When we consider the influence exerted on the study of nature during the last few centuries, by the extension of geographical knowledge and by means of scientific expeditions to remote regions of the earth, we are at once made sensible of the various character of this influence, according as the investigations have been directed to the forms of the organic world, the study of the inorganic crust of the earth, or to the knowledge of rocks, their relative ages, and their origin. Different vegetable and animal developments exist in every division of the earth, whether it be on the plains, where, on a level with the sea, the temperature varies with the latitude and with the various inflections of the isothermal lines, or on the steep declivity of mountain ranges, warmed by the direct rays of the sun. Organic nature imparts to every region of the globe its own characteristic physiognomy. But this does not apply to the inorganic crust of the earth divested of its vegetable covering, for everywhere, in both hemispheres, from the equator to the poles, the same rocks are found grouped with some relation to each other, either of attraction or repulsion. In distant lands, surrounded by strange forms of vegetation, and beneath a sky beaming with other stars than those to which his eye had been accustomed, the mariner often recognises, with joyful surprise, argillaceous schists and rocks familiar to him in his native land.

This independence of geological relations on the actual condition of climates does not diminish the beneficial influence exercised on the progress of mineralogy and physical geognosy by the numerous observations instituted in distant regions of the earth, but simply gives a particular direction to them. Every expedition enriches natural history with new genera of plants and animals. At one time we acquire a knowledge of new organic forms which are allied to types long familiar to us, and which not unfrequently, by furnishing links till then deficient, enable us to establish, in all its original perfection, an uninterrupted chain of natural structures. At another time we become acquainted with isolated structures, which appear either as the remains of extinct genera, or members of unknown groups, the discovery of which stimulates further research. It is not, however, from the investigation of the earth’s crust that we acquire these manifold additions to our knowledge, for here we meet rather with an uniformity in the constituent parts, in the superposition of dissimilar masses, and in their regular recurrence, which cannot fail to excite the surprise and admiration of the geologist. In the chain of the Andes, as in the mountains of Central Europe, one formation appears, as it were, to call forth another. Masses identical in character assume the same forms; basalt and dolerite compose twin mountains; dolomite, sandstone, and porphyry form abrupt rocky walls; while vitreous trachyte, containing a large proportion of feldspar, rises in bell-shaped and high-vaulted domes. In the most remote regions large crystals are separated in a similar manner from the compact texture of the fundamental mass, and, blending and grouping together into subordinate strata, frequently announce the commencement of new and independent formations. It is thus that the inorganic world may be said to reflect itself, more or less distinctly, in every mountain of any great extent. It is necessary, however, in order perfectly to understand the most important phenomena of the composition, relative age, and origin of formations, to compare together the observations made in regions of the earth most widely remote from each other. Problems which have long baffled the geologist in his own northern region, find their solution in the vicinity of the equator. If, as we have already observed, remote regions do not present us with new formations, that is to say, with unknown groupings of simple substances, they at least help us to unravel the great and universal laws of nature, by showing how different strata of the crust of the earth are mutually superimposed on, and intersect, each other in the form of veins, or rise to different elevations in obedience to elastic forces.

Although our geological knowledge may be thus extensively augmented by researches over vast regions, it can hardly be a matter of surprise that the class of phenomena constituting the principal subject of this address should have been so long examined in an imperfect manner, since the means of comparison were of difficult, and almost, it may be said, of laborious access.

Until towards the close of the eighteenth century all that was known of the form of volcanos and of the action of their subterranean forces was derived from observations made on two volcanic mountains of Southern Italy, Vesuvius and Etna. As the former of these was the more accessible, and (like all volcanos of slight elevation) had frequent eruptions, a hill became to a certain degree the type according to which a whole world—the mighty volcanos of Mexico, South America, and the Asiatic Islands—was supposed to be formed. Such a mode of reasoning involuntarily calls to mind Virgil’s shepherd, who believed that in his own humble cot he saw the image of the eternal city, Imperial Rome.

