VOLCANOES.

THE word volcano is derived from Volcanus, the name of the Roman god of fire, who was supposed to have his subterranean forges at the roots of the mountain Etna. It is applied to any conical mound, hill, or mountain, formed of materials which have been erupted from beneath the surface. When it is active it emits gases, steam, water, mud, dust, stones, or molten rock from its summit, or from fissures or other openings on its sides. The term volcanic action is used to describe all the kinds of work done by a volcano. A volcano may be dormant when it remains a long time without giving any of the usual signs of volcanic action; and it is said to be extinct when, though its external form may be retained, it has never been known to be active and does not seem to have been so for a great many centuries.

The size of a volcano varies from a mere little mound a few yards in diameter, like some of the mud-volcanoes around the Caspian Sea, up to a giant mountain like Cotopaxi, which rises among the Andes to a height of 18,887 feet above the sea, its upper 4,000 feet forming a smooth snow-covered cone with an orifice at the top whence hot ashes and stones are scattered far and wide over the surrounding country.

At the top of a volcano lies a basin-shaped hollow called the crater, from the bottom of which the pipe or

shaft descends, whereby the volcanic products are brought to the surface. Vast showers of fine dust and stones are frequently thrown out from most volcanic craters. These materials falling down the slopes of the cone gradually increase its diameter and height. In like manner streams of molten rock called lava, issuing either from the lowest part of the lip of the crater or from some fissure or orifice on the side of the hill, pour down the slope and harden there, thus still further augmenting the bulk of the volcano.

As a volcano increases in size, and cracks are formed in weak parts of the cone, smaller cones are piled up on its flanks by the emission of dust, stones, and lava from these fissures. A large volcanic mountain, like Etna, or the peak of Teneriffe, is thus sometimes loaded with small volcanoes which often reach a height of five or six hundred feet.

At the beginning of a volcanic eruption rumblings are heard like the muttering of distant thunder, while the ground is felt to tremble slightly. These noises and tremors increase in intensity, successive loud explosions take place in the pipe of the volcano, and at last clouds of fine dust and steam are hurled with prodigious force far up into the air. The steam rapidly condenses into rain, which falls in torrents down the outer slopes of the mountain. The fine dust is sometimes given out in such quantities as to darken the sky for many miles around. In the famous eruption of Vesuvius, which in the year 79 destroyed the Roman cities, Herculaneum, Pompeii, and Stabiæ, the air was

as dark as midnight for twelve or fifteen miles round, and a thick deposit of fine ashes and stones fell on this whole district. The finer dust carried up into a strong upper current of air, is sometimes transported for hundreds of miles before sinking to the ground.

Besides the light dust vast numbers of white-hot blocks and smaller fragments of stone are ejected from the crater. Many of these strike against each other as they rise and fall, producing at night a wonderful spectacle as their sparks and flashes light up the darkness. Of the force with which the stones are occasionally discharged some notion may be formed from the feats of the volcano Cotopaxi, which is said to have thrown a block, estimated to weigh 200 tons, to a distance of nine miles. The volcano of Antuco in Chili has sent stones flying to a distance of thirty-six miles.

It is not difficult to understand why in the earlier stages of a violent eruption so vast an amount of fragmentary materials should be ejected. During the successive explosions the walls of the crater and the accumulated lava and other volcanic ejections which had gradually choked up the funnel are rent and finally blown into fragments. The water which, under enormous pressure, has been retained in the liquid form at a temperature far above the boiling-point, and in that condition has, perhaps, saturated the molten and solid rocks in the pipe of the volcano, instantly flashes into steam when in its upward ascent it can overcome the influence of the superincumbent pressure. So violent

is the conversion of such red-hot or even white-hot water into steam that the molten lava is thrown out in huge jets and actually blown into the finest dust. It is the liberation of successive portions of this highlyheated water into the gaseous state that produces the explosions which form so magnificent a part in a great volcanic eruption. At each explosion a vast ball of steam shoots up into the air, at once condensing into white clouds and rolling over in huge folds, which either dissolve and float away in the higher atmosphere or are further condensed and fall as rain.

We can perceive, too, why it is that the upper part of a cone during the early part of a tremendous eruption should sometimes disappear. It is shattered by the explosions and blown up into fragments, which fall back either into the crater or down the outer slopes of the mountain. Mount Vesuvius has supplied some excellent illustrations of this kind of destruction. Prior to the first century of the Christian era that mountain was a dormant volcano, from which no explosions had ever been known to come, but which had an enormous crater on its summit, overgrown then with brushwood and wild vines Suddenly in the year 79, when the great eruption took place which destroyed Pompeii, the south-western side of the cone was blown away, and a new cone of much smaller dimensions was formed inside the circuit of the former crater. In the drawing of Vesuvius (p. 367) as seen from the sea, the semicircle left of the old crater appears behind the modern diminished cone.

It is no doubt by the enormous expansive force of the imprisoned water and steam, that the molten lava is forced up the pipe of the volcano. After the first explosions, lava is seen to flow either from the top or from one or more points on the side of the cone. Should the sides of the mountain be solid enough to resist the enormous pressure of the ascending column of lava, the latter will, of course, find no escape until it

fills up the crater to the level of the lowest part of its rim, over which it will pour down the mountain. More usually, however, there are weak parts, such as rents, caused by the previous explosions, through which the lava finds egress to the outer slopes of the cone.

Few sights in nature are more terrible than that of a lava torrent as it issues and pours down the side of a mountain. At first it glows with a white light, and