The _lava_ formations comprehend both extinct and active volcanoes. “The
term,” says Lyell, “has a somewhat vague signification, having been
applied to all melted matter observed to flow in streams from volcanic
vents. When this matter consolidates in the open air, the upper part is
usually scoriaceous, and the mass becomes more and more stony as we
descend, or in proportion as it has consolidated more slowly and under
greater pressure.”[21]
[21] “Elements of Geology,” p. 596.
The formation of extinct volcanoes is represented in France by the
volcanoes situated in the ancient provinces of Auvergne, Velay, and the
Vivarais, but principally by nearly seventy volcanic cones of various
sizes and of the height of from 500 to 1,000 feet, composed of loose
scoriæ, lava, and pozzuolana, arranged upon a granitic table-land, about
twelve miles wide, which overlooks the town of Clermont-Ferrand, and
which seem to have been produced along a longitudinal fracture in the
earth’s crust, running in a direction from north to south. It is a range
of volcanic hills, the “chain of _Puys_” nearly twenty miles in length,
by two in breadth. By its cellular and porous structure, which is also
granular and crystalline, the felspathic or pyroxenic lava which flowed
from these volcanoes is readily distinguishable from the analogous lavas
which belong to the basaltic or trachytic formations. Their surface is
irregular, and bristles with asperities, formed by heaped-up angular
blocks.
The volcanoes of the chain of _Puys_, represented on opposite page (PL.
III.) are so perfectly preserved, their lava is so frequently superposed
on sheets of basalt, and presents a composition and texture so distinct,
that there is no difficulty in establishing the fact that they are
posterior to the basaltic formation, and of very recent age.
Nevertheless, they do not appear to belong to the historic ages, for no
tradition attests their eruption. Lyell places these eruptions in the
Lower Miocene period, and their greatest activity in the Upper Miocene
and Pliocene eras. “Extinct quadrupeds of those eras,” he says,
“belonging to the genera mastodon, rhinoceros, and others, were buried
in ashes and beds of alluvial sand and gravel, which owe their
preservation to overspreading sheets of lava.”[22]
[22] Ibid, p. 677.
[Illustration: Fig. 9.--Section of a volcano in action.]
All volcanic phenomena can be explained by the theory we have already
indicated, of fractures in the solid crust of the globe resulting from
its cooling. The various phenomena which existing volcanoes present to
us are, as Humboldt has said, “the result of every action exercised by
the interior of a planet on its external crust.”[23] We designate as
volcanoes all conduits which establish a permanent communication between
the interior of the earth and its surface--a conduit which gives passage
at intervals to eruptions of _lava_, and in Fig. 9 we have represented,
in an ideal section, the geological mode of action of volcanic
eruptions. The volcanoes on the surface of the globe, known to be in an
occasional state of activity, number about three hundred, and these may
be divided into two classes: the _isolated_ or _central_, and the
_linear_ or those volcanoes which belong to a _series_.[24]
[23] “Cosmos,” vol. i., p. 25. Bohn.
[24] “Cosmos,” vol. i., p. 237.
The first are active volcanoes, around which there may be established
many secondary active mouths of eruption, always in connection with some
principal crater. The second are disposed like the chimneys of furnaces,
along fissures extending over considerable distances. Twenty, thirty,
and even a greater number of volcanic cones may rise above one such rent
in the earth’s crust, the direction of which will be indicated by their
linear course. The Peak of Teneriffe is an instance of a central
volcano; the long rampart-like chain of the Andes, presents, from the
south of Chili to the north-west coast of America, one of the grandest
instances of a continental volcanic chain; the remarkable range of
volcanoes in the province of Quito belong to the latter class. Darwin
relates that on the 19th of March, 1835, the attention of a sentry was
called to something like a large star which gradually increased in size
till about three o’clock, when it presented a very magnificent
spectacle. “By the aid of a glass, dark objects, in constant succession,
were seen in the midst of a great glare of red light, to be thrown up
and to fall down. The light was sufficient to cast on the water a long
bright reflection--it was the volcano of Osorno in action.” Mr. Darwin
was afterwards assured that Aconcagua, in Chili, 480 miles to the north,
was in action on the same night, and that the great eruption of
Coseguina (2,700 miles north of Aconcagua), accompanied by an earthquake
felt over 1,000 miles, also occurred within six hours of this same time;
and yet Coseguina had been dormant for six-and-twenty years, and
Aconcagua most rarely shows any signs of action.[25] It is also stated
by Professor Dove that in the year 1835 the ashes discharged from the
mountain of Coseguina were carried 700 miles, and that the roaring noise
of the eruption was heard at San Salvador, a distance of 1,000 miles.