This imperfect mode of studying nature might indeed have been obviated by a more attentive examination of the whole Mediterranean, and especially of its eastern islands and littoral districts, where mankind first awoke to intellectual culture and to a higher standard of feeling. Among the Sporades, trachytic rocks have risen from the bottom of the sea, and have formed lands similar to those of the Azores, which in the course of three centuries have appeared periodically at three almost equal intervals of time. Between Epidaurus and Trœzene, near Methone, in the Peloponnesus, there is a Monte Nuovo, described by Strabo and since by Dodwell. Its elevation is greater than that of the Monte Nuovo of the Phlegræan fields near Baiæ, and perhaps even than that of the new volcano of Xorullo, in the plains of Mexico, which I found to be surrounded by many thousand small basaltic cones, upheaved from the earth, and still emitting smoke. It is not only in the basin of the Mediterranean, that volcanic fires escape from the permanent craters of isolated mountains having a constant communication with the interior of the earth, as Stromboli, Vesuvius, and Etna; for at Ischia, and on Mount Epomeus, and also, according to the accounts of the ancients, in the Lelantine plain, near Chalcis, lavas have flowed from fissures which have suddenly opened on the surface of the earth. Besides these phenomena, which fall within historical periods, that is, within the narrow bounds of authentic tradition, and which Ritter purposes collecting and explaining in his masterly work on geography, the shores of the Mediterranean present numerous remains of the earlier action of fire. The south of France exhibits in Auvergne a distinct and peculiar system of volcanos, linearly arranged, trachytic domes alternating with cones of eruption, emitting lava streams in the form of bands. The plains of Lombardy, which are on a level with the sea, and constitute the innermost bay of the Adriatic, inclose the trachyte of the Euganean Hills, where rise domes of granular trachyte, obsidian, and pearl-stone. These masses are developed from each other, and break through the lower chalk formations and nummulitic limestone, but have never been emitted in narrow streams. Similar evidence of former revolutions of our earth, is afforded in many parts of the Greek Continent and in Western Asia, countries which will undoubtedly some day yield the geologist ample materials for investigation, when the light of knowledge shall again shine on those lands whence it first dawned on our western world, and when oppressed humanity shall cease to groan beneath the weight of Turkish barbarism.

I allude to the geographical proximity of such numerous and various phenomena in order to show that the basin of the Mediterranean, with its series of islands, might have enabled the attentive observer to note all those phenomena which have recently been discovered under various forms and structures in South America, Teneriffe, and in the Aleutian islands, near the Polar region. The materials for observation were, no doubt, accumulated within a narrow compass; but it was yet necessary that travels in distant countries and comparisons between extensive tracts of land, both in and out of Europe, should be undertaken, in order to obtain a correct idea of the resemblance between volcanic phenomena and of their dependence on each other.

Language, which so frequently imparts permanence and authority to first, and often also erroneous views, but which points, as it were, instinctively to the truth, has applied the term volcanic to all eruptions of subterranean fire and molten matter; to columns of smoke and vapour which ascend sporadically from rocks, as at Colares, after the great earthquake of Lisbon; to Salses, or argillaceous cones emitting moist mud, asphalt, and hydrogen, as at Girgenti in Sicily, and at Turbaco in South America; to hot Geyser springs, which rise under the pressure of elastic vapours; and, in general, to all operations of impetuous natural forces which have their seat deep in the interior of our planet. In Central America (Guatimala) and in the Philippine Islands, the natives even formally distinguish between Volcanes de agua y de fuego, volcanos emitting water, and those emitting fire; designating by the former appellation, mountains from which subterranean waters burst forth from time to time, accompanied by a dull hollow sound and violent earthquakes.

Without denying the connection, which undoubtedly exists among the phenomena just referred to, it would seem advisable to apply more definite terms to the physical as well as to the mineralogical portion of the science of geology, and not at one time to designate by the word volcano a mountain terminating in a permanent fire-emitting mouth, and at another to apply it to any subterranean cause, be it what it may, of volcanic action. In the present condition of our earth, the form of isolated conical mountains (as those of Vesuvius, Etna, the Peak of Teneriffe, Tunguragua and Cotopaxi) is certainly the shape most commonly observed in volcanos. I have myself seen such volcanos varying in height from the most inconsiderable hill to an elevation of more than 19,000 feet above the level of the sea. Besides such conical forms, however, we continually meet with permanent fire-emitting mouths, in which the communication with the interior of the earth is maintained on far-extended jagged ridges, and not even always from the centre of their mural summits, but at their extremity towards their slope. Such, for instance, is Pichincha, situated between the Pacific and the city of Quito, which has acquired celebrity from Bouguer’s earliest barometric formulæ, and such are the volcanos on the Steppe de los Pastos, situate at more than 10,000 feet above the level of the sea. All these variously shaped summits consist of trachyte, formerly known as trap-porphyry; a granular stone full of narrow fissures, composed of different kinds of feldspar (labradorite, oligoklase, and albite), augite, hornblende, and sometimes interspersed mica, and even quartz. Wherever the evidences of the first eruption, the ancient structures—if I may use the expression—remain complete, the isolated cone is surrounded, circus-like, with a high wall of rock consisting of different superimposed strata, encompassing it like an outer sheath. Such walls or circular inclosures are termed craters of elevation, and constitute a great and important phenomenon, upon which that eminent geologist, Leopold von Buch, from whose writings I have borrowed many facts advanced in this treatise, presented so remarkable a paper to our Academy five years ago.

