VESUVIUS IN ERUPTION
At intervals of ten to twenty years the best-known volcano in the
world–Vesuvius, on the Bay of Naples–has in the last two centuries
burst into eruption, and the probability of the recurrence of this
violent state of activity, at no distant date, render some account of
my own acquaintance with that great and wonderful thing seasonable.
We inhabitants of the West of Europe have little personal experience
of earthquakes, and still less of volcanoes, for there is not in the
British Islands even an "extinct" volcanic cone to remind us of the
terrible forces held down beneath our feet by the crust of the earth.
In regions as near as the Auvergne of Central France and the Eiffel,
close to the junction of the Moselle with the Rhine, there are complete
volcanic craters whose fiery origin is recognized even by the local
peasantry. They are, however, regarded by these optimist folk as the
products of ancient fires long since burnt out. The natives have as
little apprehension of a renewed activity of their volcanoes as we have
of the outburst of molten lava and devastating clouds of ashes and
poisonous vapour from the top of Primrose Hill. Nevertheless, the hot
springs and gas issuing from fissures in the Auvergne show that the
subterranean fires are not yet closed down, and may at any day burst
again into violent activity.
Such also was the happy indifference with which from time immemorial
the Greek colonists and other earlier and later inhabitants of the
rich and beautiful shores of the Neapolitan bay before the fateful year
A.D. 79, had regarded the low crater-topped mountain called Vesuvius
or Vesbius, as well as the great circular forest-grown or lake-holding
cups near Cumæ and the Cape Misenum, at the northern end of the
bay–known to-day as the Solfatara, Astroni, Monti Grillo, Barbaro, and
Cigliano–and the lakes Lucrino, Averno, and Agnano. These together with
the Monte Nuovo–which suddenly rose from the sea near Baiæ in 1538 and
as suddenly disappeared–constitute "the Phlegræan fields." Vesuvius was
loftier than any one of the Phlegræan craters, and the gentle slope
by which it rose from the sea level to a height of nearly 3700 ft.
had, as now, a circumference of ten miles. It did not terminate in a
"cone," as in later ages, but in a depressed, circular, forest-covered
area measuring a mile across, which was the ancient crater. A drawing
showing the shape of the mountain at this period is the work of the
late Prof. Phillips of Oxford (Fig. 30). The soil formed around and
upon the ancient lava-streams of Vesuvius appears to have been always
especially fertile, so that flourishing towns and villages occupied
its slopes, and the ports of Herculaneum, Pompeii, and Stabiæ were
the seats of a busy and long-established population. The existence of
active volcanoes at no great distance from Vesuvius was, however, well
known to the ancient Greeks and Romans. The great Sicilian mountain,
Etna–more than 10,000 ft. in height, rising from a base of ninety
miles in circumference–and the Lipari Islands, such as Stromboli and
Volcano, were for many centuries in intermittent activity before the
first recorded eruption of Vesuvius–that of A.D. 79–and great eruptions
are recorded as having occurred in the mountain mass of the island of
Ischia, close to the Bay of Naples, in the fifth, third, and first
centuries B.C.
[Illustration: FIG. 30.–Vesuvius as it appeared before the eruption of
August 24, A.D. 79. From a sketch by Prof. Phillips, F.R.S.]
Nevertheless, the outburst of Vesuvius in A.D. 79 and its re-entrance
into a state of activity came upon the unfortunate population around it
as an absolutely unexpected thing. At least a thousand years–probably
several thousand years–had passed since Vesuvius had become "extinct."
All tradition of its prehistoric activity had disappeared, though
the learned Greek traveller Strabo had pointed out the indications
it presented of having been once a seat of consuming fire. From
A.D. 63 there were during sixteen years frequent earthquakes in its
neighbourhood, which, as we know by records and inscriptions, caused
serious damage to the towns around it, and then suddenly, on the night
of Aug. 24, A.D. 79, vast explosions burst from its summit. A huge
black cloud of fine dust and cinders, lasting for three days, spread
from it for twenty miles around, streams of boiling mud poured down
its sides, and in a few hours covered the city of Herculaneum, whilst
a dense shower of hot volcanic dust completely buried the gay little
seaside resort known as Pompeii. Many thousand persons perished,
choked by the vapours or overwhelmed by the hot cinders or engulfed in
the boiling mud.