[25] Darwin’s “Journal,” p. 291, 2nd edition.
In the sea the _series_ of volcanoes show themselves in groups of
islands disposed in longitudinal series.
Among these may be ranged the volcanic series of Sunda, which, according
to the accounts of the matter ejected and the violence of the eruptions,
seem to be among the most remarkable on the globe; the series of the
Moluccas and of the Philippines; those of Japan; of the Marianne
Islands; of Chili; of the double series of volcanic summits near Quito,
those of the Antilles, Guatemala, and Mexico.
Among the central, or isolated volcanoes, we may class those of the
Lipari Islands, which have _Stromboli_, in permanent activity, for
their centre; _Etna_, _Vesuvius_, the volcanoes of the _Azores_, of the
_Canaries_, of the _Cape de Verde_, of the _Galapagos_ Islands, the
_Sandwich_ Islands, the _Marquesas_, the _Society_ Islands, the
_Friendly_ Islands, _Bourbon_, and, finally, _Ararat_.
[Illustration: Fig. 10.--Existing crater of Vesuvius.]
The mouths of volcanic chimneys are, almost always, situated near the
summit of a more or less isolated conical mountain; they usually consist
of an opening in the form of a funnel, which is called the _crater_, and
which descends into the interior of the volcanic chimney. But in the
course of ages the crater becomes extended and enlarged, until, in some
of the older volcanoes, it has attained almost incredible dimensions. In
1822 the crater of Vesuvius was 2,000 feet deep, and of a very
considerable circumference. The crater of Kilauea, in the Sandwich
Islands group, is an immense chasm 1,000 feet deep, with an outer
circle no less than from two to three miles in diameter, in which lava
is usually seen, Mr. Dana tells us, to boil up at the bottom of a lake,
the level of which varies continually according to the active or
quiescent state of the volcano. The cone which supports these craters,
and which is designated the _cone of ejection_, is composed for the most
part of lava or _scoriæ_, the products of eruption. Many volcanoes
consist only of a _cone of scoriæ_. Such is that of Barren Isle, in the
Bay of Bengal. Others, on the contrary, present a very small cone,
notwithstanding the considerable height of the volcanic chain. As an
example we may mention the new crater of Vesuvius, which was produced in
1829 within the former crater (Fig. 10).
[Illustration: Fig. 11.--Fissures near Locarno.]
The frequency and intensity of the eruptions bear no relation to the
dimensions of the volcanic mountain. The eruption of a volcano is
usually announced by a subterranean noise, accompanied by shocks,
quivering of the ground, and sometimes by actual earthquakes. The noise,
which usually proceeds from a great depth, makes itself heard, sometimes
over a great extent of country, and resembles a well-sustained fire of
artillery, accompanied by the rattle of musketry. Sometimes it is like
the heavy rolling of subterranean thunder. Fissures are frequently
produced during the eruptions, extending over a considerable radius, as
represented in the woodcut on page 57 of the fissures of Locarno (Fig.
11), where they present a singular appearance; the clefts radiating from
a centre in all directions, not unlike the starred fracture in a cracked
pane of glass. The eruption begins with a strong shock, which shakes the
whole interior of the mountain; masses of heated vapour and fluids begin
to ascend, revealing themselves in some cases by the melting of the snow
upon the flanks of the cone of ejection; while simultaneously with the
final shock, which overcomes the last resistance opposed by the solid
crust of the ground, a considerable body of gas, and more especially of
steam, escapes from the mouth of the crater.
