APRIL...SEPTEMBER 1828.

Essay on the Structure and Action of Volcanoes in different regions of the Earth.

By Baron Humboldt .

Translated from the Tableaux de la Nature, par Humboldt, t. ii.

When we reflect upon the influence which, for many ages, has been exercised upon the study of nature, by the improvements of geography, and by scientific journeys made into distant regions, we quickly perceive how different this influence has been, according as the researches have been directed toward the forms of the organic world, or toward the inanimate mass of the earth.

Different forms of plants and animals enliven the earth's surface in each zone, however much the heat of the atmosphere may change, whether according to the geographical latitude, or the numerous curves of the isothermal lines, in the extended plains, level as the surface of the sea, or in an almost vertical direction on the steep slopes of the mountain chains.

Organic nature gives to each region of the earth the peculiar physiognomy by which it is distinguished.

The case is different with inorganic nature in the places where the solid envelope of the earth is deprived of vegetation.

The same species of rocks, attracting and repelling each other by groups, disclose themselves in the two hemispheres, from the equator to the poles.

In a distant isle, surrounded by unknown plants, in a clime where the stars to which his eye is habituated no longer shine, the voyager often recognises with joy the granite of his native country, 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 beneficial influence which numerous observations, made in distant countries, have upon the progress of geognosy; it only gives them a particular direction.

Each succeeding expedition enriches natural history with new species of animals and plants.

Sometimes organic forms are discovered which connect themselves with types long known, and which present in its original perfection the regularly woven, and often apparently interrupted, net-work of animated natural forms.

Sometimes the discoveries consist of forms which present themselves isolated, like the remains of extinct races; sometimes of members of yet unknown groups.

The examination of the solid crust of the earth exhibits no such diversity.

On the contrary, it discloses, in the constituent parts, in the relative position, and in the periodical recurrence of the different masses, a similarity which strikes the geologist with astonishment.

In the chain of the Andes, as in the central mountains of Europe, one formation seems, as it were, to recall another.

Masses of the same name assume similar forms; the basalt and greenstone form twin mountains; dolomite, white sandstone and porphyry, form masses broken 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 of the primitive mass, form themselves into groups, appear as subordinate masses, and often announce the vicinity of independent new formations.

In this manner the whole inorganic world is evidently pictured in every mountain chain of any extent.

To become perfectly acquainted, however, with the most important phenomena of the composition, relative age, and origin of the formations, it is necessary to compare, with each other, observations made in countries the most widely separated, problems which have long seemed enigmatical to geologists living in the north, find their solution near the equator.

If, as has been observed, the distant zones do not furnish us with new formations, that is to say, unknown groups of simple substances, they yet enable us to understand the uniform laws of nature, by which the various strata support each other, penetrate into each other's substance in the form of veins, or raise each other in obedience to elastic powers.

If it be true that our geognostical knowledge derives the greatest advantage from researches made over vast expanses of country, it ought not to excite surprise that the class of phenomena which forms the principal object of this memoir should, till lately, have been examined in a very imperfect manner, because the points of comparison are very difficult, and may even be said laborious, to find.

Until the end of the eighteenth century, all that was known of the form of volcanoes, and of the action of their subterranean powers, was derived from two mountains in the south of Italy, Vesuvius and Etna.

The former being the most accessible, and, like all volcanoes of inferior elevation, having more frequent eruptions, a small hill became, in some measure, the type according to which a whole distant world was represented, containing the great volcanoes of Mexico, South America, and the Asiatic Isles.

This mode of reasoning might naturally bring to our recollection Virgil's shepherd, who, in his humble cabin, imagined he saw the image of the eternal city.

An attentive examination of the whole Mediterranean, especially its islands and eastern shores, where the human race has begun to rise in the progress of intellect, and in the cultivation of generous feelings, might, however, reform this imperfect manner of studying nature.

Among the Sporades, trachyte rocks have risen from the bottom of the sea, and formed islands, like that among the Azores, which, in the space of three centuries, has shewn itself at nearly equal intervals.

Between Epidaurus and Trezene, near Methone, in the Peleponnesus, there occurs a Monte Nuovo, which was described by Strabo, and has been seen again by Dodwell. It is higher than the Monte Nuovo of the Phlegrean Fields, near Baiae, perhaps even higher than the new Volcano of Jorullo, in the Plains of Mexico, which I found surrounded with many thousands of small basaltic cones, that had issued from the ground, and were still smoking.

