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Alexander von Humboldt: „On the Constitution and Mode of Action of Volcanoes, in different Parts of the Earth“, in: ders., Sämtliche Schriften digital, herausgegeben von Oliver Lubrich und Thomas Nehrlich, Universität Bern 2021. URL: <https://humboldt.unibe.ch/text/1823-Ueber_den_Bau-02> [abgerufen am 25.04.2024].
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| Titel | On the Constitution and Mode of Action of Volcanoes, in different Parts of the Earth | ||||
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| Jahr | 1823 | ||||
| Ort | London | ||||
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Nachweis in: The Annals of Philosophy 6:2 (August 1823), S. 121–135.
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| Sprache | Englisch | ||||
| Typografischer Befund | Antiqua; Auszeichnung: Kursivierung; Fußnoten mit Asterisken; Schmuck: Kapitälchen. | ||||
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When we consider the influence which scientific travels intodistant regions, and a more extended geographical knowledge, havefor some centuries past exerted upon the study of nature, we soondiscover how this influence has varied according to the objects ofinquiry, which have been, on the one hand, the forms of the organicworld, and, on the other, the inanimate formation of the earth; —the knowledge of rocks, their relative ages, and origin. Differ-ent forms of plants and animals enliven the earth in every zone,as well in the plains, where the heat of the atmosphere is deter-mined by the geographical latitude and the different inflexions ofthe isothermal lines, as where it changes suddenly on the steepdeclivities of the mountains. Organic nature gives a peculiarphysiognomical character to every zone, which is not the casewith the inorganic world where the solid crust of the earth isdivested of its vegetable covering. The same rocks approaching
* Read before the Royal Academy of Sciences of Berlin, Jan. 24, 1823.|122| to and receding from each other in groups occur in both hemi-spheres, from the equator to the poles. On a distant island,surrounded by strange plants, under a sky where the well-known stars do not shine, the sailor recognises, often with gladsurprise, the clayslate which is the common rock of his nativecountry.This independence of the geognostical relations of places onthe present constitution of their climate, does not diminish, butonly gives a particular direction to the favourable effect upon theprogress of geology and physical geognosy, which is producedby numerous observations made in foreign countries. Everyexpedition enriches natural history with new plants, and newgenera of animals; at one time they are organic forms rangingthemselves with well-known types, and representing to us, in itsoriginal perfection, a regularly woven, though often apparentlyinterrupted texture of animated creatures; at another, they areforms which appear to be isolated, as vestiges of genera whichhave been destroyed, or as surprising members of groups still tobe discovered. Such a variety is not presented by the examina-tion of the solid crust of the earth; it rather reveals to us anagreement, which excites the admiration of the geognost,between the parts of which it is composed, in the superpositionof masses of different natures, and in their periodical repetition.In the chain of the Andes, as well as in the central mountainsof Europe, one formation seems, as it were, to occasion theexistence of another; masses of the same character assumesimilar forms: * mountains are formed by basalt anddolerite; steep declivities by dolomite, porphyry, and quader-sandstein; bell-shaped eminences and high-vaulted domes byvitreous trachyte rich in felspar.In the most distant zones, larger crystals, as it were by inter-nal evolution out of the more compact texture of the greatermass, aggregate into subordinate beds, and thus frequentlyannounce the vicinity of a new and independent formation.Thus is the whole inorganic world reflected, more or less clearly,in every mountain of considerable extent; but in order to ascer-tain completely the most important phænomena respecting thecomposition, the relative age, and the origin of the differentspecies of rocks, observations from the most distant parts of theearth must be compared together. Problems which had appearedenigmatical to the geognost in his mother country are solvednear the equator. If distant zones do not furnish new speciesof rocks, that is to say, unknown arrangements of simple sub-stances, as has already been remarked, they yet teach us how todiscover the great laws which are every where the same, andaccording to which, the different strata of the earth support each
* In an imperfect translation of this paper, which has been forwarded to the Editorfrom the Continent, a word here occurs which cannot be decyphered; and on account ofother inaccuracies which it has been necessary to correct, unaided by the original, thetranslation, as now given, is not to be regarded as exact in every particular.|123| other, appear in the form of veins, or are elevated by elasticpowers.We need not be surprised, that, notwithstanding the greatassistance which our geological information derives from inqui-ries, having whole countries for their object, an extensive classof phænomena (with which I venture to entertain this assembly),has been treated, during so long a period, in a confined manner;the points of comparison being more difficult, and, I might say,more troublesome to find. Whatever we believed we knew,until the end of the last century, respecting the form of volca-noes, and the action of their subterraneous forces, had beenderived from two mountains of the south of Italy,—from Ætna,and from Vesuvius. The first being more accessible, and hav-ing, like all low volcanoes, more frequent eruptions, has servedfor a type, according to which a whole distant world,—thepowerful volcanoes of Mexico, South America, and the AsiaticIslands, has been considered. Such a method recalls to ourremembrance the shepherd of Virgil, who expected his narrowcottage to contain the ideal of the eternal city, imperial Rome.A careful examination of the whole Mediterranean, and princi-pally of its easterly islands and shores, where mankind firstawakened to mental culture, and to noble feelings, might cer-tainly have dispelled such a narrow idea of nature. Out of thedeep bed of the sea, among the Sporades, rocks of trachyte havearisen, like the Azoric island, which has thrice reappeared dur-ing three centuries, the intervening periods being almost equal.Between Epidaurus and Troezene, near Methone, the Pelopon-nesus has a Monte Nuovo which has been described by Strabo,and seen by Dodwell, higher than the Monte Nuovo of theCampi Phlegræi, near Baia; perhaps higher than the new vol-cano of Xorullo in the plains of Mexico, which I have foundamong a thousand basaltic cones, raised out of the earth, andstill smoking. In the bason of the Mediterranean Sea also, thevolcanic fire bursts forth, and not only from permanent craters,from isolated mountains which preserve a lasting communicationwith the interior of the earth, like Stromboli, Vesuvius, and Ætna;—on Ischia, near the Epomæus, and also, as it would appear fromthe reports of the ancients, near Chalcis in the Lelantic plains, haslava flowed out of fissures which have suddenly opened. Besidesthese phænomena, which have taken place in the period of historywithin the narrow limits of certain traditions, and which Ritterwill collect and explain in his masterly Geography, the shores ofthe Mediterranean contain abundant remains of more ancientigneous effects. The south of France shows, in Auvergne, arange of hills, in which bells of trachyte occur alternately withcones of eruption, from which currents of lava have descended.The Lombardic plain, which forms the innermost bay of theAdriatic Sea, surrounds the trachyte of the Euganean Hills,where domes of granular trachyte, of obsidian, and of pearlstone,rise, which, passing into each other, break through the Jura|124| limestone, but never occur in narrow streams which have flowed.Similar evidences of former revolutions may be found in manyparts of the Grecian continent, and in Asia Minor, countrieswhich will afford the geognost copious subjects for examination,when the light once returns to the land whence it first beamedover the western world—when tormented mankind ceases tosink under the savage lethargy of the Ottoman.I mention the geographical neighbourhood of so many phæno-mena, in order to prove, that the bed of the Mediterranean,with all its chains of islands, might have afforded to the atten-tive observer, every thing that has been discovered, in latterperiods, under the most varied forms, in South America, onTeneriffe, or on the Aleutian islands, near the polar regions.There were accumulated objects for observation, but tours intodistant regions, and the comparison of large tracts of countrywithin and beyond Europe, were necessary, in order to discoverwhat was common to all these phænomena, and to learn,clearly, their dependence on each other.By the usage of language, which often gives stability andrespect to the first erroneous views of things, but often, as itwere, by instinct, distinguishes the truth, we apply the termvolcanic to all eruptions of subterranean and melted matter; tocolumns of smoke and steam, which rise sporadically out ofrocks, as at Colares after the great earthquake at Lisbon; toSalsæ, or conical hills of clay which emit mud, asphaltum, andhydrogen, as those near Girgenti, in Sicily, and near Turbaco,in South America; to hot Geyser springs which rise by thepressure of elastic vapours; and, in general, to all violent powersof nature which have their seat deep in the interior of our planet.In the Spanish main of America, and in the Philippine islands,the inhabitants make a distinction between igneous and aqueousvolcanoes, vulcanes de agua y de fuego: they apply the first nameto mountains, which, during violent earthquakes, from time totime, eject subterraneous water, and with a dull noise.Without denying the connexion between the different phæno-mena just mentioned, it seems advisable to give a distinctlanguage to the physical as well as to the oryctognostic branchof geognosy; and not to apply the term volcano in one instanceto a