The steam, it is important to remark, is essentially the cause of the
terrible mechanical effects which accompany volcanic eruptions.
Granitic, porphyritic, trachytic, and sometimes even basaltic matters,
have reached the surface without producing any of those violent
explosions or ejections of rocks and stones which accompany modern
volcanic eruptions; the older granites, porphyries, trachytes, and
basalts were discharged without violence, because steam did not
accompany those melted rocks--a sufficient proof of the comparative calm
which attended the ancient as compared with modern eruptions. Well
established by scientific observations, this is a fact which enables us
to explain the cause of the tremendous mechanical effects attending
modern volcanic eruptions, contrasted with the more tranquil eruptions
of earlier times.
During the first moments of a volcanic eruption, the accumulated masses
of stones and ashes, which fill the crater, are shot up into the sky by
the suddenly and powerfully developed elasticity of the steam. This
steam, which has been disengaged by the heat of the fluid lava, assumes
the form of great rounded bubbles, which are evolved into the air to a
great height above the crater, where they expand as they rise, in clouds
of dazzling whiteness, assuming that appearance which Pliny the Younger
compared to a stone pine rising over Vesuvius. The masses of clouds
finally condense and follow the direction of the wind.
These volcanic clouds are grey or black, according to the quantity of
_ashes_, that is, of pulverulent matter or dust, mixed with watery
vapour, which they convey. In some eruptions it has been observed that
these clouds, on descending to the surface of the soil, spread around an
odour of hydrochloric or sulphuric acid, and traces of both these acids
are found in the rain which proceeds from the condensation of these
clouds.
The fleecy clouds of vapour which issue from the volcanoes are streaked
with lightning, followed by continuous peals of thunder; in condensing,
they discharge disastrous showers, which sweep the sides of the
mountain. Many eruptions, known as _mud volcanoes_, and _watery
volcanoes_, are nothing more than these heavy rains, carrying down with
them showers of ashes, stones, and scoriæ, more or less mixed with
water.
Passing on to the phenomena of which the crater is the scene at the time
of an eruption, it is stated that at first there is an incessant rise
and fall of the lava which fills the interior of the crater. This double
movement is often interrupted by violent explosions of gas. The crater
of Kilauea, in the Island of Hawaii, contains a lake of molten matter
1,600 feet broad, which is subject to such a double movement of
elevation and depression. Each of the vaporous bubbles as it issues from
the crater presses the molten lava upwards, till it rises and bursts
with great force at the surface. A portion of the lava, half-cooled and
reduced to scoriæ, is thus projected upwards, and the several fragments
are hurled violently in all directions, like those of a shell at the
moment when it bursts.
The greater number of the fragments being thrown vertically into the
air, fall back into the crater again. Many accumulating on the edge of
the opening add more and more to the height of the cone of eruption. The
lighter and smaller fragments, as well as the fine ashes, are drawn
upwards by the spiral vapours, and sometimes transported by the winds
over almost incredible distances.
In 1794 the ashes from Vesuvius were carried as far as the extremity of
Calabria. In 1812 the volcanic ashes of Saint Vincent, in the Antilles,
were carried eastward as far as Barbadoes, spreading such obscurity over
the island, that, in open day, passengers could not see their way.
Finally, some of the masses of molten lava are shot singly into the air
during an eruption with a rapid rotatory motion, which causes them to
assume the rounded shape in which they are known by the name of
_volcanic bombs_.
We have already remarked that the lava, which in a fluid state fills the
crater and the internal vent or chimney of the volcano, is forced
upwards by gaseous fluids, and by the steam which has been generated
from the water, entangled with the lava. In some cases the mechanical
force of this vapour is so great as to drive the lava over the edge of
the crater, when it forms a fiery torrent, spreading over the sides of
the mountain. This only happens in the case of volcanoes of
inconsiderable height; in lofty volcanoes it is not unusual for the lava
thus to force an outlet for itself near the base of the mountain,
through which the fiery stream discharges itself over the surrounding
country. In such circumstances the lava cools somewhat rapidly; it
becomes hard and presents a scoriaceous crust on the surface, while the
vapour escapes in jets of steam through the interstices. But under this
superficial crust the lava retains its fluid state, cooling slowly in
the interior of the mass, while the thickening stream moves sluggishly
along, impeded in its progress by the fragments of rock which this
burning river drives before it.
