Volume I Part 5 (1/2)
We had yet to scale the steepest part of the mountain, the Piton, which forms the summit. The slope of this small cone, covered with volcanic ashes, and fragments of pumice-stone, is so steep, that it would have been almost impossible to reach the top, had we not ascended by an old current of lava, the debris of which have resisted the ravages of time. These debris form a wall of scorious rock, which stretches into the midst of the loose ashes. We ascended the Piton by grasping these half-decomposed scoriae, which often broke in our hands. We employed nearly half an hour to scale a hill, the perpendicular height of which is scarcely ninety toises. Vesuvius, three times lower than the peak of Teneriffe, is terminated by a cone of ashes almost three times higher, but with a more accessible and easy slope. Of all the volcanoes which I have visited, that of Jorullo, in Mexico, is the only one that is more difficult to climb than the Peak, because the whole mountain is covered with loose ashes.
When the Sugar-loaf (el Piton) is covered with snow, as it is in the beginning of winter, the steepness of its declivity may be very dangerous to the traveller. M. Le Gros showed us the place where captain Baudin was nearly killed when he visited the Peak of Teneriffe. That officer had the courage to undertake, in company with the naturalists Advenier, Mauger, and Riedle, an excursion to the top of the volcano about the end of December, 1797. Having reached half the height of the cone, he fell, and rolled down as far as the small plain of Rambleta; happily a heap of lava, covered with snow, hindered him from rolling farther with accelerated velocity. I have been told, that in Switzerland a traveller was suffocated by rolling down the declivity of the Col de Balme, over the compact turf of the Alps.
When we gained the summit of the Piton, we were surprised to find scarcely room enough to seat ourselves conveniently. We were stopped by a small circular wall of porphyritic lava, with a base of pitchstone, which concealed from us the view of the crater.* (*
Called La Caldera, or the caldron of the peak, a denomination which recalls to mind the Oules of the Pyrenees.) The west wind blew with such violence that we could scarcely stand. It was eight in the morning, and we suffered severely from the cold, though the thermometer kept a little above freezing point. For a long time we had been accustomed to a very high temperature, and the dry wind increased the feeling of cold, because it carried off every moment the small atmosphere of warm and humid air, which was formed around us from the effect of cutaneous perspiration.
The brink of the crater of the peak bears no resemblance to those of most of the other volcanoes which I have visited: for instance, the craters of Vesuvius, Jorullo, and Pichincha. In these the Piton preserves its conic figure to the very summit: the whole of their declivity is inclined the same number of degrees, and uniformly covered with a layer of pumice-stone very minutely divided; when we reach the top of these volcanoes, nothing obstructs the view of the bottom of the crater. The peaks of Teneriffe and Cotopaxi, on the contrary, are of very different construction. At their summit a circular wall surrounds the crater; which wall, at a distance, has the appearance of a small cylinder placed on a truncated cone. On Cotopaxi this peculiar construction is visible to the naked eye at more than 2000 toises distance; and no person has ever reached the crater of that volcano. On the peak of Teneriffe, the wall, which surrounds the crater like a parapet, is so high, that it would be impossible to reach the Caldera, if, on the eastern side, there was not a breach, which seems to have been the effect of a flowing of very old lava. We descended through this breach toward the bottom of the funnel, the figure of which is elliptic. Its greater axis has a direction from north-west to south-east, nearly north 35 degrees west. The greatest breadth of the mouth appeared to us to be 300 feet, the smallest 200 feet, which numbers agree very nearly with the measurement of MM. Verguin, Varela, and Borda.
It is easy to conceive, that the size of a crater does not depend solely on the height and ma.s.s of the mountain, of which it forms the princ.i.p.al air-vent. This opening is indeed seldom in direct ratio with the intensity of the volcanic fire, or with the activity of the volcano. At Vesuvius, which is but a hill compared with the Peak of Teneriffe, the diameter of the crater is five times greater. When we reflect, that very lofty volcanoes throw out less matter from their summits than from lateral openings, we should be led to think, that the lower the volcanoes, their force and activity being the same, the more considerable ought to be their craters. In fact, there are immense volcanoes in the Andes, which have but very small openings; and we might establish as a geological principle, that the most colossal mountains have craters of little extent at the summits, if the Cordilleras did not present many instances to the contrary.* (* The great volcanoes of Cotopaxi and Rucupichincha have craters, the diameters of which, according to my measurements, exceed 400 and 700 toises.) I shall have occasion, in the progress of this work, to cite a number of facts, which will throw some light on what may be called the external structure of volcanoes. This structure is as varied as the volcanic phenomena themselves; and in order to raise ourselves to geological conceptions worthy of the greatness of nature, we must set aside the idea that all volcanoes are formed after the model of Vesuvius, Stromboli, and Etna.
