Among the effects of great earthquakes are fissures in the crust of the earth, both In volcanic regions and in distant situations. In 1811 and 1812 the movements of the ground in the valley of the JSississippi And in Caracas caused vast depremions and elevations, some of which remained, so as pertnanently to affect the drainage and change the form of the surface. Some of the numerous fissures produced In Calabria by the earthquake of 1783 assumed a radiating form, and it is con jectured by Lycll that in these situations the ground was perma nently raised. In 1669 the flanks of :Etna were fissured, and through the opening the Monte Rossi was raised, by ejection of miles, &c., to the height of 450 feet. In 1759 the new volcano of Jorullo was formed on the plains west of Mexico by the accumulation of ejected materials (as is now known) Into a mountain I Gar, feet high.
By these instances, taken from situations far from other volcanoes at points in the vicinity of active and extinct volcanoes (the Monte Nuevo), and on the elopes of a frequently burning cone (the Monte Heusi). it appears that generally the earliest observable fact in the history of volcanic phenomena is the opening of the gromni.— a. Along a line of fissure. 6. In a system of intersecting fissures. Such openings, when happening on land,00nstitnto sub-serial volcanoes; and alien occurring in the bed of the Rea they produce submarine volcanoes.
The preceding statements relative to the connection of earthquakes and volcanoes, and the alleged elevation of land by the former, are in agreement with the inferences generally drawn from both classes of phenomena prior to the publication of the receut inductive researches on earthquakes, of which, and of their results, we have given some account in the articles EAHTUQUAKE and SEISMOLOGY. Seismic and volcanic energy are evidently the same in origin, manifested in the former in a manner purely mechanical, and in the latter in a thermal (if not chemical) and also mechanical manner. But we must now admit that these phenomena are not related as cause and effect. Earthquakes do not produce or commence volcanoes, or initiate, though they may be simultaneous with, volcanic eruptions ; nor do volcanoes cause earthquakes, though there is a common cause, or chain of causes, for both. Mr. It. Mallet has shown that fissures even are never produced by the direct passage of the wave of shock in an earthquake, but are a mere secondary effect, conformed to and determined by the dip or elope of the subjacent beds of rock, and are no more than incipient landslips. The movements of the ground in Caracas and in the valley cf the Mississippi, mentioned above, were in all probability of this description. In like manner, having deduced from theory, as well as inferred from observation. that earthquakes cannot produce elevations (although the Latter have been known to have taken place about the same time as earthquakes and in the same region), he examined with care more than 150 miles of sea-coast, as well as river.courses, for
evidence of any permanent elevation of laud having taken place even concurrently with the earthquake of the lath of December, 1857,—the greatest that has occurred in Italy since that of 1733, referred to above,—but found none.
The production of elevations and true fissures appears In fact to be a third great effect, or class of effects, of the common chain of muses of earthquakes and volcanoes,—to be a distinct consequence from them of "the tendency of the globe to swell into froth at the surface," to use the emphatic and comprehensive expression of Sir F. \V. Herschel That a volcano begins by the production of a fissure, through which the ejections constituting an eruption are subsequently to be discharged by the continuance of the force originating the fissure, is most probable; though it may often happen that the elevation of such materials from below falls short of an eruption, and merely fills the fissure (afterwards to be observed as a dyke), or that the materials elevated are of a different description, and are not otherwise related to volcanic pheno mena than as being effects of a common primary cause —subterranean heat. We shall again have to notice the light thrown upon the philosophy of volcanoes by the recent investigations of earthquake phenomena.
Eruptions.—When by some movement of the ground a channel is opened from the interior to the surface of the earth, a paroxysm of volcanic excitement follows, and an eruption happens through the new opening. There tnay be a slow outpouring of melted rock, pressed upwards against gravity by an internal force ; or a violent upburst of clouds of scoriae and ashes, mixed with larger stones ; or a torrent of the same materials mixed with water, and constituting mud ; or volumes of steam and gases of different sorts ; but permanently gaseous matter appears to be much inferiof in quantity to aqueous vapour, and frequently, if not generally, to be the least important product or .duct of an eruption. These are exactly the products, singly or in cembiva tion, which are delivered by longest...blushed vents, and, as far as we can judge, the same have been yielded by volcanoes which probably became extinct before the historic era of the human race; moreover, the volcanoes of all regions agree gencrally,in this respect. Evidently, therefore, the condition of the interior parts of the earth, which aro under the influence of volcanic excitement, is of a general and continuous nature, and must Iss stippoeed capable of interpretation by examination of the products and the circumstances of their extrication.