VOLCA'NOES are conical mountains which vomit flame and smoke, and occasionally throw out showers of ashes and stone, or eject melted rock on the surface of the earth. Volcanoes may have their origin on flat plains on the surface of the earth, or even at the bottom of the sea; but the gradual accumulation of the ejected material around the vent, through which it has been poured, forms in time a mountain, if it is allowed to remain The waves swept away the cone of Grahame's island (q.v.), which in 1831 appeared In the Meditenanean, scattering the lava and scorim of which it was composed at the tom of the sea. When, however, the ejected materials are sufficiently compact to resist i the action of the waves, a permanent island is produced, and sometimes increases in height with a rapidity that can scarcely be imagined. In 1796 a volume of smoke was seen to rise from the Pacific ocean about 30 in. to the ii. of Unalaska. The ejected materials having raised the crater above the level of the water, flames issued from the islet, which illuminated the country for 10 in. around. Six years afterward, when a few hunters landed oil the new island, they found the soil in some places so hot that they could not walk upon it. Repeated eruptions have increased the dimensions of the island, until now it is several thousand feet in height, and between 2 and 3 m. in circumference. Iu the same region is the volcanic island of Klintseliewsk, which rises at once from the sea to the enormous height of 15,000 feet.
The lava, scorite, and ashes which are thrust out of the crater form highly inclined and more or less regular beds on the surface of the mountain, extending from the crater mouth to varying distances down the sides of the volcano. This method of increase gives the uniform conical outline to volcanoes, without the terraces or breaks which are found in almost all other mountains. The sides are often furrowed longitudinally by straight narrow ravines, which increase in number toward the base. These are pro duced by the action of running water obtained from rain or from melting snows during an eruption. The rapidity with which floods rush down the steep sides of a volcano gives a prodigious force, which the loose scorke and ashes, and even the solid lava, can not resist.
The grayish color of volcanic mountains is produced by the ash and scoria?, which, though in composition the same as the dark lava, have this lighter color from the minute subdivision of their particles. When a particular series of rocks remain on the surface,
and are not covered by the products of more recent eruptions, they weather and decom pose, and produce a very fertile soil, which is speedily clothed with vegetation, and thus change the whole aspect of the formerly bare and uniformly-colored mountain.
The vent through which the materials are vomited forth is called the crater. This is a more or less circular opening, communicating with the source from which the ejected materials are obtained. The crater has generally one side much lower than the other— that from which the prevailing wind blows, winch carries with it the showers of ashes to the opposite side of the mountain. In many cases, the cone is truncated; a wide hollow of immense extent, and often of great depth, in the base of which the crater is situated, occupies the summit. The Spanish name caldera is technically applied to these hol lows. Their origin has been a subject of considerable controversy. Von Buell and others maintain that they are craters of elevation; that is, that the rocks were originally spread out in nearly horizontal deposits, and then upheaved into a dome-shaped moun tain, with the hollow caldera in the center of its summit. The more satisfactory expla nation is that the original cone. formed by the alternate deposition of the lava and ashes ejected from the crater, has, from the great heat of the molten lava rising in the tube of the volcano, or from gaseous explosions, given way, and fallen in. The cones both of Etna and Vesuvius have frequently fallen in and been reproduced. In 1822 the summit of Vesuvius was reduced by 800 feet. The immense size of some calderas seems, how ever, opposed to this theory. That of the island of Palma, one of the Canaries, is from 3 to 4 geographical m. in diameter, and the precipices which surround the cavity are from 1500 to 2,000 ft. in vertical height. They form an unbroken wall, except at the south-western end, where a deep gorge permits the passage of the torrent which drains the caldera. The precipices are traversed by numerous vertical dikes, and exhibit all the appearances which would be produced by the falling-in of the huge summit of this once enormous volcano.