In the basin of the Mediterranean, not only does the volcanic fire escape from permanent craters of isolated mountains, which have a constant communication with the interior of the earth, as Stromboli, Vesuvius, and Etna; but at Ischia, on Mount Epomee; and, according to the accounts of the ancients, in the Plains of Lelantis, near Chalcis, lavas have flowed from fissures which have suddenly opened at the surface of the ground.

Independently of these phenomena which belong to historical times, to the limited domain of sure tradition, the shores of the Mediterranean contain numerous remains of more ancient effects of the action of fire.

The south of France, in Auvergne, displays a particular and entire system of volcanoes, arranged in series, of trachytic domes, alternating with cones perforated with craters, from which torrents of lava have flowed in narrow stripes.

The Plain of Lombardy, which, smooth as the surface of the waters, forms the most remote gulf of the Adriatic Sea, surrounds the trachyte of the Euganean Hills, in which there rise domes of granular trachyte, obsidian, and perlite, forming three masses proceeding from each other, which have forced their way through the Juraic limestone, filled with flints, but which have never run in narrow torrents.

Similar evidences of ancient revolutions of the earth occur in various parts of the Continent of Greece and of Asia Minor, a country which will one day present rich materials for geological research, when light shall have returned to those countries whence it began to shine on the west, when outraged humanity shall no longer groan beneath the savage barbarity of the Ottomans.

I bring forward the geographical proximity of these numerous phenomena, to shew that the basin of the Mediterranean, with its islands, is capable of presenting to the attentive observer all that has recently been discovered, under various forms, in South America, in Teneriffe, or in the Aleutian Isles, in the vicinity of the polar regions.

The objects to be observed were united together; but travels into distant regions, and comparisons of extensive countries in Europe and out of it, were necessary for clearly shewing the mutual resemblance of volcanic phenomena, and their dependence upon one another.

Common language, which often gives consistency and duration 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 subterranean fires and melted substances; to the columns of smoke and vapour which issue from the heart of rocks, as at Colares, after the great earthquake at Lisbon; to the salses or cones of clay which vomit mud, asphaltes, and hydrogen, as at Girgenti, in Sicily, and at Turbaco, in South America; to the hot springs of the Geyser, which, impelled by elastic vapours, rise to an immense height; in a word, to all the effects of the mighty powers of nature, which have their seat in the interior of our planet.

In central America, or in the country of Guatemala, and in the Philippine Isles, the natives make an essential difference between water 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 time to time.

Without denying the connection of the phenomena just mentioned, it would yet appear expedient to give a more precise language to the physical and oryctognostical department of geognosy, in order to prevent the application of the name of Volcano, sometimes to a mountain which is terminated by a permanent furnace, and sometimes to each subterranean cause of volcanic phenomena.

In the present state of the terrestrial globe, the most common form of volcanoes, in all parts of the world, is that of an isolated cone, such as Vesuvius, Etna, the Peak of Teyde, Tunguragua, and Cotopaxi. I have observed them rising from the size of the lowest hills to 17,700 feet above the level of the sea.

But close to these conical mountains, there also occur permanent apertures, forming regular communications with the interior of the earth, on long serrated chains, not at the middle of their mural summit, but at their extremity, and near the declivity.

Of this kind is Pichincha, which rises between the great ocean and the city of Quito, and which Bouguer's barometrical formulae have long rendered celebrated.

Such also are the volcanoes which rise on the Steppe de los Pastos, which is 10,000 feet high.

All these summits, of varied forms, are composed of trachyte, formerly named trap porphyry, a granular fissured rock, formed of glassy felspar and hornblende, and in which augite, mica, laminar felspar, and quartz, also occur.

In places where the evidences of the first eruption I might say of the ancient volcanic scaffolding, are preserved entire, the isolated conical mountain is surrounded, in the form of an amphitheatre, with a great wall, constructed of rocky strata, superimposed upon each other.

These walls or circumvallations are the remains of craters of elevation, a phenomenon worthy of attention, respecting which the first geologist of our times, M. Leopold Von Buch, in his writings, from which I have borrowed several ideas stated in the present memoir, has presented such interesting views.

