths by the loss of heat parted
with by radiation. All volcanic phenomena are probably the result of
a communication either permanent or transient between the interior
and exterior of the globe. Elastic vapours press the molten oxydising
substances upwards through deep fissures. Volcanos might thus be termed
intermitting springs or fountains of earthy substances; _i. e._ of
the fluid mixture of metals, alkalis, and earths which solidify into
lava currents and flow softly and tranquilly, when being upheaved they
find a passage by which to escape. In a similar manner the Ancients
represented (according to Plato’s Phædon) all volcanic fiery currents
as streams flowing from the Pyriphlegethon.
To these considerations and views let me be permitted to add another
more bold. May we not find in this internal heat of our globe,--(a heat
indicated by thermometric experiments on the waters of springs rising
from different depths,[40] as well by our observations on volcanos),--a
cause which may explain one of the most wonderful phænomena with
which the study of fossils has made us acquainted? Tropical forms of
animals, and, in the vegetable kingdom, arborescent ferns, palms,
and bambusaceæ, are found buried in the cold regions of the North.
Everywhere the ancient world shews a distribution of organic forms at
variance with our present climates. To resolve so important a problem,
recourse has been had to several hypotheses; such as the approach of
a comet, a change in the obliquity of the Ecliptic, and a different
degree of intensity in the solar light. None of these explanations
are satisfactory at once to the astronomer, the physicist, and the
geologist. For my part I willingly leave the axis of the Earth in its
place, and suppose no change in the light of the solar disk (from whose
spots a celebrated astronomer was inclined to explain the favourable or
unfavourable harvests of particular years); I am disposed to recognise
that in each planet there exist, independently of its relations to
the central body of the system to which it belongs, and independently
of its astronomical position, various causes for the development of
heat;--processes of oxydation, precipitations and chemical changes in
the capacity of bodies, by increase of electro-magnetic intensity, and
communications opened between the internal and external portions of the
planet.
It may be that in the Ancient World, exhalations of heat issuing forth
through the many openings of the deeply fissured crust of the globe
may have favoured, perhaps for centuries, the growth of palms and
tree-ferns and the existence of animals requiring a high temperature,
over entire countries where now a very different climate prevails.
According to this view of things (a view already indicated by me in a
work entitled “Geological Essay on the Superposition of Rocks in both
Hemispheres”) the temperature of volcanos would be that of the interior
of the earth, and the same cause which, operating through volcanic
eruptions, now produces devastating effects, might in primeval ages
have clothed the deeply fissured rocks of the newly oxydised earth in
every zone with the most luxuriant vegetation.
If, with a view to explain the distribution of tropical forms whose
remains are now discovered buried in northern regions, it should be
assumed that the long-haired species of Elephant now found enclosed
in ice was originally indigenous in cold climates, and that forms
resembling the same leading type may, as in the case of lions and
lynxes, have been able to live in wholly different climates, still this
manner of solving the difficulty presented by fossil remains cannot be
extended so as to apply to vegetable productions. From reasons with
which the study of vegetable physiology makes us acquainted, Palms,
Musaceæ, and arborescent Monocotyledones, are incapable of supporting
the deprivation of their appendicular organs which would be caused by
the present temperature of our northern regions; and in the geological
problem which we have to examine, it appears to me difficult to
separate vegetable and animal remains from each other. The same mode of
explanation ought to comprehend both.
I have permitted myself at the conclusion of the present discussion
to connect with facts collected in different and widely separated
countries some uncertain and hypothetical conjectures. The
philosophical study of Nature rises beyond the requirements of a simple
description of Nature: it does not consist in a sterile accumulation of
isolated facts. It may sometimes be permitted to the active and curious
mind of man to stretch forward from the present to the still obscure
future; to divine that which cannot yet be clearly known; and thus to
take pleasure in the ancient myths of geology reproduced in our own
days in new and varied forms.
ANNOTATIONS AND ADDITIONS.
[38] p. 226.--“_A more complete determination of the height of all
parts of the margin of the crater._”
Oltmanns, my astronomical fellow labourer, of whom, alas! science
has been early deprived, re-calculated the barometric measurements
of Vesuvius referred to in the preceding memoir (of the 22d and 25th
of November and of the 1st of December, 1822), and has compared the
results with the measurements which have been communicated to me in
manuscript by Lord Minto, Visconti, Monticelli, Brioschi, and Poulett
Scrope.
A. _Rocca del Palo, the highest and northern margin of the Crater of
Vesuvius._
Toises. Eng. ft.
Saussure, barometric measurement computed in
1773, probably by Deluc’s formula 609 3894
Poli, 1794, barometric 606 3875
Breislak, 1794, barometric (but, like Poli, the formula
employed uncertain) 613 3920
Gay-Lussac, Leopold von Buch, and Humboldt, 1805,
barometric, computed by Laplace’s formula, as
are also all the barometric results which follow 603 3856
Brioschi, 1810, trigonometric 638 4080
Visconti, 1816, trigonometric 622 3977
Lord Minto, 1822, barometric, often repeated 621 3971
Poulett Scrope, 1822, barometric, somewhat uncertain
from the proportion between the diameters
of the tube and cistern being unknown 604 3862
Monticelli and Covelli, 1822 624 3990
Humboldt, 1822 629 4022
Most probable result 317 toises, or 2027 English feet, above the
Hermitage; or 625 toises, or 3996 English feet, above the level of the
sea.