Imágenes de páginas
PDF
EPUB

had been so fortunate as to have escaped, we should have had something like a scientific account of what occurred. As it is, we have only the letters, referring to the circumstances of his uncle's death, written by the younger Pliny to Tacitus, some years after the event. The Plinies were at Misenum, and about 1 p.m. on the 24th of August, A.D. 79, the mother of the younger one called attention to a curious cloud hanging over the Vesuvian region. From the distance, it was not clear from which mountain the cloud proceeded, but it was “ like a pine-tree, for it shot up to a great height in the form of a trunk, which extended itself at the top into a sort of branches.” Many times since has the famous “pine-tree cloud” hung in terrific beauty over the landscape, but this was its first appearance in the historic period; and we cannot wonder that the elder Pliny, who was in command of the Roman fleet at Misenum, at once ordered a light vessel to be got ready, that he might go nearer and examine the strange phenomenon. His. nephew's preference for stopping at home with his books seems unaccountable, unless we ascribe it to fear, notwithstanding his explicit declarations that no such feeling possessed him, at a later period, when flying with his mother from a destruction which he consoled himself with thinking threatened the whole world.

As the elder Pliny was passing out of the house, he received despatches from Retina—the site of the modern Resina, not far from Herculaneum-soliciting his aid, as there was no escape from the fiery perils, except by sea. He proceeded at once with his ships towards the coast, but the sudden retreat of the sea threatened to leave them aground, and the showers of hot cinders and stones made it impossible to take the direction he intended along the coast by Herculaneum and Retina. He then ordered the pilot to carry him to Pomponianus, at Stabiæ, south of Pompeii, and nearly double the distance of that city from the mountain. The eruption continued with great violence, the court which led to the apartment in which he retired to rest became filled with stones and ashes, while violent concussions shook the houses. Pliny, Pomponianus, and the rest of the company went into the open country, with pillows tied with napkins on their heads. They walked towards the shore, intending to re-embark, but the waves rendered this impossible, and the younger Pliny states that flames and noxious vapours dispersed the party, and speedily caused his uncle's suffocation. Professor Phillips doubts the accuracy of this description of the closing scene. He thinks flames and sulphurous vapours could hardly be present at Stabiæ, ten miles from the centre of the eruption.

The difference between modern and ancient times is very strikingly shown in the paucity of information which has come down to us concerning this tremendous eruption. No scientific travellers, or unscientific, but graphic “special correspondents,” hastened to collect particulars. Young Pliny does not say a word about the fate of the two cities, although he gives a vivid picture of the lesser horrors at Misenum, which affected his mother and himself. Martial, writing a few years after the event, makes a passing allusion to both the devastated towns, and Dion Cassius long after (A.D. 230) speaks of them briefly as buried under an "inexpressible quantity of dust."

Pompeii was overwhelmed with dry ashes, while Herculaneum was either buried in erupted mud, or what may perhaps be more likely, under dust converted into mud by torrents of rain. Sir William Hamilton was convinced that the city was covered with “liquid mud” issuing from Vesuvius, and he saw the head of a statue dug out, and leaving a perfect impression in the tufa, which had encased it like a mould.

Previous to the eruption of 79, the mountain appears to have presented the form of a single cone, truncated and hollowed out at the top. In the year 203 what Dion Cassius calls a "mighty conflagration” occurred, confined to the middle of the mountain, and from his description Professor Phillips concludes that nothing like the modern cone of Vesuvius was then known; but that some idea was preserved of a mountain top more elevated and more contracted than that left after the eruption of A.D. 79.

In 472 there was a great outpouring of ashes, spreading as far as Constantinople; 512, 685, and 993 were also years of eruption, and in 1036, Francis Scot, in his “Itinerary of Italy,” relates that it happened not only from the top, but its sides, and that its burning products ran into the sea. In 1049 more lava currents are described as running to the sea. An eruption is also mentioned in 1138, and in 1139 Vesuvius was reported to have flamed for eight days, and to have ejected so much dust and stones for thirty days, that the whole interior was consumed, and the crater is stated to have remained empty till 1631, though volcanic activity was manifested in 1306 and 1500.

In December A.D. 1631 occurred the great convulsion, whose memorials are written widely on the western face of Vesuvius in ruined villages, and left in layers of ashes over hundreds of miles of country, or in heaps of mud swept down by hot water floods from the crater. The crater itself was dissipated in the convulsion.”

This great commotion occurred sixteen centuries after the Plinian eruption, and “since then the mountain has never been at rest."

Professor Phillips gives a table of the eruptions of Etna and Lipari, Vesuvius, and the volcanoes of the Phlegræan tract known to have occurred since the sixth century, B.C., in which none were recorded. In the fifth century, B.C., Etna and Lipari made two eruptions, and one occurred in the Phlegræan fields. In the second century, B.c., there were five Etna and Lipari eruptions, and two in the first century, together with one Phlegræan outburst. In the first century, A.D., one took place in Etna, and one great one in Vesuvius. In the third century, A.D., Etna and Vesuvius had an eruption each, and Vesuvius did not make two in a century until the eleventh, A.D., was reached. It was not till the seventeenth century that more than two Vesuvian eruptions occurred, and in that century there were four, and fourteen of Etna and Lipari. The eighteenth century witnessed twenty-three Vesuvian and fifteen Etnean eruptions, and in the nineteenth century we have already had twentyfour Vesuvian and ten Etnean outbursts. It is remarkable that the Icelandic eruptions seem to have reached a maximum in point of number and in the eighteenth century, and taking the European volcanoes altogether, it would seem that "not less than 2000 years is the average interval between two epochs of maximum frequency in the combined systems of active European volcanoes, and that these apparently separate systems may have a common dependence on some generally recurring condition more extensive than the whole triangular area within which they are placed.”