mountain that terminates in a permanent crater; and inanother, to every subterranean cause of volcanic phænomena.In the present state of the earth, the most common from ofvolcanic eminences is that of isolated cones; such are Vesuvius,Ætna, the Peak of Teneriffe, Tunguragna, and Cotopaxi. Ihave seen them of every magnitude, from the lowest hills tomountains rising to the height of 17,700 feet above the level ofthe sea. Besides these conical mountains, there are other cra-ters, permanently communicating with the interior of the earth,situated upon lengthened craggy ranges of mountains, notalways in the middle of their wall-like summits, but towards theend, and near their declivities. Such is Pichincha which rises|125| between the Pacific Ocean and the town of Quito, and whichhas become celebrated by Bouguer’s earliest formula for thebarometer; such also are the volcanoes that rise in the plain delos Pastos, at the elevation of 10,000 feet.All these differently formed summits consist of trachyte, ortrap-porphyry, a granular rock, full of cracks and fissures, andcomposed of glassy felspar and hornblende, but often containingin addition, augite, mica, laminar felspar, and quartz.Where the evidence of the first eruption, and where the firstscaffolding, I might say, has been entirely preserved, the isolatedconical hills are surrounded by a high wall of rocks forming acircus, consisting of superposed strata; such walls, or annularsurrounding masses, are called craters of elevation; of these veryimportant phænomena, Leopold von Buch, the first geognost ofour times, from whose works I have taken several views con-tained in this paper, read a remarkable account, five years ago.The volcanoes which communicate with the atmosphere bymeans of craters, and the conical hills of basalt and bell-shapedtrachytic hills without craters, the latter either low like Sarcouy,or high like Chimborazo, form different groups. A geographicalcomparison shows, in one place, small Archipelagi, or, as it were,classed systems of mountains, either with craters and currentsof lava, as in the Canaries and Azores, or devoid of craters andreal currents of lava in the Euganeans, and the Siebengebirgenear Bonn; or it shows, in other places, single and doublechains of volcanoes, connected with each other, and formingtracts of many hundred miles in length, which are eitherparallel to the direction of the mountains, as in Guatimala,Peru, and Java, or in directions perpendicular to their axis, asin the land of the Aztekes, where none but volcanic trachyte-mountains attain the limits of eternal snow, and those, probably,have been thrust out of a fissure nearly 500 miles in length,which divides the whole continent, from the Pacific Ocean tothe Atlantic.This aggregation of volcanoes either in single round groups,or in double ranges, affords the most determinate proof thatvolcanic effects do not depend upon slight causes existing nearthe surface of the earth, but that they are great and deeplyfounded phænomena. The whole eastern part of the Americancontinent, which is poor in metals, is at present without craters,without trachyte, probably even without basalt. All the volca-noes are situated in the part opposite to Asia, in the meridianline of the Andes chain, 1800 geographical miles long; thewhole of the elevated district of Quito is nothing but a singlevolcanic hearth, the summits of which are Pichincha, Cotopaxi,and Tunguragua. The volcanic fire now bursts forth from one,and then from another of these apertures, which we are accus-tomed to consider as separate volcanoes.The progressive motion of the fire here, in the space of threecenturies, turned from north to south. The earthquakes with|126| which this part of the world is so terribly visited, furnish remark-able evidences of the existence of subterraneous communication,not only between countries without volcanoes, as was knownlong ago, but even between craters which are far distant fromeach other. Thus the volcano of Pasto, situated to the east ofthe river Guaytara, uninterruptedly vomited a high column ofsmoke, during three months of the year 1797; and this columndisappeared at the very moment, when, at the distance ofnearly 300 miles, the great earthquake of Riobamba andthe mud eruption of the Moya, killed from 30,000 to 40,000Indians. The sudden appearance of the Azoric island Sabrina,on the 30th of January, 1811, was the forerunner of those dread-ful shocks, which, further to the west, shook, almost uninterrupt-edly, from the month of May, 1811, to that of June, 1813, firstthe Antilles, afterwards the plains of the Ohio and the Missis-sippi, and at last the opposite coast of Venezuela. Thirty daysafter the complete destruction of the town of Caraccas, the erup-tion of the volcano of St. Vincent in the neighbouring Antillestook place; at the same moment when this explosion happened,on the 30th of April, 1811, a subterranean noise was heardthroughout a country of 2200 geographical square miles, or47,900 English square miles, in extent.The inhabitants near the Apure, where it is joined by theRio Nula, as well as those of the most distant part of the coast,compared this noise to that of artillery. From where the RioNula falls into the Apure, through which river I came into theOrinoco, to the volcano of St. Vincent, the distance, in a directline, is 731 English miles. The noise just alluded to, whichcertainly was not communicated through the air, must, there-fore, have had a deep internal cause. Its intensity on the coastof the Antillic sea was scarcely greater than in the interior of thecountry.It would be useless to augment the number of examples, butfor the purpose of recalling to memory a phenomenon which hasbecome historically interesting to Europe, I will mention theearthquake at Lisbon. At the same time with this, on the 1stof November, 1755, not only were the Swiss lakes, and the seaon the Swedish shores violently agitated, but even in the easterlyAntilles, around Martinique, Antigua, and Barbadoes, where thetide never exceeds 28 inches, it suddenly rose to 20 feet. Allthese phænomena prove, that the subterranean powers act eitherdynamically, by producing tension and vibration, as in earth-quakes; or chemically, by producing or altering substances, asin volcanoes. They prove, likewise, that these powers do notact from superficial causes, from the exterior crust of the earth;but from deeply-seated causes, from the interior of our planet;extending their simultaneous effects to the most distant parts ofthe earth, through fissures and empty veins.The more different the structure of volcanoes; that is to say,of those raised masses which surround the canal through which|127| the melted substances proceed from the interior of the earth toits surface, the more important is it to become thoroughlyacquainted with that structure, by exact measurement. Theinterest attached to this measurement, which has been a parti-cular object of my examination in another part of the world, isheightened by the consideration, that that which is to be mea-sured is a variable magnitude. The physiognomy of nature con-sists in the change of phænomena tending to connect the presentwith the past. In order to ascertain a periodical return, or thelaws of progressive natural changes in general, certain fixedpoints are necessary; and observations carefully made at statedperiods, may serve for numerical comparison. Had the meantemperature of the atmosphere in different latitudes beenobserved for a few thousand years, and the mean height of thebarometer at the level of the sea, we might now know in whatproportion the heat of different climates has increased, or dimi-nished, and whether the height of the atmosphere has undergoneany changes. Similar points for comparison are required, forthe variation and the declination of the magnetic needle, andfor the intensity of the electromagnetic power, upon which twoexcellent philosophers of this Academy have thrown so muchlight. If it be a praiseworthy undertaking of learned societiesto inquire assiduously into the changes of temperature undergoneby the globe, into those which take place in the pressure of the atmo-sphere, and in the magnetic variation, — it is the duty of a travellinggeognost, in ascertaining the inequality of the earth’s surface, toconsider, principally, the variable height of the volcanoes. WhatI formerly attempted on the mountains of Mexico, on the TolucaNauhiampatepetl and Xorullo, and in the Andes of Quito, onthe Pichincha, I have found opportunity, since my return toEurope, to repeat at different periods on Vesuvius. Saussuremeasured this mountain in 1773, at the time when both sides ofthe crater, the south-eastern and north-western, appeared to beof equal altitude; he found their height to be 609 toises (3894feet) above the level of the sea. The eruption of 1794 occa-sioned a fall on the south side, which even the unaccustomedeye discovers at a great distance. In 1805, I measured Vesu-vius three times, in conjunction with M. von Buch, and M. Gay-Lussac; we found the elevation of the northern edge, oppositeto Monte Somma, la Rocca del Palo, to be exactly the same asSaussure had before determined it; the southern edge we found71 toises (454 feet) lower than it was in 1773; the total heightof the volcano on the side opposite Torre del Greco (towardswhich side the fire seems to have acted the most powerfully,during the last 30 years), had diminished one-ninth part.The cone of ashes on Vesuvius bears the proportion of one-third to the height of the whole mountain, that on Pichincha isas 1 to 10, and that on the Peak of Teneriffe as 1 to 22; Vesu-vius has, therefore, the largest cone of ashes in proportion,|128| because, probably, as a low volcano, it has acted principallythrough its summit. A few months ago, I succeeded not onlyin repeating my former measurements on Vesuvius, but also inascertaining the elevation of all the edges of the crater. Thiswork, perhaps, deserves some consideration, for the periods atwhich it was executed include those of the great eruptions from 1805to 1822, and it is, perhaps, the only admeasurement yet publishedof any volcano which may be compared in all its parts. It provesthat the edges of the craters, not only where they evidently consistof trachyte, as in the volcanoes of the Andes, but likewise every-where else, are much more constant phænomena