The rate at which a current of lava moves along depends upon its mass,
upon its degree of fluidity, and upon the inclination of the ground. It
has been stated that certain streams of lava have traversed more than
3,000 yards in an hour; but the rate at which they travel is usually
much less, a man on foot being often able to outstrip them. These
streams, also, vary greatly in dimensions. The most considerable stream
of lava from Etna had, in some parts, a thickness of nearly 120 feet,
with a breadth of a geographical mile and a half. The largest
lava-stream which has been recorded issued from the Skaptár Jokul, in
Iceland, in 1783. It formed two currents, whose extremities were twenty
leagues apart, and which from time to time presented a breadth of from
seven to fifteen miles and a thickness of 650 feet.
A peculiar effect, and which only simulates volcanic activity, is
observable in localities where _mud volcanoes_ exist. Volcanoes of this
class are for the most part conical hills of low elevation, with a
hollow or depression at the centre, from which they discharge the mud
which is forced upwards by gas and steam. The temperature of the ejected
matter is only slightly elevated. The mud, generally of a greyish
colour, with the odour of petroleum, is subject to the same alternating
movements which have been already ascribed to the fluid lava of
volcanoes, properly so called. The gases which force out this liquid
mud, mixed with salts, gypsum, naphtha, sulphur, sometimes even of
ammonia, are usually carburetted hydrogen and carbonic acid. Everything
leads to the conclusion that these compounds proceed, at least in great
part, from the reaction produced between the various elements of the
subsoil under the influence of infiltrating water between bituminous
marls, complex carbonates, and probably carbonic acid, derived from
acidulated springs. M. Fournet saw in Languedoc, near Roujan, traces of
some of these formations; and not far from that neighbourhood is the
bituminous spring of Gabian.
[Illustration: IV.--Mud volcano at Turbaco, South America.]
Mud volcanoes, or _salses_, exist in rather numerous localities. Several
are found in the neighbourhood of Modena. There are some in Sicily,
between Aragona and Girgenti. Pallas observed them in the Crimea--in the
peninsula of Kertch, and in the Isle of Tamàn. Von Humboldt has
described and figured a group of them in the province of Cartagena, in
South America. Finally, they have been observed in the Island of
Trinidad and in Hindostan. In 1797 an eruption of mud ejected from
Tunguragua, in Quito, filled a valley 1,000 feet wide to a depth of 600
feet. On the opposite page is represented the mud volcano of Turbaco, in
the province of Cartagena (PLATE IV.), which is described and figured by
Von Humboldt in his “Voyage to the Equatorial Regions of America.”
In certain countries we find small hillocks of argillaceous formation,
resulting from ancient discharges of mud volcanoes, from which all
disengagement of gas, water, and mud has long ceased. Sometimes,
however, the phenomenon returns and resumes its interrupted course with
great violence. Slight shocks of earthquakes are then felt; blocks of
dried earth are projected from the ancient crater, and new waves of mud
flow over its edge, and spread over the neighbouring ground.
To return to ordinary volcanoes, that is to say, those which eject lava.
At the end of a lava-flow, when the violence of the volcanic action
begins to subside, the discharge from the crater is confined to the
disengagement of vaporous gases, mixed with steam, which make their
escape in more or less abundance through a multitude of fissures in the
ground.
The great number of volcanoes which have thus become extinct form what
are called _solfataras_. The sulphuretted hydrogen, which is given out
through the fissures in the ground, is decomposed by contact with the
air, water being formed by the action of the oxygen of the atmosphere,
and sulphur deposited in considerable quantities on the walls of the
crater, and in the cracks of the ground. Such is the geological source
of the sulphur which is collected at Pozzuoli, near Naples, and in many
other similar regions--a substance which plays a most important part in
the industrial occupations of the world. It is, in fact, from sulphur
extracted from the ground about the mouths of extinct volcanoes, that is
to say from the products of _solfataras_, that sulphuric acid is
frequently made--sulphuric acid being the fundamental agent, one of the
most powerful elements, of the manufacturing productions of both worlds.