The great naturalist Pliny was in command of the fleet at Cape Misenum,
and went by ship across the bay to render assistance to the inhabitants
of the towns at the foot of Vesuvius. Pliny's nephew wrote two letters
to the historian Tacitus, giving an account of these events and of
the remarkable courage and coolness of his uncle, who, after sleeping
the night at Stabiæ, was suffocated by the sulphurous vapours as he
advanced into the open country near the volcano. The friends who were
with him left him to his fate and made their escape. The younger Pliny
had prudently remained, out of danger, with his mother at Misenum.
The alternating periods of activity and of rest exhibited by volcanoes
seem to us capricious, and even at the present day are not sufficiently
well understood to enable us to discern any order or regularity in
their succession. Vesuvius is a thousand centuries old, and we have
only known it for thirty. We cannot expect to get the time-table of its
activities on so brief an acquaintance. Strangely enough, Vesuvius,
having, after immemorial silence, spasmodically burst into eruption
and spread devastation around it, resumed its slumber for many years.
There is no mention of its activity for 130 years after A.D. 79. Then
it growled and sent forth steam and cinder-dust to an extent sufficient
to attract attention again; its efforts were thereafter recorded once
or so in a century, though little, if any, harm was done by it. In A.D.
1139 there was a great throwing-up of dust and stones, with steam,
which reflected the light of molten lava within the crater, and looked
like flames. And then for close on 500 years there was little, if any,
sign of activity. The "eruptions" between that of A.D. 79 and that of
A.D. 1139 had been ejections of steam and cinders, unaccompanied by
any flow or stream of lava. Then suddenly the whole business shut up
for 500 years, and after that–also quite suddenly–in 1631, a really big
eruption took place, exceeding in volume the catastrophe of Pliny's
date. Not only were columns of dust and vapour ejected to a height of
many miles, but several streams of white-hot lava overflowed the edge
of the crater and reached the seacoast, destroying towns and villages
on the way. Some of these lava-streams were five miles broad, and can
be studied at the present day. As many as 18,000 persons were killed.
There were three more eruptions in the seventeenth century, and from
that date there set in a period of far more frequent outbursts, which
have continued to our own times. In the eighteenth century there were
twenty-three distinct eruptions, lasting each from a few hours to two
or three days, and of varying degrees of violence–a vast steam-jet
forcing up cinders and stones from the crater into the air, usually
accompanied by the outflow of lava, from cracks in sides of the crater,
in greater or less quantity. In the nineteenth century there were
twenty-five distinct eruptions, the most formidable of which were those
of 1822, 1834, and 1872. All of the eruptions of Vesuvius in the last
280 years have been carefully described, and most of them recorded
in coloured pictures (a favourite industry of the Neapolitans),
showing the appearance of the active volcano both by day and night and
its change of shape in successive years. Sir William Hamilton, the
British Ambassador at the Court of Naples at the end of the eighteenth
century (of whose great folio volumes I am the fortunate possessor),
largely occupied himself in the study and description of Vesuvius,
and published illustrations of the kind mentioned above, showing the
appearance of the mountain at various epochs. Since his day there has
been no lack of descriptions of every succeeding eruption, and now we
have the records of photography.
The crater or basin formed by a volcano starts with the opening
of a fissure in the earth's surface communicating by a pipe-like
passage with very deeply-seated molten matter and steam. Whether the
molten matter thus naturally "tapped" is only a local, though vast,
accumulation, or is universally distributed at a given depth below the
earth's crust, and at how many miles from the surface, is not known.