than hashitherto been believed. Simple angles of elevation ascertainedfrom the same points are more proper for these examinations thanbarometrical and trigonometrical measurements. According tomy last determination, the north-western edge of the crater ofVesuvius has not changed its form in the least since Saussure’stime, a period of 49 years. The south-eastern edge towardsBosche tre Case, which became about 450 feet lower in 1794,has sunk very little since that time.If in the description of great eruptions, in the public papers,the completely changed form of Vesuvius has frequently beenmentioned, if this opinion often seems to be corroborated by thepicturesque views of the mountain made at Naples, the cause ofthis mistake may be found in the circumstance, that the outlinesof the edges of the crater have been confounded with those ofthe cone of eruption which is accidentally formed in the middleof the crater, upon a bottom that has been raised by vapours.Such a cone of eruption, consisting of rapilli and slags looselyheaped together, has become visible over the south-eastern edgeof the crater, since 1816 and 1818. The eruption of February,1822, had so much increased it that it had become from 70 to80 feet higher than the north-eastern edge of the crater, Roccadel Palo. This remarkable cone, which, at Naples, they wereaccustomed to consider as the true summit of Vesuvius, fell inwith a tremendous noise, during the eruption of the 22d ofOctober, so that the bottom of the crater, which had been unin-terruptedly accessible from the year 1811, now lies 850 English feetbeneath the northern edge, and about 213 feet deeper than thesouthern edge of the volcano. The variable form and relativesituation of the crater of eruption, the opening of which mustnot be taken for the real crater of the volcano, as frequently hasbeen done, gives, at different times, a peculiar physiognomy toVesuvius; and the historiographer of that volcano, from the mereoutline of the summit, and the relative height of the northernor southern side of the mountain, as it is drawn in Hackert’sViews in the palace of Portici, would guess the year in whichthe artist made the sketch of his picture.In the night between the 23d and 24th of October, one dayafter the fall of the cone of slags 400 feet in height, when small|129| but numerous currents of lava had already flowed, the fieryeruption of ashes and rapilli began. It continued uninterrupt-edly for twelve days, but was most violent during the first four.During this time the detonations in the interior of the volcanowere so violent, that the mere concussion of the air (no earth-quake had been observed) caused the roofs to burst in the palaceof Portici. In the surrounding villages of Resina, Torre delGreco, Torre del Annonciata, and Bosche tre Case, an interest-ing phænomenon was observed; the atmosphere was so thicklyfilled with ashes, that the most intense darkness overspread thewhole country for several hours in the middle of the day. Thepeople walked in the streets with lanterns, as is often done atQuito when Pichincha is in eruption. The flight of the inhabi-tants was never more general; currents of lava were less fearedthan a fall of ashes, a phenomenon which was unknown therewith such violence, and in consequence of the relations respect-ing the destruction of Herculaneum, Pompeii, and Stabiæ, filledthe minds of the people with frightful images.The hot steam which rose from the crater during the eruptionand passed into the atmosphere, formed on cooling a thick massof clouds, around the column of ashes and fire, 9000 feet inheight. This sudden condensation of steam, and, as Gay-Lussac has shown, the very formation of the clouds, increasesthe electric tension. Lightnings burst forth in all directionsfrom the column of ashes, and the rolling thunder might clearlybe distinguished from the interior noise of the volcano. At noformer eruption had the play of electric charges been so sur-prising.On the morning of the 26th of October, a singular accountwas circulated, that a current of boiling water had issued fromthe crater, and rushed down from the cone of ashes. Monticelli,the zealous and learned observer of the volcano, soon discernedthat the rumour had been occasioned by an optical deception.The supposed current of water was nothing but a dry mass ofashes, which flowed down, like quicksand, from a fissure in thesuperior edge of the crater. A drought, which had completelydesolated the fields, preceded the eruption, but the volcanicthunderstorm occasioned, towards its termination, a very heavyand continued rain. Such a phenomenon characterizes theconclusion of an eruption in every zone. On account of the coneof ashes being generally covered with clouds during this time,and likewise because the torrents of rain are heaviest in itsneighbourhood, currents of mud flow down on all sides. Theaffrighted peasant considers it to be water which has risen fromthe interior of the crater, and the deceived geognost conceivesthat he recognizes in it either sea-water, or mud-like volcanicproductions, which are called eruptions boueuses, or, as the oldFrench systematic writers termed them, products of a fiery-aqueous liquefaction.|130| When the summits of volcanoes (as is generally the case inthe chain of the Andes), extend into the region of eternal snow,or even to double the height of Ætna, the melted snow rendersthe inundations amazingly frequent and destructive. They arephænomena meteorologically connected with volcanic eruptions,and are multifariously modified by the altitude of the mountains,the extent of their summits covered with eternal snow, and thecalefaction of the sides of the cone of ashes; but they shouldnever be considered as real volcanic phænomena. Subterraneanlakes, in connexion with alpine rivers, are formed both on theslopes and at the feet of the mountains. When the earthquakeswhich precede every eruption in the chain of the Andes, shakewith mighty force the entire mass of the volcano, the subterra-nean vaults are opened, and emit, at the same time, water, fishes,and tufa-mud. This is the singular phenomenon that fur-nishes the fish pimelodes cyclopum, which the inhabitants ofthe high lands of Quito call preñadilla, and which was describedby me soon after my return. When the summit of the mountainCarguairazo, to the north of Chimborazo, and 18,000 feethigh, fell, in the night between the 19th and 20th of June,1698, the surrounding fields, to the extent of about 43 Eng-lish square miles, were covered with mud and fishes. Thefever which raged in the town of Ibarra, seven years before, hadbeen ascribed to a similar eruption of fishes from the volcanoImbaburu. I recur to these facts, because they throw somelight on the difference between the eruption of ashes, and thatof mud-like masses of tufa and trass, which contain wood,coal, and shells.The quantity of ashes ejected by Vesuvius in the late erup-tions, like all other things which are connected with great andappalling phænomena, has been enormously exaggerated in thepublic papers; and two Neapolitan chemists, Vincenzo Pepe,and Giuseppe di Nobili, have affirmed, that they contain goldand silver, notwithstanding the contradiction of Monticelli andCovelli.* According to my examination, the stratum of asheswhich had fallen in twelve days, towards Bosche tre Case, onthe slope of the cone, where rapilli were mixed with it, wasonly three feet in thickness, and in the plain, it did notexceed from 15 to 18 inches. Measurements of this kindmust not be made in places where the ashes have been driftedby wind, like snow, or sand, nor in those where they have beenaccumulated by water. The times are past in which we soughtonly for the marvellous in volcanic phænomena, and, like Ctesias,made the ashes of Ætna fly to the Indian peninsula. Some ofthe Mexican gold and silver mines are certainly in trachyticporphyry, but in the ashes of Vesuvius which I collected, andwhich, at my desire, have been analyzed by Henry Rose, of
* See Annals, v. 236.|131| Berlin, an excellent chemist, no traces of either metal, could bediscovered.However great may be the discrepancy between the results thatI have here given, but which agree with Monticelli’s more exactobservations, and those which have been circulated duringseveral months past, yet the eruption of ashes from Vesuvius,from the 24th to the 28th of October, still remains the most re-markable of which we have any certain account since the deathof the elder Pliny. Its quantity, perhaps, was three times asgreat as that of all the ashes, collectively, which have beenobserved to fall, during the time in which volcanic phænomenahave been attentively considered. A stratum of from 15 to 18inches in thickness, seems at first view unimportant, if comparedto the mass with which we find Pompeii to be covered; butwithout speaking of the torrents and inundations which certainlymay have increased this mass for centuries, without renewingthe violent dispute concerning the cause of the destruction ofthe Campanian towns, which has been carried on with so muchscepticism on the other side of the Alps, it may be affirmed thatthe eruptions of one and the same volcano at distant periods canby no means be compared with respect to their intensity. Allconclusions founded on analogy are insufficient, when the ques-tion is about quantitative proportions,—the quantity of ashesand lava, the height of the column of smoke, or the violence ofthe detonation.From the geographical description of Strabo, and from anopinion of Vitruvius concerning the volcanic origin of pumice,we see that until the year in which Vespasian died, that is tosay, until the eruption which overwhelmed Pompeii, Vesuviuswas more like an extinguished volcano than a solfatara.When after long rest the subterranean powers suddenlyopen new passages, and again break through beds of pri-mitive rocks and of trachyte, effects must necessarily takeplace, for which all the phænomena subsequently observed donot afford any standard of comparison. It may be clearly seenfrom the well-known letter in which the younger Pliny announcesthe death of his uncle to Tacitus, that the recommencement ofthe eruptions, I might say, the awakening of the dormant vol-cano, began with an eruption of ashes. The same circumstancewas observed at Xorullo, in Sept. 1759, when the new volcano,breaking through beds of syenite and trachyte, suddenly arosein the plain. The peasants fled, because they found in theirhuts, ashes that had been ejected from the fissures of the earth,which was burst in every place. Every partial eruption, in theperiodical general eruptions of volcanoes terminates with a showerof ashes.There is a passage in Pliny’s letter, which shows, that the dryashes which had fallen from the air had already attained a heightof from four to five feet, in the commencement of the eruption,|132| and without the effect of accumulation by water. “The courtwhich led to his [uncle’s] apartment,” he says, “being nowalmost filled with ashes and pumice, it would have been impos-sible for him, if he had continued there any longer, to have madehis way out.” In the narrow space of a court, the wind couldnot have had any great effect in accumulating the ashes.I have ventured to interrupt my comparative view of volca-noes by observations solely on Vesuvius, partly on account ofthe great interest which the last eruption has excited, and partlybecause every great fall of ashes almost involuntarily reminds usof the classic ground of Pompeii and Herculaneum.