CHAPTER II.
ETNA.
(a.) Structure of the Mountain.—Etna, unlike Vesuvius, has ever been a burning mountain; hence it was well known as such to classic writers before the Christian era. The structure and features of this magnificent mountain have been abundantly illustrated by Elie de Beaumont,[1] Daubeny,[2] Baron von Waltershausen,[3] and Lyell,[4] of whose writings I shall freely avail myself in the following account, not having had the advantage of a personal examination of this region.
Structure of Etna.—So large is Etna that it would enclose within its ample skirts several cones of the size of Vesuvius. It rises to a height of nearly 11,000 feet above the waters of the Mediterranean,[5] and is planted on a floor consisting of stratified marine volcanic matter, with clays, sands, and limestones of newer Pliocene age. Its base is nearly circular, and has a circumference of 87 English miles. In ascending its flanks we pass successively over three well-defined physical zones: the lowest, or fertile zone, comprising the tract around the skirts of the mountain up to a level of about 2500 feet, being well cultivated and covered by dwellings surrounded by olive groves, fields, vineyards, and fruit-trees; the second, or forest zone, extending to a level of about 6270 feet, clothed with chestnut, oak, beech, and cork trees, giving place to pines; and the third, extending to the summit and called "the desert region," a waste of black lava and scoriæ with mighty crags and precipices, terminating in a snow-clad tableland, from which rises the central cone, 1100 feet high, emitting continually steam and sulphurous vapours, and in the course of almost every century sending forth streams of molten lava.
The forest zone is remarkable for the great number of minor craters which rise up from the midst of the foliage, and are themselves clothed with trees. Sartorius von Waltershausen has laid down on his map of Etna about 200 of these cones and craters, some of which, like those of Auvergne, have been broken down on one side. Many of these volcanoes of second or third magnitude lie outside the forest zone, both above and below it; such as the double hill of Monti Rossi, near Nicolosi, formed in 1659, which is 450 feet in height, and two miles in circumference at its base. Sir C. Lyell observes that these minor crater-cones present us with one of the most delightful and characteristic scenes in Europe. They occur of every variety of height and size, and are arranged in picturesque groups. However uniform they may appear when seen from the sea or the plains below, nothing can be more diversified than their shape when we look from above into their ruptured craters. The cones situated in the higher parts of the forest zone are chiefly clothed with lofty pines; while those at a lower elevation are adorned with chestnuts, oaks, and beech trees. These cones have from time to time been buried amidst fresh lava-streams descending from the great crater, and thus often become obliterated.
| Fig. 10.—Ideal Section through Etna. (After Lyell.)—A. Axis of present cone of eruption; B. Axis of extinct cone of eruption; a. Older lavas, chiefly trachytic; b. Newer lavas, erupted (with a) before origin of the Val del Bove; c. Scoria and lava of recent age; T. Tertiary strata forming the foundation to the volcanic rocks. The position of the Val del Bove before its formation is shown by the lightly-shaded portion above B. |
(b.) Val del Bove.—The most wonderful feature of Mount Etna is the celebrated Val del Bove (Valle del Bue), of which S. von Waltershausen has furnished a very beautiful plate[6]—a vast amphitheatre hewn out of the eastern flank of the mountain, just below the snow-mantled platform. It is a physical feature somewhat after the fashion of Monte Somma in Vesuvius, but exceeds it in magnitude as Etna exceeds Vesuvius. The Val del Bove is about five miles in diameter, bounded throughout three-fourths of its circumference by precipitous walls of ashes, scoriæ, and lava, traversed by innumerable dykes, and rising inwards to a height of between 3000 and 4000 feet. Towards the east the cliffs gradually fall to a height of about 500 feet, and at this side the vast chasm opens out upon the slope of the mountain. At the head of the Val del Bove rises the platform, surmounted by the great cone and crater. It will thus be seen that by means of this hollow we have access almost to the very heart of the mountain.
What is very remarkable about the structure of this valley is that the beds exhibit "the quâ-quâ versal dip"—in other words, they dip away on all sides from the centre—which has led to the conclusion that in the centre is a focus of eruption which had become closed up antecedently to the formation of the valley itself. Lyell has explained this point very clearly by showing that this focus had ceased to eject matter at some distant period, and that the existing crater at the summit of the mountain had poured out its lavas over those of the extinct orifice. This was prior to the formation of the Val del Bove itself; and the question remains for consideration how this vast natural amphitheatre came to be hollowed out; for its structure shows unquestionably that it owes its form to some process of excavation.
In the first place, it is certainly not the work of running water, as in the case of the cañons of Colorado; the porous matter of which the mountain is formed is quite incapable of originating and supporting a stream of sufficient volume to excavate and carry away such enormous masses of matter within the period required for the purpose. We must therefore have recourse to some other agency. Numerous illustrations are to be found of the explosive action of volcanoes in blowing off either the summits of mountains, or portions of their sides. For example, there is reason for believing that the first result of the renewed energy of Vesuvius was to blow into the air the upper surface of the mountain. Again, so late as 1822, during a violent earthquake in Java, a country which has been repeatedly devastated by earthquakes and volcanic eruptions, the mountain of Galongoon, which was covered by a dense forest, and situated in a fertile and thickly-peopled region, and had never within the period of tradition been in activity, was thus ruptured by internal forces. In the month of July 1822, after a terrible earthquake, an explosion was heard, and immense columns of boiling water, mixed with mud and stones, were projected from the mountain like a water-spout, and in falling filled up the valleys, and covered the country with a thick deposit for many miles, burying villages and their inhabitants. During a subsequent eruption great blocks of basalt were thrown to a distance of seven miles; the result of all being that an enormous semicircular gulf was formed between the summit and the plain, bounded by steep cliffs, and bearing considerable resemblance to the Val del Bove. Other examples of the power of volcanic explosions might be cited; but the above are sufficient to show that great hollows may thus be formed either on the summits or flanks of volcanic mountains. Chasms may also be formed by the falling in of the solidified crust, owing to the extrusion of molten matter from some neighbouring vent of eruption; and it is conceivable that by one or other of these processes the vast chasm of the Val del Bove on the flanks of Etna may have been produced.
(c.) The Physical History of Etna.—The physical history of Etna seems to be somewhat as follows:—