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Show 468 MOST VOLCANOS NEAll TilE SEA. repress them. If we regard earthquakes as abortive volcanic eruptions at a great depth, we must expect them to succeed each other for an indefinite number of times in the same place, for the same reason that eruptions do ; and it is easy to conceive that, if the matter has failed several times to reach the surface, the consolidation of the lava first raised and congealed will strengthen the earth's crust, and become an additional obstacle to the protrusion of other fused matter during subsequent convulsions. As most volcanos are in islands or maritime tracts, the neighbourhood of the sea seems one of the conditions necessary for the ascent of lava to great heights. Even those volcanos which lie inland form part of a chain of volcanic hills, and may be supposed to have a subterranean communication with the ex· tremities of the chain which are in the neighbourhood of large masses of salt-water. Thus Jorullo, in Mexico, though itself 110 less than forty leagues from the nearest ocean, seems, nevertheless, connected with the volcano of Tuxtla on the one hand, and that of Colima on the other, the one bordering on the Atlantic, the other on the Pacific ocean. This communication is rendered the more probable by the parallelism that exists between these and several volcanic hills intermediate*. Perhaps the quantity of water which percolates from the surface of the land is sufficient to contribute to the violence of earthquakes, without producing so much steam as is required to brinO' on a volcanic eruption. But when the sea overlies a mas; of incandescent lava, and the intermediate crust of the "lobe is shaken and fissured by earthquakes, it may well be =upposed that a convulsion of a different kind will ensue. If an open fissure be caused like that which traversed the plain of S. Lio, on Etna, in 1669, so that the water descends at once llpon a mass of melted lava, eruptions will probably burst forth alonO' the line of this aperture, the steam rushing up, together with 0 gaseous emanations from the lava, and carrying up scorire with it. But from what we know of the wave-like motion of the ground during earthquakes, there is good reason to conclude that a continuous communication will rarely be formed betweer. the sea and a bed of lava at great depth below, because the alternate "' See Daubeny's remarks on this suhject,-11 Volcanos/' p. 308, CONSTANT DISCIIARGE OF SUBTERRANEAN IIEAT. 469 risin cr ancl falling f tl 1 1° . . 0 le cart 1 causes chasms to open and aO'ain !0 cliose 111 vw 1 lently. In the same manner, therefore as ya5wn-mcr 1ssures s 1ut a(J'ain f 1 . ' o o a tcr engu phmcr trees and houses so great masses of water may be swall 0d l I ' I· mme dt' ate1 y a f terwards be excluded oweS up, anch t 1e seaI ma.y Vent to b e once wl' rme d b b · · uppos.e t en a Yo came 1 h d ']) y a su marme eruptiOn, all the water engu p e WI ' on penetr~ting to subterranean reservoirs of .h eated lava, be converted mto stearn , and tl u·s st cam rna 1u ·n O' 1ts wa~ thhro~gh th 1 e same channels by which elastic fluid~ escape ~n t e mterva s between eruptions, will drive melted lava before 1t. Successive . eruptions will have a tendency to seek the same vent, especially if the peak of a cone is raised above the water; for then there will probably be no more than the .Pressure of the atmosphere in a great part of the duct leadmg to the crater. Volcano~ exhale, during. er.upti~ns, besides aqueous vapour, the followmg gases: munattc actd, sulphur combined with hydrogen o.r oxygen, carbonic acid and nitrogen, the greater part of wluch ~ould result fro~ the decomposition of salt~ va.ter, a fact winch, when taken .m conjunction with the proxImity of nearly two hundred active vents to the sea and their absence in the interior of large continents, is almost' conclusive as to the co-operation of water and fire in the raising of lava to t!1e surface. .W. e have b:fore ~uggcsted the great probability that, in extstmg volcamc regiOns, there are enormous masses of matter in a constant state of fusion far below the surface : this opinion is confirmed by numerous phenomena. Perennial supplies of hot vapour and aeriform fluids rise to certain craters, as in Stromboli for example, and Nicaragua, which are in a ~tate of. ceaseless eruption. San gay in Quito, Popocatepetl m Mexico, and the volcano of the isle of Bourbon, have continued in incessant activity for periods of sixty or one hundred and fifty years. Numerous solfataras, evolving the same gases as volcanos, serve as permanent vents of heat generated in the subterranean regions. 'l'he plentiful evolution also, of carbonic acid, from springs and fissures throuO'hou~ hundreds of square leagues, is another regular source of 5communication between the interior and the surface. Steam, often above the boiling temperature, is emitted for nges without |