The volcanoes which communicate with the atmosphere by permanent apertures, the basaltic cones or domes of trachyte, destitute of crater, sometimes low like Sarcouy, and sometimes elevated like Chimborazo, form various groups.

Comparative geography shews us, on the one hand, small archipelagoes, and entire systems of volcanic mountains, with their oraters and currents of lava, resembling those of the Canary Islands, and the Azores; and, on the other, mountains without craters, and without currents of lava, properly so called, as the Euganeans, and the (Siebengebirge) seven mountains of Bonn.

Moreover, it shews us volcanoes arranged in single or double lines, and extending to several hundreds of leagues, sometimes parallel to the axis of the chain, as in Guatemala, Peru, and Java; sometimes cutting it perpendicularly, as in the country of the Azteques, where trachytic mountains, which vomit fire, alone attain the height of perpetual snow, and are probably situated upon a crevice traversing the whole continent, over an extent of 105 geographical leagues from the Pacific Ocean to the Atlantic.

This association of volcanoes, whether in isolated and rounded groups, or in longitudinal bands, demonstrates, in the most decisive manner, that volcanic effects do not depend upon slight causes existing near the surface of the earth, but are phenomena whose origin is to be found at a great depth in the interior of the globe.

The whole eastern part of the American continent, which is poor in metals, is, in its present state, destitute of volcanic mountains, of masses of trachyte, and probably even basalt, with olivine.

All the American volcanoes are collected together in the chain of the Andes, which is situated in the part of that continent opposite to Asia, and which extends, in the direction of the meridians, over a space of 1800 leagues.

The whole plain of Quito, of which Pichincha, Cotopaxi, and Tunguraqua form the cymes, is a volcanic focus.

The subterranean fire escapes, sometimes by one, sometimes by another, of those apertures which it has been customary to consider as distinct volcanoes.

The progressive march of the fire in them has, for the last three centuries, been from north to south.

The very earthquakes, which produce such terrible ravages in this part of the world, afford remarkable proofs of the existence of subterranean communications, not only with countries destitute of volcanoes, which has been long known, but also between ignivomous mountains placed at very great distances from each other.

Thus, in 1797, the volcano of Pasto, to the east of the course of the Guaytara, vomited, unremittingly, for three months, a high column of smoke.

This column disappeared at the very moment, when, at a distance of sixty leagues, the great carthquake of Riobamba, and the muddy eruption of Moya, destroyed about forty thousand Indians.

The sudden appearance of the Island of Sabrina, to the east of the Azores, on the 30th January 1811, was announced by the dreadful earthquake, which, at a much greater distance to the west, from May 1811 to June 1812, shook, almost without intermission, first the West India Islands, then the plains of the Ohio and Missisippi, and, lastly, the coasts of Venezuela, situated on the opposite side.

Thirty days after the total destruction of the city of Caraccas, the explosion of the volcano of St Vincent, in the Lesser Antilles, took place at a distance of 130 leagues.

At the same moment when this eruption happened, on the 30th April 1811, a subterranean noise was propagated, and carried terror over an extent of country of 2200 square leagues.

The inhabitants of the banks of the Apure, at the confluence of the Rio Nula, as well as those of the sea coast, compared the noise to that produced by the discharge of large pieces of artillery.

Now, from the confluence of the Rio Nula and Apure, by which I arrived at the Oronocco, to the volcano of St Vincent, the distance is 157 leagues in a straight line.

This noise, which assuredly was not propagated 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 to recall to mind a phenomenon which has acquired a historical importance with reference to Europe, I shall now mention the famous earthquake of Lisbon.

It took place on the 1st November 1755.

Not only were the waters of the Swiss Lakes, and of the sea on the coasts of Sweden, violently agitated; but also those of the sea around the eastern Antilles.

At Martinique, Antigua, and Barbadoes, where the tide does not commonly rise more than eighteen inches, it suddenly rose twenty feet.

All these phenomena prove, that the subterranean powers manifest themselves, either dynamically, by earthquakes, or chemically, by occasioning changes in the form of volcanic eruptions.

They also demonstrate, that these powers act, not superficially in the outer crust of the earth, but at immense depths in the interior of our planet, by crevices and unfilled veins, which lead to points of the earth's surface, at the greatest distances from each other.