The last phase of volcanic activity is the disengagement of carbonic
acid gas without any increase of temperature. In places where these
continued emanations of carbonic acid gas manifest themselves, the
existence of ancient volcanoes may be recognised, of which these
discharges are the closing phenomenon. This is seen in a most remarkable
manner in Auvergne, where there are a multitude of acidulated springs,
that is to say, springs charged with carbonic acid. During the time when
he was opening the mines of Pontgibaud, M. Fournet had to contend with
emanations which sometimes exhibited themselves with explosive power.
Jets of water were thrown to great heights in the galleries, roaring
with the noise of steam when escaping from the boiler of a locomotive
engine. The water which filled an abandoned mine-shaft was, on two
separate occasions, upheaved with great violence--half emptying the
pit--while vast volumes of the gas overspread the whole valley,
suffocating a horse and a flock of geese. The miners were compelled to
fly in all haste at the moment when the gas burst forth, holding
themselves as upright as possible, to avoid plunging their heads into
the carbonic acid gas, which, from its low specific gravity, was now
filling the lower parts of the galleries. It represented on a small
scale the effect of the _Grotto del Cane_, which excites such surprise
among the ignorant near Naples; passing, also, for one of the marvels of
Nature all over the world. M. Fournet states that all the minute
fissures of the metalliferous gneiss near Clermont are quite saturated
with free carbonic acid gas, which rises plentifully from the soil
there, as well as in many parts of the surrounding country. The
components of the gneiss, with the exception of the quartz, are softened
by it; and fresh combinations of the acid with lime, iron, and manganese
are continually taking place. In short, long after volcanoes have become
extinct, hot springs, charged with mineral ingredients, continue to flow
in the same area.
The same facts as those of the _Grotto del Cane_ manifest themselves
with even greater intensity in Java, in the so-called Valley of Poison,
which is an object of terror to the natives. In this celebrated valley
the ground is said to be covered with skeletons and carcases of tigers,
goats, stags, birds, and even of human beings; for asphyxia or
suffocation, it seems, strikes all living things which venture into this
desolate place. In the same island a stream of sulphurous water, as
white as milk, issues from the crater of Mount Idienne, on the east
coast; and on one occasion, as cited by Nozet in the _Journal de
Géologie_, a great body of hot water, charged with sulphuric acid, was
discharged from the same volcano, inundating and destroying all the
vegetation of a large tract of country by its noxious fumes and
poisonous properties.
[Illustration: V.--Great Geyser of Iceland.]
It is known that the alkaline waters of Plombières, in the Vosges, have
a temperature of 160° Fahr. For 2,000 years, according to Daubrée,
through beds of concrete, of lime, brick, and sandstone, these hot
waters have percolated until they have originated calcareous spar,
aragonite, and fluor spar, together with siliceous minerals, such as
opal, which are found filling the interstices of the bricks and mortar.
From these and other similar statements, “we are led,” says Sir Charles
Lyell,[26] “to infer that when in the bowels of the earth there are
large volumes of molten matter, containing heated water and various
acids, under enormous pressure, these subterraneous fluid masses will
gradually part with their heat by the escape of steam and various gases
through fissures producing hot springs, or by the passage of the same
through the pores of the overlying and injected rocks.” “Although,” he
adds,[27] “we can only study the phenomena as exhibited at the surface,
it is clear that the gaseous fluids must have made their way through the
whole thickness of the porous or fissured rocks, which intervene between
the subterraneous reservoirs of gas and the external air. The extent,
therefore, of the earth’s crust which the vapours have permeated, and
are now permeating, may be thousands of fathoms in thickness, and their
heating and modifying influence may be spread throughout the whole of
this solid mass.”
[26] “Elements of Geology,” p. 732.
