Volume I Part 6 (2/2)

The objections against the volcanic origin of obsidians, founded on their speedy loss of colour, and their swelling by a slow fire, have been shaken by the ingenious experiments of Sir James Hall.

These experiments prove, that a stone which is fusible only at thirty-eight degrees of Wedgwood's pyrometer, yields a gla.s.s that softens at fourteen degrees; and that this gla.s.s, melted again and unvitrified (glastenized), is fusible again only at thirty-five degrees of the same pyrometer. I applied the blowpipe to some black pumice-stone from the volcano of the isle of Bourbon, which, on the slightest contact with the flame, whitened and melted into an enamel.

But whether obsidians be primitive rocks which have undergone the action of volcanic fire, or lavas repeatedly melted within the crater, the origin of the pumice-stones contained in the obsidian of the Peak of Teneriffe is not less problematic. This subject is the more worthy of being investigated, since it is generally interesting to the geology of volcanoes; and since that excellent mineralogist, M. Fleuriau de Bellevue, after having examined Italy and the adjacent islands with great attention, affirms, that it is highly improbable that pumice-stone owes its origin to the swelling of obsidian.

The experiments of M. da Camara, and those I made in 1802, tend to support the opinion, that the pumice-stones adherent to the obsidians of the Peak of Teneriffe do not unite to them accidentally, but are produced by the expansion of an elastic fluid, which is disengaged from the compact vitreous matter. This idea had for a long time occupied the mind of a person highly distinguished for his talents and reputation at Quito, who, unacquainted with the labours of the mineralogists of Europe, had devoted himself to researches on the volcanoes of his country. Don Juan de Larea, one of those men lately sacrificed to the fury of faction, had been struck with the phenomena exhibited by obsidians exposed to a white heat. He had thought, that, wherever volcanoes act in the centre of a country covered with porphyry with base of obsidian, the elastic fluids must cause a swelling of the liquified ma.s.s, and perform an important part in the earthquakes preceding eruptions. Without adopting an opinion, which seems somewhat bold, I made, in concert with M. Larea, a series of experiments on the tumefaction of the volcanic vitreous substances at Teneriffe, and on those which are found at Quinche, in the kingdom of Quito. To judge of the augmentation of their bulk, we measured pieces exposed to a forge-fire of moderate heat, by the water they displaced from a cylindric gla.s.s, enveloping the spongy ma.s.s with a thin coating of wax. According to our experiments, the obsidians swelled very unequally: those of the Peak and the black varieties of Cotopaxi and of Quinche increased nearly five times their bulk.

The colour of the pumice-stones of the Peak leads to another important observation. The sea of white ashes which encircles the Piton, and covers the vast plain of Retama, is a certain proof of the former activity of the crater: for in all volcanoes, even when there are lateral eruptions, the ashes and the rapilli issue conjointly with the vapours only from the opening at the summit of the mountain. Now, at Teneriffe, the black rapilli extend from the foot of the Peak to the sea-sh.o.r.e; while the white ashes, which are only pumice ground to powder, and among which I have discovered, with a lens, fragments of vitreous feldspar and pyroxene, exclusively occupy the region next to the Peak. This peculiar distribution seems to confirm the observations made long ago at Vesuvius, that the white ashes are thrown out last, and indicate the end of the eruption. In proportion as the elasticity of the vapours diminishes, the matter is thrown to a less distance; and the black rapilli, which issue first, when the lava has ceased running, must necessarily reach farther than the white rapilli. The latter appear to have been exposed to the action of a more intense fire.

