Sidor som bilder
PDF
ePub

Fig. 29.

Few of the pieces of stone thrown out from Graham Island exceeded a foot in diameter. Some fragments of dolomitic limestone were intermixed; but these were the only non-volcanic substances. During the month of August, there occurred on the S. W. side of the new island a violent ebullition and agitation of the sea, accompanied by the constant ascension of a column of dense white steam, indicating the existence of a second vent at no great depth from the surface. Towards the close of October, no vestige of the crater remained, and the island was nearly levelled with the surface of the ocean, with the exception, at one point, of a small monticule of sand and scoriae. It was reported that, at the commencement of the year following (1832), there was a depth of 150 feet where the island had been : but this account was quite erroneous; for in the early part of that year Captain Swinburne found a shoal and discoloured water there, and towards the end of 1833 a dangerous reef existed, of an oval figure, about three-fifths of a mile in extent. In the centre was a black rock, of the diameter of about twenty-six sathoms, from nine to eleven feet under water; and round this rock are banks of black volcanic stones and loose sand. At the distance of sixty fathoms from this central mass, the depth increased rapidly. There was also a second shoal at the distance of 450 feet S. W. of the great reef, with fis. teen feet water over it, also composed of rock surrounded by deep sea.

* In the annexed sketch (Fig. 29.), drawn by M. Joinville, who accompanied M. C. Prevost, the beds seem to slope towards the centre of the crater; but l am informed by Mr. Prevost that these lines were not intended by the artist to represent the dip of the beds.

[graphic]

We can scarcely doubt that the rock in the middle of the larger reef is solid lava which rose up in the principal crater, and that the second shoal marks the sight of the submarine eruption observed in August, 1831, to the S.W. of the island. From the whole of the facts above detailed, it appears that a hill 800 feet or more in height was formed by a submarine volcanic vent, of which the upper part (only about 200 feet high) emerged above the waters, so as to form an island. This cone must have been equal in size to one of the largest of the lateral volcanos on the flanks of Etna, and about half the height of the mountain Jorullo in Mexico, which was formed in the course of nine months, in 1759. In the centre of the new volcano a large cavity was kept open by gaseous discharges, which threw out scoriae; and fluid lava probably rose up in this cavity. It is not uncommon for small subsidiary craters to open near the summit of a cone, and one of these may have been formed in the case of Graham Island; a vent, perhaps, connected with the main channel of discharge which gave passage in that direction to elastic fluids, scoriae, and melted lava. It does not appear that, either from this duct, or from the principal vent, there was any overflowing of lava; but melted rock may have flowed from the flanks or base of the cone (a common occurrence on land), and may have spread in a broad sheet over the bottom of the sea.

The dotted lines in the annexed figure are an imaginary restoration of the upper part of the cone, now removed by the waves: the strong lines represent the part of the volcano which is still under water. In the centre is a great column, or dike, of solid lava, 200 feet in diameter, supposed to fill the space by which the gaseous fluids rose; and on each side of the dike is a stratified mass of scorie and fragmentary lava. The solid nucleus of the reef where the black rock is now found, withstands the movements of the sea; while the surrounding loose tuffs are cut away to a somewhat lower level. In this manner the lava, which was the lowest part of the island, or to speak more correctly, which scarcely ever rose above the level of the sea when the island existed, has now become the highest point in the reef.

No appearances observed, either during the eruption or since the island disappeared, give the least support to the opinion promulgated by some writers, that part of the ancient bed of the sea had been lifted up bodily.

[graphic]

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. 31.4); but their mode of origin is thus explained: Beds of pumice, breccia, trachyte, basalt, scoriae, and other substances were first accumulated in a horizontal position, and then listed 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 1 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 Dorand 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 * - a deep empty cavity, but an irregular openFig. 32. ing where-many rents converged, and these

wa. b rents would be seen to break through the

N 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 saithful 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 losty 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.

[graphic]

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 surrowed by numerous minor ravines, in which beds of red and yellow scoriae 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 asterwards 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 list 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

[graphic][graphic]
« FöregåendeFortsätt »