It seems to be certain that the great pressure of the crust of the
earth (from five to twenty-five miles thick) must prevent the heated
matter below it from becoming either liquid or gaseous, whether the
heat of that mass be due to the cracking of the earth's crust and the
friction of the moving surfaces as the crust cools and shrinks, or is
to be accounted for by the original high temperature of the entire mass
of the terrestrial globe. It is only when the gigantic pressure is
relieved by the cracking or fissuring of the closed case called "the
crust of the earth" that the enclosed deep-lying matter of immensely
high temperature liquefies, or even vaporizes, and rushes into the
up-leading fissure. Steam and gas thus "set free" drive everything
before them, carrying solid masses along with them, tearing, rending,
shaking "the foundations of the hills," and issuing in terrific
jets from the earth's surface, as through a safety valve, into the
astonished world above. Often in a few hours they choke their own path
by the destruction they produce and the falling in of the walls of
their briefly-opened channels. Then there is a lull of hours, days,
or even centuries, and after that again, a movement of the crust, a
"giving" of the blockage of the deep, vertical pipe, and a renewed rush
and jet of expanding gas and liquefying rock.
The general scheme of this process and its relations to the structure
and properties of the outer crust and inner mass of the globe is
still a matter of discussion, theory and verification; but whatever
conclusions geologists may reach on these matters, the main fact of
importance is that steam and gases issue from these fissures with
enormous velocity and pressure, and that "a vent" of this kind,
once established, continues, as a rule, to serve intermittently for
centuries, and, indeed, for vast periods to which we can assign no
definite limits. The solid matter ejected becomes piled up around
the vent as a mound, its outline taking the graceful catenary curves
of rest and adjustment to which are due the great beauty of volcanic
cones. The apex of the cone is blown away at intervals by the violent
blasts issuing from the vent, and thus we have formed the "crater,"
varying in the area enclosed by its margin and in the depth and
appearance of the cup so produced. At a rate depending on the amount
of solid matter ejected by the crater, the mound will grow in the
course of time to be a mountain, and often secondary craters or
temporary openings, connected at some depth with the main passage
leading to the central vent, will form on the sides of the mound or
mountain. Sometimes the old crater will cease to grow in consequence
of the blocking of its central vent and the formation of one or more
subsidiary vents, the activity of which may blast away or smother the
cup-like edge of the first crater.
[Illustration: FIG. 31.–Five successive stages in the change of form
of Vesuvius (after Phillips' "Vesuvius," Oxford, 1869). In the oldest
(lowest figure) we see the mountain with its still earlier outline
completed by the cone drawn in dotted line. Within the period of
historic record that cone had not been seen. The mountain had, so far
as men knew, always been truncated as shown here and in Fig. 30. The
next figure above shows the further lowering of the mountain by the
first eruption on record–that which destroyed Pompeii in A.D. 79. The
commencing formation of a new ash-cone is indicated by a dotted line.
In the three upper figures we trace the gradual growth of the new cone
from 1631 to 1868. In 1872 the top of the new ash-cone was blown away,
and the mountain reverted to the shape of 1822. Now (1920) the cone has
accumulated once more and is higher than it was in 1868.]
Such a history has been that of Vesuvius shown in outline in Fig. 31.
In geologic ages–perhaps some thousands of centuries ago–Vesuvius was
probably a perfect cone (its outline is shown at the bottom of p. 62)
some 7000 ft. high, rising by a characteristically accelerated upgrowth
from a circle of ten miles or more in diameter to its delicate central
peak, hollowed out at the summit by a small crater a couple of hundred
yards across. Its eruptions at that time were neither excessive nor
violent. Then came a period of greatly increased energy–the steam-jet
blew with such violence that it shattered and dispersed the cone,
lowering the mountain to 3700 ft. in height, truncating it and leaving
a proportionately widened crater of a mile and a half in diameter.
And then the mountain reposed for long centuries. We do not know how
long this period of extinction was, for we do not know when it began,
but we know that this was the state of the mountain when in A.D. 79
it once more burst into life. In recent years–that is, since the
seventeenth century A.D., a curious change took place in the mountain:
the vent or orifice of the conducting channel by which eruptive
matters were brought to the surface ceased to be in the centre of the
wide broken-down crater of Pliny's time, and a vent was formed a few
hundred yards to the south of the centre of the old crater, nearer to
the south side of the old crater's wall. From this ashes or cinders
issued, and were piled up to form a new cone, which soon added 600 ft.
to the height of the mountain and covered in the southern half of the
old crater's lip, whilst leaving the northern half or semicircle free.
