OCR Text |
Show 96 NEWER PLTOCRNg PERIOD· [Ch. VIII. It is recorded, as we hare already seen in our. history of eart1 1 q uak e s, tl1 a t l'n the year 177~ an immense. subside. nce took p 1a ce on P apan d ayan g , the Iarror est volcano m the Island of Java, and that, during the catastrophe, an extent of ground, fifteen miles in length and six in breadth, gave way, so that no less than forty villages were engulphed, and the cone lost no less than four thousand feet of its height*· . N · 1 t 1 ' 1ow we 1mg1 nacor1 'ne a similar event, or a se.n es of sub-sidences to have formerly occurred on the eastern side of Etna, although such catastrophes have not been witnessed i~ modern times, or only on a very trifling scale. A narrow rav.me, a~o~t a mile long, twenty feet wide, and from twenty to tlurty-s1x m depth, has been formed, within the histori~al era, on the fla~ks of the volcano, near the town of MascaluCia ; and a small cn·cular tract, called the Cisterna, near the summit, sank down in the year 179~, to the depth of about forty feet, and left on all sides of the chasm a vertical section of the beds, exactly resemblincr those which are seen in the precipices of the Val del b • 1 Bove. At some remote periods, therefore, we m1g 1t suppose more extensive portions of the mountain to have fallen in during great earthquakes. But some geologists will, perhaps, incline to the opinion, that the removed mass was blown up by paroxysmal explosions, such as that which, in the year 79, destroyed the ancient cone of Vesuvius, and gave rise to the escarpment of Somma. The Val del Bove, it will be remembered, lies within the zone of lateral eruptions, so that a repetition of volcanic explosions might have taken place, after which the action of running water may have contributed powerfully to degrade the rocks, and to transport the materials to the sea. We have before alluded to the effects of a violent flood, which swept through the Val del Dove in the year 1755, when a fiery torrent of lava. had suddenly overflowed a great depth of snow in wintet· t. In the present imperfect state of our knowledge of the his- * Vol. i. chap. nv. t See vol. i. chap. xxi. Ch. VIII.J ANTIQUITY OF THE CONE OF ETNA. 97 tory of volcanos, we have some difficulty in decidincr on the relative probability of these hypotheses; but if we embrace the theory of explosions from below) the cavity would not constitute a crater in the ordinary acceptation of that termJ still less would it accord with the notion of the so-called 'elevation craters.' ANTIQUITY OF TilE CONE OF ETNA. W c have stated in a former volume, that confined notions in regard to the quantity of past timeJ have tended, more than any other prepossessions, to retard the progress of sound ~heoretical views in Geology ; the inadequacy of our conceptiOns of the earth's antiquity having cramped the freedom of our speculations in this science, very much in the same way as a belief in the existence of a vaulted firmament once retarded the progress of astronomy. It was not until Descartes assumed the indefinite extent of the celestial spaces, and rem~ v~d the supposed boundaries of the universe, that just opmwns began to be entertained of the relative distances of the heavenly bodies; and until we habituate ourselves to contemplate the possibility of an indefinite lapse of acres havin b . d . h' 0 g een ,co~pr1se wit m each of the more modern periods of the earth 8 history, we shall be in danger of formincr most erro-neous and partial views in Geology. 5 Mode of computing the age of volcanos.-If history had bequeathed to us a faithful record of the eruptions of Etna, and a h~ndred other of the principal active volcanos of the globe, dunng the last three thousand years,-if we had an exact acc~unt of the volume of lava and matter ejected during that period, and the times of their production,-we mirrht, perhaps be able to for · o ' m a correct estimate of the average rate of the growth of a volcani F · . f c cone. or we might obtam a mean result rom the comp ari·s on o f t h e erupti·o ns of so great a number of ;rents, however irregular might be the development of the lO'neous t' · do ,. · ac .10n m any one of them, if contemplated singly unng a hl'lef period. Vot, III. H |