Digitale Ausgabe

When we reflect upon the influence which, for many ages,has been exercised upon the study of nature, by the improve-ments of geography, and by scientific journeys made into dis-tant regions, we quickly perceive how different this influence

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* Translated from the Tableaux de la Nature, par Humboldt, t. ii.|223|has been, according as the researches have been directed towardthe forms of the organic world, or toward the inanimate massof the earth. Different forms of plants and animals enliven theearth’s surface in each zone, however much the heat of the at-mosphere may change, whether according to the geographicallatitude, or the numerous curves of the isothermal lines, in theextended plains, level as the surface of the sea, or in an almostvertical direction on the steep slopes of the mountain chains.Organic nature gives to each region of the earth the peculiarphysiognomy by which it is distinguished. The case is differentwith inorganic nature in the places where the solid envelope ofthe earth is deprived of vegetation. The same species of rocks,attracting and repelling each other by groups, disclose them-selves in the two hemispheres, from the equator to the poles.In a distant isle, surrounded by unknown plants, in a climewhere the stars to which his eye is habituated no longer shine,the voyager often recognises with joy the granite of his nativecountry, and the rocks which he has been accustomed to see. This independence upon the present constitution of climates,which is peculiar to inorganic nature, does not diminish the be-neficial influence which numerous observations, made in distantcountries, have upon the progress of geognosy; it only givesthem a particular direction. Each succeeding expedition en-riches natural history with new species of animals and plants.Sometimes organic forms are discovered which connect them-selves with types long known, and which present in its originalperfection the regularly woven, and often apparently interrupt-ed, net-work of animated natural forms. Sometimes the dis-coveries consist of forms which present themselves isolated, likethe remains of extinct races; sometimes of members of yet un-known groups. The examination of the solid crust of the earthexhibits no such diversity. On the contrary, it discloses, in theconstituent parts, in the relative position, and in the periodicalrecurrence of the different masses, a similarity which strikes thegeologist with astonishment. In the chain of the Andes, as inthe central mountains of Europe, one formation seems, as itwere, to recall another. Masses of the same name assume simi-lar forms; the basalt and greenstone form twin mountains;dolomite, white sandstone and porphyry, form masses broken|224|into cliffs; trachyte, rich in vitreous felspar, rises into domes.In the most distant zones, large crystals separate similarly,as by an internal development, from the compact texture ofthe primitive mass, form themselves into groups, appear assubordinate masses, and often announce the vicinity of indepen-dent new formations. In this manner the whole inorganic worldis evidently pictured in every mountain chain of any extent.To become perfectly acquainted, however, with the most im-portant phenomena of the composition, relative age, and originof the formations, it is necessary to compare, with each other,observations made in countries the most widely separated, pro-blems which have long seemed enigmatical to geologists living inthe north, find their solution near the equator. If, as has beenobserved, the distant zones do not furnish us with new forma-tions, that is to say, unknown groups of simple substances, theyyet enable us to understand the uniform laws of nature, bywhich the various strata support each other, penetrate into eachother’s substance in the form of veins, or raise each other inobedience to elastic powers. If it be true that our geognostical knowledge derives thegreatest advantage from researches made over vast expanses ofcountry, it ought not to excite surprise that the class of pheno-mena which forms the principal object of this memoir should,till lately, have been examined in a very imperfect manner, be-cause the points of comparison are very difficult, and may evenbe said laborious, to find. Until the end of the eighteenthcentury, all that was known of the form of volcanoes, and of theaction of their subterranean powers, was derived from two moun-tains in the south of Italy, Vesuvius and Etna. The formerbeing the most accessible, and, like all volcanoes of inferior ele-vation, having more frequent eruptions, a small hill became, insome measure, the type according to which a whole distantworld was represented, containing the great volcanoes of Mexi-co, South America, and the Asiatic Isles. This mode of rea-soning might naturally bring to our recollection Virgil’s shep-herd, who, in his humble cabin, imagined he saw the image ofthe eternal city. An attentive examination of the whole Mediterranean, espe-cially its islands and eastern shores, where the human race has|225|begun to rise in the progress of intellect, and in the cultivationof generous feelings, might, however, reform this imperfect man-ner of studying nature. Among the Sporades, trachyte rockshave risen from the bottom of the sea, and formed islands, likethat