* We havehitherto considered the form and the effects of those volcanoeswhich are in permanent communication with the interior of theearth, by means of a crater. Their summits are raised massesof trachyte and lava, intersected by numerous veins; the dura-tion of their effects causes us to believe that they have a verystable and undisturbed structure. They possess, I may say, amore individual character, which remains the same during longperiods. Neighbouring mountains often furnish completelydifferent products, leucite-lava, and felspar-lava; obsidian, withpumice, and basaltic masses containing olivine. They belongto the newer phænomena of the earth, pass generally through allthe strata of secondary rocks, and their eruptions and currentsof lava are of later origin than our valleys. Their life, if I mayuse that expression, depends upon the manner and duration oftheir connexion with the interior of the earth. They often restfor centuries, suddenly take fire again, and terminate as solfa-taras, which emit steam, gases, and acids. Sometimes, as onthe Peak of Teneriffe, their summit has already become such adepository of reproduced sulphur, while mighty currents of lavaflow from the sides of the mountain, like basalt below, andabove, where the pressure is less, like obsidian with pumice.Independently of these with permanent craters, volcanicphænomena of another kind exist, which have been observedless frequently, but are principally interesting in geognosy, andremind us of the primitive world; that is to say, of the earliestrevolutions of our earth. Mountains of trachyte suddenlyopen, eject lava and ashes, and close again, perhaps, forever: thus was it with the mighty Antisana; and thus with theEpomæus, in Ischia, in 1302. Such an eruption sometimestakes place even in the plain, as in the high lands of Quito; inIceland, far from Hecla; and in Eubœa, in the Lelantic fields.Many of the islands which have been raised up are owing to thesetemporary phænomena. In these cases the communication withthe interior of the earth is not permanent, and the effect ceasesas soon as the fissure, which is the communicating channel, is
* The author here mentions a paper on the data of his measurements at Vesuvius,which was unsuitable for reading; and then proceeds to notice a collection of mineralsthat he brought with him, and which will be added to the Royal Museum at Berlin.|133| closed again. The veins of basalt, dolerite, and porphyry,which, in different parts of the world, pass through every forma-tion; and those of syenite, augite-porphyry, and amygdaloid,which are characteristic of the newest strata of the transitionformation, and of the oldest rocks of the secondary strata, haveprobably been formed in a similar manner. In the first age ofour planet, the yet liquid substances penetrated through thecrust of the earth, which was every where intersected byfissures, and assumed the form of granular rocks, either in veins,or spreading over and expanding themselves in strata. The rocksstrictly volcanic which the primitive ages have afforded us, havenot flowed in currents like the lava of our insulated conical hills;the same mixture of augite, titaniferous iron, glassy felspar, andhornblende, may have existed at different periods, but at onetime it may have approached nearer to basalt, and at others totrachyte; the chemical substances may have combined in acrystalline form, in distinct proportions, as we are taught byM. Mitscherlich’s new and important labours, and by the ana-logy of artificial products of fire: we find that substancessimilarly formed have arrived at the surface of the earthin very different ways; they have either been merely raised, orprotruded by temporary fissures through the older strata; thatis to say, through the already oxidized surface of the earth; orthey have flowed, as currents of lava, from conical hills with apermanent crater. By confounding such different phænomenatogether, the geognosy of volcanoes is carried back to that dark-ness from which a great number of comparative observations arebeginning to extricate it.The question has often been asked, What is it that burns involcanoes? What was it that excited the heat by which earthsand metals were melted? Modern chemistry answers, thatthe substances which melt are the metals of the earths andalkalies. The solid crust of the earth, already oxidized, se-parated the surrounding air with its oxygen, from the com-bustible unoxidized substances of the interior of our planet.The observations which have been made in mines and cavesin every zone, and which, in conjunction with M. Arago, I havecollected in a particular paper, demonstrate that the heat of themass of the earth is yet much greater than the mean temperatureof the atmosphere at the same place. Such a remarkable andalmost generally proved fact, is closely connected with thosewhich are proved by volcanic phænomena. Laplace has evengone so far as to endeavour to calculate the depth at which thebody of the earth may be considered to be a melted mass.Whatever doubts may be entertained, notwithstanding the vene-ration due to so great a name, with respect to the numericalcertainty of such a calculation, thus much remains probable;that all volcanic phænomena originate in a very simple cause, in|134| a permanent or in a variable communication between the interiorand the exterior of our planet.The pressure of elastic vapour forces the melted substancesupwards through deep fissures while they are undergoing oxida-tion; volcanoes, if I may so speak, are intermitting springs ofthe earth; the liquid mixtures of metals, alkalies, and earths,which on cooling become currents of lava, flow quietly whenthey are raised, and find a vent. The ancients imagined,according to Plato’s Phædon, that all volcanic currents of fireflowed, in a similar way, from the Periphlegeton.It may be permitted me, perhaps, to add to these considerationsone which is still more hazardous. In this interior heat of theearth, indicated by experiments with the thermometer, and byobservations on volcanoes, the cause, perhaps, may be found, ofone of the most wonderful phænomena which the examination offossils presents to us. Tropical forms of animals, arboriform ferns,palms, and bamboo-like plants, lie interred in the cold north.The primitive world every where shows a distribution of organicforms at variance with the then existing nature of the cli-mate. In order to solve this important problem, several hypo-theses have been invented; as the neighbourhood of a comet,the altered inclination of the ecliptic, the increased intensityof the solar light. Neither of these has been sufficient tosatisfy at once the astronomer, the natural philosopher, andthe geognost. For my part, I leave the axis of the earth un-altered, as well as the light of the solar disc, by the spotson which, a celebrated astronomer has explained both the ferti-lity and the unfruitfulness of the fields; but I believe, that inevery planet, independently of its relation to a central body, andof its astronomical situation, various causes exist of the produc-tion of heat; oxidation, precipitation, and a change in thecapacity of bodies; by increase of electromagnetic charge, bythe opening of a communication between the interior and theexterior part of the earth.Where the deeply cleft crust of the earth in the primitiveworld radiated heat from its fissures, whole countries, perhaps,could produce for centuries, palms and arborescent ferns, and sus-tain all the animals of the torrid zone. According to this view, towhich I have already alluded in a work just published, “EssaiGéognostique sur le Gissement des Roches dans les deuxHemispheres,” the temperature of volcanoes would be that of theinterior of the earth itself, and the same cause which now occa-sions such dreadful destruction, would once have occasioned,on the newly oxidated crust of the earth, upon the deeply cleftstrata of rocks, the most luxuriant growth of plants in everyzone.Even if any one should be inclined to suppose, in order toexplain the marvellous distribution of tropical forms in their|135| ancient graves, that shaggy animals of the elephant tribe nowimbedded in icebergs, were once peculiar to a northern climate,and that similar forms belonging to the same primary types, likelions and lynxes, could live in very different climates, such anexplanation could not, however, be extended to the products ofvegetation. For reasons which the physiology of plantsexplains, palms, and arboriform monocotyledones cannot sustainthe northern cold, and in the geological problem we here speakof, it seems difficult to me to separate plants and animals. Thesame explanation must be applied to both.Towards the end of this paper, I have combined uncertainhypothetical suppositions with facts collected from the mostdifferent parts of the world. The philosophical knowledge ofnature rises above a mere description of nature. It does notconsist in a sterile aggregation of isolated observations. It maysometimes be allowed, therefore, to the curious and ever-activemind of man, to look back upon the past, to imagine what cannotbe clearly known, and to amuse himself with the ancient, and,under many forms, returning mysteries of geogony.
