The study of volcanoes may be divided into two distinct branches; one, simply mineralogical, is directed to the examination of the stony strata, altered or produced by the action of fire; from the formation of the trachytes or trap-porphyries, of basalts, phonolites, and dolerites, to the most recent lavas: the other branch, less accessible and more neglected, comprehends the physical relations which link volcanoes together, the influence of one volcanic system on another, the connection existing between the action of burning mountains and the commotions which agitate the earth at great distances, and during long intervals, in the same direction. This study cannot progress till the various epochs of simultaneous action, the direction, the extent, and the force of the convulsions are carefully noted; till we have attentively observed their progressive advance to regions which they had not previously reached; and the coincidence between distant volcanic eruptions and those noises which the inhabitants of the Andes very expressively term subterraneous thunders, or roarings.* (* Bramidos y truenos subterraneos.) All these objects are comprehended in the domain of the history of nature.
Though the narrow circle within which all certain traditions are confined, does not present any of those general revolutions which have heaved up the Cordilleras and buried myriads of pelagian animals; yet Nature, acting under our eyes, nevertheless exhibits violent though partial changes, the study of which may throw light on the most remote epochs. In the interior of the earth those mysterious powers exist, the effects of which are manifested at the surface by the production of vapours, of incandescent scoriae, of new volcanic rocks and thermal springs, by the appearance of new islands and mountains, by commotions propagated with the rapidity of an electric shock, finally by those subterranean thunders,* heard during whole months, without shaking the earth, in regions far distant from active volcanoes. (* In the town of Guanaxuato, in Mexico, these thunders lasted from the 9th of January till the 12th of February, 1784. Guanaxuato is situated forty leagues north of the volcano of Jorullo, and sixty leagues north west of the volcano of Popocatepetl. In places nearer these two volcanoes, three leagues distant from Guanaxuato, the subterranean thunders were not heard. The noise was circumscribed within a very narrow space, in the region of a primitive schist, which approaches a transition-schist, containing the richest silver mines of the known world, and on which rest trap-porphyries, slates, and diabasis (grunstein.))
In proportion as equinoctial America shall increase in culture and population, and the system of volcanoes of the central table-land of Mexico, of the Caribbee Islands, of Popayan, of los Pastos, and Quito, shall be more attentively observed, the connection of eruptions and of earthquakes, which precede and sometimes accompany those eruptions, will be more generally recognized. The volcanoes just mentioned, particularly those of the Andes, which rise above the enormous height of two thousand five hundred toises, present great advantages for observation. The periods of their eruptions are singularly regular. They remain thirty or forty years without emitting scoriae, ashes, or even vapours. I could not perceive the smallest trace of smoke on the summit of Tunguragua or Cotopaxi. A gust of vapour issuing from the crater of Mount Vesuvius scarcely attracts the attention of the inhabitants of Naples, accustomed to the movements of that little volcano, which throws out scoriae sometimes during two or three years successively. Thence it becomes difficult to judge whether the emission of scoriae may have been more frequent at the time when an earthquake has been felt in the Apennines. On the ridge of the Cordilleras everything assumes a more decided character. An eruption of ashes, which lasts only a few minutes, is often followed by a calm of ten years. In such circumstances it is easy to mark the periods, and to observe the coincidence of phenomena.
If, as there appears to be little reason to doubt, that the destruction of Cumana in 1797, and of Caracas in 1812, indicate the influence of the volcanoes of the West India Islands* on the commotions felt on the coasts of Terra Firma, it may be desirable, before we close this chapter, to take a cursory view of this Mediterranean archipelago.
(* The following is the series of the phenomena:—
27th of September, 1796. Eruption in the West India Islands. (Volcano of Guadaloupe).
November, 1796. The volcano of Pasto began to emit smoke.
14th of December, 1796. Destruction of Cumana.
4th of February, 1797. Destruction of Riobamba.
30th of January, 1811. Appearance of Sabrina Island, in the Azores. The island enlarged very considerably on the 15th of June, 1811.