When we reflect on the influence which, for some centuries past, the progress of geography and the multiplication of distant voyages and travels have exercised on the study of nature, we are not long in perceiving how different this influence has been, according as the researches were directed to organic forms on the one hand, or on the other to the study of the inanimate substances of which the earth is composed—to the knowledge of rocks, their relative ages, and their origin. Different forms of plants and animals enliven the surface of the earth in every zone, whether the temperature of the atmosphere varies in accordance with the latitude and with the many inflections of the isothermal lines on plains but little raised above the level of the sea, or whether it changes rapidly in ascending in an almost vertical direction the steep declivities of mountain-chains. Organic nature gives to each zone of the earth a peculiar physiognomy; but where the solid crust of the earth appears unclothed by vegetation, inorganic nature imparts no such distinctive character. The same kinds of rocks, associated in groups, appear in either hemisphere, from the equator to the poles. In a remote island, surrounded by exotic vegetation, beneath a sky where his accustomed stars no longer shine, the voyager often recognises with joy the argillaceous schists of his birth-place, and the rocks familiar to his eye in his native land.

This absence of any dependence of geological relations on the present constitution of climates does not preclude or even diminish the salutary influence of numerous observations made in distant regions on the advance and progress of geological science, though it imparts to this progress something of a peculiar direction. Every expedition enriches natural history with new species or new genera of plants and animals: there are thus presented to us sometimes forms which connect themselves with previously long known types, and thus permit us to trace and contemplate in its perfection the really regular though apparently broken or interrupted network of organic forms: at other times shapes which appear isolated,—either surviving remnants of extinct genera or orders, or otherwise members of still undiscovered groups, stimulating afresh the spirit of research and expectation. The examination of the solid crust of the globe does not, indeed, unfold to us such diversity and variety; it presents to us, on the contrary, an agreement in the constituent particles, in the superposition of the different kinds of masses, and in their regular recurrence, which excites the admiration of the geologist. In the chain of the Andes, as in the mountains of middle Europe, one formation appears, as it were, to summon to itself another. Rocks of the same name exhibit the same outlines; basalt and dolerite form twin mountains; dolomite, sandstone, and porphyry, abrupt precipices; and vitreous feldspathic trachyte, high dome-like elevations. In the most distant zones large crystals separate themselves in a similar manner from the compact texture of the primitive mass, as if by an internal development, form groups in association, and appear associated in layers, often announcing the vicinity of new independent formations. Thus in any single system of mountains of considerable extent we see the whole inorganic substances of which the crust of the earth is composed represented, as it were, with more or less distinctness; yet, in order to become completely acquainted with the important phenomena of the composition, the relative age, and mode of origin of rocks, we must compare together observations from the most varied and remote regions. Problems which long perplexed the geologist in his native land in these northern countries, find their solution near the equator. If, as has been already remarked, new zones do not necessarily present to us new kinds of rock (i. e. unknown groupings or associations of simple substances), they, on the other hand, teach us to discern the great and every where equally prevailing laws, according to which the strata of the crust of the earth are superposed upon each other, penetrate each other as veins or dykes, or are upheaved or elevated by elastic forces.

If, then, our geological knowledge is thus promoted by researches embracing extensive parts of the earth’s surface, it is not surprising that the particular class of phenomena which form the subject of the present discussion should long have been regarded from a point of view the more restricted as the points of comparison were of difficult, I might almost say arduous and painful, attainment and access. Until the close of the last century all real or supposed knowledge of the structure or form of volcanos, and of the mode of operation of subterranean forces, was taken from two mountains of the South of Europe, Vesuvius and Etna. The former of these being the easiest of access, and its eruptions, as is generally the case in volcanos of small elevation, being most frequent in their occurrence, a hill of minor elevation became the type which regulated all the ideas formed respecting phænomena exhibited on a far larger scale in many vast and distant regions, as in the mighty volcanos arranged in linear series in Mexico, South America, and the Asiatic Islands. Such a proceeding might not unnaturally recall Virgil’s shepherd, who thought he beheld in his humble cottage the type of the eternal City, Imperial Rome.

