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Show CHAPTER XXVI. Magnitude of the subterranean changes produced by earthquakes at great depths below tho surface-Obscurity of geological phenomena no proof of want of uniformity in the system, because subterramlan processes arc but little understood-Reasons for presuming tho earthquake and volcano to have a. common origin-Probable analogy between tho agency of steam in the Icelandic geysers, and in volcanos during eruptions-Effects of hydrostatic pressure of high columns of lava-Of the condensation of vapours in the interior of the earth-That some earthquakes may be abortive eruptionsWhy all volcanos are in islands or mal'itime tracts-Gases evolved from volcanos-Regular discharge of heat and of gaseous an(l earthy matter from tho subterranean regions-Cause of the wave-like motion and of the retreat of tho sea during earthquakes-Difference of circumstances of heat and pressure at great depths-Inferences from the superficial changes brought about by earthquakes-In what matter the repair of land destroyeu by aqueous causes takes place-Proofs that the sinking in of tho earth's crust somewhat exceeds the forcing out by earthquakes-Geological consequences of this bypothesis, that there is no ground for presuming that the degree of force exerted by subteiTanean movements in a given time has diminished-Concluding remarks. 'iVHEN we consider attentively the changes brought about by earthquakes during the last century, and. reflec~ on the light which they already throw on the ancient history of the globe, we cannot but regret that investigations into the effects of this powerful cause have hitherto been prosecuted with so little zeal. The disregard of this important subject may be attributed to the general persuasion, that former rev?lutions of the earth were not brought about by causes now m operation,~a theory which, if true, would fully justify a geologist in neglecting the study of such phenomena. We may say of the superficial alterations arising from subterranean movements as we have ah·eady declared of the visible effects of ' . active volcanos, that, important as they are m themselves~ they arc still more so as indicative of far greater changes m the interior of the earth's crust. That both the chemical and mechanical changes in the subterranean regions must often be UNIFORMITY OF TilE ACTION OF EARTIIQUAICES. 461 of ll kind to wh' h ~ . 1 . IC no counterpart can possibly be found in pr~gred,:,S Wlt 1dm the reach of our observation, may be confidently 101erre · an 8 1 · d ' . pecu atlOns on these subjects ou()'ht not to be dis-courage , smce a o-reat st · · 'd 'f 0 · f h 0 ep Is game 1 they render us more ~?n~CI~us o It e extent of our inability to define the amount and m ~ .resu .ts to wh.ich ordinary subterranean operations are now grmg ~Ise. It ~s no longer disputed that a great series of convu Slons ave earned up deposits once formed on the bottom of the ocean to the hei o·h t of sever 1 .1 b . · · d'ffi 0 a mi es a ove Its level· and It Is not 1 cult to perceive that th ' ' . I e same movements must 111 numerous p aces have raised rocks to 1 . b f I . e evatwns a ove the level o t Ie sea, wluch were once formed at th d h f m1'1 es m· t h e b ow.e ls of the earth If th eth ept o several d. bl l . I . ' en, ere were no spots 1scovera e w 11c 1 exhibited signs of ext . ·d' h . d h · 1 taor mary mec amcal a. n c emica chang. es, the effects at some 1.v0 1• m er per1•0 d f 0 Immense pressure, mtense heat and oth d' · .1! • .1! ' er con ItiOns 1ar dif- J.ercnt from those developed on the surface it . h b · h · · ' m1g t e urged a~ a tnump ant. argument against those who are dissatisfied with the proofs lutherto adduced in favour of the mutabilit of the course of Nature. y In or<.ler to set this in a clear light let the read h. lf · . ' er suppose 1mse acquamte<.l with just one-tenth part of tl1 d f 1 . . 1 e wor s o some Ivmg ~nguage, and that he is presented with several books purportmg to be written in the same tongue ten centuries ago. If ~e should n.nd that he comprehends a tenth part of the terms. m the ancient volumes, and that he cannot divine the meamng of the other nine-tenths, would he not be stmno·l disp~sed to believe that, for a thousand years, the language hfs remame.d ttnaltcred? Could he, without great labour and stu~y, mterpret the greater part of what is written in the ~nt.I~ue doc 1 umfents, he m~st feel at once convinced that, in 1e Interva o ten centul'les, a great revolution in the Jangu~ ge had taken place. lie might, undoubte<.lly, by comparmg the co~ventional signs already known to him, with those not previOusly acquired, and by oLservino- the analoaies and associations of terms in many of the old books com: at len_g~h to discover the true import of much of th~ ancient wn.tmgs •. and guess at the meaning of nearly all the rest; but if he Is entirely shut out from all communication with those who now use the same language, he will never fully understand the |