The solid products, says Dr. John Davy, whether they consisted of sand, light cinders, or vesicular lava, differed more in form than in composition. The lava contained augite; and the specific gravity was 2.07 and 2.70. When the light spongy cinder, which floated on the sea, was reduced to fine powder by trituration, and the greater part of the entangled air got rid of, it was found to be of the specific gravity 2.64; and that of some of the sand which fell in the eruption was 2.75;* so that the materials equalled ordinary granites in weight and solidity. The only gas evolved in any considerable quantity was carbonic acid.t Theory of Elevation Craters.+-Before quitting the subject of submarine volcanos, it will be necessary to say something of an opinion which has been promulgated by Leopold Von Buch, respecting what he has termed elevation craters (Erhebungscratere.) He has attempted to explain, by a novel hypothesis, the origin of certain large cavities, and the peculiar conical disposition of the masses of volcanic matter which surround them. According to this view, such cones as the ancient Vesuvius (or Somma), and the greater part even of the modern Vesuvius, as well as the nucleus of Etna, and many other mountains of similar form, have not derived the actual arrangement of their materials from successive eruptions as above described (p. 314); but their mode of origin is thus explained: Beds of pumice, breccia, trachyte, basalt, scoriæ, and other substances were first accumulated in a horizontal position, and then lifted up by the force of pent-up vapours, which burst open a cavity in the middle of the upraised mass. By this elevation the beds were so tilted as to dip outwards, in every direction from the central cavity or crater, at various angles of between twelve and thirty-five degrees. In this way, says Von Buch, Monte Nuovo itself originated, being formed of the same marine pumiceous tuff which occurs at Posilippo and the country round Naples. He supposes that, previously to 1538, this tuff stretched uninterruptedly to the site of Monte Nuovo in nearly horizontal beds, until, at that period, it was upheaved and made to constitute a hill more than 400 feet in height, with a crater of nearly equal depth in the centre. In the unbroken walls surrounding the crater appear the upper ends of the beds of tuff, which are there seen to be inclined every where from within outwards. § * Phil. Trans. 1832, p. 243. + Ibid. p. 249. The view which I now give of the theory of elevation craters, although more full, is substantially the same which I published in the first edition, printed in 1829, after I had examined Vesuvius and Etna, and compared them with Mont Dor and the Plomb du Cantal. The late Professor Hoffmann of Berlin set out on his travels through Italy and Sicily in 1829, with a strong expectation of finding every where the clearest illustrations of the "Erhebungscratere;" but when he had explored the Lake Albano, near Rome, as well as Vesuvius, Etna, Stromboli, and the other Lipari Islands, he was compelled reluctantly to abandon the doctrine. (Bulletin de la Soc. Géol. de France, tom. iii. p. 170.) An examination of the same countries led M. C. Prevost, as it had done Mr. Scrope and myself, to similar conclusions. § Poggendorf's Annalen, 1836, p. 181. Before the publication of these opinions it had always been inferred, from the accounts of eye-witnesses, that Monte Nuovo was produced, in 1538, in the same manner as Graham Island in 1831. Those who beheld the eruption relate that a gulf opened on the site of the small town of Tripergola, near Puzzuoli, close to the sea, from which jets of mud, mingled with pumice and stones, were vomited for a day and a night. These substances, falling down on all sides of the vent, caused a conical hill, on which several persons ascended a few days after the eruption, and found a deep funnel-shaped crater on the summit. (See p. 309, and Fig. 22, of Monte Nuovo.) There is no difficulty in conceiving that the pumiceous mud, if so thrown out, may have set into a kind of stone on drying, just as some cements, composed of volcanic ashes, are known to consolidate with facility. One of the first objections which naturally suggest themselves to the notion of a cone like Monte Nuovo being the effect of the sudden uplifting of horizontal beds of rock, has been well stated by Mons. C. Prevost, who remarks, that if beds of solid and non-elastic materials had yielded to a violent pressure directed from below upwards, we should find not simply Fig. 32. a deep empty cavity, but an irregular opening where many rents converged, and these rents would be seen to break through the walls of the crater. They would also be widest at top and diminish downwards. (See Fig. 32. a. b.)* But not a single fissure of this kind is observable in the interior of Monte Nuovo, where the walls of the crater are quite continous and entire. Isle of Palma.-As the theory of elevation craters was first invented for the Canary Islands, it will be desirable to give them our first consideration; and when treating of this subject we must not forget how much we are indebted to the talents and zeal of Leopold Von Buch for his faithful description of these islands, as well as for his numerous other works on Geology. Fig. 33. Nearly in the centre of Palma is an immense circular cavity, called the Caldera or basin, which forms the hollow axis of the entire island. A lofty mountain ridge runs round this axis, and presents in all directions, towards the Caldera, a perpendicular precipice of no less than four thousand feet in height, while on the outside the slope is gentle towards the sea. The middle of the Caldera is more than 2000 feet above the level of the Isle of Palma. ocean; the surrounding borders ("cumbre," or "crest" in Spanish,) are Mém. de la Soc Géol. de France, tom. ii. p. 91. View of the Isle of Palma, and of the Caldera in its centre. of various heights, attaining at one point an elevation of 7234 feet. The diameter of the Caldera is about six miles; and so steep are the cliffs by which it is environed, that there is not a single pathway down the rocks; and the only entrance is by the ravine, or "baranco," which runs from the great circus down to the sea, intersecting all the rocks of which the island is composed. In this section are exposed strata of tuff, alternating with beds of basalt; and below are conglomerates, composed of fragments. of granite, quartz, syenite, and other crystalline rocks, some of which appear in one place in situ. Volcanic dikes, or veins, are seen cutting through all these formations in the precipice on each side of the baranco, and these increase in number as we pass up the gorge, and approach