A more careful examination of the whole of the Mediterranean, and especially of those islands and coasts where men awoke to the noblest intellectual culture, might, however, have dispelled views formed from so limited a consideration of nature. Among the Sporades, trachytic rocks have been upraised from the deep bottom of the sea, forming islands resembling that which, in the vicinity of the Azores, appeared thrice periodically, at nearly equal intervals, in three centuries. The Peloponnesus has, between Epidaurus and Trœzene, near Methone, a Monte Nuovo described by Strabo and seen again by Dodwell, which is higher than the Monte Nuovo of the Phlegræan Fields near Baiæ, and perhaps even higher than the new volcano of Jorullo in the plains of Mexico, which I found surrounded by several thousand small basaltic cones which had been protruded from the earth and were still smoking. In the Mediterranean and its shores, it is not only from the permanent craters of isolated mountains having a constant communication with the interior, as Stromboli, Vesuvius, and Etna, that volcanic fires break forth: at Ischia, on the Monte Epomeo, and also, as it would appear by the accounts of the ancients, in the Lelantine plain near Chalcis, lavas have flowed from fissures which have suddenly opened at the surface of the earth. Besides these phænomena, which fall within the historic period, or within the restricted domain of well-assured tradition, and which Carl Ritter will collect and elucidate in his masterly work on Geography,—the shores of the Mediterranean exhibit numerous remains of more ancient volcanic action. In the south part of France, in Auvergne, we see a separate complete system of volcanos arranged in lines, trachytic domes alternating with cones of eruption, from which streams of lava have flowed in narrow bands. The plain of Lombardy, as level as the surface of the sea, and forming an inner Gulf of the Adriatic, surrounds the trachyte of the Euganean Hills, where rise domes of granular trachyte, obsidian, and pearl-stone, masses connected by a common origin, which break through the lower cretaceous rock and nummulitic lime-stone, but have never flowed in narrow streams. Similar evidences of ancient revolutions of nature are found in several parts of the mainland of Greece and in Asia Minor, countries which will one day offer a rich field for geological investigation, when intellectual light shall revisit the seats from which it has radiated to the western world, and when oppressed humanity shall no longer be subject to the barbarism of Turkish rule.

I recall the geographical proximity of these various phænomena, in order to shew that the basin of the Mediterranean, with its series of islands, might have offered to an attentive observer much that has been recently discovered, under various forms, in South America, Teneriffe, and the Aleutian Islands near the polar circle. The objects to be observed were assembled within a moderate distance; yet distant voyages, and the comparison of extensive regions in and out of Europe, have been required for the clear perception and recognition of the resemblance between volcanic phænomena and their dependence on each other.

Our ordinary language, which often gives permanency and apparent authority to the first-formed erroneous views of natural phænomena, but which also often points instinctively to the truth,—our ordinary language, I repeat, applies the term “volcanic” to all eruptions of subterranean fires or molten substances; to columns of smoke and vapour rising from rocks, as at Colares after the great earthquake of Lisbon; to “Salses” or mud volcanos, argillaceous cones emitting mud, asphalte, and hydrogen, as at Girgenti in Sicily, and at Turbaco in South America; to the Geysers, hot springs in which, as in those of Iceland, the waters, pressed by elastic vapours, rise in jets to a considerable altitude; and, in general, to all operations of natural forces having their seat in the interior of our planet. In Central America (Guatimala), and in the Philippine Islands, the natives even distinguish formally between water- and fire-volcanos, Volcanes de agua y de fuego, giving the former name to those mountains from which subterranean waters issue from time to time with violent earthquake shocks and a hollow noise.

Not denying the connexion of the different phenomena which have been referred to, it yet appears desirable to give greater precision to the terms employed in the physical as well as in the mineralogical part of geology, and not to apply the word “volcano” at one moment to a mountain terminating in a permanent igneous opening or fiery crater, and at another to every subterranean cause of volcanic phenomena. In the present state of our planet the most ordinary form of volcanos is indeed in all parts of the globe that of an isolated conical mountain, such as Vesuvius, Etna, the Peak of Teneriffe, Tunguragua, and Cotapaxi. I have myself seen such volcanos varying in size from the smallest hill to an elevation of 18000 (19184 English) feet above the sea. But besides these isolated cones there are also permanent openings or craters, having established channels of communication with the interior of the earth, which are situated on long chains of mountains with serrated crests, and not even always on the middle of the ridge, but sometimes at its extremity: such is Pichincha, situated between the Pacific and the city of Quito, and which acquired celebrity in connection with Bouguer’s earliest barometric formulæ, and such are the volcanos which rise in the elevated Steppe de los Pastos, itself ten thousand (10657 English) feet high. All these summits, which are of various shapes, consist of trachyte, formerly called Trap-porphyry: a granular vesicular rock composed of different kinds of feldspar (Labradorite, Oligoklase, and Albite), augite, hornblende, and sometimes interspersed mica, and even quartz. In cases where the evidence of the first outburst or eruption, or I might say where the ancient structure or scaffolding remain entire, the isolated conical mount is surrounded by an amphitheatre or lofty circular rampart of rocky strata superimposed upon each other. Such walls or ring-formed ramparts are called “craters of elevation,” a great and important phenomenon, concerning which a memorable treatise was presented to our Academy five years ago (i. e. in 1818), by the first geologist of our time, Leopold von Buch, from whose writings I have borrowed several of the views contained in the present discussion.