Volcanos which communicate with the atmosphere by means of fire-emitting mouths, such as conical basaltic hills, and dome-like craterless trachytic mountains, (the latter being sometimes low, like the Sarcouy, and sometimes high, like the Chimborazo,) form various groups. Comparative geography draws our attention, at one time, to small Archipelagos or independent mountain-systems, with craters and lava streams, like those in the Canary Isles and the Azores, and without craters or true lava streams, as in the Euganean hills, and the Siebengebirge near Bonn; at another time, it makes us acquainted with volcanos arranged in single or double chains, and extending for many hundred miles in length, either running parallel with the main direction of the range, as in Guatimala, Peru, and Java, or intersecting its axis at right angles, as in tropical Mexico. In this land of the Aztecs fire-emitting trachytic mountains alone attain the high snow limit: they are ranged in the direction of a parallel of latitude, and have probably been upheaved from a chasm extending over upwards of 420 miles, intersecting the whole continent from the Pacific to the Atlantic.

This crowding together of volcanos, either in rounded groups or double lines, affords the most convincing proof that their action does not depend on slight causes located near the surface, but that they are great and deep-seated phenomena. The whole of the eastern portion of the American continent, which is poor in metals, has in its present condition no fire-emitting openings, no trachytic masses, and perhaps no basalt containing olivine. All the volcanos of America are united in the portion of the continent opposite to Asia, along the chain of the Andes, which runs nearly due north and south over a distance of more than 7200 miles.

The whole elevated table-land of Quito, which is surmounted by the high mountains of Pichincha, Cotopaxi, and Tunguragua, constitutes one sole volcanic hearth. The subterranean fire bursts sometimes from one and sometimes from another of these openings, which have generally been regarded as independent volcanos. The progressive movement of the fire has, for three centuries, inclined from north to south. Even the earthquakes, which so fearfully devastate this portion of the globe, afford striking evidence of the existence of subterranean communications, not only between countries where there are no volcanos—as has long been known—but likewise between volcanic apertures situated at a distance from each other. Thus the volcano of Pasto, east of the river Guaytara, continued during three months of the year 1797, to emit, uninterruptedly, a lofty column of smoke, until it suddenly ceased at the moment of the great earthquake of Riobamba, (at a distance of 240 miles,) and the mud eruption of the “Moya,” in which from thirty to forty thousand Indians perished.

The sudden appearance, on the 30th of January, 1811, of the island of Sabrina, in the group of the Azores, was the precursor of the dreadful earthquakes which, further westward, shook, from May, 1811, to June, 1813, almost uninterruptedly, first the Antilles, then the plains of the Ohio and Mississippi, and lastly, the opposite coasts of Venezuela or Caracas. Thirty days after the total destruction of the beautiful capital of the province, there was an eruption of the long inactive volcano of St. Vincent, in the neighbouring islands of the Antilles. A remarkable phenomenon accompanied this eruption: at the moment of this explosion, which occurred on the 30th of April, 1811, a terrible subterranean noise was heard in South America, over a district of more than 35,000 square miles. The inhabitants of the banks of the Apure, at the confluence of the Rio Nula, and those living on the remote sea-coast of Venezuela, agreed in comparing this sound to the noise of heavy artillery. The distance from the confluence of the Rio Nula with the Apure (by which I entered the Orinoco) to the volcano of St. Vincent, measured in a straight line, is no less than 628 miles. This noise was certainly not propagated through the air, and must have arisen from some deep-seated subterranean cause; its intensity was, moreover, hardly greater on the shores of the Caribbean sea, near the seat of the raging volcano, than in the interior of the country in the basin of the Apure and the Orinoco.