among the Azores, which, in the space of three centuries,has shewn itself at nearly equal intervals. Between Epidaurusand Trèzéne, near Methone, in the Peleponnesus, there occursa Monte Nuovo, which was described by Strabo, and has beenseen again by Dodwell. It is higher than the Monte Nuovo ofthe Phlegrean Fields, near Baiæ, perhaps even higher than thenew Volcano of Jorullo, in the Plains of Mexico, which I foundsurrounded with many thousands of small basaltic cones, thathad issued from the ground, and were still smoking. In thebasin of the Mediterranean, not only does the volcanic fire es-cape from permanent craters of isolated mountains, which havea constant communication with the interior of the earth, asStromboli, Vesuvius, and Etna; but at Ischia, on Mount Epo-mée; and, according to the accounts of the ancients, in the Plainsof Lelantis, near Chalcis, lavas have flowed from fissures whichhave suddenly opened at the surface of the ground. Independently of these phenomena which belong to historicaltimes, to the limited domain of sure tradition, the shores of theMediterranean contain numerous remains of more ancient effectsof the action of fire. The south of France, in Auvergne, dis-plays a particular and entire system of volcanoes, arranged inseries, of trachytic domes, alternating with cones perforated withcraters, from which torrents of lava have flowed in narrowstripes. The Plain of Lombardy, which, smooth as the surfaceof the waters, forms the most remote gulf of the Adriatic Sea,surrounds the trachyte of the Euganean Hills, in which thererise domes of granular trachyte, obsidian, and perlite, formingthree masses proceeding from each other, which have forcedtheir way through the Juraic limestone, filled with flints, butwhich have never run in narrow torrents. Similar evidences ofancient revolutions of the earth occur in various parts of theContinent of Greece and of Asia Minor, a country which willone day present rich materials for geological research, whenlight shall have returned to those countries whence it began to|226|shine on the west, when outraged humanity shall no longergroan beneath the savage barbarity of the Ottomans. I bring forward the geographical proximity of these numerousphenomena, to shew that the basin of the Mediterranean, withits islands, is capable of presenting to the attentive observer allthat has recently been discovered, under various forms, in SouthAmerica, in Teneriffe, or in the Aleutian Isles, in the vicinityof the polar regions. The objects to be observed were unitedtogether; but travels into distant regions, and comparisons ofextensive countries in Europe and out of it, were necessary forclearly shewing the mutual resemblance of volcanic phenomena,and their dependence upon one another. Common language, which often gives consistency and dura-tion to ideas arising from the most erroneous views of things,but which also frequently indicates the truth instinctively,gives the name of Volcanic to all the eruptions of subterraneanfires and melted substances; to the columns of smoke and va-pour which issue from the heart of rocks, as at Colares, after thegreat earthquake at Lisbon; to the salses or cones of clay whichvomit mud, asphaltes, and hydrogen, as at Girgenti, in Sicily,and at Turbaco, in South America; to the hot springs of theGeyser, which, impelled by elastic vapours, rise to an immenseheight; in a word, to all the effects of the mighty powers of na-ture, which have their seat in the interior of our planet. Incentral America, or in the country of Guatemala, and in thePhilippine Isles, the natives make an essential difference betweenwater volcanoes and fire volcanoes (volcanes de agua y de fuego).By the former name they designate the mountains, from which,amid violent earthquakes, subterranean waters issue from timeto time. Without denying the connection of the phenomena just men-tioned, it would yet appear expedient to give a more precise lan-guage to the physical and oryctognostical department of geo-gnosy, in order to prevent the application of the name of Volcano,sometimes to a mountain which is terminated by a permanentfurnace, and sometimes to each subterranean cause of volcanicphenomena. In the present state of the terrestrial globe, themost common form of volcanoes, in all parts of the world, isthat of an isolated cone, such as Vesuvius, Etna, the Peak of|227|Teyde, Tunguragua, and Cotopaxi. I have observed themrising from the size of the lowest hills to 17,700 feet above thelevel of the sea. But close to these conical mountains, there al-so occur permanent apertures, forming regular communicationswith the interior of the earth, on long serrated chains, not at themiddle of their mural summit, but at their extremity, and nearthe declivity. Of this kind is Pichincha, which rises betweenthe great ocean and the city of Quito, and which Bouguer’s ba-rometrical formulæ have long rendered celebrated. Such alsoare the volcanoes which rise on the Steppe de los Pastos, whichis 10,000 feet high. All these summits, of varied forms, arecomposed of trachyte, formerly named trap porphyry, a granu-lar fissured rock, formed of glassy felspar and hornblende, andin which augite, mica, laminar felspar, and quartz, also occur.In places where the evidences of the first eruption I might sayof the ancient volcanic scaffolding, are preserved