Imágenes de páginas
PDF
EPUB

trees habitually grow most on the side on which the most favoring influences predominate. On the sea-coast the trees naturally grow most freely on the land side.

The following gentlemen were elected Resident Fellows, viz. :

Professor Henry W. Torrey of Cambridge, in Class III., Section 3.

Rev. N. L. Frothingham, in Class III., Section 4.
Benjamin A. Gould, in Class III., Section 2.
E. A. Sophocles, in Class III., Section 2.
Dr. C. H. F. Peters, in Class I., Section 2.
Henry James Clark, in Class II., Section 3.

Four hundred and thirty-third meeting.

December 9th, 1856. - ADJOURNED QUARTERLY MEETING.

The Academy met at the house of the President. The President in the chair.

The Corresponding Secretary read letters from the Rev. N. L. Frothingham, accepting the Fellowship; from the Imperial Academy of Sciences, Vienna, March 10th and April 15th, acknowledging the receipt of the Academy's publications ; from the Zoological and Botanical Association, Vienna, May 10th; the Royal Society of Sciences at Upsal, November 16th, 1855; the Royal Prussian Academy of Sciences, Berlin, March 6th ; the Natural History Association of the Prussian Rhine Countries and Westphalia, Bonn, January 12th; the Imperial Geological Society, Vienna, March 20th ; the Imperial Academy of Sciences, Vienna, May 23d and July 16th, presenting their various publications ; from the Society of Physics and Natural History, Geneva, March 11th, in acknowledgment of the receipt of the Academy's publications, and presenting its own, with a circular, offering the fifth annual botanical prize on the foundation of De Candolle.

The President read a paper on the probable cause and nature of the death of Pliny the Elder, taking the ground, in opposition to the commonly received opinion, that he died of apoplexy, and not of suffocation.

A discussion arising upon the nature of the effluvia, ashes, &c., emitted by volcanoes, Dr. Hayes remarked, that eruptions were of a mixed character, distinguished by lava overflows, sublimations, and chloridic and aqueous exhalations in some cases, while in others the presence of atmospheric air and vapor of water in large quantity gave rise to sulphurous acid fumes, and sulphydric gases, with sulphur depositions. Regarding the account of the death of the elder Pliny as remarkably explicit in details, he thought the statement in relation to emission of sulphurous fumes at that time as in accordance with present knowledge on this subject, and yet as in no wise opposing the interesting view which has just now been presented of the cause of his death. The abundant source of sulphur fumes and sulphydric gases is the solfa-taras, which, generally in action, exhibit during eruptions their highest activity; and these existed then, and now exist, in the low grounds in the vicinity of Vesuvius, changing their places as the decompositions on which they are dependent proceed. Solfa-tara action can hardly be classed with true volcanic action, although primarily dependent on it. It is a slow combustion, taking place among the aggregates formed at the time of previous volcanic action, requiring the presence of water for continuing it. The earliest history of Vesuvius presents it as a solfa-tara consuming the lava rocks of an earlier period, and its cones of later dates have been craters of elevation, subject to degradation, which has several times occurred. Pliny, at the time of his death, was at Stabiæ, where were hot volcanic waters along the shores, marking the points of solfa-tara action. The sulphur of commerce is derived from the deposits formed by solfa-taras, and its immense quantity affords some estimate of their extent and antiquity.

Mr. Everett followed, illustrating the changes in locality of the solfa-taras, from his own observations, after an interval of a few years had elapsed. He also stated as his experience, VOL. II.

43

that the crevices of the lavas, in the vicinity of the crater of Vesuvius, emitted sulphur fumes; the napkin containing eggs, while being cooked, was coated with sulphur.

Dr. Pickering remarked, that, so far as his observations extended, sulphur vapors were abundant in lava crevices, wherever vapor was emitted. Referring to the great lava lake of the volcano of Hawaii, he said there was no perceptible smell of sulphur near its surface.

The President, Dr. Peters, and Professor Horsford also took part in the discussion.

The President expressed an interest in the question of the origin of volcanic ashes, referring to the fact of their being carried by winds and dispersed over extended areas at the time of eruptions ; falling on vessels at sea far from any land.