On the Constitution and Mode of Action of Volcanoes, in differ-ent Parts of the Earth. By Alexander Von Humboldt.*
* Read before the Royal Academy of Sciences of Berlin, Jan. 24, 1823.|122| to and receding from each other in groups occur in both hemi-spheres, from the equator to the poles. On a distant island,surrounded by strange plants, under a sky where the well-known stars do not shine, the sailor recognises, often with gladsurprise, the clayslate which is the common rock of his nativecountry.This independence of the geognostical relations of places onthe present constitution of their climate, does not diminish, butonly gives a particular direction to the favourable effect upon theprogress of geology and physical geognosy, which is producedby numerous observations made in foreign countries. Everyexpedition enriches natural history with new plants, and newgenera of animals; at one time they are organic forms rangingthemselves with well-known types, and representing to us, in itsoriginal perfection, a regularly woven, though often apparentlyinterrupted texture of animated creatures; at another, they areforms which appear to be isolated, as vestiges of genera whichhave been destroyed, or as surprising members of groups still tobe discovered. Such a variety is not presented by the examina-tion of the solid crust of the earth; it rather reveals to us anagreement, which excites the admiration of the geognost,between the parts of which it is composed, in the superpositionof masses of different natures, and in their periodical repetition.In the chain of the Andes, as well as in the central mountainsof Europe, one formation seems, as it were, to occasion theexistence of another; masses of the same character assumesimilar forms: * mountains are formed by basalt anddolerite; steep declivities by dolomite, porphyry, and quader-sandstein; bell-shaped eminences and high-vaulted domes byvitreous trachyte rich in felspar.In the most distant zones, larger crystals, as it were by inter-nal evolution out of the more compact texture of the greatermass, aggregate into subordinate beds, and thus frequentlyannounce the vicinity of a new and independent formation.Thus is the whole inorganic world reflected, more or less clearly,in every mountain of considerable extent; but in order to ascer-tain completely the most important phænomena respecting thecomposition, the relative age, and the origin of the differentspecies of rocks, observations from the most distant parts of theearth must be compared together. Problems which had appearedenigmatical to the geognost in his mother country are solvednear the equator. If distant zones do not furnish new speciesof rocks, that is to say, unknown arrangements of simple sub-stances, as has already been remarked, they yet teach us how todiscover the great laws which are every where the same, andaccording to which, the different strata of the earth support each
* In an imperfect translation of this paper, which has been forwarded to the Editorfrom the Continent, a word here occurs which cannot be decyphered; and on account ofother inaccuracies which it has been necessary to correct, unaided by the original, thetranslation, as now given, is not to be regarded as exact in every particular.|123| other, appear in the form of veins, or are elevated by elasticpowers.We need not be surprised, that, notwithstanding the greatassistance which our geological information derives from inqui-ries, having whole countries for their object, an extensive classof phænomena (with which I venture to entertain this assembly),has been treated, during so long a period, in a confined manner;the points of comparison being more difficult, and, I might say,more troublesome to find. Whatever we believed we knew,until the end of the last century, respecting the form of volca-noes, and the action of their subterraneous forces, had beenderived from two mountains of the south of Italy,—from Ætna,and from Vesuvius. The first being more accessible, and hav-ing, like all low volcanoes, more frequent eruptions, has servedfor a type, according to which a whole distant world,—thepowerful volcanoes of Mexico, South America, and the AsiaticIslands, has been considered. Such a method recalls to ourremembrance the shepherd of Virgil, who expected his narrowcottage to contain the ideal of the eternal city, imperial Rome.A careful examination of the whole Mediterranean, and princi-pally of its easterly islands and shores, where mankind firstawakened to mental culture, and to noble feelings, might cer-tainly have dispelled such a narrow idea of nature. Out of thedeep bed of the sea, among the Sporades, rocks of trachyte havearisen, like the Azoric island, which has thrice reappeared dur-ing three centuries, the intervening periods being almost equal.Between Epidaurus and Troezene, near Methone, the Pelopon-nesus has a Monte Nuovo which has been described by Strabo,and seen by Dodwell, higher than the Monte Nuovo of theCampi Phlegræi, near Baia; perhaps higher than the new vol-cano of Xorullo in the plains of Mexico, which I have foundamong a thousand basaltic cones, raised out of the earth, andstill smoking. In the bason of the Mediterranean Sea also, thevolcanic fire bursts forth, and not only from permanent craters,from isolated mountains which preserve a lasting communicationwith the interior of the earth, like Stromboli, Vesuvius, and Ætna;—on Ischia, near the Epomæus, and also, as it would appear fromthe reports of the ancients, near Chalcis in the Lelantic plains, haslava flowed out of fissures which have suddenly opened. Besidesthese phænomena, which have taken place in the period of historywithin the narrow limits of certain traditions, and which Ritterwill collect and explain in his masterly Geography, the shores ofthe Mediterranean contain abundant remains of more ancientigneous effects. The south of France shows, in Auvergne, arange of hills, in which bells of trachyte occur alternately withcones of eruption, from which currents of lava have descended.The Lombardic plain, which forms the innermost bay of theAdriatic Sea, surrounds the trachyte of the Euganean Hills,where domes of granular trachyte, of obsidian, and of pearlstone,rise, which, passing into each other, break through the Jura|124| limestone, but never occur in narrow streams which have flowed.Similar evidences of former revolutions may be found in manyparts of the Grecian continent, and in Asia Minor, countrieswhich will afford the geognost copious subjects for examination,when the light once returns to the land whence it first beamedover the western world—when tormented mankind ceases tosink under the savage lethargy of the Ottoman.I mention the geographical neighbourhood of so many phæno-mena, in order to prove, that the bed of the Mediterranean,with all its chains of islands, might have afforded to the atten-tive observer, every thing that has been discovered, in latterperiods, under the most varied forms, in South America, onTeneriffe, or on the Aleutian islands, near the polar regions.There were accumulated objects for observation, but tours intodistant regions, and the comparison of large tracts of countrywithin and beyond Europe, were necessary, in order to discoverwhat was common to all these phænomena, and to learn,clearly, their dependence on each other.By the usage of language, which often gives stability andrespect to the first erroneous views of things, but often, as itwere, by instinct, distinguishes the truth, we apply the termvolcanic to all eruptions of subterranean and melted matter; tocolumns of smoke and steam, which rise sporadically out ofrocks, as at Colares after the great earthquake at Lisbon; toSalsæ, or conical hills of clay which emit mud, asphaltum, andhydrogen, as those near Girgenti, in Sicily, and near Turbaco,in South America; to hot Geyser springs which rise by thepressure of elastic vapours; and, in general, to all violent powersof nature which have their seat deep in the interior of our planet.In the Spanish main of America, and in the Philippine islands,the inhabitants make a distinction between igneous and aqueousvolcanoes, vulcanes de agua y de fuego: they apply the first nameto mountains, which, during violent earthquakes, from time totime, eject subterraneous water, and with a dull noise.Without denying the connexion between the different phæno-mena just mentioned, it seems advisable to give a distinctlanguage to the physical as well as to the oryctognostic branchof geognosy; and not to apply the term volcano in one instanceto a mountain that terminates in a permanent crater; and inanother, to every subterranean cause of volcanic phænomena.In the present state of the earth, the most common from ofvolcanic eminences is that of isolated cones; such are Vesuvius,Ætna, the Peak of Teneriffe, Tunguragna, and Cotopaxi. Ihave seen them of every magnitude, from the lowest hills tomountains rising to the height of 17,700 feet above the level ofthe sea. Besides these conical mountains, there are other cra-ters, permanently communicating with the interior of the earth,situated upon lengthened craggy ranges of mountains, notalways in the middle of their wall-like summits, but towards theend, and near their declivities. Such is Pichincha which rises|125| between the Pacific Ocean and the town of Quito, and whichhas become celebrated by Bouguer’s earliest formula for thebarometer; such also are the volcanoes that rise in the plain delos Pastos, at the elevation of 10,000 feet.All these differently formed summits consist of trachyte, ortrap-porphyry, a granular rock, full of cracks and fissures, andcomposed of glassy felspar and hornblende, but often containingin addition, augite, mica, laminar felspar, and quartz.Where the evidence of the first eruption, and where the firstscaffolding, I might say, has been entirely preserved, the isolatedconical hills are surrounded by a high wall of rocks