[27] Ibid, p. 733.
The fountains of boiling water, known under the name of _Geysers_, are
another emanation connected with ancient craters. They are either
continuous or intermittent. In Iceland we find great numbers of these
gushing springs--in fact, the island is one entire mass of eruptive
rock. Nearly all the volcanoes are situated upon a broad band of
trachyte, which traverses the island from south-west to north-east. It
is traversed by immense fissures, and covered with masses of lava, such
as no other country presents. The volcanic action, in short, goes on
with such energy that certain paroxysms of Mount Hecla have lasted for
six years without interruption. But the Great Geyser, represented on the
opposite page (PLATE V.), is, perhaps, even more an object of curiosity.
This water-volcano projects a column of boiling water, eight yards in
diameter, charged with silica, to the height, it has been said, of about
150 feet, depositing vast quantities of silica as it cools after
reaching the earth.
* * * * *
The volcanoes in actual activity are, as we have said, very numerous,
being more than 200 in number, scattered over the whole surface of the
globe, but mostly occurring in tropical regions. The island of Java
alone contains about fifty, which have been mapped and described by Dr.
Junghahn. Those best known are Vesuvius, near Naples; Etna, in Sicily;
and Stromboli, in the Lipari Islands. A rapid sketch of a few of these
may interest the reader.
Vesuvius is of all volcanoes that which has been most closely studied;
it is, so to speak, the classical volcano. Few persons are ignorant of
the fact that it opened--after a period of quiescence extending beyond
the memory of living man--in the year 79 of our era. This eruption cost
the elder Pliny his life, who fell a sacrifice to his desire to witness
one of the most imposing of natural phenomena. After many mutations the
present crater of Vesuvius consists of a cone, surrounded on the side
opposite the sea by a semicircular crest, composed of pumiceous matter,
foreign to Vesuvius properly speaking, for we believe that Mount
Vesuvius was originally the mountain to which the name of _Somma_ is now
given. The cone which now bears the name of Vesuvius was probably formed
during the celebrated eruption of 79, which buried under its showers of
pumiceous ashes the cities of Pompeii and Herculaneum. This cone
terminates in a crater, the shape of which has undergone many changes,
and which has, since its origin, thrown out eruptions of a varied
character, together with streams of lava. In our days the eruptions of
Vesuvius have only been separated by intervals of a few years.
The Lipari Isles contain the volcano of Stromboli, which is continually
in a state of ignition, and forms the natural lighthouse of the
Tyrrhenian Sea; such it was when Homer mentioned it, such it was before
old Homer’s time, and such it still appears in our days. Its eruptions
are incessant. The crater whence they issue is not situated on the
summit of the cone, but upon one of its sides, at nearly two-thirds of
its height. It is in part filled with fluid lava, which is continually
subjected to alternate elevation and depression--a movement provoked by
the ebullition and ascension of bubbles of steam which rise to the
surface, projecting upwards a tall column of ashes. During the night
these clouds of vapour shine with a magnificent red reflection, which
lights up the whole isle and the surrounding sea with a lurid glow.
Situated on the eastern coast of Sicily, Etna appears, at the first
glance, to have a much more simple structure than Vesuvius. Its slopes
are less steep, more uniform on all sides; its vast base nearly
represents the form of a buckler. The lower portion of Etna, or the
cultivated region of the mountain, has an inclination of about three
degrees. The middle, or forest region, is steeper, and has an
inclination of about eight degrees. The mountain terminates in a cone
of an elliptical form of thirty-two degrees of inclination, which bears
in the middle, above a nearly horizontal terrace, the cone of eruption
with its circular crater. The crater is 10,874 feet high. It gives out
no lava, but only vomits forth gas and vapour, the streams of lava
issuing from sixteen smaller cones which have been formed on the slopes
of the mountain. The observer may, by looking at the summit, convince
himself that these cones are disposed in rays, and are based upon clefts
or fissures which converge towards the crater as towards a centre.