The more numerous the diversities in the structure of volcanoes, or in other words, of the elevations surrounding the canals by which the melted masses of the interior of the globe arrive at its surface, so much the more important is it to submit this structure to accurate measurements.

The interest of these measurements, which, in another part of the world, have formed the object of my researches, increases if we consider that the magnitude to be measured varies in several points.

The philosophical examination of nature applies itself, in the vicissitude of phenomena, to connect the present with the past.

To establish a periodical return, or to fix the laws of progressive and variable phenomena, it is necessary to have some well determined points of departure, observations made with care, and which, being conneoted with determined epochs, may furnish numerical comparisons.

Had only the mean temperature of the atmosphere, and of the earth in different latitudes, or the mean temperature of the barometer on the edge of the sea, been determined from one century to another, we should have known in what proportion the heat of climates has increased or diminished, and whether or not the height of the atmosphere has undergone changes.

These points of comparison are required for the declination and inclination of the magnetic needle, as well as for the intensity of the electro-magnetic forces. If it be a praiseworthy occupation for societies to follow, with assiduity, the cosmic vicissitudes of heat, of the pressure of the air, and of the magnetic direction and intensity; it is, on the other hand, the duty of the geologist, in determining the inequalities of the earth's surface, to take into consideration the change of height of volcanoes.

What I attempted at the time, in the mountains of Mexico, at Toluca, Nauhiampatepetl and Jorullo, and in the Andes of Quito at Pichincha, I have had an opportunity, since my return to Europe, of repeating several times at Vesuvius.

In 1773, Saussure measured that mountain at a period when the two edges of the crater, the north-west and south-west, appeared to him of equal height.

He found their elevation 609 toises above the level of the sea.

The eruption of 1794 occasioned a falling in of the southern part, and an inequality of the edges of the crater which the most inexperienced eye distinguishes at a considerable distance.

In 1805, M. von Buch, M. Gay Lussac and myself, measured Vesuvius three times.

The result of our operations was, that the height of the north edge, the Rocca del Palo, which is opposite the Somma, agreed with Saussure's measurement, but that the south edge was 75 toises lower than in 1773.

The total elevation of the volcano, towards the Torre del Greco, the side towards which the fire had principally directed its action for thirty years, had diminished an eighth part.

The cone of ashes is, to the total height of the mountain, on Vesuvius, as one to ten; on the Peak of Teneriffe as one to twenty-two.

Vesuvius, therefore, has the cone of ashes proportionally higher, probably because, as a volcano of little height, it has acted principally by its summit.

I succeeded lately not only in repeating my barometrical measurements on Vesuvius, but also in ascending that mountain three times, in order to take a complete survey of all the edges of the crater.

This undertaking is perhaps deserving of some interest, because it embraces the period of the great eruptions from 1805 to 1822; and because it affords, perhaps, the only measurement of the volcano, made with reference to all its parts, that has hitherto been published.

It shews that the edges of the crater, not only in the places where they are visibly composed of trachyte, as in the Peak of Teneriffe, and in all the volcanoes of the chain of the Andes, but also every where else, present a phenomenon much more constant than had previously been supposed from observations hastily made.

Simple angles of height, determined from the same point, answer much better for researches of this kind than trigonometrical and barometrical measurements, otherwise very complete.

According to my last determination, the northwest edge of Vesuvius has not perhaps undergone any diminution of height since the time of Saussure, that is to say for the last forty-nine years, and the south-east edge, on the Bosche Tre- Case side, which, in 1794, was 400 feet lower than the preceding, has undergone a diminution of 10 toises.

If the public journals, in describing the great eruptions, very frequently relate that the form of Vesuvius has totally changed, and if these assertions are confirmed by the picturesque views of that mountain which are painted at Naples, the cause of error exists in the circumstance that the contour of the edges of the crater is confounded with those of the heaps of scoriae which are accidentally formed in the centre of the crater, on the bottom of the ignivomous mouth raised up by vapours.

One of these heaps, consisting of rapilli and scoriae, became gradually visible in 1816 and 1818, above the south-east edge of the crater.

The eruption of February 1822 increased it to such a degree, that it even exceeded the Rocca del Palo, or the northwest edge of the crater, by 100 or 110 feet.