Volcanos which communicate with the atmosphere through permanent openings, conical basaltic hills, and craterless trachytic domes, sometimes as low as Sarcouy, sometimes as lofty as the Chimborazo, form various groups. Comparative geography shows us sometimes small clusters or distinct systems of mountains, with craters and lava-currents in the Canaries and the Azores, and without craters and without lava-currents, properly so-called, in the Euganean hills and the Siebengebirge near Bonn;—and at other times the same study describes to us volcanos arranged in single or double lines extending through many hundred leagues in length, these lines being either parallel to the direction of a great chain of mountains, as in Guatimala, in Peru, and in Java, or cutting it transversely or at right angles, as in tropical Mexico. In this land of the Aztecs the fire-emitting trachytic mountains are the only ones which attain the elevation of the lofty region of perpetual snow; they are ranged in the direction of a parallel of latitude, and have probably been raised from a fissure 420 English geographical miles long, traversing the continent from the Pacific to the Atlantic Ocean.

These assemblages of volcanos, whether in rounded groups or in double lines, show in the most conclusive manner that the volcanic agencies do not depend on small or restricted causes, in near proximity to the surface of the earth, but that they are great phænomena of deep-seated origin. The whole of the eastern part of the American continent, which is poor in metals, is, in its present state, without fire-emitting mountains, without masses of trachyte, and perhaps even without basalt containing olivine. All the American volcanos are on the side of the continent which is opposite to Asia, in the chain of the Andes which runs nearly in the direction of a meridian, and extends over a length of 7200 geographical miles.

The whole plateau or high-land of Quito, of which Pichincha, Cotopaxi, and Tunguragua form the summits, is to be viewed as a single volcanic furnace. The subterranean fire breaks forth sometimes through one and sometimes through another of these openings, which it has been customary to regard as separate and distinct volcanos. The progressive march of the subterranean fire has been here directed for three centuries from North to South. Even the earthquakes which occasion such dreadful ravages in this part of the world afford remarkable proofs of the existence of subterranean communications, not only between countries where there are no volcanos (a fact which had long been known), but also between fire-emitting openings situated at great distances asunder. Thus in 1797 the volcano of Pasto, east of the Guaytara River, emitted uninterruptedly for three months a lofty column of smoke, which column disappeared at the instant when, at a distance of 240 geographical miles, the great earthquake of Riobamba and the immense eruption of mud called “Moya” took place, causing the death of between thirty and forty thousand persons.

The sudden appearance of the Island of Sabrina near the Azores, on the 80th of January, 1811, was the precursor of the terrible earthquake movements which, much farther to the west, shook almost incessantly, from the month of May 1811 to June 1813, first the West Indian Islands, then the plain of the Ohio and Mississipi, and lastly, the opposite coast of Venezuela or Caraccas. Thirty days after the destruction of the principal city of that province, the long tranquil volcano of the Island of St. Vincent burst forth in an eruption. A remarkable phenomenon accompanied this eruption: at the same moment when the explosion took place, on the 30th of April, 1811, a loud subterranean noise was heard in South America, which spread terror and dismay over a district of 2200 (German) geographical square miles (35200 English geographical square miles). The dwellers on the banks of the Apure near the confluence of the Rio Nula, and the most distant inhabitants of the sea coast of Venezuela, alike compared the sound to that of the discharge of great pieces of ordnance. Now from the confluence of the Nula with the Apure (by which latter river I arrived on the Orinoco) to the volcano of St. Vincent is a distance in a straight line of 628 English geographical miles. The sound, which certainly was not propagated through the air, must have proceeded from a deep-seated subterranean cause; for its intensity was scarcely greater on the sea coast nearest to the volcano where the eruption was taking place, than in the interior of the country, in the basin of the Apure and the Orinoco.

It would be unnecessary to multiply examples by citing other instances which I have collected, but, to recall a phenomenon of European historical importance, I will only farther mention the celebrated earthquake of Lisbon. Simultaneously with that event, on the 1st of November, 1755, not only were the Swiss lakes and the sea near the coast of Sweden violently agitated, but even among the eastern West Indian Islands, Martinique, Antigua, and Barbadoes, where the tide never exceeds thirty inches, the sea suddenly rose more than twenty feet. All these phenomena show the operation of subterranean forces, acting either dynamically in earthquakes, in the tension and agitation of the crust; or in volcanos, in the production and chemical alteration of substances. They also show that these forces do not act superficially, in the thin outermost crust of the globe, but from great depths in the interior of our planet, through crevices or unfilled veins, affecting simultaneously widely distant points of the earth’s surface.

