The contrast so often presented between a striking agreement of physiognomy and the greatest diversity in the inflorescence and fructification,—between the external aspect as determined by the appendicular or leaf-system, and the reproductive organs on which the groups of the natural systems of botany are founded,—is a remarkable and surprising phenomenon. We should have been inclined beforehand to imagine that the shape of what are exclusively termed the vegetative organs (for example, the leaves) would have been less independent of the structure of the organs of reproduction; but in reality such a dependence only shows itself in a small number of families,—in Ferns, Grasses and Cyperaceæ, Palms, Coniferæ, Umbelliferæ, and Aroideæ. In Leguminosæ the agreement in physiognomic character is scarcely to be recognised until we divide them into the several groups (Papilionaceæ, Cæsalpinineæ, and Mimoseæ). I may name, of types which, when compared with each other, shew considerable accordance in physiognomy with great difference in the structure of the flowers and fruit, Palms and Cycadeæ, the latter being more nearly allied to Coniferæ; Cuscuta, one of the Convolvulacæ, and the leafless Cassytha, a parasitical Laurinea; Equisetum (belonging to the great division of Cryptogamia), and Ephedra, closely allied to Coniferæ. On the other hand, our common gooseberries and currants (Ribes) are so closely allied by their inflorescence to the Cactus, i. e. to the family of Opuntiaceæ, that it is only quite recently that they have been separated from it! One and the same family (that of Asphodeleæ) comprises the gigantic Dracæna draco, the common asparagus, and the Aletris with its coloured flowers. Not only do simple and compound leaves often belong to the same family, but they even occur in the same genus. We found in the high plains of Peru and New Granada, among twelve new species of Weinmannia, five with “foliis simplicibus,” and the rest with pinnate leaves. The genus Aralia shews still greater independence in the form of the leaves: “folia simplicia, integra, vel lobata, digitata et pinnata.” (Compare Kunth, Synopsis Plantarum quas in itinere collegerunt, Al. de Humboldt et Am. Bonpland, T. iii, p. 87 and 360.)
Pinnated leaves appear to me to belong chiefly to families which are in the highest grade of organic development, namely, the Polypetalæ; and among these, in the Perigynic class, to the Leguminosæ, Rosaceæ, Terebinthaceæ, and Juglandeæ; and in the Hypogynic, to the Aurantiaceæ, Cedrelaceæ, and Sapindaceæ. The beautiful doubly-pinnated leaves which form one of the principal ornaments of the torrid zone, are most frequent among the Leguminosæ, in Mimoseæ, also in some Cæsalpinieæ, Coulterias, and Gleditschias; never, as Kunth remarks, in Papilionaceæ. “Folia pinnata” and “folia composita” are never found in Gentianeæ, Rubiaceæ, and Myrtaceæ. In the morphological development presented by the abundance and variety of form in the appendicular organs of Dicotyledones, we can at present discern only a small number of general laws.
ON THE
STRUCTURE AND MODE OF ACTION
OF
VOLCANOS,
IN DIFFERENT PARTS OF THE GLOBE.
[This dissertation was read in a public assembly of the Academy at Berlin, on the 24th of January, 1823.]
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.