In the last eruption, the remarkable cone, which was usually considered as the true summit of Vesuvius, fell down with a terrible noise, so that the bottom of the crater, which, since 1811, was always accessible, is now 750 feet lower than the northern edge of the volcano, and 200 feet lower than the southern.

The variable form and relative position of the cones of eruption, whose aperture ought not, as is too often done, to be confounded with the crater of the volcano, give a particular aspect to Vesuvius at different periods, and the historiographer of this volcano might, from the contours of the summit, and from the simple inspection of the landscapes painted by Hackert, which are at Portici, according as the northern or southern side of the mountain is represented higher or lower, guess the year in which the artist made the drawing from which he composed his picture.

A day after the cone of scoriae, 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 eruption of ashes and rapilli.

It lasted twelve days without interruption; but it was more intense during the first four.

All this time, the detonations in the interior of the volcano were so violent, that the mere concussion of the air (for no commotion was observed in the earth), cracked the ceilings of the apartments in the palace of Portici.

The villages of Resina, Torre-del-Greco, Torre del Anunziata, and Bosche-Tre-Case, which are close upon the mountain, witnessed a remarkable phenomenon.

The atmosphere was so filled with ashes, that the whole district was for several hours in the middle of the day enveloped in profound darkness.

People used lanterns in the streets, as often happens at Quito, during the eruptions of Pichincha.

The inhabitants never fled in such numbers.

The torrents of lava were much less dreaded than an eruption of ashes,--a phenomenon which had not before been known to such a degree, and which, from the obscure tradition of the manner in which Herculaneum, Pompeii and Stabiae were destroyed, filled the imagination of men with terrifying images.

The watery and hot vapour which shot up from the crater during the eruption, and diffused itself in the atmosphere, formed, on cooling, a thick cloud round the column of ashes and flame which rose to the height of 9000 feet.

So rapid a condensation of the vapours, and, as M.

Gay Lussac has shewn, the very formation of the cloud, augmented the electrical intensity.

Flashes issued from the column of ashes in all directions, and the thunder, which was easily distinguished from the noises of the volcano, was distinctly heard.

In no other eruption was the manifestation of the electric powers so astonishing.

On the morning of the 26th October, a surprising noise was heard, which seemed to arise from a torrent of boiling water that was ejected from the crater, and descended along the declivity of the cone of the ashes.

Monticelli, the learned and zealous observer of the volcano, immediately discovered that an optical illusion had occasioned this erroneous rumour.

The supposed torrent was a great heap of dry ashes, which issued from a crevice in the upper edge of the crater.

A drought which spread desolation in the fields, had preceded the eruption of Vesuvius.

Toward the end of this phenomenon, the volcanic thunder storm which we have just been describing, occasioned an extremely heavy and long continued rain.

In all countries, the cessation of an eruption is characterized by a similar meteor.

So long as the present one lasted, the cone of ashes being generally enveloped with clouds, and the rain being heaviest in its vicinity, torrents of mud were seen flowing on all sides.

The affrighted husbandman thought it was water, that, after ascending from the bottom of the volcano, issued by the crater.

The geologist thought he discovered in it sea water, or muddy productions of the volcano, or, to use the expression of the French old systematic writers, products of an igno-aqueous liquefaction.

When the summit of the volcano, as is almost always the case in the Andes, rises above the region of snow, or attains a height double that of Etna, the snow, by melting and flowing toward the lower regions, produces frequent and disastrous inundations.

These are phenomena which the meteors connect with the eruptions of volcanoes, and which are variously modified by the height of the mountain, the extent of its summit covered with perpetual snows, and the heating of the walls of the cone of cinders.

They cannot at all be regarded as true volcanic phenomena, being merely the effects of such phenomena.

In vast cavities, sometimes on the declivity, sometimes at the foot of volcanoes, are found subterranean lakes which communicate in various ways with the alpine torrents.

When the commotions of the earth which always precede all the igneous eruptions in the chain of the Andes, have violently shaken the whole mass of the volcano, then the subterranean gulfs open, and there issue at the same time water, fishes, and clay tufa.

Such is the singular phenomenon which brings to light the Pimelodes cyclopum, a fish to which the inhabitants of the plain of Quito gave the name of Prenadilla, and which I described shortly after my return.