The greater the variety of structure in volcanos, or in the elevations which surround the channel through which the molten masses of the interior of the earth reach its surface, the greater the importance of submitting this structure to strict investigation and measurement. The interest attaching to these measurements, which formed a particular object of my researches in another quarter of the globe, is enhanced by the consideration that at many points the magnitude to be measured is found to be a variable quantity. The philosophical study of nature endeavours, in the vicissitudes of phenomena, to connect the present with the past.

If we desire to investigate either the fact of a periodical return, or the law of progressive variations or changes in phenomena, it is essential to obtain, by means of observations carefully made and connected with determinate epochs, certain fixed points which may afford a base for future numerical comparisons. If we only possessed determinations made once in each period of a thousand years, of the mean temperature of the atmosphere and of the earth in different latitudes, or of the mean height of the barometer at the level of the sea, we should know whether, and in what ratio, the temperature of different climates had increased or decreased, or whether the height of the atmosphere had undergone changes. Such points of comparison are also needed for the inclination and declination of the magnetic needle, as well as for the intensity of the magneto-electric forces, on which, within the circle of this Academy, two excellent physicists, Seebeck and Erman, have thrown so much light. As it is an honourable object for the exertions of scientific societies to trace out perseveringly the cosmical variations of temperature, atmospheric pressure, and magnetic direction and intensity, so it is the duty of the geological traveller, in determining the inequalities of the earth’s surface, to attend more particularly to the variable height of volcanos. The endeavours made by me for this object in the Mexican mountains, in respect to the Volcan de Toluca, the Popocatepetl, the Cofre de Perote or Nauhcampatepetl, and the Jorullo, and also the volcano of Pichincha in the Andes of Quito, have been continued since my return to Europe at different epochs on Vesuvius. Where complete trigonometric or barometric measurements are wanting, accurate angles of altitude, taken at points which are exactly determined, may be substituted for them; and for a comparison of determinations made at different epochs, angles of altitude so measured may even be often preferable to the complication of circumstances which more complete operations may involve.

Saussure had measured Mount Vesuvius, in 1773, when the two margins of the crater, the north-western and the south-eastern, appeared to him be of equal height. He found their height above the level of the sea 609 toises, 3894 English feet. The eruption of 1794 occasioned a breaking down of the margin of the crater on the southern side, and a consequent inequality between the height of the two edges which the most unpractised eye does not fail to distinguish even at a considerable distance. In 1805, Leopold von Buch, Gay-Lussac, and myself, measured the height of Vesuvius three times, and found the northern margin opposite to La Somma, (the Rocca del Palo), exactly as given by Saussure, but the southern margin 75 toises, or 450 French or 479 English feet, lower than he had found it in 1773. The whole elevation of the volcano on the side of Torre del Greco (the side towards which, for the last thirty years, the igneous action has, as it were, been principally directed,) had at that time diminished one-eighth. The height of the cone of ashes, as compared with the whole height of the mountain, is in Vesuvius as 1 to 3; in Pichincha, as 1 to 10; and in the Peak of Teneriffe, as 1 to 22. In these three volcanic mountains, the cone of ashes is therefore, relatively speaking, highest in Vesuvius; probably because, being a low volcano, the action has been principally by the summit.

A few months ago (in 1822) I was enabled not only to repeat my former barometric measurements of the height of Vesuvius, but also, during the course of three visits to the summit, to make a more complete determination of all the edges of the crater[38]. These determinations may not be without interest, since they include the long period of great eruptions between 1805 and 1822, and constitute perhaps the only known examination and measurement of a volcano at different epochs, in which the different parts of the examination are all truly comparable with each other. We learn from it that the margins of craters are a phenomenon of far more permanent character than had been previously inferred from passing observations, and this not only where (as in the Peak of Teneriffe, and in all the volcanos of the chain of the Andes,) they are visibly composed of trachyte, but also elsewhere. According to my last determinations, the north-west edge of Vesuvius has, perhaps, not altered at all since the time of Saussure, an interval of 49 years; and the south-eastern side, on the side towards Bosche Tre Case, which, in 1794, had become 400 French (426 English) feet lower, has since then hardly altered 10 toises (60 French or 64 English feet).