It would be useless to multiply examples of this nature, by adducing others which I have collected: I will therefore only refer to one further instance, namely, the memorable earthquake of Lisbon, an important phenomenon in the annals of Europe. Simultaneously with this event, which took place on the 1st of November, 1755, not only were the Lakes of Switzerland and the sea off the Swedish coasts violently agitated, but in the eastern portion of the Antilles, near the islands of Martinique, Antigua, and Barbadoes, the tide, which never exceeds thirty inches, suddenly rose upwards of twenty feet. All these phenomena prove, that subterranean forces are manifested either dynamically, expansively, and attended by commotion, in earthquakes; or possess the property of producing, or of chemically modifying substances in volcanos; and they further show, that these forces are not seated near the surface in the thin crust of the earth, but deep in the interior of our planet, whence through fissures and unfilled veins they act simultaneously at widely distant points of the earth’s surface.

The more varied the structure of volcanos, that is to say, of elevations inclosing a channel through which the molten masses of the interior of the earth reach the surface, the more important it is to form a correct idea of these structures by careful measurement. The interest derived from measurements of this kind, which I made a special subject of inquiry in the western hemisphere, is increased by the consideration, that the objects to be measured vary in magnitude at different points. A philosophical study of nature seeks, in considering the changes of phenomena, to connect the present with the past.

In order to ascertain the periodic recurrence, or the laws of the progressive changes in nature, we require certain fixed points, and carefully conducted observations, which, by their connection with definite epochs, may serve as a basis for numerical comparisons. If the mean temperature of the atmosphere and of the earth in different latitudes, or the mean height of the barometer at the sea level, had been determined only once in every thousand years, we should know to what extent the heat of climates has increased or diminished, and whether any changes have taken place in the height of the atmosphere. Such points of comparison are especially required to determine the inclination and declination of the magnetic needle, and the intensity of those electro-magnetic forces on which Seebeck and Erman, two admirable physicists belonging to this Academy, have thrown so much light. If it be a meritorious undertaking on the part of learned societies to investigate with perseverance the cosmical changes in the heat and pressure of the atmosphere, and particularly the magnetic direction and intensity, it is no less the duty of the travelling geologist to direct attention to the varying height of volcanos in determining the inequalities of the earth’s surface. The observations which I formerly made in the Mexican mountains, at the volcano of Toluca, at Popocatepetl, at the Cofre de Perote, or Nauhcampatepetl, and Xorullo, and in the Andes of Quito at Pichincha, I have had opportunities since my return to Europe of repeating, at different periods, on Mount Vesuvius. Where complete trigonometric or barometric measurements are wanting, their place may be supplied by angles of altitude laid down with precision, and taken at points accurately determined. The comparison of such determinations, made at different periods of time, may sometimes be even preferable to the complication of more complete operations.

Saussure measured Vesuvius in 1773, and at that time both the north-western and south-eastern margins of the crater appeared to him to be equal in height. He found their elevation above the level of the sea to be 3894 feet. The eruption of 1794 occasioned a falling in towards the south, and an inequality in the margins of the crater, which may be distinguished from a considerable distance even by the most unpractised eye. Leopold von Buch, Gay Lussac, and myself, measured Mount Vesuvius three times in the year 1805, and found that the elevation of the northern margin, la Rocca del Palo, opposite the Somma, was exactly as it had been given by Saussure, while the southern margin was 479 feet lower than it had been in 1773. The elevation of the volcano itself towards Torre del Greco (the side towards which, for thirty years, the volcanic action has been principally directed) had, at that time, decreased one-eighth. The cone of cinders bears to the total height of Vesuvius the relation of 1 : 3; in Pichincha, the ratio is as 1 : 10, and at the Peak of Teneriffe, as 1 : 22. Of these three volcanic mountains, Vesuvius has, therefore, comparatively, the highest cone of cinders; probably because, being a volcano of inconsiderable height, it has chiefly acted through its summit.

A few months ago, in the year 1822, I succeeded not only in repeating my earlier barometric measurements of Mount Vesuvius, but also in determining more completely all the margins of the crater^[108] during three ascents of the mountain.

These determinations are, perhaps, deserving of some degree of attention, since they embrace the long period of the great eruptions between 1805 and 1822, and are probably the only measurements hitherto published of any volcano which admit of comparison in all their parts. They prove, that the margins of the crater should be regarded as a much more permanent phenomenon than has hitherto been supposed, from the hasty observations made on the subject; and that this character appertains to them everywhere, and not merely in those instances where, as at the Peak of Teneriffe, and in all the volcanos of the Andes, they evidently consist of trachyte. According to my latest determinations it would seem, that since the time of Saussure, a period of forty-nine years, the north-western margin of Vesuvius has probably not changed at all, and that the south-eastern one, in the direction of Bosche Tre Case, which in 1794 had become 426 feet lower, has since then only altered about 64 feet.

