An enormous hollow, covering an area of ten square miles, and more than 3,000 feet in depth, occupies a portion of the western slope of the volcano. This is the Val di Bove, a vast amphitheatre of explosion, the bottom of which is dotted over with subsidiary craters, and which rises in gigantic steps, over which, when the mountain is in a state of eruption, pour fiery cascades of lava. Lyell has shown that this Val di Bove is the ancient crater of Mount Etna, but that, at some period not known to us, the existing terminal vent opened a couple of miles farther west. The steep sides of the Val di Bove enable us to gain a considerable insight into the history of the volcano, for the various layers of lava may be studied there at leisure. The cliffs upon which stands the town of Aci Reale afford a similar opportunity for embracing at one glance a long period of its history. These cliffs, over 300 feet in height, consist of seven distinct layers of lava, successively poured forth from the bowels of Mount Etna. Each layer consists nearly throughout of a compact mass, affording no hold for the roots of plants, but their surfaces have invariably been converted into tufa, or even mould, owing to atmospheric agencies which operated for centuries after each eruption. It has likewise been proved not only that these cliffs increased in height in consequence of successive eruptions, but that they were also repeatedly upheaved from below. Lines of erosion resulting from the action of the waves can be distinctly traced at various elevations above the present level of the Me­di­ter­ra­nean. The lavas, too, have undergone a change of structure since they were poured forth, as is proved by beautiful caverns enclosed by prismatic columns of basalt, and by the islet of the Cyclops, near Aci Trezza.

During the last two thousand years Mount Etna has had more than a hundred eruptions, some of them continuing for a number of years. Hitherto it has not been possible to trace any regularity in these eruptions. They appear to occur at irregular intervals, and the quantity of lava poured forth from the principal or any subsidiary cone varies exceedingly. The most considerable stream of lava of which we have any record was that which overwhelmed the city of Catania in 1669. It first converted the fields of Nicolosi into a fiery lake, then enveloped a portion of the hill of Monpilieri, which for a time arrested its progress, and finally divided into three separate streams, the principal of which descended upon Catania. It swept away a part of that town, filled up its port, and formed a promontory in its stead. The quantity of lava poured forth on that occasion has been estimated at 3,532 millions of cubic feet; and nearly 40 square miles of fertile land, supporting a population of 20,000 souls, were converted into a stony waste. The double cone of Monti Rossi, with its beautiful crater now grown over with golden-flowered broom, was formed by the ashes ejected during that great eruption. More than 700 subsidiary cones, similar to the Monti Rossi, are scattered over the exterior slopes of Mount Etna, and bear witness to as many eruptions. The most ancient amongst them have been nearly obliterated in the {313} course of ages, or buried beneath streams of lava, but the others still retain their conical shape, and rise to a height of many hundred feet. Several amongst them are now covered with forests, and the craters of others have been converted into gardens—delightful cup-shaped hollows, where villas shine like gems set in verdure.

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Fig. 115.—THE LAVA STREAM OF CATANIA.

Scale 1 : 200,000.

Most of these subsidiary cones lie at an elevation of between 3,300 and 6,500 feet above the sea, and it is there the internal forces make themselves most strongly felt. As a rule the subterranean activity is less violent near the summit, and during most of the eruptions the great terminal crater merely serves as a vent, through which the aqueous vapours and gases make their escape. Fumaroles surrounding it convert the soil into a kind of pap, and the substances which escape from them streak the scoriæ with brilliant colours—scarlet, yellow, and emerald green. The internal heat makes itself felt on many parts of the exterior slopes. It converts loose rocks into a compact mass, far less difficult to climb than are the loose cinders of Mount Vesuvius. Travellers ascending the mountain need fear nothing from volcanic bombs. Showers of stone are occasionally ejected from the principal vent, but this is quite an exceptional occurrence. If it were not so, the small structure above the precipices of the Val di Bove, which dates from the {314} time of the Romans, and is known as the “Philosopher’s Tower,” would long ago have been buried beneath débris. A meteorological observatory might therefore be established with safety on the summit of this mountain, and no better station could be found for giving warning of approaching storms.

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Fig. 116.—SUBSIDIARY CONES OF MOUNT ETNA.