If the public journals, in describing great eruptions, often state the shape of Vesuvius to have undergone an entire change, and if these assertions appear to be confirmed by picturesque views sketched at Naples, the cause of the error consists in the outlines of the margin of the crater having been confounded with those of the cones of eruption accidentally formed in the middle of the crater on its floor or bottom which has been upheaved by vapours. Such a cone of eruption, consisting of loosely heaped-up rapilli and scoriæ, had in the course of the years 1816-1818 gradually risen so as to be seen above the south-eastern margin of the crater; and the eruption of the month of February 1822 augmented it so much, that it even became from 100 to 110 (about 107 to 117 English) feet higher than the north-western margin of the crater (the Rocca del Palo). This remarkable cone, which it had become customary in Naples to regard as the true summit of the mountain, fell in, with a dreadful noise, in the last eruption, on the night of the 22d of October (1822): so that the floor of the crater, which had been constantly accessible since 1811, is now 750 (almost 800 English) feet lower than the northern, and 200 (213 English) feet lower than the southern edge of the volcano. Variations in the form and relative position of the cones of eruption,—the openings of which ought not to be confounded, as they often are, with the crater of the volcano itself,—give to Vesuvius at different epochs a different appearance, which would enable a person well acquainted with the history of the volcano, on a mere inspection of Hackert’s paintings in the palace of Portici, to tell from the outlines of the summit, according as the northern or the southern side of the mountain is represented as the highest, in what year the artist had taken the sketch from which the picture was made.

In the last eruption, in the night of the 23d to the 24th of October, twenty-four hours after the falling in of the great cone of scoriæ which has been mentioned, and when the small but numerous currents of lava had already flowed off, the fiery eruption of ashes and rapilli commenced: it continued without intermission for twelve days, but was greatest in the first four days. During this period the detonations in the interior of the volcano were so violent that the mere concussion of the air, (for no earthquake movement was perceived), rent the ceilings of the rooms in the palace of Portici. In the neighbouring villages of Resina, Torre del Greco, Torre del Annunziata, and Bosche Tre Case, a remarkable phenomenon was witnessed. Throughout the whole of that part of the country the air was so filled with ashes as to cause in the middle of the day profound darkness, lasting for several hours: lanterns were carried in the streets, as has so often been done at Quito during the eruptions of Pichincha. The flight of the inhabitants had never been more general: lava currents are regarded by those who dwell near Vesuvius with less dread than an eruption of ashes, a phenomenon which had never been known to such a degree in modern times; and the obscure tradition of the manner in which the destruction of Herculaneum, Pompeii, and Stabiæ took place, filled the imaginations of men with appalling images.

The hot aqueous vapours which rose from the crater during the eruption and spread themselves in the atmosphere, formed, in cooling, a dense cloud, surrounding the column of fire and ashes, which rose to a height of between nine and ten thousand feet. So sudden a condensation of vapour, and even, as Gay-Lussac has shewn, the formation of the cloud itself, augmented the electric tension. Flashes of forked lightning, issuing from the column of ashes, darted in every direction; and the rolling thunders were distinctly heard, and distinguished from the sounds which proceeded from the interior of the volcano. In no other eruption had the play of the electric forces formed so striking a feature.

On the morning of the 26th of October, a surprising rumour prevailed, to the effect that a torrent of boiling water was gushing from the crater, and pouring down the slope of the cone of ashes. The learned and zealous observer of the volcano, Monticelli, soon discovered that this erroneous rumour had arisen from an optical illusion. The supposed torrent of water was in reality a flow of dry ashes, which, being as loose and moveable as shifting sands, issued in large quantities from a crevice in the upper margin of the crater. The cultivated fields had suffered much from a long-continued drought which had preceded the eruption; towards its close the “volcanic thunder-storm” which has been described produced an exceedingly violent and abundant fall of rain. This phenomenon is associated in all climates with the close of a volcanic eruption. As during the eruption the cone of ashes is generally enveloped in cloud, and as it is in its immediate vicinity that the rain is most violent, torrents of mud are seen to descend from it in all directions, which the terrified husbandman imagines to consist of waters which have risen from the interior of the volcano and overflowed the crater; while geologists have erroneously thought they recognised in them either sea-water or muddy products of the volcano, “Eruptions boueuses,” or, in the language of some old French systematists, products of an igneo-aqueous liquefaction.

