Page images
PDF
EPUB

records of the American consul, by which it appears that eighty thousand boxes is the exportation of a very productive year, which is somewhat less than the produce of two trees by Ashe's method of computation. The cultivation of fruit appears to be the only considerable branch of agriculture, except that of Indian corn, of which three hundred thousand bushels are annually exported. No wine is made beyond what is required for home consumption. The tenure by which lands are held is unfavourable to the encouragement of agriculture, which, combined with the indolent habits of the people, and their prejudices against innovations, however beneficial, reduce the surplus produce of the island vastly below what the fertility of its soil, and the excellence of its climate, make it capable of yielding. This is an affair of much more importance to the people of the west of Europe and to us, than it is to the inhabitants of the island; for, beside being a most convenient and admirable fruit garden for both countries, the commerce which is carried on with it affords employment to a very considerable number of their ships and men.

The remainder of the volume is entirely geological, and we congratulate the author upon being the first to furnish this needed information, and thus to fill up one of the chasms in the history of volcanos. It does not appear that these islands were observed to be volcanic, until a long time after their discovery. Berland, who visited them in 1589, says nothing of it; neither does Linschoten, who was there in 1592, although both of them speak of the earthquakes by which they are convulsed. One of these, according to the latter authority, must have been more violent than any that has been felt since. The shock began July 26, 1591, and lasted till August 12, during which time, the land in some places rose up, and the cliffs removed from one place to another, and some hills were levelled and made even with the ground; and such thunder and noise were heard under the earth, as if all the devils in hell had been assembled together there. This is very like Kircher's description of the earthquake which preceded the submarine volcanic eruption in 1638, when a new island sprung up from the sea, near St Michael : and it is in the work which contains this account, his Mundus Subterraneus, first published in 1660, that we find the earliest mention of the volcanic character of the Azores. Another island made a like sudden appearance between St Michael and Terceira, November 1720, New Series, No. 9.

6

which again brought them into discussion, and the fact of their being volcanos was as clearly established as any scientific facts ever were in those days. Separate accounts of this phenomenon were published in the London Philosophical Transactions for 1722, and in the Histoire de l'Academie des Sciences of the same year, and it is worthy of remark, that in the former the event is said to bave happened eighteen days earlier than in the latter. The English account, however, is most entitled to credit, as it came from an eye witness, John Robinson, master of the pink snow Richard and Elizabeth, from Piscataqua in New England. Since that time these islands have always been set down as volcanos, but nothing more is found in any writer on the subject than the bare fact. They are recognized as such by Faujas de St Fond, in his · Recherches sur les Volcans ;' but in bis Mineralogie des Volcans,' he never refers to them as localities of any of the volcanic substances described by him, nor does Ordinaire, in his Histoire naturelle des Volcans,' give any particulars of their history. Hebbe, a Swedish traveller, spent the winter of 1802 in Fayal, and on his return to Sweden published an account of that island, with some remarks on the other Azores, but he did not anticipate Dr Webster in any part of his geological information : still less did Capt. Ashe, notwithstanding he went so deeply into the subject, as to satisfy himself that the subterraneous conflagration was travelling westward, and that in a short time they [the people of St Michael] would be relieved from all apprehensions of volcanos and earthquakes.' In fact, the examination of volcanos and volcanic phenomena in situ is altogether a modern study ; for, although it began with Pliny, it was laid aside again until the time of Sir William Hamilton, to both of whom it was suggested by the same circumstance,-a residence in the neighbourhood of Vesuvius. Sir William's example was soon followed by Dolomieu, Ferber, Spallanzani, Dietrich, Born, Gioeni, Raspe, Fortis, and Faujas de St Fond; and more latterly a new interest has been given to the inquiry by the influence ascribed by Dr Hutton and Mr Playfair to the great volanic agent in the formation of the present exterior crust of the earth. Breislak, Von Buch, Montlosier, Lacoste, sir G. Mackenzie, Cordier, and Humboldt have collected new facts from widely distant regions, in which these grand operations of nature are exhibited; but the mystery is not yet solved, and the remark of Humboldt still remains true, that notwith

standing all the researches that have recently been made on volcanos, there is nothing but the study of volcanic products, which has made any progress. As to the nature of the combustibles which nourish those subterranean fires, we are very far from being able to give an explication, which, without being contrary to the rules of chemistry and of physics can account for the great phenomena, which volcanic explosions present.'

But it is time to see what new light our author has elicited from the hitherto neglected Azores. These islands are represented as exhibiting at every step marks of a comparatively recent formation. St Michael, the one which he examined, is bounded by high precipitous shores, and broken by a range of hills and mountains running through it nearly in an east and west direction. These mountains are not in one continuous chain, but are for the most part completely separated at their bases, having the usual volcanic conical form, the summits being sometimes sharply acuminated, and sometimes truncated and hollowed into basins, some of which are several miles in circuit, and filled with water so as to form considerable lakes. The vallies and level lands of the island are all of the same volcanic character, the surfaces, as deep as could be examined, being either lava unchanged or decomposed, and converted into breccia, or beds of pumice, or tuff, or scoriæ, in a word some one of the various substances ejected from the craters. We know not, therefore, upon what this volcanic mass rests; it gives us no other insight into its history, than what its products afford. Among these there are very few, if any, which distinguish it from the volcanos before known, but it presents us with nearly all the substances, which have been collected from the others, as well those which are still in activity as those which have been long extinguished.