This latter uncovered part was called by the Italians "Monte Somma,"
and the new cone low down in the southern side of which the rest of
the old crater-lip could be traced, was henceforth spoken of as "the
ash-cone" and sometimes misleadingly as "the true" Vesuvius. Clearly
it was not "the true Vesuvius" since it was a new growth. The original
old Vesuvius was crowned by a crater formed by the cliffs of Monte
Somma and their continuation round to the south side, now more or less
completely concealed by the new ash-cone.
In the course of various eruptions during the last two centuries the
new ash-cone thus formed was blown away more or less completely, and
gradually grew up again. During the nineteenth century it was a
permanent feature of the mountain, though a good deal cut down in 1822,
and later grew so high as to give a total elevation from the sea-level
of 4300 ft. The crater at the top of the ash-cone has varied during
the past century in width and depth, according to its building up or
blowing away by the central steam jet. In 1822 it is reported to have
been funnel-like and 2000 ft. deep, tapering downwards to the narrow
fissures which are the actual vent. At other times it has been largely
filled by débris, and only 200 ft. deep. Molten lava has often issued
from fissures in the sides of the ash-cone, and even lower down on the
sides of the mountain, and a very small secondary crater has sometimes
appeared on the side of the ash-cone 100 ft. or 200 ft. from the
terminal crater which "finishes off" the cone.
Such was the condition of the mountain when I first saw it in the
autumn of 1871. Six months later I witnessed the most violent eruption
of the nineteenth century. Vesuvius kept up a continuous roar like that
of a railway engine letting off steam when at rest in a covered station
only a thousandfold bigger. Its vibrations shook with a deep musical
note, for twenty-four hours, the house nine miles distant in Naples in
which I was staying. My windows commanded a view of the mountain, and
when the noise ceased and the huge steam-cloud cleared away, I saw a
different Vesuvius, the higher part of the ash-cone was gone, and a
huge gap in it had been formed by the blowing away of its northern side.
In October 1871, when I joined my friend Anton Dohrn at Naples in order
to study the marine creatures of the beautiful bay, Vesuvius was in the
proud possession of a splendid cone, completing its graceful outline.
A little steam-cloud hung about one side of the cone during the day,
and as night came on Vesuvius used, as we said, to "light his cigar."
In fact, a very small quantity of molten lava was at that time flowing
from the side of the ash-cone, about 100 ft. from its summit, and this
gave a most picturesque effect as we watched it from our balcony high
up on Pausilippo, when the sun set. It was a friendly sort of beacon,
far away on the commanding mountain's top, which was answered by the
lighting up of a thousand lamps along the coast, and by innumerable
flaming faggots in the fishermen's boats moving across the bay, drawing
to their light strange fishes, to be impaled by the long tridents of
the skilful spearmen. That little beacon light on Vesuvius increased
in volume in the course of three weeks, and was supplemented by other
flaming streams and by showers of red-hot stones from the crater. This
small "eruption" was the precursor by six months of the great eruption
of the end of April 1872, and I spent a night on Vesuvius during its
progress, and looked into the crater from which the glowing masses of
rock were being belched forth.
Not long before I went, in 1871, to Naples I had spent some weeks in
visiting the extinct volcanoes of the Auvergne and of the Eiffel, and
I was eager to examine the still living Vesuvius. In the first week
of October I made an excursion to the crater of Vesuvius in company
with the son of a Russian admiral, whose name, "Popoff," was under the
circumstances unpleasantly suggestive. We examined some black slaglike
masses of old lava-streams, and struggled up the loose sandy ash-cone
(there was no "funicular" in those days), and prodded with our sticks
the few yards of molten lava which emerged from the side of the cone
about 100 ft. from the summit. On Nov. 1 my friend Anton Dohrn (who was
then negotiating with the Naples Municipality for a site in the Villa
Nazionale on which to erect the great Zoological Station and Aquarium,
now so well known) was with me and some Neapolitan acquaintances
looking at Vesuvius across the bay from Pausilippo, where we had
established ourselves, when we noticed that a long line of steam was
rising from the lower part of the ash-cone and that puffs of steam were
issuing at intervals from the crater. "Dio mio! Dio di Dio!" cried the
Neapolitans in terror, and expressed their intention of leaving Naples
without an hour's delay. As night fell a new glowing line of fire
appeared far down near the base of the ash-cone, whilst what looked
in the distance like sparks from a furnace, but were really red-hot
stones–each as big as a Gladstone bag–were thrown every two or three
minutes from the crater.