Where, as is generally the case in the Andes, the summit of the volcano rises into the region of perpetual snow, (even attaining, in some cases, an elevation twice as great as that of Etna), the melting of the snows renders such inundations as have been described far more abundant and disastrous. The phenomena in question are meteorologically connected with the eruptions of volcanos, and are variously modified by the height of the mountain, the dimensions of that part of it which is always covered with snow, and the extent and degree to which the sides of the cone of cinders become heated; but they are not to be regarded as volcanic phenomena properly so called. Vast cavities also often exist on the slope or at the foot of volcanos which, communicating through many channels with the mountain torrents, form large subterranean lakes or reservoirs of water. When earthquake shocks, which, in the Andes, usually precede all igneous eruptions, convulse the entire mass of the volcano, these subterranean reservoirs are opened, and there issue from them water, fishes, and tufaceous mud. This is the singular phenomenon which brings to light an otherwise unknown fish, the Pimelodes Cyclopum, called by the inhabitants of the highlands of Quito “Preñadilla,” and which I described soon after my return. When, on the night of the 19th of June, 1698, the summit of a mountain situated to the north of Chimborazo, the Carguairazo, above 19000 English feet high, fell in, the country for nearly thirty English geographical square miles round was covered with mud and fishes; and seven years earlier a putrid fever, in the town of Ibarra, was ascribed to a similar eruption of fish from the volcano of Imbaburu.

I recall these facts, because they throw some light on the difference between the eruption of dry ashes and miry inundations of tufa and trass, carrying with them wood, charcoal, and shells. The quantity of ashes emitted by Vesuvius in the recent eruption, like every thing connected with volcanos and other great natural phenomena of a character to excite terror, has been exceedingly exaggerated in the public papers; and two Neapolitan chemists, Vicenzo Pepe and Giuseppe di Nobili, notwithstanding the statements of Monticelli and Covelli to the contrary, even describe the ashes as containing silver and gold. According to the results of my researches and inquiries, the thickness of the bed of ashes formed by the twelve days’ shower was but little above three feet, towards Bosche Tre Case, on the slope of the cone where rapilli were mingled with them; and in the plain, from 15½ to 19 inches at the utmost. Such measurements ought not to be taken in places where the ashes have been heaped up by the action of wind, like drifted snow or sand, or have accumulated from being carried thither by water. The times are passed for seeking only the marvellous in volcanic phenomena, in the manner of the ancients among whom Ctesias made the ashes of Etna to be conveyed as far as the Indian peninsula. There are in Mexico veins of gold and silver in trachytic porphyry; but in the ashes of Vesuvius which I brought back with me, and which an excellent chemist, Heinrich Rose, has examined at my request, no traces of either gold or silver have been discovered.

Although the above mentioned results, which are quite in accordance with the exact observations of Monticelli, differ much from the accounts which have been current during the short interval which has elapsed, it is nevertheless true that the eruption of ashes from Vesuvius from the 24th to the 28th of last October (1822) is the most memorable of any of which we possess an authentic account, since that which occasioned the death of the elder Pliny. The quantity of ashes is, perhaps, three times as great as has ever been seen to fall since volcanic phenomena have been attentively observed in Italy. A stratum of ashes, from 16 to 19 inches thick, appears at first sight insignificant compared with the mass which we find covering Pompeii; but, not to speak of the increase which that mass has probably received by the effects of heavy rains and other causes during the centuries which have since elapsed, and without renewing the animated debate respecting the causes of the destruction of the Campanian towns, and which, on the other side of the Alps, has been carried on with a considerable degree of scepticism, it should here be recalled to recollection that the eruptions of a volcano, at widely separated epochs, do not well admit of comparison, as respects their intensity. All inferences derived from analogy are inadequate where quantitative relations are concerned; as the quantity of lava and ashes, the height of the column of smoke, and the loudness or intensity of the detonations.

From the geographical description of Strabo, and from an opinion given by Vitruvius respecting the volcanic origin of pumice, we perceive that, up to the year of the death of Vespasian, i. e. previous to the eruption which overwhelmed Pompeii, Vesuvius had more the appearance of an extinct volcano than of a Solfatara. When, after long repose, the subterranean forces suddenly opened for themselves new channels, and again broke through the beds of primitive and trachytic rocks, effects must have been produced for which subsequent ones do not furnish a standard. From the well-known letter in which the younger Pliny informs Tacitus of his uncle’s death, it may be clearly seen that the renewal of volcanic outbursts, or what might be called the revival of the slumbering volcano, began with an eruption of ashes. The same thing was observed at Jorullo when, in September 1759, the new volcano, breaking through beds of syenite and trachyte, rose suddenly in the plain. The country-people took flight on finding their huts strewed with ashes which had been emitted from the everywhere opening ground. In the ordinary periodical manifestations of volcanic activity, on the contrary, the shower of ashes marks the termination of each particular eruption. There is a passage in the letter of the younger Pliny which shews clearly that, at a very early stage of the eruption, the dry ashes which had fallen had reached a thickness of four or five feet, without accumulation from drift or other extraneous cause. He writes, in the course of his narrative, “the court which had to be crossed to reach the room in which Pliny was taking his noon-day repose was so filled with ashes and pumice, that, if he had longer delayed coming forth, he would have found the passage stopped.” In an enclosed space like a court, the action of wind in drifting the ashes can scarcely have been very considerable.

