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Show 106 NEWER PLIOCENE PERIOD• [Ch. IX, 1 . 1 t . fficiently great to overcome the sive force of vo came 1ea IS su . . . f 1 f 1 va several miles or leagues m hetght, resistance o co umns o a ' . fl . . f. . t depths and causmg the md forcmg them up rom grea ' . . fl th surface To 1magme, therefore, matter to ow out upon e . . . . . . h' 11 is so frequently exerted m dlf-that tlus same power, w 1c f tl l be should occasionally propel a column ferent parts o 1e go ' . of lava to a cons1'cl e ra ble beio·ht yet be unable to force 1t o ' . throu· gh the superm. cum b ent roc 1 is qmte natural. {S, • \1\Thenever th e sup erl.tnposed masses happe.n to be of a y1eld· J·O O' an d e1 a sti·c na tur·e , they will bend, and mstead of break- • 0 wg, so as to aJuro r d an escape to the melted. matter throug.h . a fi tl Vl'll allow it to accumulate m large quantities ssure, . 1ey ' beneath the surface, sometimes in amorphous masses, and someti·m es m· hor·1'zontal sheets. So long as such sheets of matter re t a1· n thet'r· fluidity ' and communicate with the column of lava which is still urged upwards, they must exert an enormous llydl·ostatic pressure on the overlying mass, tend· inO' to elevate it, and an equal force on the subjacent beds 0 press·m g tllem down ' and probably r. endering them more com-pact. If we consider how great Is the volume ~f lava that sometimes flows out on the surface from volcamc vents, we must expect that it will produce great changes of level so often as its escape is impeded. Let us only reflect on the magnitude of Iceland~ an island two hundred and sixty miles long by two hundred m breadth, and which rises, at some points, to the height of six thousand feet above the level of the sea. Nearly the entire mass is represented to be of vo1canic origin; but even if we suppose some ts to consist of aqueous deposits, still that portion may be par l'l more than compensated by the great volume of lava w uc 1 must have been poured out upon the bottom of the surroundin()' sea during the growth of the entire island; for we know th:t submarine eruptions have been considerable near the coast during the historical era. Now if the whole of this lava had been prevented from reaching the surface, by the ~eigh~ and tenacity of certain overlying rocks, it might have g1ven r1se to Ch, IX.] SUBTERRANEAN LAVAS~ 107 the gradual elevation of a tract of land nearly as large as Iceland. We say nearly, because the lava which cooled down beneath the surface, and under consid~rable pressure, would be more compact than the same when poured out in the open air, or in a sea of moderate depth, or shot up into the atmq,sphcre by the explosive force of elastic vapours, and thus converted into sand and scorire. According to this theory, we must suppose the action of the upheaving power to be intermittent, and, like ordinary volcanic eruptions, to be reiterated again and again in the same region, at unequal intervals of time and with unequal degrees of force. If we follow this train of induction, which appears so easy and natural, to what important conclusions are we led! The reader will bear in mind that the tertiary strata have attained in the central parts of Sicily, as at Castrogiovanni, for example, an elevation of about three thousand feet above the level of the sea, and a height of from fifty to two thousand feet in different parts of the Val di Noto. In this country, therefore, we must suppose a solid support of igneous rock to have been successively introduced into part of the earth's crust immediately subjacent, equal in volume to the upraised tract, and this generation of subterranean rock must have taken place durin()' the 1atter part of the newer Pliocene period, 'rhe dimension~ of the Etnean cone shrink into insignificance, in comparison to the volume of this subterranean lava; and, however staO'O'ering the inference might at first appear, that the oldest foL~:dations of Etna were laid subsequently to the period when the Mediterranean became inhabited by the living species of testacea and zoophytes, yet we may be reconciled to such conclusions, when we find incontestable proofs of still greater revolutions beneath the surface within the same modern period. Probable stn:ct·u~e of the recent subter·ranean 1·ocks of fusion. -Let us now mqmre what form these unerupted newer Pliocen: lavas of Sicily have assumed? For reasons already explamed, we may infer that they cannot have been converted • |