The external edges of the Caldera are almost perpendicular. Their appearance is somewhat like the Somma, seen from the Atrio dei Cavalli. We descended to the bottom of the crater on a train of broken lava, from the eastern breach of the enclosure. The heat was perceptible only in a few crevices, which gave vent to aqueous vapours with a peculiar buzzing noise. Some of these funnels or crevices are on the outside of the enclosure, on the external brink of the parapet that surrounds the crater. We plunged the thermometer into them, and saw it rise rapidly to 68 and 75 degrees. It no doubt indicated a higher temperature, but we could not observe the instrument till we had drawn it up, lest we should burn our hands. M. Cordier found several crevices, the heat of which was that of boiling water. It might be thought that these vapours, which are emitted in gusts, contain muriatic or sulphurous acid; but when condensed, they have no particular taste; and experiments, which have been made with re-agents, prove that the chimneys of the peak exhale only pure water. This phenomenon, a.n.a.logous to that which I observed in the crater of Jorullo, deserves the more attention, as muriatic acid abounds in the greater part of volcanoes, and as M. Vauquelin has discovered it even in the porphyritic lavas of Sarcouy in Auvergne.
I sketched on the spot a view of the interior edge of the crater, as it presented itself in the descent by the eastern break. Nothing is more striking than the manner in which these strata of lava are piled on one another, exhibiting the sinuosities of the calcareous rock of the higher Alps. These enormous ledges, sometimes horizontal, sometimes inclined and undulating, are indicative of the ancient fluidity of the whole ma.s.s, and of the combination of several deranging causes, which have determined the direction of each flow. The top of the circular wall exhibits those curious ramifications which we find in c.o.ke. The northern edge is most elevated. Towards the south-west the enclosure is considerably sunk and an enormous ma.s.s of scorious lava seems glued to the extremity of the brink. On the west the rock is perforated; and a large opening gives a view of the horizon of the sea. The force of the elastic vapours perhaps formed this natural aperture, at the time of some inundation of lava thrown out from the crater.
The inside of this funnel indicates a volcano, which for thousands of years has vomited no fire but from its sides. This conclusion is not founded on the absence of great openings, which might be expected in the bottom of the Caldera. Those whose experience is founded on personal observation, know that several volcanoes, in the intervals of an eruption, appear filled up, and almost extinguished; but that in these same mountains, the crater of the volcano exhibits layers of scoriae, rough, sonorous, and s.h.i.+ning.
We observe hillocks and intumescences caused by the action of the elastic vapours, cones of broken scoriae and ashes which cover the funnels. None of these phenomena characterise the crater of the peak of Teneriffe; its bottom is not in the state which ensues at the close of an eruption. From the lapse of time, and the action of the vapours, the inside walls are detached, and have covered the basin with great blocks of lithoid lavas.
The bottom of the Caldera is reached without danger. In a volcano, the activity of which is princ.i.p.ally directed towards the summit, such as Vesuvius, the depth of the crater varies before and after each eruption; but at the peak of Teneriffe the depth appears to have remained unchanged for a long time. Eden, in 1715, estimated it at 115 feet; Cordier, in 1803, at 110 feet. Judging by mere inspection, I should have thought the funnel of still less depth.
Its present state is that of a solfatara; and it is rather an object of curious investigation, than of imposing aspect. The majesty of the site consists in its elevation above the level of the sea, in the profound solitude of these lofty regions, and in the immense s.p.a.ce over which the eye ranges from the summit of the mountain.
