Aspects of nature, in different lands and different climates (Vol. 2 of 2) / with scientific elucidations

by Alexander von Humboldt

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.

ANNOTATIONS AND ADDITIONS.

[38] p. 226.—“A more complete determination of the height of all parts of the margin of the crater.”

Oltmanns, my astronomical fellow labourer, of whom, alas! science has been early deprived, re-calculated the barometric measurements of Vesuvius referred to in the preceding memoir (of the 22d and 25th of November and of the 1st of December, 1822), and has compared the results with the measurements which have been communicated to me in manuscript by Lord Minto, Visconti, Monticelli, Brioschi, and Poulett Scrope.

A. Rocca del Palo, the highest and northern margin of the Crater of Vesuvius.