I have now examined the exterior structure of the Peak, and the composition of its volcanic productions, from the region of the coast to the top of the Piton:--I have endeavoured to render these researches interesting, by comparing the phenomena of the volcano of Teneriffe with those that are observed in other regions, the soil of which is equally undermined by subterranean fires. This mode of viewing Nature in the universality of her relations is no doubt adverse to the rapidity desirable in an itinerary; but it appears to me that, in a narrative, the princ.i.p.al end of which is the progress of physical knowledge, every other consideration ought to be subservient to those of instruction and utility. By isolating facts, travellers, whose labours are in every other respect valuable, have given currency to many false ideas of the pretended contrasts which Nature offers in Africa, in New Holland, and on the ridge of the Cordilleras. The great geological phenomena are subject to regular laws, as well as the forms of plants and animals. The ties which unite these phenomena, the relations which exist between the varied forms of organized beings, are discovered only when we have acquired the habit of viewing the globe as a great whole; and when we consider in the same point of view the composition of rocks, the causes which alter them, and the productions of the soil, in the most distant regions.

Having treated of the volcanic substances of the isle of Teneriffe, there now remains to be solved a question intimately connected with the preceding investigation. Does the archipelago of the Canary Islands contain any rocks of primitive or secondary formation; or is there any production observed, that has not been modified by fire? This interesting problem has been considered by the naturalists of Lord Macartney's expedition, and by those who accompanied captain Baudin in his voyage to the Austral regions.

Their opinions are in direct opposition to each other; and the contradiction is the more striking, as the question does not refer to one of those geological reveries which we are accustomed to call systems, but to a positive fact.

Doctor Gillan imagined that he observed, between Laguna and the port of Orotava, in very deep ravines, beds of primitive rocks.

This, however, is a mistake. What Dr. Gillan calls somewhat vaguely, mountains of hard ferruginous clay, are nothing but an alluvium which we find at the foot of every volcano. Strata of clay accompany basalts, as tufas accompany modern lavas. Neither M.

Cordier nor myself observed in any part of Teneriffe a primitive rock, either in its natural place, or thrown out by the mouth of the Peak; and the absence of these rocks characterizes almost every island of small extent that has an unextinguis.h.i.+ed volcano. We know nothing positive of the mountains of the Azores; but it is certain, that the island of Bourbon as well as Teneriffe, exhibits only a heap of lavas and basalts. No volcanic rock rears its head, either on the Gros Morne, or on the volcano of Bourbon, or on the colossal pyramid of Cimandef, which is perhaps more elevated than the Peak of the Canary Islands.

Bory St. Vincent nevertheless a.s.serted, that lavas including fragments of granite have been found on the elevated plain of Retama; and M. Broussonnet informed me, that on a hill above Guimar, fragments of mica-slate, containing beautiful plates of specular iron, had been found. I can affirm nothing respecting the accuracy of this latter statement, which it would be so much the more important to verify, as M. Poli, of Naples, is in possession of a fragment of rock thrown out by Vesuvius,* which I found to be a real mica-slate. (* In the valuable collection of Dr. Thomson, who resided at Naples till 1805, is a fragment of lava enclosing a real granite, which is composed of reddish feldspar with a pearly l.u.s.tre like adularia, quartz, mica, hornblende, and, what is very remarkable, lazulite. But in general the ma.s.ses of known primitive rocks, (I mean those which perfectly resemble our granites, our gneiss, and our mica-slates) are very rare in lavas; the substances we commonly denote by the name of granite, thrown out by Vesuvius, are mixtures of nepheline, mica, and pyroxene. We are ignorant whether these mixtures const.i.tute rocks sui generis placed under granite, and consequently of more ancient date; or simply form either intermediate strata on veins, in the interior of the primitive mountains, the tops of which appear at the surface of the globe.) Every thing that tends to enlighten us with respect to the site of the volcanic fire, and the position of rocks subject to its action, is highly interesting to geology.

It is possible, that at the Peak of Teneriffe, the fragments of primitive rocks thrown out by the mouth of the volcano may be less rare than they at present appear to be, and may be heaped together in some ravine, not yet visited by travellers. In fact, at Vesuvius, these same fragments are met with only in one single place, at the Fossa Grande, where they are hidden under a thick layer of ashes. If this ravine had not long ago attracted the attention of naturalists, when ma.s.ses of granular limestone, and other primitive rocks, were laid bare by the rains, we might have thought them as rare at Vesuvius, as they are, at least in appearance, at the Peak of Teneriffe.