We hired a carriage, drove to Resina (built above buried Herculaneum),
and walked up towards the Observatory in order to spend the night
on the burning mountain. We found that two white-hot streams, each
about twenty yards broad at the free end, were issuing from the base
of the cone. The glowing stones thrown up by the crater were now
separately visible; a loud roar accompanied each spasmodic ejection.
The night was very clear, and a white firmly-cut cloud, due to the
steam ejected by the crater, hung above it. At intervals we heard a
milder detonation–that of thunder which accompanied the lightning which
played in the cloud, giving it a greenish illumination by contrast with
the red flame colour reflected on to it by red-hot material within
the crater. The flames attributed to volcanoes are generally of this
nature, but actual flames do sometimes occur in volcanic eruptions by
the ignition of combustible gases. The puffs of steam from the crater
were separated by intervals of about three minutes. When an eruption
becomes violent they succeed one another at the rate of many in a
second, and the force of the steam jet is gigantic, driving a column
of transparent super-heated steam with such vigour that as it cools
into the condition of "cloud" an appearance like that of a gigantic
pine-tree seven miles high (in the case of Vesuvius) is produced.
We made our way to the advancing end of one of the lava-streams (like
the "snout" of a glacier), which was 20 ft. high, and moved forwards
but slowly, in successive jerks. Two hundred yards farther up, where it
issued from the sandy ashes, the lava was white-hot and running like
water, but it was not in very great quantity and rapidly cooled on the
surface and became "sticky." A cooled skin of slag was formed in this
way, which arrested the advancing stream of lava. At intervals of a
few minutes this cooled crust was broken into innumerable clinkers by
the pressure of the stream, and there was a noise like the smashing
of a gigantic store of crockery ware as the pieces or "clinkers" fell
over one another down the nearly vertical "snout" of the lava-stream,
whilst the red-hot molten material burst forward for a few feet, but
immediately became again "crusted over" and stopped in its progress.
We watched the coming together and fusion of the two streams and the
overwhelming and burning up of several trees by the steadily, though
slowly, advancing river of fire. Then we climbed up the ash-cone,
getting nearer and nearer to the rim of the crater, from which showers
of glowing stones were being shot. The deep roar of the mountain at
each effort was echoed from the cliffs of the ancient mother-crater,
Monte Somma, and the ground shook under our feet as does a ship at
sea when struck by a wave. The night was very still in the intervals.
The moon was shining, and a weird melancholy "ritornelle" sung by
peasants far off in some village below us came to our ears with strange
distinctness. It might have been the chorus of the imprisoned giants of
Vulcan's forge as they blew the sparks with their bellows and shook the
mountains with the heavy blows of their hammers.
As we ascended the upper part of the cone the red-hot stones were
falling to our left, and we determined to risk a rapid climb to the
edge of the crater on the right or southern side, and to look into it.
We did so, and as we peered into the great steaming pit a terrific
roar, accompanied by a shuddering of the whole mountain, burst from it.
Hundreds of red-hot stones rose in the air to a height of 400 ft., and
fell, happily in accordance with our expectation, to our left. We ran
quickly down the sandy side of the cone to a safe position, about 300
ft. below the crater's lip, and having lit our pipes from one of the
red-hot "bombs," rested for a while at a safe distance and waited for
the sunrise. A vast horizontal layer of cloud had now formed below us,
and Vesuvius and the hills around Naples appeared as islands emerging
from a sea. The brilliant sunlight was reassuring after this night
of strange experiences. The fields and lanes were deserted in the
early morning as we descended to the sea-level. On our way we met a
procession of weird figures clad in long white robes, enveloping the
head closely but leaving apertures for the eyes. They were a party of
the lay-brothers of the Misericordia carrying a dead man to his grave.
Then we found our carriage, and drove quickly back to Naples and sleep!