entire, the iso-lated conical mountain is surrounded, in the form of an amphi-theatre, with a great wall, constructed of rocky strata, super-imposed upon each other. These walls or circumvallations arethe remains of craters of elevation, a phenomenon worthy ofattention, respecting which the first geologist of our times, M.Leopold Von Buch, in his writings, from which I have borrowedseveral ideas stated in the present memoir, has presented suchinteresting views. The volcanoes which communicate with the atmosphere bypermanent apertures, the basaltic cones or domes of trachyte,destitute of crater, sometimes low like Sarcouy, and sometimeselevated like Chimborazo, form various groups. Comparativegeography shews us, on the one hand, small archipelagoes, andentire systems of volcanic mountains, with their oraters and cur-rents of lava, resembling those of the Canary Islands, and theAzores; and, on the other, mountains without craters, and with-out currents of lava, properly so called, as the Euganeans, andthe (Siebengebirge) seven mountains of Bonn. Moreover, itshews us volcanoes arranged in single or double lines, and extend-ing to several hundreds of leagues, sometimes parallel to the axisof the chain, as in Guatemala, Peru, and Java; sometimes cuttingit perpendicularly, as in the country of the Azteques, where tra-chytic mountains, which vomit fire, alone attain the height of|228|perpetual snow, and are probably situated upon a crevice tra-versing the whole continent, over an extent of 105 geographicalleagues from the Pacific Ocean to the Atlantic. This association of volcanoes, whether in isolated and roundedgroups, or in longitudinal bands, demonstrates, in the most de-cisive manner, that volcanic effects do not depend upon slightcauses existing near the surface of the earth, but are phenomenawhose origin is to be found at a great depth in the interior ofthe globe. The whole eastern part of the American continent,which is poor in metals, is, in its present state, destitute of vol-canic mountains, of masses of trachyte, and probably even basalt,with olivine. All the American volcanoes are collected togetherin the chain of the Andes, which is situated in the part of thatcontinent opposite to Asia, and which extends, in the directionof the meridians, over a space of 1800 leagues. The wholeplain of Quito, of which Pichincha, Cotopaxi, and Tunguraquaform the cymes, is a volcanic focus. The subterranean fireescapes, sometimes by one, sometimes by another, of those aper-tures which it has been customary to consider as distinct vol-canoes. The progressive march of the fire in them has, for thelast three centuries, been from north to south. The very earth-quakes, which produce such terrible ravages in this part of theworld, afford remarkable proofs of the existence of subterraneancommunications, not only with countries destitute of volcanoes,which has been long known, but also between ignivomousmountains placed at very great distances from each other.Thus, in 1797, the volcano of Pasto, to the east of the course ofthe Guaytara, vomited, unremittingly, for three months, a highcolumn of smoke. This column disappeared at the very mo-ment, when, at a distance of sixty leagues, the great carthquakeof Riobamba, and the muddy eruption of Moya, destroyedabout forty thousand Indians. The sudden appearance of theIsland of Sabrina, to the east of the Azores, on the 30th Janu-ary 1811, was announced by the dreadful earthquake, which,at a much greater distance to the west, from May 1811 to June1812, shook, almost without intermission, first the West IndiaIslands, then the plains of the Ohio and Missisippi, and, lastly,the coasts of Venezuela, situated on the opposite side. Thirtydays after the total destruction of the city of Caraccas, the ex-|229|plosion of the volcano of St Vincent, in the Lesser Antilles,took place at a distance of 130 leagues. At the same momentwhen this eruption happened, on the 30th April 1811, a sub-terranean noise was propagated, and carried terror over an ex-tent of country of 2200 square leagues. The inhabitants of thebanks of the Apuré, at the confluence of the Rio Nula, as wellas those of the sea coast, compared the noise to that producedby the discharge of large pieces of artillery. Now, from theconfluence of the Rio Nula and Apure, by which I arrived at theOronocco, to the volcano of St Vincent, the distance is 157leagues in a straight line. This noise, which assuredly was notpropagated by the air, must have had its cause deep in the earth.Its intensity was scarcely greater on the shores of the Antilles,near the volcano in action, than in the interior of the country. It would be useless to multiply examples; but in order torecall to mind a phenomenon which has acquired a historicalimportance with reference to Europe, I shall now mention thefamous earthquake of Lisbon. It took place on the 1st Novem-ber 1755. Not only were the waters of the Swiss Lakes, andof the sea on the coasts of Sweden, violently agitated; but alsothose of the sea around the eastern Antilles. At Martinique,Antigua, and Barbadoes, where the tide does not commonly risemore than eighteen inches, it suddenly rose twenty feet. Allthese phenomena prove, that the subterranean powers manifestthemselves, either dynamically, by earthquakes, or chemically,by occasioning changes in the form of volcanic