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. Footnote 17: ‘The Polar World,’ p. 58. 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.