The wall of compact lava, forming the enclosure of the Caldera, is snow-white at its surface. The same colour prevails in the inside of the Solfatara of Puzzuoli. When we break these lavas, which might be taken at some distance for calcareous stone, we find in them a blackish brown nucleus. Porphyry, with basis of pitch-stone, is whitened externally by the slow action of the vapours of sulphurous acid gas. These vapours rise in abundance; and what is rather remarkable, through crevices which seem to have no communication with the apertures that emit aqueous vapours. We may be convinced of the presence of the sulphurous acid, by examining the fine crystals of sulphur, which are everywhere found in the crevices of the lava. This acid, combined with the water with which the soil is impregnated, is transformed into sulphuric acid by contact with the oxygen of the atmosphere. In general, the humidity in the crater of the peak is more to be feared than the heat; and they who seat themselves for a while on the ground find their clothes corroded. The porphyritic lavas are affected by the action of the sulphuric acid: the alumine, magnesia, soda, and metallic oxides gradually disappear; and often nothing remains but the silex, which unites in mammillary plates, like opal. These siliceous concretions,* (* Opalartiger kieselsinter. The siliceous gurh of the volcanoes of the Isle of France contains, according to Klaproth, 0.72 silex, and 0.21 water; and thus comes near to opal, which Karsten considers as a hydrated silex.) which M. Cordier first made known, are similar to those found in the isle of Ischia, in the extinguished volcanoes of Santa Fiora, and in the Solfatara of Puzzuoli. It is not easy to form an idea of the origin of these incrustations. The aqueous vapours, discharged through great spiracles, do not contain alkali in solution, like the waters of the Geyser, in Iceland. Perhaps the soda contained in the lavas of the peak acts an important part in the formation of these deposits of silex. There may exist in the crater small crevices, the vapours of which are not of the same nature as those on which travellers, whose attention has been directed simultaneously to a great number of objects, have made experiments.
Seated on the northern brink of the crater, I dug a hole of some inches in depth; and the thermometer placed in this hole rose rapidly to 42 degrees. Hence we may conclude what must be the heat in this solfatara at the depth of thirty or forty fathoms. The sulphur reduced into vapour is condensed into fine crystals, which however are not equal in size to those M. Dolomieu brought from Sicily. They are semi-diaphanous octahedrons, very brilliant on the surface, and of a conchoidal fracture. These ma.s.ses, which will one day perhaps be objects of commerce, are constantly bedewed with sulphurous acid. I had the imprudence to wrap up a few, in order to preserve them, but I soon discovered that the acid had consumed not only the paper which contained them, but a part also of my mineralogical journal. The heat of the vapours, which issue from the crevices of the caldera, is not sufficiently great to combine the sulphur while in a state of minute division, with the oxygen of the atmospheric air; and after the experiment I have just cited on the temperature of the soil, we may presume that the sulphurous acid is formed at a certain depth,* in cavities to which the external air has free access. (* An observer, in general very accurate, M. Breislack, a.s.serts that the muriatic acid always predominates in the vapours of Vesuvius. This a.s.sertion is contrary to what M. Gay-Lussac and myself observed, before the great eruption of 1805, and while the lava was issuing from the crater.
The smell of the sulphurous acid, so easy to distinguish, was perceptible at a great distance; and when the volcano threw out scoriae, the smell was mingled with that of petroleum.)
The vapours of heated water, which act on the fragments of lava scattered about on the caldera, reduce certain parts of it to a state of paste. On examining, after I had reached America, those earthy and friable ma.s.ses, I found crystals of sulphate of alumine.
MM. Davy and Gay-Lussac have already made the ingenious remark, that two bodies highly inflammable, the metals of soda and potash, have probably an important part in the action of a volcano; now the potash necessary to the formation of alum is found not only in feldspar, mica, pumice-stone, and augite, but also in obsidian.
This last substance is very common at Teneriffe, where it forms the basis of the tephrinic lava. These a.n.a.logies between the peak of Teneriffe and the Solfatara of Puzzuoli, might no doubt be shown to be more numerous, if the former were more accessible, and had been frequently visited by naturalists.
An expedition to the summit of the volcano of Teneriffe is interesting, not solely on account of the great number of phenomena which are the objects of scientific research; it has still greater attractions from the picturesque beauties which it lays open to those who are feelingly alive to the majesty of nature. It is a difficult task to describe the sensations, which are the more forcible, inasmuch as they have something undefined, produced by the immensity of the s.p.a.ce as well as by the vastness, the novelty, and the mult.i.tude of the objects, amidst which we find ourselves transported. When a traveller attempts to describe the loftiest summits of the globe, the cataracts of the great rivers, the tortuous valleys of the Andes, he incurs the danger of fatiguing his readers by the monotonous expression of his admiration. It appears to me more conformable to the plan I have proposed to myself in this narrative, to indicate the peculiar character that distinguishes each zone: we exhibit with more clearness the physiognomy of the landscape, in proportion as we endeavour to sketch its individual features, to compare them with each other, and to discover by this kind of a.n.a.lysis the sources of the enjoyments, furnished by the great picture of nature.