nearer to the Caldera. The veins often cross one another, and at length form a perfect net-work. In the cliffs encircling the Caldera itself are various volcanic rocks, traversed by basaltic dikes, most of which are perpendicular, and appear to hold together the more incoherent masses through which they cut. The sloping sides of the island, which has much the appearance of a flattened and hollow cone, are furrowed by numerous minor ravines, in which beds of red and yellow scoriæ are exposed to view. The ravines are deep near the sea, but they terminate before reaching the Caldera. From this description I find it impossible to draw any other inference than that we have here the remains of a great volcanic mountain, formed by successive eruptions, the first of which burst through granitic rocks. A great cone having, in the course of ages, been built up, the higher parts of it were afterwards destroyed, and the central crater enlarged by gaseous explosions; at the same time that a falling in, or engulfment, of large masses may have taken place. But, according to the theory of "elevation craters," we are called upon to suppose, that a series of horizontal beds of volcanic matter were first accumulated over each other, to the enormous depth of more than 4000 thousand feet, after which the expansive force was directed on a given point with such extraordinary energy, as to lift up bodily the whole mass, so that it rose in some parts to the height of 7000 feet above the sea, while a great void or cavity was formed in the middle. Yet, notwithstanding this prodigious effort of gaseous explosions, concentrated on so small a point, the beds, instead of being shattered, contorted, and thrown into the utmost disorder, have acquired that regular and symmetrical arrangement which characterizes the flanks of the large cone of Etna! Earthquakes, when they act on extensive tracts of country, may certainly elevate and depress them without deranging considerably the relative position of hills, valleys, and ravines. But if the aeriform fluids should break through a mere point, as it were, of the earth's crust, and that, too, where the beds were not composed of soft yielding clay, or incoherent sand, but in great part of solid trachyte and basalt, thousands of feet thick, is it possible to conceive that such masses of rock could be heaved up, so as to attain the height of 7000 feet, or more, without being fissured and fractured in every direction? But there is another difficulty which the advocates of "elevation craters" appear to have overlooked. Numerous dikes or veins of igneous rock are observed in the walls of the craters of Palma, Somma, Stromboli, and other volcanic masses. It is agreed on all hands that such dikes were once fissures, at first empty and afterwards filled up with melted matter. It must also be conceded that the fluid was introduced at different periods, for after the cooling and consolidation of some dikes, new rents have occasionally been made into which other lavas have entered and solidified. Now these phenomena imply the successive rise of lava from the interior towards the surface, precisely in the region where the height of the volcanic mountain is greatest, and where, in perfect accordance with the eruption theory, the quantity of igneous rock and tuff are in excess. It cannot be said in reply, that the dikes were all produced at once during the upheaving of the mass, or, in other words, that fissures were both caused and filled at the moment when the uplifting force was exerted, and when the cone and crater were formed; for had this been the case, there would have been a large quantity of melted matter ready to flow down into the crater, which would then have been partially choked up, at the same time that the fissures would have been left partially empty. Great Canary.-The form of the Great Canary is very analogous to that of Palma, there being here also a caldera and a principal ravine leading out of it, on the south side. The rocks are tuff, conglomerate, basalt, and trachyte. In some of the borders of the island are marls and conglomerates containing recent marine shells, from 300 to 400 feet above the level of the sea, and presenting an appearance, says Von Buch, as if the level of the ocean had subsided at successive periods. These are doubtless the effects of elevation, and at the base of Etna marine strata are in like manner discoverable; but their occurrence does not prove an upheaving of that kind from which cones and craters would result. It is also stated that between masses of basalt in the Great Canary, marine shells have been found, 500 or 600 feet above the sea, all of which is what we should now expect to arise on the flanks of Stromboli. For a great number of alternating beds of lava, and deposits containing shells mixed with volcanic sand and scoriæ, may have accumulated on the flanks of that half-submerged cone, and may, one day, be raised up in the same manner as continents and islands have risen from the deep. Teneriffe.-The Peak of Teneriffe rises out of a valley surrounded by precipitous cliffs, which vary in height from 1000 to 1800 feet, and which are given as an exemplification of the "Erhebungscratere." The Peak stands, says Von Buch, like a tower encircled by its fosse and bastion. The volcanic rocks resemble, in general, those found in the other Canary Islands. Barren Island.-Barren Island, in the Bay of Bengal, is also proposed as a striking illustration of the Erhebungscratere; and here, it is said, we have the advantage of being able to contrast the ancient crater of elevation with a cone and crater of eruption in its centre. When seen from the ocean, this island presents, on almost all sides, a surface of bare rocks, rising, with a moderate acclivity, towards the interior; but at one point there is a cleft, by which we can penetrate into the centre, and there discover that it is occupied by a great circular basin, filled by the Fig. 35. Cone and Crater of Barren Island, in the Bay of Bengal. waters of the sea, and bordered all around by steep rocks, in the midst of which rises a volcanic cone, very frequently in eruption. The summit of this cone is 1690 French feet in height, corresponding to that of the circular border which incloses the basin; so that it can be seen from the sea only through the ravine, which precisely resembles the deep gorge of the caldera of the Isle of Palma, and of which an equivalent, more or less decided in its characters, is said to occur in all elevation craters. It is most probable that the exterior inclosure of Barren Island, c, d, (Fig. 36.) * See Berthelot and Webb, cited by De Beaumont, Descrip. Géol. de la France, tom. iii. p. 254. VOL. I.-2 V |