If, in the newspaper reports of great eruptions, we often find assertions made of an entire change of form in Mount Vesuvius, and if these assertions appear to be confirmed by the picturesque views of the volcano made at Naples, the cause of the error arises from the outlines of the margins of the crater having been confounded with those of the cones of eruption accidentally formed in its centre, the bottom of which has been raised by the force of vapours. A cone of eruption of this kind, formed by the accumulation of masses of rapilli and scoriæ, gradually came to view, above the south-eastern margin of the crater, between the years 1816 and 1818. The eruption in the month of February, 1822, increased this cone to such an elevation, that it projected from 107 to 117 feet above the north-western margin of the crater (the Rocca del Palo). This remarkable cone, which was at length regarded at Naples as the actual summit of Vesuvius, fell in with a fearful crash at the last eruption, on the night of the 22nd of October; in consequence of which, the bottom of the crater, which had continued uninterruptedly accessible from the year 1811, is now nearly 800 feet below the northern and 213 feet below the southern margin of the volcano. The varying form and relative position of the cones of eruption, the apertures of which must not, as they sometimes are, be confounded with the crater of the volcano, give to Vesuvius at different epochs a peculiar physiognomy; so much so, that the historiographer of this volcano, by a mere inspection of Hackert’s landscapes in the Palace of Portici, might guess the exact year in which the artist had made his sketch, by the outline of the summit of the mountain, according as the northern or southern side is represented in respect to height.

Twenty-four hours after the fall of the cone of scoriæ, which was 426 feet high, and when the small but numerous streams of lava had flowed off, on the night between the 23rd and 24th of October, there began a fiery eruption of ashes and rapilli, which continued uninterruptedly for twelve days, but was most violent during the first four days. During this period the explosions in the interior of the volcano were so loud that the mere vibrations of the air caused the ceilings to crack in the Palace of Portici, although no shocks of an earthquake were then or had previously been experienced. A remarkable phenomenon was observed in the neighbouring villages of Resina, Torre del Greco, Torre del’ Annunziata, and Bosche Tre Case. Here the atmosphere was so completely saturated with ashes that the whole region was enveloped in complete darkness during many hours in the middle of the day. The inhabitants were obliged to carry lanterns with them through the streets, as is often done in Quito during the eruptions of Pichincha. Never had the flight of the inhabitants been more general, for lava streams are less dreaded even than an eruption of ashes, a phenomenon unknown here in any degree of intensity, and one which fills the imaginations of men with images of terror from the vague tradition of the manner in which Herculaneum, Pompeii, and Stabiæ were destroyed.

The hot aqueous vapour which issued from the crater during the eruption, and diffused itself through the atmosphere, formed, on cooling, a dense cloud, which enveloped the column of ashes and fire, that rose to an elevation of between 9000 and 10,000 feet above the level of the sea. So sudden a condensation of vapour, and, as Gay Lussac has shown, the formation of the cloud itself, tended to increase electric tension. Flashes of forked lightning darted in all directions from the column of ashes, while the rolling thunder might be clearly distinguished from the deep rumbling sounds within the volcano. In no other eruption had the play of the electric forces been so powerfully manifested as on this occasion.

On the morning of the 26th of October the strange report was circulated that a stream of boiling water was gushing from the crater, and pouring down the cone of cinders. Monticelli, the zealous and learned observer of the volcano, soon perceived that this erroneous report originated in an optical illusion, and that the supposed stream of water was a great quantity of dry ashes which issued like drift sand from a crevice in the highest margin of the crater. The long drought, which had parched and desolated the fields before this eruption of Vesuvius, was succeeded, towards the termination of the phenomenon, by a continued and violent rain, occasioned by the volcanic storm which we have just described. A similar phenomenon characterizes the termination of an eruption in all zones of the earth. As the cone of cinders is usually wrapped in clouds at this period, and as the rain is poured forth with most violence near this portion of the volcano, streams of mud are generally observed to descend from the sides in all directions. The terrified peasant looks upon them as streams of water that rise from the interior of the volcano and overflow the crater, while the deceived geologist believes that he can recognise in them either sea-water or muddy products of the volcano, the so-called eruptions boueuses, or, in the language of the old French systematisers, products of an igneo-aqueous liquefaction.

Where, as is generally the case in the chain of the Andes, the summit of the volcano penetrates beyond the snow-line, attaining sometimes an elevation twice as great as that of Mount Etna, the inundations we have described are rendered very frequent and destructive, owing to the melting and permeating snow.