Travellers have learned by experience, that views from the summits of very lofty mountains are neither so beautiful, picturesque, nor so varied, as those from heights which do not exceed that of Vesuvius, Righi, and the Puy-de-Dome. Colossal mountains, such as Chimborazo, Antisana, or Mount Rosa, compose so large a ma.s.s, that the plains covered with rich vegetation are seen only in the immensity of distance, and a blue and vapoury tint is uniformly spread over the landscape. The peak of Teneriffe, from its slender form and local position, unites the advantages of less lofty summits with those peculiar to very great heights. We not only discern from its top a vast expanse of sea, but we perceive also the forests of Teneriffe, and the inhabited parts of the coasts, in a proximity calculated to produce the most beautiful contrasts of form and colour. We might say, that the volcano overwhelms with its ma.s.s the little island which serves as its base, and it shoots up from the bosom of the waters to a height three times loftier than the region where the clouds float in summer. If its crater, half extinguished for ages past, shot forth flakes of fire like that of Stromboli in the Aeolian Islands, the peak of Teneriffe, like a lighthouse, would serve to guide the mariner in a circuit of more than 260 leagues.
When we were seated on the external edge of the crater, we turned our eyes towards the north-west, where the coasts are studded with villages and hamlets. At our feet, ma.s.ses of vapour, constantly drifted by the winds, afforded us the most variable spectacle. A uniform stratum of clouds, similar to that already described, and which separated us from the lower regions of the island, had been pierced in several places by the effect of the small currents of air, which the earth, heated by the sun, began to send towards us.
The port of Orotava, its vessels at anchor, the gardens and the vineyards encircling the town, shewed themselves through an opening which seemed to enlarge every instant. From the summit of these solitary regions our eyes wandered over an inhabited world; we enjoyed the striking contrast between the bare sides of the peak, its steep declivities covered with scoriae, its elevated plains dest.i.tute of vegetation, and the smiling aspect of the cultured country beneath. We beheld the plants divided by zones, as the temperature of the atmosphere diminished with the elevation of the site. Below the Piton, lichens begin to cover the scorious and l.u.s.trous lava: a violet,* (* Viola cheiranthifolia.) akin to the Viola dec.u.mbens, rises on the slope of the volcano at 1740 toises of height; it takes the lead not only of the other herbaceous plants, but even of the gramina, which, in the Alps and on the ridge of the Cordilleras, form close neighbourhood with the plants of the family of the cryptogamia. Tufts of retama, loaded with flowers, adorn the valleys hollowed out by the torrents, and enc.u.mbered with the effects of the lateral eruptions. Below the retama, lies the region of ferns, bordered by the tract of the arborescent heaths. Forests of laurel, rhamnus, and arbutus, divide the ericas from the rising grounds planted with vines and fruit trees. A rich carpet of verdure extends from the plain of spartium, and the zone of the alpine plants even to the groups of the date tree and the musa, at the feet of which the ocean appears to roll.
I here pa.s.s slightly over the princ.i.p.al features of this botanical chart, as I shall enter hereafter into some farther details respecting the geography of the plants of the island of Teneriffe.*
(* See below.)
The seeming proximity, in which, from the summit of the peak, we behold the hamlets, the vineyards, and the gardens on the coast, is increased by the prodigious transparency of the atmosphere.
Notwithstanding the great distance, we could distinguish not only the houses, the sails of the vessels, and the trunks of the trees, but we could discern the vivid colouring of the vegetation of the plains. These phenomena are owing not only to the height of the site, but to the peculiar modifications of the air in warm climates. In every zone, an object placed on a level with the sea, and viewed in a horizontal direction, appears less luminous, than when seen from the top of a mountain, where vapours arrive after pa.s.sing through strata of air of decreasing density. Differences equally striking are produced by the influence of climate. The surface of a lake or large river is less resplendent, when we see it at an equal distance, from the top of the higher Alps of Switzerland, than when we view it from the summit of the Cordilleras of Peru or of Mexico. In proportion as the air is pure and serene, the solution of the vapours becomes more complete, and the light loses less in its pa.s.sage. When from the sh.o.r.es of the Pacific we ascend the elevated plain of Quito, or that of Antisana, we are struck for some days by the nearness at which we imagine we see objects which are actually seven or eight leagues distant. The peak of Teyde has not the advantage of being situated in the equinoctial region; but the dryness of the columns of air which rise perpetually above the neighbouring plains of Africa, and which the eastern winds convey with rapidity, gives to the atmosphere of the Canary Islands a transparency which not only surpa.s.ses that of the air of Naples and Sicily, but perhaps exceeds the purity of the sky of Quito and Peru. This transparency may be regarded as one of the chief causes of the beauty of landscape scenery in the torrid zone; it heightens the splendour of the vegetable colouring, and contributes to the magical effect of its harmonies and contrasts.