With respect to the fragments of granite, gneiss, and mica-slate, found on the sh.o.r.es of Santa Cruz and Orotava, they were probably brought in s.h.i.+ps as ballast. They no more belong to the soil where they lie, than the feldsparry lavas of Etna, seen in the pavements of Hamburg and other towns of the north. The naturalist is exposed to a thousand errors, if he lose sight of the changes, produced on the surface of the globe by the intercourse between nations. We might be led to say, that man, when expatriating himself; is desirous that everything should change country with him. Not only plants, insects, and different species of small quadrupeds, follow him across the ocean; his active industry covers the sh.o.r.es with rocks, which he has torn from the soil in distant climes.

Though it be certain, that no scientific observer has. .h.i.therto found at Teneriffe primitive strata, or even those trappean and ambiguous porphyries, which const.i.tute the bases of Etna, and of several volcanoes of the Andes, we must not conclude from this isolated fact, that the whole archipelago of the Canaries is the production of submarine fires. The island of Gomera contains mountains of granite and mica-slate; and it is, undoubtedly, in these very ancient rocks, that we must seek there, as well as on all other parts of the globe, the centre of the volcanic action.

Amphibole, sometimes pure and forming intermediate strata, at other times mixed with granite, as in the basanites or basalts of the ancients, may, of itself, furnish all the iron contained in the black and stony lavas. This quant.i.ty amounts in the basalt of the modern mineralogists only to 0.20, while in amphibole it exceeds 0.

30.

From several well-informed persons, to whom I addressed myself, I learned that there are calcareous formations in the Great Canary, Forteventura, and Lancerota.* (* At Lancerota calcareous stone is burned to lime with a fire made of the alhulaga, a new species of th.o.r.n.y and arborescent Sonchus.) I was not able to determine the nature of this secondary rock; but it appears certain, that the island of Teneriffe is altogether dest.i.tute of it; and that in its alluvial lands it exhibits only clayey calcareous tufa, alternating with volcanic breccia, said to contain, (near the village of La Rambla, at Calderas, and near Candelaria,) plants, imprints of fishes, buccinites, and other fossil marine productions. M. Cordier brought away some of this tufa, which resembles that in the environs of Naples and Rome, and contains fragments of reeds. At the Salvages, which islands La Perouse took at a distance for ma.s.ses of scoriae, even fibrous gypsum is found.

I had seen, while herborizing between the port of Orotava and the garden of La Paz, heaps of greyish calcareous stones, of an imperfect conchoidal fracture, and a.n.a.logous to that of Mount Jura and the Apennines. I was informed that these stones were extracted from a quarry near Rambla; and that there were similar quarries near Realejo, and the mountain of Roxas, above Adexa. This information led me into an error. As the coasts of Portugal consist of basalts covering calcareous rocks containing sh.e.l.ls, I imagined that a trappean formation, like that of the Vicentin in Lombardy, and of Harutsh in Africa, might have extended from the banks of the Tagus and Cape St. Vincent as far as the Canary Islands; and that the basalts of the Peak might perhaps conceal a secondary calcareous stone. These conjectures exposed me to severe animadversions from M. G.A. de Luc, who is of opinion that every volcanic island is only an acc.u.mulation of lavas and scoriae. M. de Luc declares it is impossible that real lava should contain fragments of vegetable substances. Our collections, however, contain pieces of trunks of palm-trees, enclosed and penetrated by the very liquid lava of the isle of Bourbon.

Though Teneriffe belongs to a group of islands of considerable extent, the Peak exhibits nevertheless all the characteristics of a mountain rising on a solitary islet. The lead finds no bottom at a little distance from the ports of Santa Cruz, Orotava, and Garachico: in this respect it is like St. Helena. The ocean, as well as the continents, has its mountains and its plains; and, if we except the Andes, volcanic cones are formed everywhere in the lower regions of the globe.

