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.
While a lava-stream is flowing along, a slag-crust forms on its surface, inclosing the internal fluid matter as in a canal. But when the supply of fresh lava from the vent diminishes or entirely ceases, the still liquid interior at the central part of the current continuing for some time to flow on, often leaves behind hollow gutters, arched over by a thin and brittle roof—so thin sometimes as to yield to the weight of a person stepping on them. Such vaulted roofs have pseudo-stalactitic projections left by the subsidence of the liquid, and are coated with a glossy varnish. Sometimes, very large caverns are thus formed beneath the surface of a lava-stream, and even rival in their extent and windings the caves worn by water in limestone rocks. The famous Surtshellir,[[17]] situated near Kalmanstunga in Iceland, is a remarkable instance of a vast lava excavation owing its origin to this cause. It has very appropriately been named after Surt, the prince of darkness and fire, of the ancient Scandinavian mythology; for this gloomy deity could not possibly have chosen a fitter residence than its vast and dismal halls, once glowing with subterranean fires, and now the seat of perpetual darkness.
CHAPTER XIII.
CAVE RIVERS.
The Fountain of Vaucluse—The Fontaine-sans-fond—The Katabothra in Morea—Subterranean Rivers in Carniola—Subterranean Navigation of the Poik in the Cave of Planina—‘The Stalactital Paradise’—The Piuka Jama.
Wherever large bodies of water gush forth in a rapid stream from the bowels of the earth, they must either have flowed through wide underground channels, or they must come from extensive lake-like reservoirs, for the mere drainage of a porous stratum is evidently incapable of accounting for their production.
Thus the celebrated fountain of Vaucluse, near Avignon, which has the volume and power of a river at its very source, is undoubtedly fed by a subterranean sheet of water of considerable extent. Even when least abundant, it pours forth upwards of 13,000 cubic feet of water in a minute; and after the country has been flooded with abundant rains, this volume is increased fourfold. The environs of the fountain are extremely picturesque, and justify the praises which have been lavished upon it in the immortal strains of Petrarch. It fills a large oval basin, vaulted by a spacious cave, and its waters, which, when low, escape through subterranean channels into the deep bed of the Sorgue, rise, when high, over the rock-wall at the mouth of the grotto, and form a broad cataract, rushing down with a dreadful noise.
Near Sable in Anjou, a source, or rather a pit from eighteen to twenty-four feet in diameter, well known in the country under the name of the Bottomless Fountain (Fontaine-sans-fond) sometimes overflows its brink, and then casts forth a large quantity of fish, so that it is evidently a mere aperture in the vault of a large subterranean pool.
In the department of the Haute Saône, another pit, called the Frais Puits, presents a similar phenomenon. After abundant rains, the water gushing forth from its mouth inundates the neighbourhood, and on its retiring, pikes are not seldom found scattered over the surface of the flooded fields or meadows, a sure proof that the Puits must communicate with a large subterranean cavity.
Nothing is more common in limestone districts than the engulfment of rivers, which, after holding a subterranean course of many miles, escape again by some new outlet. The Guadiana loses itself in a flat country in the midst of an immense savannah, and hence the Spaniards, when the magnificent bridges of London or Paris are mentioned to them, boastfully reply that they have one in Estramadura on which more than a hundred thousand oxen graze at a time.