forming acircus, consisting of superposed strata; such walls, or annularsurrounding masses, are called craters of elevation; of these veryimportant phænomena, Leopold von Buch, the first geognost ofour times, from whose works I have taken several views con-tained in this paper, read a remarkable account, five years ago.The volcanoes which communicate with the atmosphere bymeans of craters, and the conical hills of basalt and bell-shapedtrachytic hills without craters, the latter either low like Sarcouy,or high like Chimborazo, form different groups. A geographicalcomparison shows, in one place, small Archipelagi, or, as it were,classed systems of mountains, either with craters and currentsof lava, as in the Canaries and Azores, or devoid of craters andreal currents of lava in the Euganeans, and the Siebengebirgenear Bonn; or it shows, in other places, single and doublechains of volcanoes, connected with each other, and formingtracts of many hundred miles in length, which are eitherparallel to the direction of the mountains, as in Guatimala,Peru, and Java, or in directions perpendicular to their axis, asin the land of the Aztekes, where none but volcanic trachyte-mountains attain the limits of eternal snow, and those, probably,have been thrust out of a fissure nearly 500 miles in length,which divides the whole continent, from the Pacific Ocean tothe Atlantic.This aggregation of volcanoes either in single round groups,or in double ranges, affords the most determinate proof thatvolcanic effects do not depend upon slight causes existing nearthe surface of the earth, but that they are great and deeplyfounded phænomena. The whole eastern part of the Americancontinent, which is poor in metals, is at present without craters,without trachyte, probably even without basalt. All the volca-noes are situated in the part opposite to Asia, in the meridianline of the Andes chain, 1800 geographical miles long; thewhole of the elevated district of Quito is nothing but a singlevolcanic hearth, the summits of which are Pichincha, Cotopaxi,and Tunguragua. The volcanic fire now bursts forth from one,and then from another of these apertures, which we are accus-tomed to consider as separate volcanoes.The progressive motion of the fire here, in the space of threecenturies, turned from north to south. The earthquakes with|126| which this part of the world is so terribly visited, furnish remark-able evidences of the existence of subterraneous communication,not only between countries without volcanoes, as was knownlong ago, but even between craters which are far distant fromeach other. Thus the volcano of Pasto, situated to the east ofthe river Guaytara, uninterruptedly vomited a high column ofsmoke, during three months of the year 1797; and this columndisappeared at the very moment, when, at the distance ofnearly 300 miles, the great earthquake of Riobamba andthe mud eruption of the Moya, killed from 30,000 to 40,000Indians. The sudden appearance of the Azoric island Sabrina,on the 30th of January, 1811, was the forerunner of those dread-ful shocks, which, further to the west, shook, almost uninterrupt-edly, from the month of May, 1811, to that of June, 1813, firstthe Antilles, afterwards the plains of the Ohio and the Missis-sippi, and at last the opposite coast of Venezuela. Thirty daysafter the complete destruction of the town of Caraccas, the erup-tion of the volcano of St. Vincent in the neighbouring Antillestook place; at the same moment when this explosion happened,on the 30th of April, 1811, a subterranean noise was heardthroughout a country of 2200 geographical square miles, or47,900 English square miles, in extent.The inhabitants near the Apure, where it is joined by theRio Nula, as well as those of the most distant part of the coast,compared this noise to that of artillery. From where the RioNula falls into the Apure, through which river I came into theOrinoco, to the volcano of St. Vincent, the distance, in a directline, is 731 English miles. The noise just alluded to, whichcertainly was not communicated through the air, must, there-fore, have had a deep internal cause. Its intensity on the coastof the Antillic sea was scarcely greater than in the interior of thecountry.It would be useless to augment the number of examples, butfor the purpose of recalling to memory a phenomenon which hasbecome historically interesting to Europe, I will mention theearthquake at Lisbon. At the same time with this, on the 1stof November, 1755, not only were the Swiss lakes, and the seaon the Swedish shores violently agitated, but even in the easterlyAntilles, around Martinique, Antigua, and Barbadoes, where thetide never exceeds 28 inches, it suddenly rose to 20 feet. Allthese phænomena prove, that the subterranean powers act eitherdynamically, by producing tension and vibration, as in earth-quakes; or chemically, by producing or altering substances, asin volcanoes. They prove, likewise, that these powers do notact from superficial causes, from the exterior crust of the earth;but from deeply-seated causes, from the interior of our planet;extending their simultaneous effects to the most distant parts ofthe earth, through fissures and empty veins.The more different the structure of volcanoes; that is to say,of those raised masses which surround the canal through which|127| the melted substances proceed from the interior of the earth toits surface, the more important is it to become thoroughlyacquainted with that structure, by exact measurement. Theinterest attached to this measurement, which has been a parti-cular object of my examination in another part of the world, isheightened by the consideration, that that which is to be mea-sured is a variable magnitude. The physiognomy of nature con-sists in the change of phænomena tending to connect the presentwith the past. In order to ascertain a periodical return, or thelaws of progressive natural changes in general, certain fixedpoints are necessary; and observations carefully made at statedperiods, may serve for numerical comparison. Had the meantemperature of the atmosphere in different latitudes beenobserved for a few thousand years, and the mean height of thebarometer at the level of the sea, we might now know in whatproportion the heat of different climates has increased, or dimi-nished, and whether the height of the atmosphere has undergoneany changes. Similar points for comparison are required, forthe variation and the declination of the magnetic needle, andfor the intensity of the electromagnetic power, upon which twoexcellent philosophers of this Academy have thrown so muchlight. If it be a praiseworthy undertaking of learned societiesto inquire assiduously into the changes of temperature undergoneby the globe, into those which take place in the pressure of the atmo-sphere, and in the magnetic variation, — it is the duty of a travellinggeognost, in ascertaining the inequality of the earth’s surface, toconsider, principally, the variable height of the volcanoes. WhatI formerly attempted on the mountains of Mexico, on the TolucaNauhiampatepetl and Xorullo, and in the Andes of Quito, onthe Pichincha, I have found opportunity, since my return toEurope, to repeat at different periods on Vesuvius. Saussuremeasured this mountain in 1773, at the time when both sides ofthe crater, the south-eastern and north-western, appeared to beof equal altitude; he found their height to be 609 toises (3894feet) above the level of the sea. The eruption of 1794 occa-sioned a fall on the south side, which even the unaccustomedeye discovers at a great distance. In 1805, I measured Vesu-vius three times, in conjunction with M. von Buch, and M. Gay-Lussac; we found the elevation of the northern edge, oppositeto Monte Somma, la Rocca del Palo, to be exactly the same asSaussure had before determined it; the southern edge we found71 toises (454 feet) lower than it was in 1773; the total heightof the volcano on the side opposite Torre del Greco (towardswhich side the fire seems to have acted the most powerfully,during the last 30 years), had diminished one-ninth part.The cone of ashes on Vesuvius bears the proportion of one-third to the height of the whole mountain, that on Pichincha isas 1 to 10, and that on the Peak of Teneriffe as 1 to 22; Vesu-vius has, therefore, the largest cone of ashes in proportion,|128| because, probably, as a low volcano, it has acted principallythrough its summit. A few months ago, I succeeded not onlyin repeating my former measurements on Vesuvius, but also inascertaining the elevation of all the edges of the crater. Thiswork, perhaps, deserves some consideration, for the periods atwhich it was executed include those of the great eruptions from 1805to 1822, and it is, perhaps, the only admeasurement yet publishedof any volcano which may be compared in all its parts. It provesthat the edges of the craters, not only where they evidently consistof trachyte, as in the volcanoes of the Andes, but likewise every-where else, are much more constant phænomena than hashitherto been believed. Simple angles of elevation ascertainedfrom the same points are more proper for these examinations thanbarometrical and trigonometrical measurements. According tomy last determination, the north-western edge of the crater ofVesuvius has not changed its form in the least since Saussure’stime, a period of 49 years. The south-eastern edge towardsBosche tre Case, which became about 450 feet lower in 1794,has sunk very little since that time.If in the description of great eruptions, in the public papers,the completely changed form of Vesuvius has frequently beenmentioned, if this opinion often seems to be corroborated by thepicturesque views of the mountain made at Naples, the cause ofthis mistake may be found in the circumstance, that the outlinesof the edges of the crater have been confounded with those ofthe cone of eruption which is accidentally formed in the middleof the