eruptions. Theyalso demonstrate, that these powers act, not superficially in theouter crust of the earth, but at immense depths in the interiorof our planet, by crevices and unfilled veins, which lead to pointsof the earth’s surface, at the greatest distances from each other. The more numerous the diversities in the structure of volca-noes, or in other words, of the elevations surrounding the canalsby which the melted masses of the interior of the globe arriveat its surface, so much the more important is it to submit thisstructure to accurate measurements. The interest of these mea-surements, which, in another part of the world, have formedthe object of my researches, increases if we consider that themagnitude to be measured varies in several points. The phi-losophical examination of nature applies itself, in the vicissitude|230|of phenomena, to connect the present with the past. To esta-blish a periodical return, or to fix the laws of progressive andvariable phenomena, it is necessary to have some well determinedpoints of departure, observations made with care, and which,being conneoted with determined epochs, may furnish numeri-cal comparisons. Had only the mean temperature of the atmo-sphere, and of the earth in different latitudes, or the mean tempe-rature of the barometer on the edge of the sea, been determinedfrom one century to another, we should have known in what pro-portion the heat of climates has increased or diminished, andwhether or not the height of the atmosphere has undergonechanges. These points of comparison are required for the decli-nation and inclination of the magnetic needle, as well as for theintensity of the electro-magnetic forces. If it be a praiseworthyoccupation for societies to follow, with assiduity, the cosmic vicis-situdes of heat, of the pressure of the air, and of the magneticdirection and intensity; it is, on the other hand, the duty of thegeologist, in determining the inequalities of the earth’s surface, totake into consideration the change of height of volcanoes. WhatI attempted at the time, in the mountains of Mexico, at To-luca, Nauhiampatepetl and Jorullo, and in the Andes of Quito atPichincha, I have had an opportunity, since my return to Eu-rope, of repeating several times at Vesuvius. In 1773, Saussure measured that mountain at a period whenthe two edges of the crater, the north-west and south-west, ap-peared to him of equal height. He found their elevation 609toises above the level of the sea. The eruption of 1794 occa-sioned a falling in of the southern part, and an inequality of theedges of the crater which the most inexperienced eye distin-guishes at a considerable distance. In 1805, M. von Buch, M.Gay Lussac and myself, measured Vesuvius three times. Theresult of our operations was, that the height of the northedge, the Rocca del Palo, which is opposite the Somma, agreedwith Saussure’s measurement, but that the south edge was 75toises lower than in 1773. The total elevation of the volcano,towards the Torre del Greco, the side towards which the firehad principally directed its action for thirty years, had diminish-ed an eighth part. The cone of ashes is, to the total height ofthe mountain, on Vesuvius, as one to ten; on the Peak of Te-|231|neriffe as one to twenty-two. Vesuvius, therefore, has the coneof ashes proportionally higher, probably because, as a volcano oflittle height, it has acted principally by its summit. I succeed-ed lately not only in repeating my barometrical measurements onVesuvius, but also in ascending that mountain three times, inorder to take a complete survey of all the edges of the crater.This undertaking is perhaps deserving of some interest, becauseit embraces the period of the great eruptions from 1805 to 1822;and because it affords, perhaps, the only measurement of thevolcano, made with reference to all its parts, that has hithertobeen published. It shews that the edges of the crater, not onlyin the places where they are visibly composed of trachyte, as inthe Peak of Teneriffe, and in all the volcanoes of the chain ofthe Andes, but also every where else, present a phenomenonmuch more constant than had previously been supposed from ob-servations hastily made. Simple angles of height, determined fromthe same point, answer much better for researches of this kindthan trigonometrical and barometrical measurements, otherwisevery complete. According to my last determination, the north-west edge of Vesuvius has not perhaps undergone any diminu-tion of height since the time of Saussure, that is to say for thelast forty-nine years, and the south-east edge, on the Bosche Tre-Case side, which, in 1794, was 400 feet lower than the preced-ing, has undergone a diminution of 10 toises. If the public journals, in describing the great eruptions, veryfrequently relate that the form of Vesuvius has totally changed,and if these assertions are confirmed by the picturesque viewsof that mountain which are painted at Naples, the cause oferror exists in the circumstance that the contour of the edges ofthe crater is confounded with those of the heaps of scoriæ whichare accidentally formed in the centre of the crater, on the bot-tom of the ignivomous mouth raised up by vapours. One ofthese heaps, consisting of rapilli and scoriæ, became graduallyvisible in 1816 and 1818, above the south-east edge of thecrater. The eruption of February 