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 Stroviholi^ in permanent activity, for

"Cosmos," vol. i., p.'237. fDanvin's "Journal," p. 291, 2nd edition..

THE WORLD BEFORE THE DELUGE.

56 their centre; Etna, Vesuvius, the volcanoes of the Azores, of the Ca7ianes, of the Cape de Verde, of the Galapagos Islands, the Sandwich Islands, the Marquesas, the Society Islands, the Friendly Islands, JBourbon, and, finally, Ararat,

The mouths of volcanic chimneys are, almost always, situated near

Fig. lo.— Existing crater of Vesuvius.

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

VOLCANIC OR LAVA FORMATIONS.

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 £0?ic of ejection, is composed for the most part of lava or scoria, the products of eruption. Many volcanoes consist only of a cone of scorice. Such is that of Barren Isle, in the Bay of Bengal. Others, on theI

■Fissures near Locarno.

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). 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 musketr}'.

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. ir), 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, 1st jrevealing themselves in some cases by the melting of the snow upon the flanks of the cone of ejection; while simultaneously with the finallal shock, which overcomes the last resistance opposed by the solid crust •of the ground, a considerable body of gas, and more especially steam, escapes from the mouth of the crater.

The steam, it is important to remark, is essentially the cause 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 of 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 ashcs^ 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 7uatcry volcanoes^ are nothing more than these heavy rains, carrying down with them showers of ashes, stones, and scorice, 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 i,6oo 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 scoriae, 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 181 2 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 rotator)^ 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 d,nd 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

6o THE WORLD BEFORE THE DELUGE.

moiintciin, through ^^'hich the fiery stream discharges itself over the surrounding countr}-. 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 Skaptar 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. Mud volcanoes, or salses^ exist in rather numerous localities. Several are found in the neighbourhood of Modena. There are

IV.—Mud volcano at Turbaco, South America.

some in Sicily, between Aragona and Girgenti. Pallas observed them in the Crimea — in the peninsula of Kertch, and in the Isle of Tamkn. 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."

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