Professor Phillips observes that, in considering the history of Vesuvius as of other volcanoes, -as indeed of other natural phenomena,we distinguish not only periods of greater or less action, but crises of violence, and epochs of unusual energy. In the series of eruptions from Vesuvius, we may fix on those of A.D. 79, 1631, 1737, 1767, 1779, 1794, 1823, 1855, 1858, as among the more remarkable for the extent of their lava currents, or the abundance of ashes, or the height and splendour of the eruptive columns, which often seemed to deserve the title of liquid fire spoated up to the clouds. The magnitude of the eruptions may be in some degree estimated by the mass of lava ejected. Thus, in A.D. 1737, the mass of lava was estimated at 10,237,096 cubic metres, and in A.D. 1794, a larger quantity flowed, estimated at 20,744,445 cubic metres, both calculations being made by Breislak. “The ashy showers” are believed “ to have carried three times as much matter from Vesuvius as the lava currents."

The phenomena associated with Vesuvius, and similar eruptions, are enumerated by Professor Phillips, as shakings and displacement of the land, retreat and return of great sea-waves, or raising seabed, the sky filled with uprushing volumes of expanded vapour, speedily condensing in clouds and rains, jets of stones, melted lava and scoriæ thrown up to great heights, and frequently falling in parobolic curves at distances of six and eight miles, and currents of melted rock, flowing over the edge of the crater, or bursting forth from fissures in the cone. Mr. Mallet's researches show that earthquakes are not deep-seated. In the Neapolitan regions, the concussions producing them appear to occur at about eight miles depth, at which, the earth's temperature, if presumed to increase in the ordinary proportion, would only be 883•6° F., less than half that of flowing lava. It would seem that in regions of volcanic activity, there is a constant supply of molten matter, ready to rush up through craters or fissures as soon as sufficient pressure is applied, and it is an interesting question whether these lava reservoirs are connected with a general mass of melted matter below the solid crust of the earth, or whether they are local stores, owing their high temperature to local conditions, and not directly deriving it from central heat.

The source of lava floods may be much deeper than that of earthquakes, without any connection with a supposed central incandescent and molten mass. Some years ago, Mr. Hopkins showed that if the interior of the globe was quite fuid from heat, the earth's crust must be at least 600 or 800 miles thick; but recently M. Delaunay has objected that the molten lava may be much more viscous than a true liquid. Professor Phillips remarks, that the interior fluid can only be of the nature of lava, which, when examined at the surface, flows like thick honey, and to such a fluid Mr. Hopkins's reasoning does not apply. But at enormous depths the heat may be sufficient to produce really fluid lava without viscosity. The earth crust cannot be supposed of uniform thickness, like the walls of a bottle. Probably, it is extremely irregular, and a deep seated, or central molten mass, if such exists, may communicate by channels, often irregular and narrow, with reservoirs of molten rock at higher levels. Under such circumstances, “convection” of heat would be very irregular, and our globe might contain molten matter, varying from simple fluidity to viscosity and pastiness. The central heat may set up .chemical actions in various localities not far removed from the surface, and those actions may, as in laboratory experiments, develope more

heat, and melt rocks in their vicinity without any further aid from central fire than that which sufficed to bring the chemical force into play.

Chemical theories of volcanoes should not be abandoned too hastily. They may require modification as science advances, and the particular views of Davy or Daubeny may not be sustainable, but it does not seem prudent to have recourse to central fire and primitive unsolidified terrestrial matter while the real condition of the earth's interior is so little understood. We quite agree with Professor Phillips that a complete theory of volcanoes should contain account of the consolidation of matter, and be in harmony with the general history of the cosmos; but we doubt whether he is entitled to say that the fluidity of silicated matter, and so forth, poured out by volcanoes is due to the “inherent heat of the globe.” It may be so, but it is not proved by Fouqué, or by any one else. Many lunar craters have the appearance of being hardened when the crust of our satellite was in a pasty state, and when it was much nearer the earlier stages of consolidation than any known portion of our earth, as at present existing. If we assume that the earth and moon passed from the nebulous or gaseous to the fluid state, and then gradually formed a solid crust, early volcanic eruptions would consist in outbursts of the central fluid through the thin walls of that crust while it was pasty, or as soon as it became solid; but if the cooling process went on until the crust was so thick that no lava could be forced up from the central molten mass, does it follow that eruptions would cease? Yes, according to Professor Phillips's views, but not so if we admit that chemical actions give rise to local fusions.

When great reservoirs of molten matter exist, the incursion of water from the sea through fissures or rocks would seem sufficient to account for earthquakes, and for the pressure necessary to elevate large columns of lava, and cause their overflow.

Among the numerous interesting questions which Professor Phillips treats in the work before us is that of the earth's contraction by gradual cooling. This cooling would necessarily take place very irregularly, and the contraction resulting from it may lead to great displacements of particular areas least able to resist the disturbing force. The crystallization of rocks also leads to powerful expansions, and Professor Phillips considers that the elevation of the Scandinavian coasts noticed by Lyell could be accounted for by the formation below it of less than fourteen feet of granite in one hundred years. He says, “to me it appears clear that on the general fact of a

« AnteriorContinuar »