These are phenomena which have a meteorological connection with the eruptions of volcanos, and are variously modified by the heights of the mountains, the circumference of the summits which are perpetually covered with snow, and the degree to which the walls of cinder cones become heated; but they cannot be regarded in the light of true volcanic phenomena. Subterranean lakes, communicating by various channels with the mountain streams, are frequently formed in deep and vast cavities, either on the declivity or at the base of volcanos. When the whole mass of the volcano is powerfully shaken by those earthquakes which precede all eruptions of fire in the Andes, the subterranean vaults open, and pour forth streams of water, fishes, and tuffaceous mud. This singular phenomenon brings to mind the Pimelodes Cyclopum, or the Silures of the Cyclops, which the inhabitants of the plateau of Quito call Preñadilla, and of which I gave a circumstantial account soon after my return to Europe. When, on the night between the 19th and 20th of June, 1698, the summit of Mount Carguairazo, situated to the north of Chimborazo, and having an elevation of more than 19,000 feet, fell in, all the country for nearly 32 square miles was covered with mud and fishes. A similar eruption of fish from the volcano of Imbaburu was supposed to have caused the putrid fever, which, seven years before this period, raged in the town of Ibarra.

I refer to these facts because they throw some light on the difference between the eruption of dry ashes and mud-like inundations of tuff and trass, investing fragments of wood, charcoal, and shells. The quantity of ashes recently erupted from Mount Vesuvius, like every phenomenon connected with volcanos and other great and fearful natural phenomena, has been greatly exaggerated in the public papers; and two Neapolitan chemists, Vicenzo Pepe and Guiseppe di Nobili, even asserted that the cinders were mixed with given proportions of gold and silver, notwithstanding the counter-statements of Monticelli and Covelli. According to my researches the stratum of ashes which fell during the twelve days was only three feet in thickness in the direction of Bosche Tre Case, on the declivity of the cone, where they were mixed with rapilli, while in the plains its greatest thickness did not exceed from 16 to 19 inches. Measurements of this kind must not be made at spots where the ashes have been drifted by the wind, like snow or sand, or where they have been accumulated in pulp-like heaps by means of water. The times are passed in which, after the manner of the ancients, nothing was regarded in volcanic phenomena save the marvellous, and when men would believe, like Ctesias, that the ashes from Etna were borne as far as the Indian peninsula. A portion of the Mexican gold and silver veins is certainly found in trachytic porphyry, but in the ashes of Vesuvius which I myself collected, and which were, at my request, examined by that distinguished chemist Heinrich Rose, no trace of either gold or silver was to be discovered.

However much these results, which perfectly correspond with the more exact observations of Monticelli, may differ from those recently announced, it cannot be denied that the eruption of ashes, which continued from the 24th to the 28th of October, is the most memorable that has been recorded, on unquestionable evidence, in reference to Mount Vesuvius, since the death of the elder Pliny. The quantity of ashes erupted on this occasion was probably three times as great as the whole quantity which has fallen since volcanic phenomena have been observed with attention in Italy. A stratum from 16 to 19 inches in thickness does certainly, at first sight, seem very inconsiderable, when compared with the mass with which we find Pompeii covered. But, without taking into account the heavy rains and the inundations which must have increased the bulk of this stratum in the course of ages, and without reviving the animated contention maintained with much scepticism on the other side of the Alps, regarding the causes of the destruction of the Campanian cities, it may, at any rate, be here observed that the eruptions of a volcano, at widely remote epochs, cannot be compared with respect to their intensity. All conclusions must be insufficient that are based on mere analogies of quantitative relations of the lava and ashes, the height of the column of smoke, and the intensity of the explosions.

We learn from the geographical description of Strabo, and from the opinion expressed by Vitruvius on the volcanic origin of pumice, that, until the year of Vespasian’s death, that is to say, until the eruption which buried Pompeii, Vesuvius appeared more like an extinct volcano than a Solfatara. When, after a long-continued repose, subterranean forces suddenly opened for themselves new channels, penetrating through strata of primitive rock and trachyte, effects must have been produced to which no analogy is afforded by those of subsequent occurrence. We clearly learn from the well-known letter in which Pliny the younger informs Tacitus of the death of his uncle, that the renewal of the eruptions, or, one might almost say, the revival of the slumbering volcano, began with an outbreak of ashes. The same phenomenon was observed at Xorullo, when the new volcano, in the month of September, 1759, breaking through strata of syenite and trachyte, was suddenly upheaved in the plain. The country people fled in terror on finding their cottages covered with ashes thrown up from the earth, which was bursting in every direction. In the ordinary periodical manifestations of volcanic activity a shower of ashes usually terminates each partial eruption. The letter of the younger Pliny contains, moreover, a passage which clearly shows that the dry ashes falling from the air immediately attained a height of four or five feet, independent of accumulation by drifts. “The court,” the narrative continues, “which led to the apartment in which Pliny took his siesta, was so filled with ashes and pumice that, had the sleeper tarried longer, he would have found the passage wholly blocked up.” Within the inclosed limits of a court the wind cannot have exercised any very considerable influence on the drifting of the ashes.

