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Show 94 NEWER PLIOCENE PERIOD. [Ch. VII. ·floods do occasionally sweep d own th e flanks of Etna when eruptw. ns take place . . t . nd when the snows are melted m wm ei' a by lava. 1 Many oftl 1 e angu1 a r f r agments may have been t 1rown out by vco lcam• c exp1 o s·w ns, which ' fallinOo' on the har. dened sur- face of mov.m g 1a va-cu rrents ' may have been carried to a con- s.i derab1 e dI' stance. .I t may also happen, that when lava .a d- vances very s1o w1 y , in the manner of the flow of 18.1 9, descnbed m. the £ rs t vo1 u me *, the angular masses resultmg . from the frequent b reak m. g of the mass as it rolls over. upon Itself, may prod uce t h ese b I ·ecci' as • It is at least certam, that the u•p per portw· n of th e 1a V"...'. -cu1•1·ents of 1811 and 1819, now consist of angular masses to the depth of many yards. . D' Aubuisson has compared the surface of one of the ancient lavas of Auvergne to that of a river sud~en~y frozen o~er .by the stoppage of immense fragments of dnft-ICe, a descr1pt1on perfectly applicable to these modern Etnean flows. '* Chap. xxi. CHAPTER VIII. Speculations on the origin of the Val del Dove on Etna-Subsidences-Antiquity of the cone of Etna-Mode of computing the nge of volcanos-Their growth analogous to that of exogenous trees-Period required fo1· the production of the lateral cones of Etna-Whether signs of Diluvial Waves are observable on Etna. ORIGIN OF THE VAL DEL DOVE. BEFOitE concluding our observations on the corie of Etna, the structure of which was considered in the last chapter, we desire to call the reader's attention to several questions :-fit·st, in regard to the probable origin of the great valley already described; secondly, whether any estimate can be made of the length of the period required for the accumulation of the great cone; and, thirdly, whether there are any signs on the surface of the older parts of the mountain, of those devastating waves which, according to the theories of some geologists, have swept again and again over our continents. Origin of the Val del Bove.-We explained our reasons in the last chapter for not assenting to the opinion, that the great cavity on the eastern side of Etna was the hollow of an immense crater, from which the volcanic masses of the surrounding walls were produced. On the other hand~ we think it impossible to ascribe the valley to the action of running water alone; for if it had been excavated exclusively by that power~ its depth would have increased in the descent; whereas, on the contrary, the precipices are most lofty at the upper extremity, and diminish gradual1y on approaching the lower region of the volcano. The structure of the surrounding walls is such as we should expect to see exhibited on any other side of Etna, if a cavity of equal depth should be causeJ, whether by subsidence, or by the blowing up of part of the flanks of the volcano~ or by either of these causes co-operating with the removing action ~f running water. |