1822 increased it to such adegree, that it even exceeded the Rocca del Palo, or the north-west edge of the crater, by 100 or 110 feet. In the last erup-tion, the remarkable cone, which was usually considered as thetrue summit of Vesuvius, fell down with a terrible noise, so that|232|the bottom of the crater, which, since 1811, was always acces-sible, is now 750 feet lower than the northern edge of the vol-cano, and 200 feet lower than the southern. The variableform and relative position of the cones of eruption, whose aper-ture ought not, as is too often done, to be confounded with thecrater of the volcano, give a particular aspect to Vesuvius atdifferent periods, and the historiographer of this volcano might,from the contours of the summit, and from the simple inspectionof the landscapes painted by Hackert, which are at Portici, ac-cording as the northern or southern side of the mountain is re-presented higher or lower, guess the year in which the artistmade the drawing from which he composed his picture. A day after the cone of scoriæ, 400 feet high, had fallen in,when already small but numerous torrents of lava had flowed,in the night of the 23d October, commenced the luminous erup-tion of ashes and rapilli. It lasted twelve days without inter-ruption; but it was more intense during the first four. All thistime, the detonations in the interior of the volcano were so vio-lent, that the mere concussion of the air (for no commotion wasobserved in the earth), cracked the ceilings of the apartments inthe palace of Portici. The villages of Resina, Torre-del-Greco,Torre del Anunziata, and Bosche-Tre-Case, which are close up-on the mountain, witnessed a remarkable phenomenon. Theatmosphere was so filled with ashes, that the whole district wasfor several hours in the middle of the day enveloped in profounddarkness. People used lanterns in the streets, as often hap-pens at Quito, during the eruptions of Pichincha. The inhabi-tants never fled in such numbers. The torrents of lava weremuch less dreaded than an eruption of ashes,—a phenomenonwhich had not before been known to such a degree, and which,from the obscure tradition of the manner in which Hercula-neum, Pompeii and Stabiæ were destroyed, filled the imaginationof men with terrifying images. The watery and hot vapour which shot up from the craterduring the eruption, and diffused itself in the atmosphere, form-ed, on cooling, a thick cloud round the column of ashes andflame which rose to the height of 9000 feet. So rapid a con-densation of the vapours, and, as M. Gay Lussac has shewn, thevery formation of the cloud, augmented the electrical intensity.|233|Flashes issued from the column of ashes in all directions, andthe thunder, which was easily distinguished from the noises ofthe volcano, was distinctly heard. In no other eruption wasthe manifestation of the electric powers so astonishing. On the morning of the 26th October, a surprising noise washeard, which seemed to arise from a torrent of boiling water thatwas ejected from the crater, and descended along the declivity ofthe cone of the ashes. Monticelli, the learned and zealous ob-server of the volcano, immediately discovered that an optical il-lusion had occasioned this erroneous rumour. The supposedtorrent was a great heap of dry ashes, which issued from acrevice in the upper edge of the crater. A drought whichspread desolation in the fields, had preceded the eruption of Ve-suvius. Toward the end of this phenomenon, the volcanicthunder storm which we have just been describing, occasioned anextremely heavy and long continued rain. In all countries, thecessation of an eruption is characterized by a similar meteor.So long as the present one lasted, the cone of ashes being gene-rally enveloped with clouds, and the rain being heaviest in itsvicinity, torrents of mud were seen flowing on all sides. Theaffrighted husbandman thought it was water, that, after ascend-ing from the bottom of the volcano, issued by the crater. Thegeologist thought he discovered in it sea water, or muddyproductions of the volcano, or, to use the expression of theFrench old systematic writers, products of an igno-aqueous li-quefaction. When the summit of the volcano, as is almost always the casein the Andes, rises above the region of snow, or attains a heightdouble that of Etna, the snow, by melting and flowing towardthe lower regions, produces frequent and disastrous inunda-tions. These are phenomena which the meteors connect withthe eruptions of volcanoes, and which are variously modified bythe height of the mountain, the extent of its summit covered withperpetual snows, and the heating of the walls of the cone ofcinders. They cannot at all be regarded as true volcanic phe-nomena, being merely the effects of such phenomena. In vastcavities, sometimes on the declivity, sometimes at the foot ofvolcanoes, are found subterranean lakes which communicate invarious ways with the alpine torrents. When the commotions|234|of the earth which always precede all the igneous eruptions in thechain of the Andes, have violently shaken the whole mass of thevolcano, then the subterranean gulfs open, and there issue at thesame time water, fishes, and clay tufa. Such is the singular phe-nomenon which brings to light the Pimelodes cyclopum, a fish towhich the inhabitants of the plain of Quito gave the name ofPrenadilla, and