When to the north of Chimborazo, in the night of the 19th June 1698. the summit of Carguaraizo, a mountain of the height of 18,000 feet, broke down, the whole country round, to the extent of nearly two square leaguges, was covered with mud and fishes.

Seven years before, a pernicious fever, which desolated the city of Iburra, was attributed to a similar eruption of fishes from the volcano of Imbaburu.

I mention these facts, because they throw some light on the difference which exists between the eruptions of dry ashes and those of mud, wood, charcoal, or shells, serving to explain the formation of tufa and trass.

The quantity of ashes thrown out by Vesuvius of late years, like all the circumstances connected with volcanoes, and other great phenomena of nature calculated to inspire terror, has been excessively exaggerated in the public journals.

Two chemists of Naples, Vicenzo Pepe and Giuseppe di Nobili, have even affirmed, notwithstanding the contrary assertions of Monticelli and Covelli, that the ashes contain gold and silver.

According to my inquiries, the bed of ashes that fell during twelve days on the Bosch-Tre-Case side, on the declivity of the cone, in the places where rapillo was mingled with them, was only three feet deep, and in the plain, did not rise higher than from fifteen to eighteen inches.

Measurements of this kind should not be taken in places where the ashes are heaped up, like snow or sand, by the wind, or aceumulated by water in the form of mud.

The times are gone when wonders only were looked for in volcanic phenomena, or when the ashes of Etna were represented as being carried by the winds as far as the peninsula of India.

Some of the gold and silver veins of Mexico certainly occur in a trachytic porphyry; but the ashes of Vesuvius, which I carried along with me, and which were analysed by an excellent chemist M. Henry Rose, afford not the slightest traces of gold or silver.

Although the results of which I speak, and which are in perfect accordance with the accurate observations of Monticelli, differ much from those published some months ago, the eruption of ashes from Vesuvius which took place on the 24th and 28th of October 1822, is undoubtedly the most remarkable of which we have any authentic accounts since the death of the elder Pliny in the year 70. The quantity of ashes which then fell was perhaps three times as great as any that has been observed since volcanic phenomena first began to be studied with attention.

A layer of fifty or eighty inches appears at first sight insignificant in comparison of the mass which covered Pompeii; but, without speaking of torrents of rain, and of the effects of detrition, which, in the course of ages, may have accumulated this mass, and without reviving the keen discussion which arose beyond the Alps, and which was conducted with a great degree of scepticism, respecting the causes of the destruction of the cities of Campania, it is perhaps to the purpose to mention here, that the eruptions of a volcano at periods very remote from each other, can by no means be compared together with reference to their intensity.

All the consequences founded upon analogies are insufficient, when the objects to be compared are such as the mass of lava and cinders, the height of the columns of smoke, and the loudness of the detonations.

The geographical description of Vesuvius by Strabo, and Vitruvius'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 mountain resembled more an extinct volcano than a solfaterra.

After a long repose, the subterranean forces opened up new paths, and penetrated through the strata of primitive rocks and trachyte.

Then must have been manifested effects of which those that have since followed could furnish no idea.

The celebrated letter, in which the younger Pliny relates to Tacitus the death of his uncle, clearly shews that the renewal of the eruptions, and it might even be said the awakening of the dormant volcano, commenced with an explosion of ashes.

The same thing was observed at Jorullo, when, in September 1759, the new volcano, piercing through the strata of syenite and trachyte, rose suddenly in the plain.

The country people fled, because they found on their huts ashes which the earth had vomited by opening up on all sides.

On the contrary, in the periodical and ordinary explosions of volcanoes, the ashes terminate each partial eruption.

Besides, the younger Pliny's letter contains a passage, which clearly shews, that, from the commencement, without the influence of any cause that could have heaped them up, the dry ashes that fell directly from above, had attained a height of four or five feet.

"The court," says he in the course of his narrative, which had to be passed in order to enter the chamber in which Pliny reposed, "was so filled with ashes and pumice, that, if he had delayed his coming out any longer, he would have found the entrance shut up."

In an inclosed space, like that of a court, the action of the wind, by which the ashes are collected, could not by any means have been very considerable.