In answer to an inquiry from the President as to the nature of volcanic ashes, Dr. Hayes replied, that they are the finely divided parts of broken down volcanic aggregates, having generally the composition of silicates of alumina, slightly contaminated by other silicates. To have a clear view of the origin of these ashes, it is necessary to consider that volcanic action, under its differing intensities, either fuses together or merely compacts assemblages of divers minerals, including sulphur compounds of metals and of earths. This action is often aqueous or hot-water action, and the rocks formed include the elements of their own destruction, on exposure subsequently to the air and moisture. Thus one of the most solid is the true Augitic Trachyte, which will not resist exposure to a New England atmosphere one year, without crumbling and disintegrating. Craters of elevation are composed in large part of this rock, often covered by true fused lavas in part. The latter, at the points near their source, are tolerably compact, but as they pass along the surface, they become tumefied and scoriaceous, and hence subject to decomposition. When the sulphurets have been engaged in the trachytes, decomposition commences soon after exposure to humidity, attended by the emission of vapors and acid fumes, which corrode and decom

pose all the superincumbent mass of rock, more or less. In craters of depression, as water can more copiously fall in, this decomposition of compound aggregates proceeds with great rapidity, and to great depths. The changes resulting from the chemical action thus established are not merely mechanical; salts more or less soluble form, and are dissolved in the escaping waters, while the rocky masses are reduced to their insoluble, finely divided, proximate elements. Few of the compound silicates resist this action; thus soda felspar, which generally is found in trachytes, and in the laboratory, decomposes slowly, but rapidly yields its silicate of soda to the acid and aqueous vapors, and solfa-tara is soon reduced to a mixture of silicic acid and pipe-clay. From this brief statement of facts, it becomes apparent that every volcanic focus becomes covered to a great depth with finely divided materials, resulting from rock decomposition, and any succeeding eruption must be preceded by a removal of this matter in the way of upheaval. The narratives of the eruptions of Vesuvius, so remarkable for graphic description, show that, after periods of repose, the first efforts of reawakened vigor are expended on the materials covering the surface. One of the most instructive examples, also, is that of the eruption, so called, of the volcano Morne Ronde, on the island of St. Vincent, in 1792, the marks of which I have examined. This, as is well known, is one of the volcanic vents of the Windward West India Islands, and its resumption of activity was preceded by no preliminary efforts.' The inhabitants of the island were roused from their slumbers at about 2 A. M. by a terrific convulsion, the earth swaying under their feet, while the atmosphere, suddenly displaced, was rushing in opposing currents from all directions, attended by deafening reports. By one sudden explosion, the top of the mountain, about three thousand feet high, lost several hundred feet in height, while, as afterwards appeared, a cavity of about eight hundred feet deep was formed. The larger masses of the covering material rolled down its base, while the more finely divided part was carried upward, falling into the sea, — nearer or farther off in proportion to the size of its fragments. The finer parts rose above the current of the trade winds, and, taking the upper and opposite flow, spread over the sea and the island of Barbadoes, obscuring the light of a tropical sun, and causing the greatest consternation on land and sea. This ash I have examined from several parts of its course, and it differed in no respect from the fine parts of the trachyte, undergoing decomposition by atmospheric agents on the spot. There followed after this explosion no flow of lava, but a shower of rude balls of half-fused, tumefied trachyte, succeeded by fragments of rocks, earths, and finally mud and water. The final action took place obviously within the crater, formed more than eight hundred feet below the surface of the top of the mountain, and resulted in the production of a regular cone of sand and gravel, which remained. Twenty years after (1812) a similar explosion took place, and the point of greatest interest is, that a new centre of action appeared. A smaller crater was formed, so near the older one that the rim of the later one breaks its continuity. The action which followed the dispersion of the disintegrated covering in this case was of a kind among the most remarkable on record. A large part of the force was expended in discharging from the crater rocks broken into fragments, from the size of a cubic inch to that of grains of sand; nearly every fragment and grain being bounded by straight lines, square or rectangular, with sharp angles and edges. As the volcanic vents of the West Indies, and indeed whole islands, have been elevated from below a deep ocean which surrounds them, they offer the best examples of that secondary effect, resulting from chemical action taking place within the aggregates formed, which I could adduce.

Professor Horsford suggested that, as the volcanic ashes are silicates of alumina, it might be possible for the mixed chlorides of aluminium and silicium to be shot as a bolt from a crater, and at a distance from that point find moisture and

« AnteriorContinuar »