which I described shortly after my return.When to the north of Chimborazo, in the night of the 19thJune 1698. the summit of Carguaraizo, a mountain of the heightof 18,000 feet, broke down, the whole country round, to the ex-tent of nearly two square leaguges, was covered with mudand fishes. Seven years before, a pernicious fever, which de-solated the city of Iburra, was attributed to a similar eruptionof fishes from the volcano of Imbaburu. I mention these facts, because they throw some light on thedifference which exists between the eruptions of dry ashes andthose of mud, wood, charcoal, or shells, serving to explain theformation of tufa and trass. The quantity of ashes thrownout by Vesuvius of late years, like all the circumstances con-nected with volcanoes, and other great phenomena of naturecalculated to inspire terror, has been excessively exaggerated inthe public journals. Two chemists of Naples, Vicenzo Pepeand Giuseppe di Nobili, have even affirmed, notwithstandingthe contrary assertions of Monticelli and Covelli, that the ashescontain gold and silver. According to my inquiries, the bed ofashes that fell during twelve days on the Bosch-Tre-Case side,on the declivity of the cone, in the places where rapillo wasmingled with them, was only three feet deep, and in the plain,did not rise higher than from fifteen to eighteen inches. Mea-surements of this kind should not be taken in places where theashes are heaped up, like snow or sand, by the wind, or aceu-mulated by water in the form of mud. The times are gonewhen wonders only were looked for in volcanic phenomena, orwhen the ashes of Etna were represented as being carried bythe winds as far as the peninsula of India. Some of the goldand silver veins of Mexico certainly occur in a trachytic por-phyry; but the ashes of Vesuvius, which I carried along withme, and which were analysed by an excellent chemist M. HenryRose, afford not the slightest traces of gold or silver. |235| Although the results of which I speak, and which are in per-fect accordance with the accurate observations of Monticelli,differ much from those published some months ago, the eruptionof ashes from Vesuvius which took place on the 24th and 28th ofOctober 1822, is undoubtedly the most remarkable of which wehave any authentic accounts since the death of the elder Plinyin the year 70. The quantity of ashes which then fell was per-haps three times as great as any that has been observed sincevolcanic phenomena first began to be studied with attention.A layer of fifty or eighty inches appears at first sight insignifi-cant in comparison of the mass which covered Pompeii; but,without speaking of torrents of rain, and of the effects of detri-tion, which, in the course of ages, may have accumulated thismass, and without reviving the keen discussion which arose be-yond the Alps, and which was conducted with a great degreeof scepticism, respecting the causes of the destruction of the citiesof Campania, it is perhaps to the purpose to mention here, thatthe eruptions of a volcano at periods very remote from eachother, can by no means be compared together with reference totheir intensity. All the consequences founded upon analogiesare insufficient, when the objects to be compared are such as themass of lava and cinders, the height of the columns of smoke,and the loudness of the detonations. The geographical description of Vesuvius by Strabo, andVitruvius’s opinion respecting the volcanic origin of pumice,shew, that, until the year of Vespasian’s death, that is to say,until the eruption which overwhelmed Pompeii, that mountainresembled more an extinct volcano than a solfaterra. After along repose, the subterranean forces opened up new paths, andpenetrated through the strata of primitive rocks and trachyte.Then must have been manifested effects of which those thathave since followed could furnish no idea. The celebrated let-ter, in which the younger Pliny relates to Tacitus the death ofhis uncle, clearly shews that the renewal of the eruptions, and itmight even be said the awakening of the dormant volcano, com-menced with an explosion of ashes. The same thing was ob-served at Jorullo, when, in September 1759, the new volcano,piercing through the strata of syenite and trachyte, rose sudden-ly in the plain. The country people fled, because they found|236|on their huts ashes which the earth had vomited by opening upon all sides. On the contrary, in the periodical and ordinaryexplosions of volcanoes, the ashes terminate each partial erup-tion. Besides, the younger Pliny’s letter contains a passage,which clearly shews, that, from the commencement, without theinfluence of any cause that could have heaped them up, the dryashes that fell directly from above, had attained a height of fouror five feet. “The court,” says he in the course of his narra-tive, which had to be passed in order to enter the chamberin which Pliny reposed, “was so filled with ashes and pumice,that, if he had delayed his coming out any longer, he wouldhave found the entrance shut up.” In an inclosed space, likethat of a court, the action of the wind, by which the ashes arecollected, could not by any means have been very considerable. I have ventured to interrupt my comparative examination ofvolcanoes by particular observations made on Vesuvius, both onaccount of the great interest which the last