As the Peak rises amid a system of basalts and old lava, and as the whole part which is visible above the surface of the waters exhibits burnt substances, it has been supposed that this immense pyramid is the effect of a progressive acc.u.mulation of lavas; or that it contains in its centre a nucleus of primitive rocks. Both of these suppositions appear to me ill-founded. I think there is as little probability that mountains of granite, gneiss, or primitive calcareous stone have existed where we now see the tops of the Peak, of Vesuvius, and of Etna, as in the plains where almost in our own time has been formed the volcano of Jorullo, which is more than a third of the height of Vesuvius. On examining the circ.u.mstances which accompanied the formation of the new island, called Sabrina, in the archipelago of the Azores;* (* At Sabrina island, near St. Michael's, the crater opened at the foot of a solid rock, of almost a cubical form. This rock, surmounted by a small elevated plain perfectly level, is more than two hundred toises in breadth. Its formation was anterior to that of the crater, into which, a few days after its opening, the sea made an irruption. At Kameni, the smoke was not even visible till twenty-six days after the appearance of the upheaved rocks.

Philosophical Transactions volume 26 pages 69 and 200, volume 27 page 353. All these phenomena, on which Mr. Hawkins collected very valuable observations during his abode at Santorino, are unfavourable to the idea commonly entertained of the origin of volcanic mountains. They are usually ascribed to a progressive acc.u.mulation of liquified matter, and the diffusion of lavas issuing from a central mouth.) on carefully reading the minute and simple narrative, given by the Jesuit Bourguignon of the slow appearance of the islet of the little Kameni, near Santorino; we find that these extraordinary eruptions are generally preceded by a swelling of the softened crust of the globe. Rocks appear above the waters before the flames force their way, or lavas issue from the crater: we must distinguish between the nucleus raised up, and the ma.s.s of lavas and scoriae, which successively increases its dimensions.

It is true that from all existing records of revolutions of this kind, the perpendicular height of the stony nucleus appears never to have exceeded one hundred and fifty or two hundred toises; even taking into the account the depth of the sea, the bottom of which had been lifted up: but when considering the great effects of nature, and the intensity of its forces, the bulk of the ma.s.ses must not deter the geologist in his speculations. Every thing indicates that the physical changes of which tradition has preserved the remembrance, exhibit but a feeble image of those gigantic catastrophes which have given mountains their present form, changed the positions of the rocky strata, and buried sea-sh.e.l.ls on the summits of the higher Alps. Doubtless, in those remote times which preceded the existence of the human race, the raised crust of the globe produced those domes of trappean porphyry, those hills of isolated basalt on vast elevated plains, those solid nuclei which are clothed in the modern lavas of the Peak, of Etna, and of Cotopaxi. The volcanic revolutions have succeeded each other after long intervals, and at very different periods: of this we see the vestiges in the transition mountains, in the secondary strata, and in those of alluvium. Volcanoes of earlier date than the sandstone and calcareous rocks have been for ages extinguished; those which are yet in activity are in general surrounded only with breccias and modern tufas; but nothing hinders us from admitting, that the archipelago of the Canaries may exhibit some real rocks of secondary formation, if we recollect that subterranean fires have been there rekindled in the midst of a system of basalts and very ancient lavas.

We seek in vain in the Periplus of Hanno or of Scylax for the first written notions on the eruptions of the Peak of Teneriffe. Those navigators sailed timidly along the coast, anchoring every evening in some bay, and had no knowledge of a volcano distant fifty-six leagues from the coast of Africa. Hanno nevertheless relates, that he saw torrents of light, which seemed to fall on the sea; that every night the coast was covered with fire; and that the great mountain, called the Car of the G.o.ds, appeared to throw up sheets of flame, which rose even to the clouds. But this mountain, situated northward of the island of the Gorilli, formed the western extremity of the Atlas chain; and it is also very uncertain whether the flames seen by Hanno were the effect of some volcanic eruption, or whether they must be attributed to the custom, common to many nations, of setting fire to the forests and dry gra.s.s of the savannahs. In our own days similar doubts were entertained by the naturalists, who, in the voyage of d'Entrecasteaux, saw the island of Amsterdam covered with a thick smoke. On the coast of the Caracas, trains of reddish fire, fed by the burning gra.s.s, appeared to me, for several nights, under the delusive semblance of a current of lava, descending from the mountains, and dividing itself into several branches.