In the following March I acted as guide to my friend Professor Huxley
in expeditions up Vesuvius, now quiescent, and to the Solfatara. Then
suddenly, in April, the great eruption of 1872 burst upon us. On the
first day of the outbreak some imprudent visitors were killed by steam
and gas ejected by the lava-stream. By the next day the violence of
the eruption was too great for any one to venture near it. The crater
sent forth no intermittent "puffs" as in the preceding November, but
a continuously throbbing jet which produced a cloud five miles high,
like an enormous cauliflower in shape, suspended above the mountain
and making it look by comparison like a mole-hill. Showers of fine
ashes, as in the days of Pompeii, fell thickly around, accumulating to
the depth of an inch in a few hours even at my house in Pausilippo,
nine miles distant across the bay. I was recovering at the time from an
attack of typhoid fever, and lay in bed, listening to the deep humming
sound and wondering at the darkness until my doctor came and told me
of the eruption. I was able to get up and see from the window the
great cauliflower-like cloud and the vacant place where the ash-cone
was, but whence it had how been scattered into the sky. (It has been
gradually re-formed by later eruptions, of which the last of any size
was in 1906.) I could also see steam rising like smoke from a long line
reaching six miles down the mountain into the flat country below. It
was the great lava-stream which had destroyed two prosperous villages
in its course.
After ten days I was able to get about, and drove over to one of these
villages and along its main street, which was closely blocked at the
end by what looked like a railway embankment some 40 ft. high. This
was the side of the great lava-stream now cooled and hardened on the
surface. It had sharply cut the houses, on each side of the street,
in half without setting them on fire, so that the various rooms were
exposed in section–pictures hanging on the walls, and even chairs and
other furniture remaining in place on the unbroken portion of the
floor. The villagers had provided ladders by which I ascended the steep
side of the embankment-like mass at the end of the street, and there a
wonderful sight revealed itself. One looked out on a great river seven
miles long, narrow where it started from the broken-down crater, but
widening to three miles where I stood, and wider still farther on as
it descended. This river, with all its waves and ripples, was turned
to stone, and greatly resembled a Swiss glacier in appearance. A foot
below the surface it was still red-hot, and a stick pushed into a
crevice caught fire. It was not safe to venture far on to the pie-crust
surface. A couple of miles away the campanile of the church of a
village called Massa di Somma stood out, leaning like that of Pisa,
from the petrified mass, whilst the rest of the village was overwhelmed
and covered in by the great stream.
The curious resemblance of the lava-stream to a glacier arose from
the fact that it was almost completely covered by a white snow-like
powder. This snow-like powder, which often appears on freshly-run
lava, is salt–common sea salt and other mineral salts dissolved in
the water ejected as steam mixed with the lava. The steam condenses,
as the lava cools, into water and evaporates slowly, leaving the salt
as crystals. Often these are not white, but contain iron salt, mixed
with the white sodium, potassium, and ammonium chlorides, which gives
them a yellow or orange colour. Salts coloured in this way have the
appearance of sulphur, and are often mistaken for it. The whole of the
interior of the crater of Vesuvius when I revisited it in 1875 was thus
coloured yellow, and I have a water-colour sketch of the scene made
by a friend who came with me for the purpose. As a matter of fact,
though small quantities of the choking gas called "sulphurous acid"
are among the vapours given off by Vesuvius, there is no deposit of
sulphur there. Some large volcanoes (in Mexico and Japan) have made
deposits of sulphur, which are dug for commercial purposes; but the
sulphur of Sicily is not, and has not been, thrown out or volatilized
by Etna. It occurs in rough masses and in splendid crystals in a
tertiary calcareous marine deposit, and its deposition was probably due
to a chemical decomposition of constituents of the sea water brought
about by minute plants, known as "sulphur bacteria." Whether the
neighbouring great volcano had any share in the process seems to be
doubtful.
It is generally supposed that sea-water makes its way in large quantity
through fissures connected with volcanic channels, and is one of the
agents of the explosions caused by the subterranean molten matter.
Gaseous water, hydrochloric acid, carbonic acid, hydrofluoric acid, and
even pure hydrogen and oxygen and argon are among the gases ejected by
volcanoes.