eruption has excit-ed, and on account of the remembrance of the catastrophe ofPompeii and Herculaneum, which every considerable fall ofashes involuntarily brings to the mind. I have brought toge-ther, in a supplement, all the elements of the barometrical mea-surements and notices respecting geological collections that Ihave had an opportunity of making, towards the end of 1822, atVesuvius and in the Phlegrean fields, near Pouzzuolo. This smallcollection, together with the rocks which I brought from theEuganean mountains, and those which M. von Buch collected ona journey to the valley of Flemme, between Cavalere and Pre-dazzo, in the southern Tyrol, are deposited in the Royal Mu-seum of Berlin, an establishment which, by its utility, perfectlycorresponds to the noble intentions of the monarch, and of which,the geognostical department, containing specimens from the mostremote regions, is, in this respect, superior to any collection ofthis kind in existence. We have been considering the form and action of those vol-canoes which keep up a regular communication with the interiorof the earth, by means of craters. Their summits are masses oftrachyte and lava, raised up by elastic powers, and traversed byveins. The permanence of their action gives rise to the conclu-sion, that their structure is very complicated. They have, so|237|to speak, an individual character, which remains always thesame through long periods. The neighbouring mountainsmost commonly afford entirely different products, lavas ofleucite and felspar, obsidian and pumice, and basaltic massescontaining olivine. They belong to the most recent forma-tions of the globe, and traverse nearly all the strata of thesecondary mountains. Their eruptions and their torrents of la-va are of a more recent origin than our valleys. Their life, ifwe may be permitted to make use of such an expression, de-pends upon the mode and duration of their communication withthe interior of the earth. They frequently remain quiet forages, suddenly kindle again, and end with being solfaterras, ex-haling aqueous vapours, gases and acids. Sometimes, as in thePeak of Teneriffe, their summit has already become a laborato-ry of regenerated sulphur; while from their sides there yet flowgreat torrents of lava, basaltic and lithoid in their lower parts,vitreous, in the form of obsidian and pumice, in their upperpart, where the pressure is less. Independently of these volcanoes provided with permanentcraters, there is another species of volcanic phenomena, which ismore rarely observed, but which is peculiarly calculated to throwlight on geology, because it recalls the primitive world, or, inother words, the most ancient revolutions of our globe. Moun-tains of trachyte, opening of a sudden, vomit forth lava andashes, and again shut perhaps for ever. This is what took place inthe gigantic Antisana, in the Chain of the Andes, and at MountEpomeus in the island of Ischia, in 1302. An eruption of thiskind sometimes takes place in the plains; for example, on theplain of Quito; in Iceland, at a distance from Hecla; in Eu-beus, in the fields of Lelantée. Many islands, suddenly ele-vated from the bottom of the sea, belong to these transitoryphenomena. In these cases, the communication with the inte-rior of the earth is not permanent; the action ceases as soon asthe aperture of the canal of communication is closed anew.Veins of basalt, greenstone, and porphyry, which in the differentzones of the earth traverse almost all the formations, massesof syenite, augite, porphyry and amygdaloid, which characte-rize the newest strata of the transition, and the oldest strata ofthe secondary rocks, have probably been formed in this manner.|238|In the early stages of our planet, the substances of the interior,still in a state of fluidity, penetrated through the envelope of theearth which was fissured in all parts; sometimes condensing asmasses of veins with a granulated texture, sometimes spreadingout into sheets and stratified torrents. The volcanic rocks whichthe primitive world has transmitted to us, have nowhere flowedin narrow bands like the lavas that issue from the volcanic conesexisting at present. The mixtures of augite, titanitic iron,glassy felspar, and hornblende, may have been the same at differ-ent periods, sometimes more allied to basalt, and sometimes totrachyte. The chemical substances, as we learn from the im-portant labours of M. Mitscherlich, and the similarity of theproducts of high furnaces, may have been united under a crys-talline form, according to definite proportions. It is not the lesstrue, that substances, composed in the same manner, have ar-rived by very different ways at the earth’s surface, whether bybeing raised up by elastic forces, or by being insinuated throughcrevices into the strata of the older rocks; in other words,through the already oxidized envelope of our planet, or by issu-ing under the form of lava from conical mountains, which havea permanent crater. If phenomena so different as these be con-founded together, the geognosy of volcanoes is thrown back in-to the darkness, from which numerous comparative experimentshave begun gradually to rescue it. The question has often been asked, What is it that burns involcanoes? What is it that produces the heat in them by whichthe earth and metals are melted and intermingled? The newchemistry