VOLCANO. The situations on the globe where subterranean fires have made or found channels to the surface of the land or to the bed of the sea, are termed volcanoes. A volcano is merely the channel of upward communication from the subterranean fires • the mountain in which the volcano ate, and the extensive mounds and masses of ashes, lava, &c. (substances hereafter described), which surround it, aro the effect and the measure of the expansive mechanical forces which are relieved by the pouring forth of the streams of melted or flowing and plastic rocks, the showers of ashes, torrents of steam mingled with gases, and of water, which constitute the eruptions. By considering the nature of these solid, liquid, and gaseous substances, their mutual relations and respective functions iu volcanic phenomena, and the circumstances of their ejection, some progress may be made towards a chemical theory of the nature and origin of the subterranean fires ; but to gain a proper notion of the mechanical forces set in action during volcanic excitement, wo must enter upon a larger Inquiry '—the con nection of earthquakes and volcanic eruptions, the relation. of one volcanic district with another, especially as to coincidence or reciprocity in the times of their violent activity or remarkable repose ; and the history not only of volcanic phenomena which are now in progress or have formerly happened in particular situations, but the general his tory of the effects of the disturbance of the internal heat during all geological periods and over all parts of the globe. Those mechanical forces, it must be remembered, are simply the correlates of this heat, and come into action, primarily, in consequence of the antagonistic action of the sun's rays upon the surface of the earth, and its more immediate results in changing the position of the matter constituting that surface.
lies this extensive inquiry been followed out so completely and methodically as to justify a belief that the true theory of volcanoes is reduced, as several other branches of the great theory of nature have been, to a plain process of induction 1 That many geologists suppose so Is evident from the decision with which their general speculations are advanced ; but tho student who desires to possess clear and systematic inferences without being troubled with contending hypo theses will find it necessary to class the phenomena as if the inquiry were very far from completion. The following views may aid his
researches into this large and interesting subject :— Succession of Volcanic Plgraomena.—A complete history of any one volcano. by showing us its origin, its alternations of rest and activity, its progress to decay. and Its final extinction, would furnish a sufficient base for a general theory of volcanic action ; for the analogies among all burning mountains, as to form, structure, composition, and asso ciated phenomena, are such as to warrant the application of a few general laws and one theory to them all, But we know not completely the succession of phenomena which have happened in any one volcano. We have indeed examples in abundance of new islands and new moun tains being raised in our own days and giving forth fire ; we have the hist iry of Vesuvius as an intermitting volcano for nearly eighteen hundred, and that of 'Etna for above two thousand three hundred, Sears ; and ae may contemplate on the banks of the Rhine, in Hun gary, and in Auvergne, the aspect of a country from which the !subter ranean fires appear to have withdrawn their forces before (though in the country last named probably not long before) the origin of history. The birth, continued activity, decay, and extinction of volcanoes are phenomena seen in separate parts of the earth's surface, and acquire unity and consistence only by being rightly combined into a correct general view of volcanic action.
L'ariliersales.—Previoua to volcanic eruptions cenerally, whether there happen in old craters or burst up in new situation?), earthquakes prevail, sometimes for a considerable period, in the vicinity of the volcano, and extend their terrors to oonsiderable distances from it. Near to the centre of future violence springs have been known to fail and others to burst forth, and unusual noises have been heard. Previous to the year 1538 the Neapolitan shore had been disturbed by earthquakes for two years; and these symptoms of subterranean dia. turbanee were succeeded by the production of the Monte Nuevo (on and over the ancient site of the Lucrine Lake) in the space of forty-eight hours.