As another illustration of the solidity of the walls of an advancing lava stream, I may mention an adventure related by Recupero, who, in 1766, had ascended a small hill formed of ancient volcanic matter, to behold the slow and gradual approach of a fiery current, two miles and a half broad; when suddenly two small threads of liquid matter issuing from a crevice detached themselves from the main stream, and ran rapidly towards the hill. He and his guide had just time to escape, when they saw the hill, which was fifty feet in height, surrounded, and in a quarter of an hour melted down into the burning mass, so as to flow on with it.
But it must not be supposed that this complete fusion of rocky matter coming in contact with lava is of universal, or even common, occurrence. It probably happens when fresh portions of incandescent matter come successively in contact with fusible materials. In many of the dikes which intersect the tuffs and lavas of Etna, there is scarcely any perceptible alteration effected by heat on the edges of the horizontal beds, in contact with the vertical and more crystalline mass. On the side of Mompiliere, one of the towns overflowed in the great eruption above described, an excavation was made in 1704; and by immense labor the workmen reached, at the depth of thirty-five feet, the gate of the principal church, where there were three statues, held in high veneration. One of these, together with a bell, some money, and other articles, were extracted in a good state of preservation from beneath a great arch formed by the lava. It seems very extraordinary that any works of art, not encased with tuff, like those in Herculaneum, should have escaped fusion in hollow spaces left open in this lava-current, which was so hot at Catania eight years after it entered the town, that it was impossible to hold the hand in some of the crevices.
Subterranean caverns on Etna.—Mention was made of the entrance of a lava-stream into a subterranean grotto, whereby the foundations of a hill were partially undermined. Such underground passages are among the most curious features on Etna, and appear to have been produced by the hardening of the lava, during the escape of great volumes of elastic fluids, which are often discharged for many days in succession, after the crisis of the eruption is over. Near Nicolosi, not far from Monti Rossi, one of these great openings may be seen, called the Fossa della Palomba, 625 feet in circumference at its mouth, and seventy-eight deep. After reaching the bottom of this, we enter another dark cavity, and then others in succession, sometimes descending precipices by means of ladders. At length the vaults terminate in a great gallery ninety feet long, and from fifteen to fifty broad, beyond which there is still a passage, never yet explored; so that the extent of these caverns remains unknown.[566] The walls and roofs of these great vaults are composed of rough and bristling scoriæ, of the most fantastic forms.
Marine strata at base of Etna.—If we skirt the fertile region at the base of Etna on its southern and eastern sides, we behold marine strata of clay sand, and volcanic tuff, cropping out from beneath the modern lavas. The marine fossil shells occurring in these strata are all of them, or nearly all, identical with species now inhabiting the Mediterranean; and as they appear at the height of from 600 to 800 feet above the sea near Catania, they clearly prove that there has been in this region, as in other parts of Sicily farther to the south, an upward movement of the ancient bed of the sea. It is fair, therefore, to infer that the whole mountain, with the exception of those parts which are of very modern origin, has participated in this upheaval.
If we view Etna from the south, we see the marine deposits above alluded to, forming a low line of hills (e, e, Fig. 47), or a steep inland slope or cliff (f), as in the annexed drawing taken from the limestone platform of Primosole. It should be observed however, in reference to this view, that the height of the volcanic cone is ten times greater than the hills at its base (e, e), although it appears less elevated, because the summit of the cone is ten or twelve times more distant from the plain of Catania than is Licodia.
View of Etna from the summit of the limestone platform of Primosole.
a, Highest cone. b, Montagnuola.
c, Monte Minardo, with smaller lateral cones above.