I have ventured to interrupt my comparative examination of volcanoes by particular observations made on Vesuvius, both on account of the great interest which the last eruption has excited, and on account of the remembrance of the catastrophe of Pompeii and Herculaneum, which every considerable fall of ashes involuntarily brings to the mind. I have brought together, in a supplement, all the elements of the barometrical measurements and notices respecting geological collections that I have had an opportunity of making, towards the end of 1822, at Vesuvius and in the Phlegrean fields, near Pouzzuolo.

This small collection, together with the rocks which I brought from the Euganean mountains, and those which M. von Buch collected on a journey to the valley of Flemme, between Cavalere and Predazzo, in the southern Tyrol, are deposited in the Royal Museum of Berlin, an establishment which, by its utility, perfectly corresponds to the noble intentions of the monarch, and of which, the geognostical department, containing specimens from the most remote regions, is, in this respect, superior to any collection of this kind in existence.

We have been considering the form and action of those volcanoes which keep up a regular communication with the interior of the earth, by means of craters.

Their summits are masses of trachyte and lava, raised up by elastic powers, and traversed by veins.

The permanence of their action gives rise to the conclusion, that their structure is very complicated.

They have, so to speak, an individual character, which remains always the same through long periods.

The neighbouring mountains most commonly afford entirely different products, lavas of leucite and felspar, obsidian and pumice, and basaltic masses containing olivine.

They belong to the most recent formations of the globe, and traverse nearly all the strata of the secondary mountains.

Their eruptions and their torrents of lava are of a more recent origin than our valleys.

Their life, if we may be permitted to make use of such an expression, depends upon the mode and duration of their communication with the interior of the earth.

They frequently remain quiet for ages, suddenly kindle again, and end with being solfaterras, exhaling aqueous vapours, gases and acids.

Sometimes, as in the Peak of Teneriffe, their summit has already become a laboratory of regenerated sulphur; while from their sides there yet flow great torrents of lava, basaltic and lithoid in their lower parts, vitreous, in the form of obsidian and pumice, in their upper part, where the pressure is less.

Independently of these volcanoes provided with permanent craters, there is another species of volcanic phenomena, which is more rarely observed, but which is peculiarly calculated to throw light on geology, because it recalls the primitive world, or, in other words, the most ancient revolutions of our globe.

Mountains of trachyte, opening of a sudden, vomit forth lava and ashes, and again shut perhaps for ever.

This is what took place in the gigantic Antisana, in the Chain of the Andes, and at Mount Epomeus in the island of Ischia, in 1302.

An eruption of this kind sometimes takes place in the plains; for example, on the plain of Quito; in Iceland, at a distance from Hecla; in Eubeus, in the fields of Lelantee.

Many islands, suddenly elevated from the bottom of the sea, belong to these transitory phenomena.

In these cases, the communication with the interior of the earth is not permanent; the action ceases as soon as the aperture of the canal of communication is closed anew.

Veins of basalt, greenstone, and porphyry, which in the different zones of the earth traverse almost all the formations, masses of syenite, augite, porphyry and amygdaloid, which characterize the newest strata of the transition, and the oldest strata of the secondary rocks, have probably been formed in this manner.

In the early stages of our planet, the substances of the interior, still in a state of fluidity, penetrated through the envelope of the earth which was fissured in all parts; sometimes condensing as masses of veins with a granulated texture, sometimes spreading out into sheets and stratified torrents.

The volcanic rocks which the primitive world has transmitted to us, have nowhere flowed in narrow bands like the lavas that issue from the volcanic cones existing at present.

The mixtures of augite, titanitic iron, glassy felspar, and hornblende, may have been the same at different periods, sometimes more allied to basalt, and sometimes to trachyte.

The chemical substances, as we learn from the important labours of M. Mitscherlich, and the similarity of the products of high furnaces, may have been united under a crystalline form, according to definite proportions.

It is not the less true, that substances, composed in the same manner, have arrived by very different ways at the earth's surface, whether by being raised up by elastic forces, or by being insinuated through crevices into the strata of the older rocks; in other words, through the already oxidized envelope of our planet, or by issuing under the form of lava from conical mountains, which have a permanent crater.

If phenomena so different as these be confounded together, the geognosy of volcanoes is thrown back into the darkness, from which numerous comparative experiments have begun gradually to rescue it.

The question has often been asked, What is it that burns in volcanoes?

What is it that produces the heat in them by which the earth and metals are melted and intermingled?