If the ma.s.s of light, which circulates about objects, fatigues the external senses during a part of the day, the inhabitant of the southern climates has his compensation in moral enjoyment. A lucid clearness in the conceptions, and a serenity of mind, correspond with the transparency of the surrounding atmosphere. We feel these impressions without going beyond the boundaries of Europe. I appeal to travellers who have visited countries rendered famous by the great creations of the imagination and of art,--the favoured climes of Italy and Greece.
We prolonged in vain our stay on the summit of the Peak, awaiting the moment when we might enjoy the view of the whole of the archipelago of the Fortunate Islands:* we, however, descried Palma, Gomera, and the Great Canary, at our feet. (* Of all the small islands of the Canaries, the Rock of the East is the only one which cannot be seen, even in fine weather, from the top of the Peak. Its distance is 3 degrees 5 minutes, while that of the Salvage is only 2 degrees 1 minute. The island of Madeira, distant 4 degrees 29 minutes, would be visible, if its mountains were more than 3000 toises high.) The mountains of Lancerota, free from vapours at sunrise, were soon enveloped in thick clouds. Supposing only an ordinary refraction, the eye takes in, in calm weather, from the summit of the volcano, a surface of the globe of 5700 square leagues, equal to a fourth of the superficies of Spain. The question has often been agitated, whether it be possible to perceive the coast of Africa from the top of this colossal pyramid; but the nearest parts of that coast are still farther from Teneriffe than 2 degrees 49 minutes, or 56 leagues. The visual ray of the horizon from the Peak being 1 degree 57 minutes, cape Bojador can be seen only on the supposition of its height being 200 toises above the level of the ocean. We are ignorant of the height of the Black Mountains near cape Bojador, as well as of that peak, called by navigators the Penon Grande, farther to the south of this promontory. If the summit of the volcano of Teneriffe were more accessible, we should observe without doubt, in certain states of the wind, the effects of an extraordinary refraction. On perusing what Spanish and Portuguese authors relate respecting the existence of the fabulous isle of San Borondon, or Antilia, we find that it is particularly the humid wind from west-south-west, which produces in these lat.i.tudes the phenomena of the mirage. We shall not however admit with M. Vieyra, ”that the play of the terrestrial refractions may render visible to the inhabitants of the Canaries the islands of Cape Verd, and even the Apalachian mountains of America.”* (* The American fruits, frequently thrown by the sea on the coasts of the islands of Ferro and Gomera, were formerly supposed to emanate from the plants of the island of San Borondon.
This island, said to be governed by an archbishop and six bishops, and which Father Feijoa believed to be the image of the island of Ferro, reflected on a fog-bank, was ceded in the 16th century, by the King of Portugal, to Lewis Perdigon, at the time the latter was preparing to take possession of it by conquest.)
The cold we felt on the top of the Peak, was very considerable for the season. The centigrade thermometer, at a distance from the ground, and from the apertures that emitted the hot vapours, fell in the shade to 2.7 degrees. The wind was west, and consequently opposite to that which brings to Teneriffe, during a great part of the year, the warm air that floats above the burning desert of Africa. As the temperature of the atmosphere, observed at the port of Orotava by M. Savagi, was 22.8 degrees, the decrement of caloric was one degree every 94 toises. This result perfectly corresponds with those obtained by Lamanon and Saussure on the summits of the Peak and Etna, though in very different seasons. The tall slender form of these mountains facilitates the means of comparing the temperature of two strata of the atmosphere, which are nearly in the same perpendicular plane; and in this point of view the observations made in an excursion to the volcano of Teneriffe resemble those of an ascent in a balloon. We must nevertheless remark, that the ocean, on account of its transparency and evaporation, reflects less caloric than the plains, into the upper regions of the air; and also that summits which are surrounded by the sea are colder in summer, than mountains which rise from a continent; but this circ.u.mstance has very little influence on the decrement of atmospherical heat; the temperature of the low regions being equally diminished by the proximity of the ocean.
It is not the same with respect to the influence exercised by the direction of the wind, and the rapidity of the ascending current; the latter sometimes increases in an astonis.h.i.+ng manner the temperature of the loftiest mountains. I have seen the thermometer rise, on the slope of the volcano of Antisana, in the kingdom of Quito, to 19 degrees, when we were 2837 toises high. M.