OCR Text |
Show 394 THEORY OF ELEVATION CRATERS. . . . f the elevation of small masses from to establish the posslbl~~ydo £ t durin()' an eruption, and during a depth of several hun I~ t :~ protr~sion of isolated masses, the emission .of lava. u aff~;ds no analogy to the supposed under such Circumstances, . tlle formation of craters of ive force m . action of the expans . after the observatiOns now . I · hardly necessary, d elevatiOn. t ls . to our section of Somma an made to refer the reader agam l'bed the formation of the ' that we ascr Vesuvius, and to say t operations precisely analogous. anc1. ent an d t 11 e modern cone . ot d out the correspon d cnce of M. Necker :l<' long ago. pmln e t distinctly the origin of the their structure, an d expl.a me. c mose re afterwards confirme d bY f S . and Ins views w . . f tl form o omma' ' . d' tlle J'uxta-positiOn o 1e B t tw1thstan mg d Mr. Scrope. u '. no tl identity of the slope an d tl ·mned cone, 1e · · 1 entire an Je I b d tl e similarity of their mmera quflqutl.-versa1 d'I p of .t he e s., 1 f both cones by porphyri•t i•c . . d the mtersectiOn o d 1 d compositiOn, an f 1 " levation" theory have ec arc dikes, the defenders o . t le f S emma were once horizontal, and that the lavas and breccias o o . 1 mass while they admit · d · n to a co mea ' . were afterwards raise 1 l been as highly inclmed as that those in Vesuvius have a ways they are now. . 1 ' theory we mjght have adduced In controvertmg Von Bucl s .' t l't that it would lead 1 · gument agams ' as the most co~c uslV~ ar with themselves, to the extravaga~t its advocates, lf consistent f Vesuvius had derived then conclusion, that the two cones o B t as these geologists are not form from very d'l St. inct cause.s . to allu its consequences, an d ha v e afraid to follow their system mfi tory of their views, it would S a as con rma . b even appealed to onnn. . . out the closest analogies etween be vain to hope, by pomtmg . t' n and " craters of eleva· f d' ry volcamc ac lO . the effects o or ma don their hypothesis. tion " to induce them to aba~ t tified with basalt, through The marine shelly strata, mt~rs ra ·e also said to have con· which the great cone of Etna n1se s, tiaot n . but when we a1 1 ud e stitutcd an ancient crater ~f e ev; Siciiy it will appear that more particularly to the geo ogyd' o o as t~ countenance in the ·t he strata ·m ques tl'O U do not b Ip. s The nearest approa ch ' least dc()'ree sueh an . hypot esis. . 1 mass by elevatiO· n f ro m b 1 . ductiOn of a comca perhaps, to t le pro Ph t d'Hist. Nat. de Mem de la Soc. de ys. c "' Mcmoirc sur le Mont Somma, . Geneve, tom. u" ., :par t I ., n.~. ·· 155. MINERAL COMPOSITION OF LAVAS. 395 below, is in the Cantal in Central France. The volcanic eruptions wl1ich produced at some remote period the volcanic mountain called the Plomb du Cantal, broke up through fresh-water strata, which must have been deposited ori~inally in . an horizontal position, on rocks of granitic sclust. Durmg the gradual formation of the great cone, beds of lava and tuff, thousands of feet in thickness, were thrown out from one or more central vents, so as to cover great part of the lacustrine strata, and these at the same time were traversed by dikes, and in parts lifted up together with the subjacent granitic rocks; so that if the igneous products could now be removed, and the marls, limestones, and fundamental schists, supported at their present elevation, they would form a kind of dome-shaped protuberance. But the outline of tl1is shattered mass would be very unlike that of a regular cone, and the dip of the beds would be often horizontal, as near Aurillac, often vertical, often reversed, nor would there be in the centre any great cavity or crater of elevation*. On the other hand, the volcanic beds of the Plom b du Can tal are arranged in a conical form, like those of Etna, not by elevation from below, but because they flowed down during successive eruptions from above. We may observe that the Fossa Grande on Vesuvius, a deep ravine washed out by the winter-torrents which descend from the Atrio del Cavallo, may represent, on a small scale, the Valley of Calanna, and its continuation, the Valley of St. Giacomo on Etna. In the Fossa Grande, a small body of water has cut through tuff, and in some parts solid beds of lava of considerable thickness; and the channel, although repeatedly blocked up by modern lavas, has always been re-excnvated. It is natural that on one side of every large hollow, such as the ct·ater of a truncated cone, there should be a channel to drain off the water; and this becoming in the course of ages a deep ravine, may have caused such gorges as exist in Palma and other isles of similar conformation. Mineral Composition of Volcanic Products.-The mineral called felspar, forms in general more than half of the mass of * .s:e a Memoir by Messrs, Mw·chison and Lyell, Sur lcs Depots Lacustres 'l'erhaues du Cantal, &c., Ann, des Sci. Nat., October, 1829. |