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).