Volcano

(võl-kah'no.) [It., from L. Vulcan us, the god of fire.] (Geol.) A conical mountain which vomits flame and smoke, and occasionally throws out showers of ashes and stone, and ejects melted rock on the surface of the earth. When the V. has long ceased to act, it is said to be extinct. The lava, sc oriæ, and ashes which are thrust out of the crater form highly inclined and more or le regular beds on the surface of the mountain, extending from the crater-mouth to varying distances down the sides of the V. The hypothesis of the nucleus of the globe being composed of matter liquefied by heat, offers a simple and general solution of the phenomena of volcanic fires. Observations made in all countries, in mines and caves, prove that, even at a small depth, the earth's heat is much superior to the temperature of the surrounding atmosphere. A fact so remarkable, and elicited from observations made in almost every part of the globe, connects itself with what we learn of the phenomena of volcanoes. The sinking of mines,Artesian wells, ., has even furnished us with data, by which we may calculate the rate at which the temperature increases as we descend into the earth. Ithas been supposed that a crust of only about 200 m. in thickne covers a globular ma of matter, which is in a state of intense ignition, sufficient to fuse with ease the most refractory substances with which we are acquainted. If such be the case, it is easy to conceive that the elastic vapors, which are generated, force to the surface from below the melted matter, and cause it to i ue from openings already in existence, or from new ones-if the others are not sufficiently large or near. There are certain regions to which volcanic eruptions, and the movements of great earthquakes, are almost confined; over the whole of vast tracts active volcanic vents are distributed at intervals, and are most commonly arranged in a linear direction. Throughout the intermediate spaces there is abundant evidence that the subterranean fire is continually at work; for the ground is convulsed, from time to time, by earthquakes; gaseous vapors, especially carbonic acid gas, are disengaged plentifully from the soil; springs often i ue at a very high temperature, and their waters are very commonly impregnated with the same mineral matters, which are discharged by volcanoes during eruptions. The Pacific Ocean is bounded by an almost unbroken line of active volcanoes. Beginning in the New South Shetlands, where there is an active volcano in S. Lat. 62° 55', we pa to Tierra del Fuego, and then on to the Andes, which are throughout their whole course volcanic, although the great centres of present action are confined to Chili, Peru, the neighborhood of Quito, Guatemala, and Mexico. The line is continued northwards by the burning mountains of North-western America, and the Aleutian Islands carry the chain acro to Kamtschatka on the Asiatic side. See EARTH- [s. 943]