Though the narratives of Hanno and Scylax, in the state in which they have reached us, contain no pa.s.sage which we can reasonably apply to the Canary Islands, it is very probable that the Carthaginians, and even the Phoenicians, had some knowledge of the Peak of Teneriffe. In the time of Plato and Aristotle, vague notions of it had reached the Greeks, who considered the whole of the coast of Africa, beyond the Pillars of Hercules, as thrown into disorder by the fire of volcanoes. The Abode of the Blessed, which was sought first in the north, beyond the Riphaean mountains, among the Hyperboreans, and next to the south of Cyrenaica, was supposed to be situated in regions that were considered to be westward, being the direction in which the world known to the ancients terminated. The name of Fortunate Islands was long in as vague signification, as that of El Dorado among the conquerors of America. Happiness was thought to reside at the end of the earth, as we seek for the most exquisite enjoyments of the mind in an ideal world beyond the limits of reality.* (* The idea of the happiness, the great civilization, and the riches of the inhabitants of the north, was common to the Greeks, to the people of India, and to the Mexicans.)

We must not be surprised that, previous to the time of Aristotle, we find no accurate notion respecting the Canary Islands and the volcanoes they contain, among the Greek geographers. The only nation whose navigations extended toward the west and the north, the Carthaginians, were interested in throwing a veil of mystery over those distant regions. While the senate of Carthage was averse to any partial emigration, it pointed out those islands as a place of refuge in times of trouble and public misfortune; they were to the Carthaginians what the free soil of America has become to Europeans amidst their religious and civil dissensions.

The Canaries were not better known to the Romans till eighty-four years before the reign of Augustus. A private individual was desirous of executing the project, which wise foresight had dictated to the senate of Carthage. Sertorius, conquered by Sylla, and weary of the din of war, looked out for a safe and peaceable retreat. He chose the Fortunate Islands, of which a delightful picture had been drawn for him on the sh.o.r.es of Baetica. He carefully combined the notions he acquired from travellers; but in the little that has been transmitted to us of those notions, and in the more minute descriptions of Sebosus and Juba, there is no mention of volcanoes or volcanic eruptions. Scarcely can we recognise the isle of Teneriffe, and the snows with which the summit of the Peak is covered in winter, in the name of Nivaria, given to one of the Fortunate Islands. Hence we might conclude, that the volcano at that time threw out no flames, if it were allowable so to interpret the silence of a few authors, whom we know only by short fragments or dry nomenclatures. The naturalist vainly seeks in history for doc.u.ments of the first eruptions of the Peak; he nowhere finds any but in the language of the Guanches, in which the word Echeyde denotes, at the same time, h.e.l.l and the volcano of Teneriffe.

Of all the written testimonies, the oldest I have found in relation to the activity of this volcano dates from the beginning of the sixteenth century. It is contained in the narrative of the voyage of Aloysio Cadamusto, who landed at the Canaries in 1505. This traveller was witness of no eruptions, but he positively affirms that, like Etna, this mountain burns without interruption, and that the fire has been seen by christians held in slavery by the Guanches of Teneriffe. The Peak, therefore, was not at that time in the state of repose in which we find it at present; for it is certain that no navigator or inhabitant of Teneriffe has seen issue from the mouth of the Peak, I will not say flames, but even any smoke visible at a distance. It would be well, perhaps, were the funnel of the Caldera to open anew; the lateral eruptions would thereby be rendered less violent, and the whole group of islands would be less endangered by earthquakes.

<script>