I have interrupted my comparative view of volcanos by different observations in relation to Vesuvius, partly on account of the great interest excited by its recent eruption, and partly because every great outpouring of ashes almost involuntarily recalls to mind the classic soil of Pompeii and Herculaneum. In a note, not adapted to be read to the audience to whom this lecture is addressed, I have collected all the elements of the barometric measurements which I made during the close of last year at Mount Vesuvius, and in the Campi Phlegræi.

We have hitherto considered the form and effects of those volcanos which are permanently connected, by means of a crater, with the interior of the earth. The summits of such volcanos are upheaved masses of trachyte and lava intersected by numerous veins. The permanency of their effects indicates a highly complex structure. They have, so to say, a certain individuality of character, which remains unaltered for long periods of time. Contiguous mountains generally yield wholly different products; for instance: leucitic and feldspathic lavas, obsidian with pumice, and basaltic masses containing olivine. They belong to the more recent phenomena of the earth, usually breaking through all the strata of the floetz formation, and their lava currents and products are of subsequent origin to our valleys. Their life, if I may be permitted to use a figurative expression, depends upon the mode and the duration of their connection with the interior of the earth. After continuing for centuries in a state of repose, their activity is often suddenly revived, and they then become converted into Solfataras, emitting aqueous vapours, gases, and acids. Occasionally, as at the Peak of Teneriffe, their summits have already become a laboratory of regenerated sulphur, while considerable lava currents, being basaltic near the base, and mixed with obsidian and pumice at greater elevations, where the pressure is less, continue to flow from the sides of the mountain^[109].

Besides volcanos which have permanent craters, there is another kind of volcanic phenomena less frequently observed than the former, but especially instructive to the geologist, as they remind us of the primitive world, that is, of the earliest revolutions of our planet. Trachytic mountains suddenly open, and after throwing up ashes and lava, close again never perhaps to re-open. Such has been the case with the mighty volcano of Antisana in the chain of the Andes, and with Mount Epomæus in Ischia, in the year 1302. Occasionally such an eruption has occurred even in the plains, as on the table-land of Quito, in Iceland at a distance from Hecla, and in the Lelantine plains of Eubœa. Many upheaved islands belong to this class of transitory phenomena. In these cases, the connection with the interior of the earth is not permanent, the action ceasing as soon as the fissure, or channel of communication, is again closed. Veins of basalt, dolerite, and porphyry, which traverse almost all formations in different parts of the earth; and the masses of syenite, augitic porphyry, and amygdaloid, which characterise the most recent strata of transition rock, and the oldest stratum of the floetz formation; have all probably been formed in a similar manner. In the youthful period of our planet, the substances that had continued in a fluid condition within the earth, broke through its crust, everywhere intersected with fissures, and became solidified as granular veins, or were spread out in broad superimposed strata. The products that may be termed exclusively volcanic, which have come down to us from the primitive ages of the world, have not flowed in streams or bands like the lava of our isolated conical mountains. The mixtures of augite, titanic iron, feldspar, and hornblende, may have been the same at different periods, sometimes allied to basalt, sometimes to trachyte; while chemical substances, (as we learn from Mitscherlich’s important labours and the analogies presented by artificial igneous products,) may have ranged themselves in layers according to some definite laws of crystallization. In all cases we perceive that substances similarly composed have come to the surface of the earth by very different means, either by being simply upheaved, or escaping through temporary fissures; and that breaking through the older rocks, that is to say, through the earlier oxidized earth’s crust, they have flowed in the form of lava streams from conical mountains having a permanent crater. If we do not sufficiently distinguish between these various phenomena, our knowledge of the geology of volcanos will again be shrouded in that obscurity, from which numerous comparative experiments are now beginning gradually to release it.