The molten matter forced up from the bowels of the earth and poured
out by volcanoes is made up of various chemical substances, differing
in different localities, and even in different eruptions of the same
volcano. It consists largely of silicates of iron, lime, magnesium,
aluminium, and the alkali metals, with possible admixture of nearly
every other element. Some volcanoes eject "pitch" or "bitumen." When
the molten matter cools, interesting crystals of various "species"
(_i.e._, of various chemical composition) usually form in the deeper
part of the mass. The lavas of Vesuvius frequently contain beautiful
opaque-white twelve-sided crystals of a siliceous mineral called
"leucite." I have collected in the lava of Niedermendig, on the Rhine,
specimens embedding bright blue transparent crystals (a mineral called
Haüynite) scattered in the grey porous rock. The lava-streams, and
even the "roots," of extinct volcanoes which are of great geologic
age, sometimes become exposed by the change of the earth's surface,
and extensive sheets of volcanic rock of various kinds are thus laid
bare. Basalt is one of these rocks, and it not unfrequently presents
itself as a mass of perpendicular six-sided columns, each column 10
ft. or more high, and often a foot or more in diameter. The "Giant's
Causeway," in the North of Ireland, and the "Pavée des Géants," in
the Ardêche of Southern France, are examples both of which I have
visited. It is not easy to explain how the molten basalt has come to
take this columnar structure on cooling. It has nothing to do with
"crystallization," but is similar to the columnar formation shown by
commercial "starch" and occasionally by "tabular flint". A theoretical
explanation of its formation has been given by Prof. J. Thompson,
brother of the late Lord Kelvin.
The varieties of volcanoes and their products make up a long story–too
long to be told here. There are from 300 to 400 active craters in
Existence to-day–mostly not isolated, but grouped along certain great
lines, as, for instance, along the Andean chain, or in more irregular
tracks. If we add to the list craters no longer active, but still
recognizable, we must multiply it by ten. Vesuvius is the only active
volcano on the mainland of Europe–Hecla, Etna, Stromboli, Volcano,
and the volcanoes of the Santorin group are on islands. The biggest
volcanoes are in South America, Mexico, Java, and Japan. Volcanoes and
the related "earthquakes" have been most carefully studied with a view
to the safety of the population in Japan. The graceful and well-beloved
volcano, Fujiyama, is more than 12,000 ft. high, but, unlike others in
those islands, it has been quiescent now for just 200 years. The most
violent volcanic eruptions of recent times, with the largest "output"
of solid matter, are those of the Soufrière of St. Vincent in 1812,
of the Mont Pelée of Martinique in 1902, and of Krakatoa in 1883. A
single moderate eruption of the great volcano Mauna Loa, in Hawaii,
nearly 14,000 feet high, throws out a greater quantity of solid matter
than Vesuvius has ejected in all the years which have elapsed since the
destruction of Pompeii. Many hundred millions of tons of solid matter
were ejected by Mont Pelée in 1902, when also a peculiar heavy cloud
descended from the mountain, hot and acrid, charged with incandescent
sand, and rolling along like a liquid rather than a vapour. It burnt
up the town of St. Pierre and its inhabitants and the shipping in the
harbour. In the eruption of the volcano of St. Vincent in 1812 three
million tons of ashes were projected on to the Bahamas Islands, 100
miles distant, besides a larger quantity which fell elsewhere. The
great explosion at Krakatoa, lasting two days, blew an island of 1400
ft. high, into the air. A good deal of it was projected as excessively
fine needlelike particles of pumice with such force as to carry it up
thirty miles into the upper regions of the atmosphere, where it was
carried by air currents all over the world, causing the "red sunsets"
of the following year. The sky over Batavia, 100 miles distant, was
darkened at midday so completely that lamps had to be used–as I heard
from my brother who was there at the time. The explosions were heard
in Mauritius, 3000 miles away. A sea wave 50 ft. high was set going by
the submarine disturbance, and reaching Java and neighbouring islands
inundated the land and destroyed 36,000 persons. This wave travelled
in reduced size over a vast tract of the ocean, and was observed and
recorded at Cape Horn, 7800 miles distant from its seat of origin.