crater, upon a bottom that has been raised by vapours.Such a cone of eruption, consisting of rapilli and slags looselyheaped together, has become visible over the south-eastern edgeof the crater, since 1816 and 1818. The eruption of February,1822, had so much increased it that it had become from 70 to80 feet higher than the north-eastern edge of the crater, Roccadel Palo. This remarkable cone, which, at Naples, they wereaccustomed to consider as the true summit of Vesuvius, fell inwith a tremendous noise, during the eruption of the 22d ofOctober, so that the bottom of the crater, which had been unin-terruptedly accessible from the year 1811, now lies 850 English feetbeneath the northern edge, and about 213 feet deeper than thesouthern edge of the volcano. The variable form and relativesituation of the crater of eruption, the opening of which mustnot be taken for the real crater of the volcano, as frequently hasbeen done, gives, at different times, a peculiar physiognomy toVesuvius; and the historiographer of that volcano, from the mereoutline of the summit, and the relative height of the northernor southern side of the mountain, as it is drawn in Hackert’sViews in the palace of Portici, would guess the year in whichthe artist made the sketch of his picture.In the night between the 23d and 24th of October, one dayafter the fall of the cone of slags 400 feet in height, when small|129| but numerous currents of lava had already flowed, the fieryeruption of ashes and rapilli began. It continued uninterrupt-edly for twelve days, but was most violent during the first four.During this time the detonations in the interior of the volcanowere so violent, that the mere concussion of the air (no earth-quake had been observed) caused the roofs to burst in the palaceof Portici. In the surrounding villages of Resina, Torre delGreco, Torre del Annonciata, and Bosche tre Case, an interest-ing phænomenon was observed; the atmosphere was so thicklyfilled with ashes, that the most intense darkness overspread thewhole country for several hours in the middle of the day. Thepeople walked in the streets with lanterns, as is often done atQuito when Pichincha is in eruption. The flight of the inhabi-tants was never more general; currents of lava were less fearedthan a fall of ashes, a phenomenon which was unknown therewith such violence, and in consequence of the relations respect-ing the destruction of Herculaneum, Pompeii, and Stabiæ, filledthe minds of the people with frightful images.The hot steam which rose from the crater during the eruptionand passed into the atmosphere, formed on cooling a thick massof clouds, around the column of ashes and fire, 9000 feet inheight. This sudden condensation of steam, and, as Gay-Lussac has shown, the very formation of the clouds, increasesthe electric tension. Lightnings burst forth in all directionsfrom the column of ashes, and the rolling thunder might clearlybe distinguished from the interior noise of the volcano. At noformer eruption had the play of electric charges been so sur-prising.On the morning of the 26th of October, a singular accountwas circulated, that a current of boiling water had issued fromthe crater, and rushed down from the cone of ashes. Monticelli,the zealous and learned observer of the volcano, soon discernedthat the rumour had been occasioned by an optical deception.The supposed current of water was nothing but a dry mass ofashes, which flowed down, like quicksand, from a fissure in thesuperior edge of the crater. A drought, which had completelydesolated the fields, preceded the eruption, but the volcanicthunderstorm occasioned, towards its termination, a very heavyand continued rain. Such a phenomenon characterizes theconclusion of an eruption in every zone. On account of the coneof ashes being generally covered with clouds during this time,and likewise because the torrents of rain are heaviest in itsneighbourhood, currents of mud flow down on all sides. Theaffrighted peasant considers it to be water which has risen fromthe interior of the crater, and the deceived geognost conceivesthat he recognizes in it either sea-water, or mud-like volcanicproductions, which are called eruptions boueuses, or, as the oldFrench systematic writers termed them, products of a fiery-aqueous liquefaction.|130| When the summits of volcanoes (as is generally the case inthe chain of the Andes), extend into the region of eternal snow,or even to double the height of Ætna, the melted snow rendersthe inundations amazingly frequent and destructive. They arephænomena meteorologically connected with volcanic eruptions,and are multifariously modified by the altitude of the mountains,the extent of their summits covered with eternal snow, and thecalefaction of the sides of the cone of ashes; but they shouldnever be considered as real volcanic phænomena. Subterraneanlakes, in connexion with alpine rivers, are formed both on theslopes and at the feet of the mountains. When the earthquakeswhich precede every eruption in the chain of the Andes, shakewith mighty force the entire mass of the volcano, the subterra-nean vaults are opened, and emit, at the same time, water, fishes,and tufa-mud. This is the singular phenomenon that fur-nishes the fish pimelodes cyclopum, which the inhabitants ofthe high lands of Quito call preñadilla, and which was describedby me soon after my return. When the summit of the mountainCarguairazo, to the north of Chimborazo, and 18,000 feethigh, fell, in the night between the 19th and 20th of June,1698, the surrounding fields, to the extent of about 43 Eng-lish square miles, were covered with mud and fishes. Thefever which raged in the town of Ibarra, seven years before, hadbeen ascribed to a similar eruption of fishes from the volcanoImbaburu. I recur to these facts, because they throw somelight on the difference between the eruption of ashes, and thatof mud-like masses of tufa and trass, which contain wood,coal, and shells.The quantity of ashes ejected by Vesuvius in the late erup-tions, like all other things which are connected with great andappalling phænomena, has been enormously exaggerated in thepublic papers; and two Neapolitan chemists, Vincenzo Pepe,and Giuseppe di Nobili, have affirmed, that they contain goldand silver, notwithstanding the contradiction of Monticelli andCovelli.* According to my examination, the stratum of asheswhich had fallen in twelve days, towards Bosche tre Case, onthe slope of the cone, where rapilli were mixed with it, wasonly three feet in thickness, and in the plain, it did notexceed from 15 to 18 inches. Measurements of this kindmust not be made in places where the ashes have been driftedby wind, like snow, or sand, nor in those where they have beenaccumulated by water. The times are past in which we soughtonly for the marvellous in volcanic phænomena, and, like Ctesias,made the ashes of Ætna fly to the Indian peninsula. Some ofthe Mexican gold and silver mines are certainly in trachyticporphyry, but in the ashes of Vesuvius which I collected, andwhich, at my desire, have been analyzed by Henry Rose, of
* See Annals, v. 236.|131| Berlin, an excellent chemist, no traces of either metal, could bediscovered.However great may be the discrepancy between the results thatI have here given, but which agree with Monticelli’s more exactobservations, and those which have been circulated duringseveral months past, yet the eruption of ashes from Vesuvius,from the 24th to the 28th of October, still remains the most re-markable of which we have any certain account since the deathof the elder Pliny. Its quantity, perhaps, was three times asgreat as that of all the ashes, collectively, which have beenobserved to fall, during the time in which volcanic phænomenahave been attentively considered. A stratum of from 15 to 18inches in thickness, seems at first view unimportant, if comparedto the mass with which we find Pompeii to be covered; butwithout speaking of the torrents and inundations which certainlymay have increased this mass for centuries, without renewingthe violent dispute concerning the cause of the destruction ofthe Campanian towns, which has been carried on with so muchscepticism on the other side of the Alps, it may be affirmed thatthe eruptions of one and the same volcano at distant periods canby no means be compared with respect to their intensity. Allconclusions founded on analogy are insufficient, when the ques-tion is about quantitative proportions,—the quantity of ashesand lava, the height of the column of smoke, or the violence ofthe detonation.From the geographical description of Strabo, and from anopinion of Vitruvius concerning the volcanic origin of pumice,we see that until the year in which Vespasian died, that is tosay, until the eruption which overwhelmed Pompeii, Vesuviuswas more like an extinguished volcano than a solfatara.When after long rest the subterranean powers suddenlyopen new passages, and again break through beds of pri-mitive rocks and of trachyte, effects must necessarily takeplace, for which all the phænomena subsequently observed donot afford any standard of comparison. It may be clearly seenfrom the well-known letter in which the younger Pliny announcesthe death of his uncle to Tacitus, that the recommencement ofthe eruptions, I might say, the awakening of the dormant vol-cano, began with an eruption of ashes. The same circumstancewas observed at Xorullo, in Sept. 1759, when the new volcano,breaking through beds of syenite and trachyte, suddenly arosein the plain. The peasants fled, because they found in theirhuts, ashes that had been ejected from the fissures of the earth,which was burst in every place. Every partial eruption, in theperiodical general eruptions of volcanoes terminates with a showerof ashes.There is a passage in Pliny’s letter, which shows, that the dryashes which had fallen from the air had already attained a heightof from four to five feet, in the commencement of the eruption,|132| and without the effect of accumulation by water. “The courtwhich led to his [uncle’s] apartment,” he says, “being nowalmost filled with ashes and pumice, it would have been impos-sible for him, if he had continued there any longer, to have madehis way out.” In the narrow space of a court, the wind couldnot have had any great effect in accumulating the ashes.I have ventured to interrupt my comparative view of volca-noes by observations solely on Vesuvius, partly on account ofthe great interest which the last eruption has excited, and partlybecause every great fall of ashes almost involuntarily reminds usof the classic ground of Pompeii and Herculaneum.