The questions have often been asked, what is it that burns in volcanos, what generates the degree of heat capable of mixing earths and metals together in a state of fusion? Modern chemistry has attempted to reply that it is the earths, metals, and alkalies themselves, that is to say, the metalloids of these substances, which burn. The solid and already oxidized crust of the earth separates the surrounding atmosphere, with the oxygen it contains, from the combustible unoxidized substances in the interior of our planet. By the contact of these metalloids with the atmospheric oxygen the disengagement of caloric ensues. The celebrated and talented chemist, who advanced this explanation of volcanic phenomena, soon himself relinquished it. The experiments which have been made in mines and caverns in all parts of the earth, and which M. Arago and myself have collected in a separate treatise, prove that even at an inconsiderable depth, the temperature of the earth is much higher than the mean temperature of the atmosphere at the same place. This remarkable, and almost universally confirmed fact, is connected with what we learn from volcanic phenomena. The depth at which we might regard the earth as a fused mass, has been calculated. The primitive cause of this subterranean heat is, as in all planets, the formative process itself, the separation of the spherically conglomerating mass from a cosmical aëriform fluid, and the cooling of the terrestrial strata at different depths by the radiation of heat. All volcanic phenomena are probably the result of a permanent or transient connection between the interior and the exterior of our planet. Elastic vapours press the fused oxidizing substances upwards through deep fissures. Volcanos therefore are intermittent earth-springs, from which the fluid mixtures of metals, alkalies, and earths, which become consolidated into lava currents, flow gently and calmly, when being upheaved they find a vent. In a similar manner, according to Plato’s Phædon, the ancients regarded all volcanic streams of fire as effusions of the Pyriphlegethon.

I would fain be permitted to add one yet bolder observation to those I have already ventured to advance. May not the cause of one of the most wonderful phenomena presented by the study of petrifactions, be dependent on the condition of the inner heat of our planet, which is indicated by thermometric experiments on springs^[110] rising from different depths, and by observations on volcanos? We find tropical animals, arborescent ferns, palms, and bamboos, buried in the cold north, and everywhere the primitive world presents a distribution of organic structures wholly at variance with existing climatic relations. Many hypotheses have been advanced in elucidation of so important a problem, such as the approximation of a comet, the altered obliquity of the ecliptic, and the increased intensity of the sun’s light; but none of these have satisfied at once the astronomer, the physicist, and the geologist. I, for my part, would willingly leave undisturbed the axis of the earth or the light of the sun’s disk, (from whose spots a celebrated astronomer explained fruitfulness and failure of crops,) yet it appears to me that in every planet there exist, independently of its relations to a central body and its astronomical position, numerous causes for the development of heat, in processes of oxidation, in precipitation, in the chemically altered capacity of bodies, the increase of electro-magnetic tension, and in the channels of communication opened between its internal and external parts.

Wherever, in the primitive world, heat was radiated from the deeply fissured crust of the earth, palms, arborescent ferns, and all the animals of the torrid zone, could perhaps have flourished for centuries over extensive tracts of land. According to this view, which I have already published in my work entitled Geognostischer Versuch über die Lagerung der Gebirgsarten in beiden Hemisphären,^[RG] the temperature of volcanos would be that of the interior of our earth itself, and the same causes which now occasion such fearfully devastating results, may have been able to produce, in every zone, the most luxuriant vegetation on the newly oxidized crust of the earth and on the deeply fissured strata of rocks.

Should it be assumed, for the purpose of explaining the wonderful distribution of tropical forms in their ancient mausolea, that the long-haired elephantine animals, which are now found embedded in ice, were once indigenous to northern latitudes, and that animals of similar forms, belonging to the same type, as, for instance, lions and lynxes, were capable of living in wholly different climates, such a mode of explanation would at all events not admit of being extended to vegetable products. From causes developed by the physiology of vegetation, palms, bananas, and arborescent monocotyledons, are unable to endure the deprivation of their appendicular organs, by the northern cold; and in the geological problem which we are here considering, it seems to me a matter of difficulty to admit any distinction between vegetable and animal structures. One and the same mode of explanation must be applied to both forms.

In concluding this treatise, I have added some uncertain and hypothetical conjectures to the facts which have been collected in widely remote regions of the earth. The philosophical study of nature rises above the requirements of mere delineation, and does not consist in the sterile accumulation of isolated facts. The active and inquiring spirit of man may therefore be occasionally permitted to escape from the present into the domain of the past, to conjecture that which cannot yet be clearly determined, and thus to revel amid the ancient and ever-recurring myths of geology.