According to Sir Charles Lyell, the sudden conversion into steam of lakes or streams of water, overwhelmed by a fiery current, may perhaps explain the formation of many of the extensive underground passages or caverns which form a common feature in the structure of a volcano. Great volumes of vapour, thus produced, may force their way through liquid lava, already coated over externally with a solid crust, and may cause the sides of such passages, as they harden, to assume a very irregular outline. The famous cave on Etna, called the Fossa della Palomba, which opens near Nicolosi, not far from Monte Rossi, has not improbably been thus formed. After reaching the bottom of a hollow 625 feet in circumference at its mouth, and 78 feet deep, the explorer enters 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 feet broad, beyond which there is still a passage never yet visited, so that the whole extent of the Fossa remains unknown. The volcanic caves of Punta Delgada in the Island of San Miguel, one of the Azores, are still more grand. Their entrance is through a narrow crevice, which soon, however, expands into an enormous hall, whose vault even the strongest torch-light is unable to illumine. In one spot, an opening in the floor shows that the lava, which is here but a foot thick, forms the roof of a second cave, situated below the first, into which even the boldest explorer has never ventured, but the noise of stones cast into the abyss proves it to be of considerable size. The first-mentioned cave leads into another, the width of which is estimated by Webster[16] at 120 feet; but the height he was unable to measure. Gradually this cave becomes narrower and lower, until, about 450 feet from the entrance, it terminates in a low vault. Black lava stalactites everywhere hang down from the roof, and the floor is so covered with sharp-sided blocks of the same volcanic material that walking among them becomes a task of the greatest difficulty. Footnote 16: ‘Description of the Island of Saint Michael.’