* We havehitherto considered the form and the effects of those volcanoeswhich are in permanent communication with the interior of theearth, by means of a crater. Their summits are raised massesof trachyte and lava, intersected by numerous veins; the dura-tion of their effects causes us to believe that they have a verystable and undisturbed structure. They possess, I may say, amore individual character, which remains the same during longperiods. Neighbouring mountains often furnish completelydifferent products, leucite-lava, and felspar-lava; obsidian, withpumice, and basaltic masses containing olivine. They belongto the newer phænomena of the earth, pass generally through allthe strata of secondary rocks, and their eruptions and currentsof lava are of later origin than our valleys. Their life, if I mayuse that expression, depends upon the manner and duration oftheir connexion with the interior of the earth. They often restfor centuries, suddenly take fire again, and terminate as solfa-taras, which emit steam, gases, and acids. Sometimes, as onthe Peak of Teneriffe, their summit has already become such adepository of reproduced sulphur, while mighty currents of lavaflow from the sides of the mountain, like basalt below, andabove, where the pressure is less, like obsidian with pumice.Independently of these with permanent craters, volcanicphænomena of another kind exist, which have been observedless frequently, but are principally interesting in geognosy, andremind us of the primitive world; that is to say, of the earliestrevolutions of our earth. Mountains of trachyte suddenlyopen, eject lava and ashes, and close again, perhaps, forever: thus was it with the mighty Antisana; and thus with theEpomæus, in Ischia, in 1302. Such an eruption sometimestakes place even in the plain, as in the high lands of Quito; inIceland, far from Hecla; and in Eubœa, in the Lelantic fields.Many of the islands which have been raised up are owing to thesetemporary phænomena. In these cases the communication withthe interior of the earth is not permanent, and the effect ceasesas soon as the fissure, which is the communicating channel, is
* The author here mentions a paper on the data of his measurements at Vesuvius,which was unsuitable for reading; and then proceeds to notice a collection of mineralsthat he brought with him, and which will be added to the Royal Museum at Berlin.|133| closed again. The veins of basalt, dolerite, and porphyry,which, in different parts of the world, pass through every forma-tion; and those of syenite, augite-porphyry, and amygdaloid,which are characteristic of the newest strata of the transitionformation, and of the oldest rocks of the secondary strata, haveprobably been formed in a similar manner. In the first age ofour planet, the yet liquid substances penetrated through thecrust of the earth, which was every where intersected byfissures, and assumed the form of granular rocks, either in veins,or spreading over and expanding themselves in strata. The rocksstrictly volcanic which the primitive ages have afforded us, havenot flowed in currents like the lava of our insulated conical hills;the same mixture of augite, titaniferous iron, glassy felspar, andhornblende, may have existed at different periods, but at onetime it may have approached nearer to basalt, and at others totrachyte; the chemical substances may have combined in acrystalline form, in distinct proportions, as we are taught byM. Mitscherlich’s new and important labours, and by the ana-logy of artificial products of fire: we find that substancessimilarly formed have arrived at the surface of the earthin very different ways; they have either been merely raised, orprotruded by temporary fissures through the older strata; thatis to say, through the already oxidized surface of the earth; orthey have flowed, as currents of lava, from conical hills with apermanent crater. By confounding such different phænomenatogether, the geognosy of volcanoes is carried back to that dark-ness from which a great number of comparative observations arebeginning to extricate it.The question has often been asked, What is it that burns involcanoes? What was it that excited the heat by which earthsand metals were melted? Modern chemistry answers, thatthe substances which melt are the metals of the earths andalkalies. The solid crust of the earth, already oxidized, se-parated the surrounding air with its oxygen, from the com-bustible unoxidized substances of the interior of our planet.The observations which have been made in mines and cavesin every zone, and which, in conjunction with M. Arago, I havecollected in a particular paper, demonstrate that the heat of themass of the earth is yet much greater than the mean temperatureof the atmosphere at the same place. Such a remarkable andalmost generally proved fact, is closely connected with thosewhich are proved by volcanic phænomena. Laplace has evengone so far as to endeavour to calculate the depth at which thebody of the earth may be considered to be a melted mass.Whatever doubts may be entertained, notwithstanding the vene-ration due to so great a name, with respect to the numericalcertainty of such a calculation, thus much remains probable;that all volcanic phænomena originate in a very simple cause, in|134| a permanent or in a variable communication between the interiorand the exterior of our planet.The pressure of elastic vapour forces the melted substancesupwards through deep fissures while they are undergoing oxida-tion; volcanoes, if I may so speak, are intermitting springs ofthe earth; the liquid mixtures of metals, alkalies, and earths,which on cooling become currents of lava, flow quietly whenthey are raised, and find a vent. The ancients imagined,according to Plato’s Phædon, that all volcanic currents of fireflowed, in a similar way, from the Periphlegeton.It may be permitted me, perhaps, to add to these considerationsone which is still more hazardous. In this interior heat of theearth, indicated by experiments with the thermometer, and byobservations on volcanoes, the cause, perhaps, may be found, ofone of the most wonderful phænomena which the examination offossils presents to us. Tropical forms of animals, arboriform ferns,palms, and bamboo-like plants, lie interred in the cold north.The primitive world every where shows a distribution of organicforms at variance with the then existing nature of the cli-mate. In order to solve this important problem, several hypo-theses have been invented; as the neighbourhood of a comet,the altered inclination of the ecliptic, the increased intensityof the solar light. Neither of these has been sufficient tosatisfy at once the astronomer, the natural philosopher, andthe geognost. For my part, I leave the axis of the earth un-altered, as well as the light of the solar disc, by the spotson which, a celebrated astronomer has explained both the ferti-lity and the unfruitfulness of the fields; but I believe, that inevery planet, independently of its relation to a central body, andof its astronomical situation, various causes exist of the produc-tion of heat; oxidation, precipitation, and a change in thecapacity of bodies; by increase of electromagnetic charge, bythe opening of a communication between the interior and theexterior part of the earth.Where the deeply cleft crust of the earth in the primitiveworld radiated heat from its fissures, whole countries, perhaps,could produce for centuries, palms and arborescent ferns, and sus-tain all the animals of the torrid zone. According to this view, towhich I have already alluded in a work just published, “EssaiGéognostique sur le Gissement des Roches dans les deuxHemispheres,” the temperature of volcanoes would be that of theinterior of the earth itself, and the same cause which now occa-sions such dreadful destruction, would once have occasioned,on the newly oxidated crust of the earth, upon the deeply cleftstrata of rocks, the most luxuriant growth of plants in everyzone.Even if any one should be inclined to suppose, in order toexplain the marvellous distribution of tropical forms in their|135| ancient graves, that shaggy animals of the elephant tribe nowimbedded in icebergs, were once peculiar to a northern climate,and that similar forms belonging to the same primary types, likelions and lynxes, could live in very different climates, such anexplanation could not, however, be extended to the products ofvegetation. For reasons which the physiology of plantsexplains, palms, and arboriform monocotyledones cannot sustainthe northern cold, and in the geological problem we here speakof, it seems difficult to me to separate plants and animals. Thesame explanation must be applied to both.Towards the end of this paper, I have combined uncertainhypothetical suppositions with facts collected from the mostdifferent parts of the world. The philosophical knowledge ofnature rises above a mere description of nature. It does notconsist in a sterile aggregation of isolated observations. It maysometimes be allowed, therefore, to the curious and ever-activemind of man, to look back upon the past, to imagine what cannotbe clearly known, and to amuse himself with the ancient, and,under many forms, returning mysteries of geogony.