Of lithoid lavas we have the several varieties produced by their two chief constituents, felspar and augite, according to the proportions in which they are found, and the greater or less abundance of their usual accessories, hornblende and oxidulous iron. The porphyritic lava, or trachyte, is the most abundant; basalt, the other principal volcanic rock, occurs but rarely. The lavas which have lost their lithoid character, and become vitrified or scorious, are seen in great quantities. The obsidian, which is for the most part trachytic, is described as less compact than that of Iceland, but quite as beautiful in colour and lustre. It is found in the ravines among the loose

rocks, sometimes imbedded in the masses of pumice, which in many parts of the island cover a great extent of surface. These two volcanic products are observed constantly passing from one into the other, leaving it scarcely possible to distinguish the line of separation, still less to doubt that they were at one time the same identical substance. All the other marks by which the mighty effects of the agency of fire are recognized, such as scoriæ, volcanic sand, ashes, túff, breccias, and lava in every state of decomposition and new combination, are spread over the whole island, so as to leave no doubt which of the elements has exercised dominion there. A farther resemblance to the volcanos heretofore known is observed in the minerals found by Dr Webster among the rocks of St Michael, which had undergone either but a slight or no change by the action of fire. Those mentioned by him are felspar, augite, hornblende, olivine, mica, haüyne, leucite, arragonite, and oxidulous iron, but he does not appear to have found either meionite, nepheline, or pleonaste, which are thus far the peculiar characteristics of Vesuvius. In many respects, however, the masses, which contain these minerals from St Michael, have a striking resemblance to those from Vesuvius, particularly a semivitrified kind of felspar, which we doubt not is the eisspath of Werner and the German mineralogists. We trace also an almost perfect similarity to the solfatara of the

Campi Phlegræi, in his description of the country around the Caldeiras :

* As we pass along the narrow road from the village to this spot,' says our author, the gradual change from a fertile to a barren soil is observed, and within a few yards of the hot springs, nearly all traces of vegetation are lost. At the extremity of the road, the ground is almost snow white, and then acquires a reddish tinge; this increases in intensity and brightness, and finally through an infinite variety of shades to a deep brown. The vicinity of the springs is indicated by the increased temperature of the earth, a sulphureous odour, and the escape of vapour or steam from every crack and fissure in the ground. The volumes of smoke and steam rolling upwards from the surface to a great height, till they are gradually diffused through the atmosphere, or mingle with the heavier clouds that crown the summit of the mountains, produce a striking effect.

A few yards from the principal Caldeira is an elevation of about fifty feet in height, and probably as many in extent, composed of alternate layers of a coarser variety of sinter and clay, including grass, ferns, and reeds in different states of petrifac

tion. Not many years since, the side of this hill fell in, and dis-
covered a deep and frightful chasm ; smoke and steam at present
issue froin it in vast quantities, accompanied by a tremendous
noise. Looking down through this opening, a body of water is seen
boiling with great violence. An appalling roar is incessantly re-
verberated from side to side within the dome, and is increased at
short intervals by sudden and violent explosions. The surface
of this hill, the sides of the cavern, and the innumerable crevices
in the ground are coated with sulphur. Every stone has been
more or less changed, while not a shrub or plant flourishes for
many yards around.'
There is not a little in this description to remind one of the

Locus exciso penitus demersus hiatu
Partlienopen inter magnæque Dicarchidos arva

Cocyta perfusus aqua,' of Petronius.

Dr Webster suspected that sulphuret of iron in the act of decomposition was the cause of the phenomena observed about these hot springs, and on digging, he had the satisfaction of finding large quantities, several feet below the surface, under loose masses of basaltic fragments, which contained, without doubt, microscopic portions of the sulphuret. Breislak found the same at Solfatara, and regards its operations in the same light. There is nothing unreasonable in supposing it to be the cause of such partial and confined effects, but we do not see how it could ever have been thought adequate to the production and enkindling of volcanic fires.

It would be thought unreasonable perhaps to complain of our author for not doing more, when he has done so much, and been so faithful in what he has done, but we should have been pleased if he had furnished us with a few more means of judging of the antiquity of these volcanos, and of the number . of eruptions they have poured out. The depth of the currents of lava might in many cases be exactly ascertained, and the fact determined whether they flowed over others more ancient, or whether the whole surface of the island above water is the effect of one mighty convulsion. We are inclined to believe, from all that we gather from his accounts, that it is the production of repeated volcanic eruptions, but that none of them ever burned for any considerable time. It is not improbable even that a process has been employed in its formation similar to that which produced the recent submarine vol

[ocr errors][ocr errors]
« ՆախորդըՇարունակել »