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Show CHAPTER XVIII. Division of igneous agents into the volcano and the earthquake-Distinct regions of subterranean disturbance-Region of the Andes-System of volcanos extending from the Aleutian isles to the Moluccas-Polynesian archipelagoVolcanic region extending from the Caspian Sea to the Azores-Former counexion of the Caspian with Lake Aral and the Sea of Azof-Low steppes skirting these seas-Tradition of Deluges on the shores of the llosphorus, Hellespont, and the Grecian archipelago-Periodical alternation of earth. uakes in Syria and Southern Italy-Western limits of the European region ~Earthquakes rarer and more feeble in proportion as we recede from the centres of volcanic action-Extinct volcanos not to be included in lines of active vents. WE have hitherto considered the changes wrought, since the times of history and tradition, by the continued action of aq~e. ous causes on the earth's surface; and we have next to examme those resultin"' from igneous agency. As the rivers and springs On·the land ~nd the tides and currents in the sea, have, with ' . some slight modifications, been fixed and constant to certam localities from the earliest periods of which we have any records, so the volcano and the earthquake have, with few exceptions, continued, during the same lapse of time, to disturb the same regions. But as there are signs, on almost every pa~t of our continent, of great power having been exerted by runmng wa.ter on the surface of the land, and by tides and currents on chffs bordering the sea, where, in mode~n ~imes, no rivet'S ~ave excavated and no tidal currents undermmed-so we find s1gns of volcani~ vents and violent subterranean movements in places where the action of fire has long been dormant. We can explain why the intensity of the force of aq~eous causes should be developed in succession in different districts. Currents, f?r example, and tides, cannot destroy our coasts, sh.a~e ou~ or silt up estuaries, break through isthmuses, and anmb1late ~sla~ds~ form shoals in one place and remove them from anothe1' with out the direction and position of their destro~i~1g and t~anspor~ ing power becoming transferred to new locaht1es. Ne1ther ca POSITION OF VOLCANIC VENTS. 313 the relative levels of the earth's crust, above and beneath the wat.crs, vary from ~ime to time, as they are admitted to have v~ned at f?rmer penods, ~nd as we shall demonstrate that they still ~o, Wlthout the c~ntments being, in the course of ages, ~od1fied, and even entirely altered, ~n their external configuration. s.uch events must clearly be accompanied by a complete change m the volume, velocity, and direction of the streams and ]and floods to which certain regions give passage. That we should find, therefore, cliffs where the sea once committed ravages, and from which it has now retired-estuaries where high tides once rose, but which are now dried up-valleys hollowed out by water, where no streams now flow ;-all these a.nd similar phen?mena at:e the necessary consequences of physical causes now m operation; and we may affirm that, if there be no instability in the laws of Nature, similar fluctuations must recur again and again in time to come. But however natural it may be that the force of running water in numerous valleys, and of tides and currents in many tracts of the sea, should now be spent, it is by no means so easy to explain why the violence of the earthquake and the fire of the volcano should also have become locally extinct, at successive periods. We can look back to the time when the marine strata, whereon the great mass of Etna rests, had no existence· and that time is extremely modern in the earth's history. Thi; alone affords ground for anticipating that the eruptions of Etna will one day cease. N ec quoo sulfureis ardet fornacibus ..t£tna Ignea semper erit, neque enim fuit ignea semper, are the memorable words which are put into the mouth of Pythag~ras by the Roman poet, and they are followed by spe~~lat10ns as to the causes of volcanic vents shifting their posJtlon. Whatever doubts the philosopher expresses as to the nature of these causes, it is assumed, as incontrovertible that the points of eruption will hereafter vary, because the; have formerly done so. We h~ve ~nd.eavoured to show, by former chapters, how utterly tlus prmctple of reasoning is set at nou"'ht by the modern schools of geology, which not only refuse 0 to conclude that great revolutions in the t•arth's surface are now in progress, or |