The new chemistry replies:

What burns is the earth, the metals, and even the alkalies, that is to say, the metaloids of these substances.

The already oxidized envelope of the earth separates the atmosphere, rich in oxygen, from the unoxidised inflammable principles which reside in the interior of our planet.

Observations made in all countries, in mines, and caves, and which, in concert with M. Arago, I have detailed in a memoir on the subject, prove that, even at a small depth, the earth's heat is much superior to the mean temperature of the surrounding atmosphere.

A fact so remarkable, and elicited from observations made in almost every part of the globe, connects itself with what we learn from the phenomena of volcanoes.

La Place has even attempted to determine the depth at which the earth may be considered as a melted mass.

Whatever doubts may be entertained, notwithstanding the respect due to so great a name, as to the numerical accuracy of such a calculation, it is not the less probable, that all volcanic phenomena arise from a single cause, which is the communication, constant or interrupted, that exists between the interior of our planet and the external atmosphere.

Elastic vapours, by their pressure, raise through deep crevices the substances which are in a state of fusion, and which are oxidized.

Volcanoes are, so to speak, intermittent springs of earthy matters.

The fluid mixtures of metals, alkalies and earths, which condense into currents of lava, flow gently and slowly, when, on being raised up, they once find an issue.

It was in this manner that, according to Plato's Phoedos, the ancients represented all the torrents of fire as emanations of the Pyriphlegeton.

To these considerations may I be permitted to add another of a bolder character.

It is perhaps in the internal heat of the earth, a heat which is indicated by experiments made with the thermometer, and the phenomena of volcanoes, that the cause of one of the most astonishing phenomena which the knowledge of petrifactions presents to us resides.

Tropical forms of animals, arborescent ferns, palms and bamboos, occur imbedded in the frozen regions of the north.

The primitive world every where discloses to us a distribution of organic forms, which is in opposition to the presently existing state of climates.

To solve so important a problem, recourse has been had to a great number of hypotheses, such as the approach of a comet, the change of obliquity of the ecliptic, the increase of intensity of the solar heat.

None of these hypotheses has been able to satisfy at the same time the astronomer, the natural philosopher and the geologist.

As to my own opinion on the subject, I leave the earth's axis in its position, I admit no change in the radiation of the solar disk, a change by which a celebrated astronomer thought he could explain the good and bad harvests of our fields; but I imagine that in each planet, independently of its relations to a central body, and independently of its astronomical position, there exist numerous causes of developement of heat, whether by the chemical processes of oxidation, or by the precipitation and changes of capacity of bodies, or by the augmentation of the electro-magnetic intensity, or the communication between the internal and external parts of the globe.

When, in the primitive world, the deeply fissured crust of the earth exhaled heat by these apertures, perhaps during many centuries, palms, arborescent ferns, and the animals of warm climates, lived in vast expanses of country.

According to this system of things, which I have already indicated in my work entitled Essai Geognostique sur le Gisement des Roches dans les deux Hemispheres, the temperature of volcanoes is the same as that of the interior of the earth, and the same cause which now produces such frightful ravages, would formerly have made the richest vegetation to spring in every zone, from the newly oxidised envelope of the earth, and from the deeply fissured strata of rocks.

If, in order to account for the distribution of the tropical forms that occur buried in the northern regions of the globe, it is assumed that elephants covered with long hair, now immersed in the polar ice, were originally natives of those climates, and that forms resembling the same principal type, such as that of lions and lynxes, may have lived at the same time in very different climates, such a mode of explanation would yet be inapplicable to the vegetable productions.

For reasons which vegetable physiology discloses, palms, bananas, and arborescent monocotyledonous plants, are unable to support the cold of the northern countries; and in the geognostical problem which we are here examining, it appears to me difficult to separate the plants from the animals; the same explanation ought to embrace the two forms.

At the end of this memoir, I have added to the facts collected in countries the most remote from each other, some purely hypothetical suppositions.

The philosophical study of nature rises above the wants of descriptive natural history; it does not consist of the mere accumulation of isolated observations.

May it one day be permitted to the curious and active mind of man, to dart from the present into the future, to interpret what cannot yet be known with precision, and amuse itself with the geognostical fables of antiquity, which are in our days reproduced under various forms.

The facts alluded to do not appear in the Appendix to the Memoir.