But the most extraordinary display of volcanic phenomena occurs in the
Pacific Ocean, in the Sandwich Islands, and in Java. Mauna Loa and Mauna
Kea, in Hawaii, are huge flattened cones, 14,000 feet high. According to
Mr. Dana, these lofty, featureless hills sometimes throw out successive
streams of lava, not very far below their summits, often two miles in
breadth and six-and-twenty in length; and that not from one vent, but in
every direction, from the apex of the cone down slopes varying from four
to eight degrees of inclination. The lateral crater of Kilauea, on the
flank of Mauna Loa, is from 3,000 to 4,000 feet above the level of the
sea--an immense chasm 1,000 feet deep, with an outer circuit two to
three miles in diameter. At the bottom lava is seen to boil up in a
molten lake, the level of which rises or falls according to the active
or quiescent state of the volcano; but in place of overflowing, the
column of melted rock, when the pressure becomes excessive, forces a
passage through subterranean communications leading to the sea. One of
these outbursts, which took place at an ancient wooded crater six miles
east of Kilauea, was observed by Mr. Coan, a missionary, in June, 1840.
Another indication of the subterranean progress of the lava took place a
mile or two beyond this, in which the fiery flood spread itself over
fifty acres of land, and then found its way underground for several
miles further, to reappear at the bottom of a second ancient wooded
crater which it partly filled up.[28]
[28] Lyell’s “Elements of Geology,” p. 617.
The volcanic mountains of Java constitute the highest peaks of a
mountain-range running through the island from east to west, on which
Dr. Junghahn described and mapped forty-six conical eminences, ranging
from 4,000 to 11,000 feet high. At the top of many of the loftiest of
these Dr. Junghahn found the active cones and craters of small size, and
surrounded by a plain of ashes and sand, which he calls the “old crater
wall,” sometimes exceeding 1,000 feet in vertical height, and many of
the semicircular walls enclosing large cavities or _calderas_, four
geographical miles in diameter. From the highest parts of many of these
hollows rivers flow, which, in the course of ages, have cut out deep
valleys in the mountain’s side.[29]
[29] Lyell’s “Elements of Geology,” p. 620.
To this rapid sketch of actually existing volcanic phenomena we may add
a brief notice of submarine volcanoes. If these are known to us only in
small numbers, the circumstance is explained by the fact that their
appearance above the bosom of the sea is almost invariably followed by a
more or less complete disappearance; at the same time such very striking
and visible phenomena afford a sufficient proof of the continued
persistence of volcanic action beneath the bed of the sea-basin. At
various times islands have suddenly appeared, amid the ocean, at points
where the navigator had not before noticed them. In this manner we have
witnessed the island called Graham’s, Ferdinanda, or Julia, which
suddenly appeared off the south-west coast Sicily in 1831, and was swept
away by the waves two months afterwards.[30] At several periods also,
and notably in 1811, new islands were formed in the Azores, which raised
themselves above the waves by repeated efforts all round the islands,
and at many other points.
[30] Ibid, p. 620.
The island which appeared in 1796 ten leagues from the northern point of
Unalaska, one of the Aleutian group of islands, is specially remarkable.
We first see a column of smoke issuing from the bosom of the ocean,
afterwards a black point appears, from which bundles of fiery sparks
seem to rise over the surface of the sea. During the many months that
these phenomena continue, the island increases in breadth and in height.
Finally smoke only is seen; at the end of four years, even this last
trace of volcanic convulsion altogether ceases. The island continued,
nevertheless, to enlarge and to increase in height, and in 1806 it
formed a cone, surmounted by four other smaller ones.
In the space comprised between the isles of Santorin, Tharasia, and
Aspronisi, in the Mediterranean, there arose, 160 years before our era,
the island of _Hyera_, which was enlarged by the upheaval of islets on
its margin during the years 19, 726, and 1427. Again, in 1773,
Micra-Kameni, and in 1707, Nea-Kameni, made their appearance. These
islands increased in size successively in 1709, in 1711, in 1712.
According to ancient writers, Santorin, Tharasia, and Aspronisi, made
their appearance many ages before the Christian era, at the termination
of earthquakes of great violence.