I have interrupted my general comparative view of volcanos by a notice of particular observations made on Vesuvius, partly on account of the great interest excited by the recent eruption, and partly on account of those recollections of the catastrophes of Pompeii and Herculaneum, which are almost involuntarily recalled to our minds by the occurrence of any considerable shower of ashes. I have recorded in a note the measurements of height made by myself and others on Vesuvius and in its vicinity.

We have hitherto been considering the structure and mode of action of those volcanos which have a permanent communication with the interior of the Earth by craters. The summits of such volcanos consist of masses of trachyte and lava upheaved by elastic forces and traversed by veins. The permanency of their action gives us reason to infer great complexity of structure. They have, so to speak, an individual character which remains unaltered for long periods of time. Neighbouring mountains often present the greatest differences in their products: leucitic and feldspathic lavas, obsidian with pumice, and masses of basalt containing olivine. They belong to the most recent terrestrial phænomena, breaking through almost all the sedimentary strata, and their products and lava currents are of later origin than our valleys. Their life, if I may permit myself to employ this figurative mode of expression, depends on the manner and permanence of their communications with the interior of the Earth. They often continue for centuries in a state of repose, are then suddenly rekindled, and end by becoming Solfataras, emitting aqueous vapours, gases, and acids; sometimes, however, as in the case of the Peak of Teneriffe, we find that their summit has already become a laboratory of regenerated sulphur; while from the sides of the mountain there still issue large torrents of lava, basaltic in the lower part, but towards the upper part, where the pressure is less,[39] presenting the form of obsidian with pumice.

Distinct from these volcanos provided with permanent craters, there is another class of volcanic phenomena more rarely observed, but particularly instructive to the geologist, as they recall the ancient world or the earliest geological revolutions of our planet. Trachytic mountains open suddenly, emit lava and ashes, and close again, perhaps never to reopen. Thus it was with the gigantic mountain of Antisana in the chain of the Andes, and with the Monte Epomeo in Ischia in 1302. Sometimes such an outbreak has even taken place in plains: as in the high plateau of Quito, in Iceland at a distance from Mount Hecla, and in Eubœa in the Lelantine Fields. Many of the upheaved islands belong to this class of transitory phænomena. In all these cases the communication with the interior of the earth is not permanent, and the action ceases as soon as the cleft or fissure forming a temporary channel closes again. Veins or dykes of basalt, dolerite, and porphyry, which in different parts of the earth traverse almost all formations, and masses of syenite, augitic porphyry, and amygdaloid, which characterise the recent transition and oldest sedimentary rocks, have probably been formed in a similar manner. In the youth of our planet, the substances of the interior being still fluid, penetrated through the everywhere fissured crust of the globe, sometimes becoming solidified in the form of rocky veins or dykes of granular texture, and sometimes spreading out in broad sheets, and resembling superimposed strata. The volcanic products or rocks transmitted to us from the earlier ages of our planet have not flowed in narrow bands like the lavas of the isolated conical volcanos of the present time. The mixtures of augite, titaniferous iron, feldspar, and hornblende, may have been the same at different epochs, sometimes approximating more to basalt and sometimes to trachyte; and, (as we learn from the important researches of Mitscherlich, and the analogy of artificial igneous products) chemical substances may have united in definite proportions in a crystalline form: in all cases we recognise that substances similar in composition have arrived at the surface of the earth by very different ways; either simply upheaved, or penetrating through temporary fissures; and that breaking through the older rocks, (i. e. the earlier oxydized crust of the globe), they have finally issued as lava currents from conical mountains having a permanent crater. To confound together phenomena so different is to throw the geological study of volcanos and volcanic action back into the obscurity from which, by the aid of numerous comparative observations and researches, it has gradually began to emerge.

