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Show 472 IIOW EARTHQUAKES REPAIR TilE LAND tl1e island of Madeira and several other places. If the upheaving of the coast of Portugal had caused the rett·eat, the motion of the water_s when propagated to Madeira would have produced a wave previous to the retreat. Nor could the motion of the waters at Madeira have been caused by a different local earthquake, for the shock travelled from Lisbon to Madeira in two hours, which agrees with the time which it 1·equired to reach other places equally distant ;~:. "\Ve shall not indulge at present in further speculations on the mode whereby subterranean heat may give rise to the pheIlomena of earthquakes and volcanos. No one, however, can fail to be convinced, if he turns his thoughts to the subject, that a great part of the reasoning of the most profound natural philosophers and chemists can be regarded as little more than mere conjecture on matters where the circumstances are so far removed from those which fall under actual observation. Many processes must be carried on in situations where the pressure exceeds as much that produced by the weight of the loftiest mountains, as the weight of the unfathomed ocean surpasses that of the atmosphere. 'l'he mechanical effects, therefore, of earthquakes at vast depths, may be sach as can never be paralleled on the surface. The intensity of heat must often be so far removed from that which we can imitate by experiments, that the elements of solid rocks or fluids may enter into combinations such as can never take place .within the limited range of our observations. Water at a certain depth may, as Michell boldly suggested, become incandescent without expanding, and rem~in at rest without any tendency to produce an earthquake. Au·, if it ever penetrate to such depths, may become a fluid. Sir James Hall's experiments prove, that, under a pressure of about one thousand seven hundred feet of sea, corresponding to that of only six hundred feet of liquid lava, limestone melts without giving off' its carbonic acid, so that it is only when calcareous lavas are forced up to within a slight distance of the surface, or into a sea of moderate depth, tl1at the carbonic acid begins to assume a gaseous form, and to assist in bringing on a volcanic eruption. Dut let us now turn our attention to those superficial changes ,.. Michel11 Phil. Trans.1 vol.li. P· 614. DESTROYED BY AQUEOUS CAUSES. 473 brouo·ht about by s f h . t 0 d h 0 many 0 t e earthquakes Within the last cen ury an . a alf, before described. Besides the undulatory movements and the · f 1:!. • · ' opemng 0 ussures, It was shewn that certam parts of the earth's crust often of considerable area both above and below the level of th h b . ' 1 1 d e sea, ave een perma-nent y e evatc or depressed. example f 1 . b . l k I . ' s o e evat10n y smcr]e eart 1qua es 1avmcr occurred to th f f 0 o , e amount o rom one to about twenty-nve feet,. and of subsidence fl'Om a few inches to about nfty feet, exclusively of those limited tracts tl ./! f A · h . . , as 1e wrest o npao, w ere a smkmg down to the amount of three hun-dred feet took place. It is evident ' that t11e f,o rce of su b ter-rane~ n movement does not operate at random, but the same contmuous tracts are agitated aO'ain and ag ... 1'n. d 1 . . o " , an 1owever mco. n.s iderable may be the alterations produced dur1· ng a pen·o d suffiCie. nt onlIy for th. e .p rodu.c tion of ten or nfteen eru ptI'o ns of an active ;o cano, It Is obv1ous that, in the time required for the formation of a lofty cone, composed of thousands of lavacurrents, shallm~ seas may be converted into lofty mountains, and low lands mto deep seas. We need, therefore, cherish ~one o~ .the apprehensi~ns entertained by Buffon, that the mequaht1es of the earth s surface, or the heicrht and area of our continents, will be reduced by the ac~ion of runnincr water; nor need we participate in the wonder of Ray, that th: dry la~d. shoul~ not lose ground more rapidly. Neither need we antic1pate with Hutton the waste of successive continents followed by the creation of others by paroxysmal convulsions. The renovating as well as the destroyinG' causes are unceasingly at work, the repair of land being as co~stant as its decay, and the deepening of seas keeping pace with the formation of shoals. If, in the course of a century, the Gancres and other great rivers have carried down to the sea a mas: of matter equal to many lofty mountains, we also find that a distt·ict in Chili, one hundred thousand square miles in area has been uplifted to the average heicrht of a foot or more and I b. b ' t 1e cu 1c contents of the granitic mass thus added in a few hours to the land, may have counterbalanced the loss effected . by the aqueous action of many rivers in a century. On the other hand, if the water displaced by fluviatile sediment cause the mean level of the ocean to rise in a slight degree, such subsidenccs of its bed, as that of Cutchin 1819, or St. Domingo |
