The pressure of the incandescent lava often forces for itself a passage to the surface before it reaches the mouth of the crater, and this is more frequently the case when the volcanic eruption is accompanied with earthquakes. Immense vertical fissures arc found radiating from the center of the volcanic action, and reaching the surface of the ground, and even rising to the summit of the mountain; these being filled with the molt en rock, winch in course of time solidifies and forms often a large portion of the moun tain mass, as is shown in the Val del Bove on Etna (q.v.). The lava sometimes pours out of these fissures instead of rising to the crater. In 1783 during a terrible eruption of Reda, a prodigious stream of lava flowed from a lateral crevice; moving slowly down the mountain-side, it reached a distance of 50 rn. in 42 days; it then branched into two main streams, the one runninr. 40 m. and the other 50 m. further toward the sea. Its depth varied from 600 to 1000 ft., and its greatest width was 15 miles. The amount of lava poured out into this stream would almost equal Mont Blanc in bulk.
The power which exhausts itself in the eruption of a volcano often shows itself by changes which it produces in the level of the country around. About a hundred years ago, a volcano appeared in the center of the great table-land of Mexico, and raised an area of nearly 4 sq.m. 5.50 ft. higher than it was before, covering it at the same time with conical hills of vat-ions heights. the highest of which is Jorulla, which is 1600 ft. high. But sometimes a subsidence takes place. In 1772 a great part of the Papanda yang, a mountain in Java, was swallowed up; the inhabitants of its declivities were sud denly alarmed by tremendous noises in the earth, and before they had time to retire, the mountain began to subside, and soon disappeared. The area thus sunk was 15 m. long and 6 broad.
A volcanic eruption is generally preceded by rumbling noises and slight move ments in the earth; then fitful puffs of gases and steam are given off. These contain much sulphur; and some volcanoes give out such quantities of car bonic acid and other mephitic gases as to destroy the animals in the neighbor hood. Sir William Hamilton picked up dead birds on Vesuvius during an eruption; in 1730, all the cattle in the island of Laucerota, one of the Canaries, were destroyed by these deleterious emanations. The ti pas valley in Java contains an extinct crater; and the certain death which overtakes every animal that penetrates the valley is due to the noxious gases given out from it, and not to the antiaris, which, though yielding a deadly poison, does not affect the atmosphere in which it grows. The eruption itself begins, perhaps, with the ejection of the finest dust, and that with such a force as to pro ject it high into the atmosphere, where, taken up by air-currents, it is often carried to enormous distances. In 1845, the dust from Hecla was in ten hours thickly de posited on some of the Orkney and Shetland islands; the ashes from Conseguina fell, in 1835, on the streets of Kingston, Jamaica, at a distance of 700 m.; and during the same eruption, the fine dust covered the ground at a distance of nearly 30 m. to the s. of the volcano, to a depth of more than 10 ft., destroying the woods and dwellings, and enveloping thousands of quadrupeds and birds.
The flames seen issuing from the crater are usually the reflection of the glow ing lava emitted from the crater, and illuminating the clouds of vapor, scoria', and ashes.
Lava and scorhe are at last vomited forth. Sir William Hamilton says that, in 1779, the jets of liquid lava from Vesuvius, mixed with scoriae and stones, were thrown to a height of 10,000 feet, giving the appearance of a column of fire. The lava, however, generally issues from openings in the side of the mountain. It pours forth in a per fectly liquid state, bright and glowing with the splendor of the sun. At first, it flows
rapidly; but as its surface becomes cooled and converted into slag, its velocity dimin ishes. It has to burst the indurated coating before it can continue its progress, and the liberated lava when it flows bears on its surface masses of scoriae, looking like the slag from an iron furnace.
The materials ejected from a volcano, though differing greatly in appearance, have the same mineral composition. The ash is merely the pumice in a very fine state of division, and the pumice also is only lava made vesicular from contact with air or water.
The theories propounded to account for volcanic action are either chemical or geo logical. Sir H. Davy suggested that if immense quantities of the metallic bases of the earths and alkalies were present in the interior of the earth, all the phenomena would be produced by their oxidization from contact with air or water. Although the distin guished author of this theory abandoned it, it has since been taken up and advocated by Dauheny and others. Bischof, assuming that the interior of the earth consists of a highly heated and fused mass, considers that the mechanical action of water, converted into steam by the great heat, would produce volcanic action. Both theorists seek support for their views from the fact, that the great majority of volcanoes are situated on or near the sea-coast. Geologists accepting also the doctrine of internal heat, and believing that at a certain depth the rocks of the earth are, partially at least, in a state of fusion, explain volcanoes by considering them as connections established between the interior of the earth and the atmosphere, the elastic force of steam being the propelling power. Darwin, from observations made in all parts of the world, believes that volcanoes a:e chiefly, and, indeed, almost only, found in those areas where subterranean motive-power has lately forced, or is now forcing upward, the crust of the earth, and are invariably absent in those where the surface has lately subsided, or is still subsiding.
Volcanic action is limited to particular regions of the earth. In these regions, the active vents are distributed at intervals, and are generally arranged in a linear direction. The Pacific ocean is bounded by an almost unbroken line of active volcanoes. Begin ning in the New South Shetlands, where there is an active volcano in lat. 62° 55' a., we pass to Terra del Fuego, and then on to the Andes, which are throughout their whole course volcanic, although the great centers of present action are confined to Chili, Peru, the neighborhood of Quito, Guatemala, and Mexico. The line is continued northward by the burning mountains of north-western America, and the Aleutian islands carry the chain across to Kamtchatka on the Asiatic side. Here, turning southward, the hue may be traced through the Kurile islands, Japan, Formosa, the Philippines, Moluccas, New Guinea, and the Salomon and New Hebrides groups, to New Zealand. From Celebes, a branch proceeds in a north-westerly direction through Java and Sumatra, to Barren island in the bay of Bengal ; and even beyond this we find a region in northern India subject to earthquakes, which may lead us, on the one hand, to the volcanic region in Tartary, or, on the other, through Asia Minor to the Greek archipelago, Sicily, Naples, and on to the Canaries and cape de Verd. According to the geological theory, the lines thus traced over the globe would represent rising lands, where the crust is less strong, and so less liable to repress the expansive powers below. There are a number of isolated volcanoes also scattered over the surface of the earth. These are supposed to have opened a star-shaped communication with the interior. The most remarkable of these isolated volcanoes are Jan Mayen. in lat. 70° 49' n., and those in Iceland in the north, and mount Erebus in s. Polarland, in ]at. 77° 32' s.