replies: What burns is the earth, the metals, and eventhe alkalies, that is to say, the metaloids of these substances.The already oxidized envelope of the earth separates the atmo-sphere, rich in oxygen, from the unoxidised inflammable princi-ples which reside in the interior of our planet. Observationsmade in all countries, in mines, and caves, and which, in concertwith M. Arago, I have detailed in a memoir on the subject,prove that, even at a small depth, the earth’s heat is much su-perior to the mean temperature of the surrounding atmosphere.A fact so remarkable, and elicited from observations made in al-most every part of the globe, connects itself with what we learn|239|from the phenomena of volcanoes. La Place has even attemptedto determine the depth at which the earth may be considered asa melted mass. Whatever doubts may be entertained, notwith-standing the respect due to so great a name, as to the numericalaccuracy of such a calculation, it is not the less probable, thatall volcanic phenomena arise from a single cause, which is thecommunication, constant or interrupted, that exists between theinterior of our planet and the external atmosphere. Elastic va-pours, by their pressure, raise through deep crevices the sub-stances which are in a state of fusion, and which are oxidized.Volcanoes are, so to speak, intermittent springs of earthy mat-ters. The fluid mixtures of metals, alkalies and earths, whichcondense into currents of lava, flow gently and slowly, when,on being raised up, they once find an issue. It was in this man-ner that, according to Plato’s Phœdos, the ancients representedall the torrents of fire as emanations of the Pyriphlegeton. To these considerations may I be permitted to add another ofa bolder character. It is perhaps in the internal heat of theearth, a heat which is indicated by experiments made with thethermometer, and the phenomena of volcanoes, that the cause ofone of the most astonishing phenomena which the knowledge ofpetrifactions presents to us resides. Tropical forms of animals,arborescent ferns, palms and bamboos, occur imbedded in thefrozen regions of the north. The primitive world every wherediscloses to us a distribution of organic forms, which is in oppo-sition to the presently existing state of climates. To solve so im-portant a problem, recourse has been had to a great number ofhypotheses, such as the approach of a comet, the change of ob-liquity of the ecliptic, the increase of intensity of the solar heat.None of these hypotheses has been able to satisfy at the sametime the astronomer, the natural philosopher and the geologist.As to my own opinion on the subject, I leave the earth’s axis inits position, I admit no change in the radiation of the solar disk,a change by which a celebrated astronomer thought he could ex-plain the good and bad harvests of our fields; but I ima-gine that in each planet, independently of its relations to a cen-tral body, and independently of its astronomical position, thereexist numerous causes of developement of heat, whether by thechemical processes of oxidation, or by the precipitation andchanges of capacity of bodies, or by the augmentation of the|240|electro-magnetic intensity, or the communication between theinternal and external parts of the globe. When, in the primitive world, the deeply fissured crust of theearth exhaled heat by these apertures, perhaps during manycenturies, palms, arborescent ferns, and the animals of warm cli-mates, lived in vast expanses of country. According to this sys-tem of things, which I have already indicated in my work en-titled Essai Geognostique sur le Gisement des Roches dans lesdeux Hemispheres, the temperature of volcanoes is the same asthat of the interior of the earth, and the same cause which nowproduces such frightful ravages, would formerly have made therichest vegetation to spring in every zone, from the newly oxi-dised envelope of the earth, and from the deeply fissured strataof rocks. If, in order to account for the distribution of the tropicalforms that occur buried in the northern regions of the globe, itis assumed that elephants covered with long hair, now immersedin the polar ice, were originally natives of those climates, andthat forms resembling the same principal type, such as that oflions and lynxes, may have lived at the same time in very dif-ferent climates, such a mode of explanation would yet be inap-plicable to the vegetable productions. For reasons which ve-getable physiology discloses, palms, bananas, and arborescentmonocotyledonous plants, are unable to support the cold of thenorthern countries; and in the geognostical problem which weare here examining, it appears to me difficult to separate theplants from the animals; the same explanation ought to embracethe two forms. At the end of this memoir, I have added to the facts collectedin countries the most remote from each other, some purely hypo-thetical suppositions*. The philosophical study of nature risesabove the wants of descriptive natural history; it does not consistof the mere accumulation of isolated observations. May it oneday be permitted to the curious and active mind of man, to dartfrom the present into the future, to interpret what cannot yet beknown with precision, and amuse itself with the geognosticalfables of antiquity, which are in our days reproduced under va-rious forms.