The question has often been propounded: What is it that burns in volcanos,—What produces the heat which melts and fuses together earths and metals? Modern chemical science has essayed to answer, that what burns are the earths, the metals, the alkalies themselves; viz. the metalloids of those substances. The solid and already-oxydised crust of the globe separates the surrounding atmosphere, with the oxygen which it contains, from the inflammable unoxydised substances in the interior of our planet: when those metalloids come in contact with the oxygen of the atmosphere there arises disengagement of heat. The great and celebrated chemist who propounded this explanation of volcanic phenomena soon himself relinquished it. Observations made in mines and caverns in all climates, and which in concert with M. Arago I have collected in a separate memoir, shew that, even at what may be considered a very small depth, the temperature of the Earth is much above the mean temperature of the atmosphere at the same place. A fact so remarkable, and so generally confirmed, connects itself with that which we learn from volcanic phenomena. The depth at which the globe may be regarded as a molten mass has been calculated. The primitive cause of this subterranean heat is, as in all planets, the process of formation itself, the separation of the spherically condensing mass from a cosmical gaseous fluid, and the cooling of the terrestrial strata at different depths by the loss of heat parted with by radiation. All volcanic phenomena are probably the result of a communication either permanent or transient between the interior and exterior of the globe. Elastic vapours press the molten oxydising substances upwards through deep fissures. Volcanos might thus be termed intermitting springs or fountains of earthy substances; i. e. of the fluid mixture of metals, alkalis, and earths which solidify into lava currents and flow softly and tranquilly, when being upheaved they find a passage by which to escape. In a similar manner the Ancients represented (according to Plato’s Phædon) all volcanic fiery currents as streams flowing from the Pyriphlegethon.

To these considerations and views let me be permitted to add another more bold. May we not find in this internal heat of our globe,—(a heat indicated by thermometric experiments on the waters of springs rising from different depths,[40] as well by our observations on volcanos),—a cause which may explain one of the most wonderful phænomena with which the study of fossils has made us acquainted? Tropical forms of animals, and, in the vegetable kingdom, arborescent ferns, palms, and bambusaceæ, are found buried in the cold regions of the North. Everywhere the ancient world shews a distribution of organic forms at variance with our present climates. To resolve so important a problem, recourse has been had to several hypotheses; such as the approach of a comet, a change in the obliquity of the Ecliptic, and a different degree of intensity in the solar light. None of these explanations are satisfactory at once to the astronomer, the physicist, and the geologist. For my part I willingly leave the axis of the Earth in its place, and suppose no change in the light of the solar disk (from whose spots a celebrated astronomer was inclined to explain the favourable or unfavourable harvests of particular years); I am disposed to recognise that in each planet there exist, independently of its relations to the central body of the system to which it belongs, and independently of its astronomical position, various causes for the development of heat;—processes of oxydation, precipitations and chemical changes in the capacity of bodies, by increase of electro-magnetic intensity, and communications opened between the internal and external portions of the planet.

It may be that in the Ancient World, exhalations of heat issuing forth through the many openings of the deeply fissured crust of the globe may have favoured, perhaps for centuries, the growth of palms and tree-ferns and the existence of animals requiring a high temperature, over entire countries where now a very different climate prevails. According to this view of things (a view already indicated by me in a work entitled “Geological Essay on the Superposition of Rocks in both Hemispheres”) the temperature of volcanos would be that of the interior of the earth, and the same cause which, operating through volcanic eruptions, now produces devastating effects, might in primeval ages have clothed the deeply fissured rocks of the newly oxydised earth in every zone with the most luxuriant vegetation.

If, with a view to explain the distribution of tropical forms whose remains are now discovered buried in northern regions, it should be assumed that the long-haired species of Elephant now found enclosed in ice was originally indigenous in cold climates, and that forms resembling the same leading type may, as in the case of lions and lynxes, have been able to live in wholly different climates, still this manner of solving the difficulty presented by fossil remains cannot be extended so as to apply to vegetable productions. From reasons with which the study of vegetable physiology makes us acquainted, Palms, Musaceæ, and arborescent Monocotyledones, are incapable of supporting the deprivation of their appendicular organs which would be caused by the present temperature of our northern regions; and in the geological problem which we have to examine, it appears to me difficult to separate vegetable and animal remains from each other. The same mode of explanation ought to comprehend both.

I have permitted myself at the conclusion of the present discussion to connect with facts collected in different and widely separated countries some uncertain and hypothetical conjectures. The philosophical study of Nature rises beyond the requirements of a simple description of Nature: it does not consist in a sterile accumulation of isolated facts. It may sometimes be permitted to the active and curious mind of man to stretch forward from the present to the still obscure future; to divine that which cannot yet be clearly known; and thus to take pleasure in the ancient myths of geology reproduced in our own days in new and varied forms.