The greatest caves known—new ones are constantly being discovered—are of limestone, and of comparatively recent origin. Geology teaches that the primary formations of caves are many, though small, being produced by the action of water coursing through the strata, and that the continuation of this process for ages creates the vast and beautiful chambers, which all of us are so fond of exploring. Sweden and Norway boast of granite vaults, especially Marienstadt, of extraordinary dimensions, though some of them have been, as yet, but partially penetrated. The vicinity of Quito contains caves of modern porphyry, and the Isle of France caves of lava. Gurtshellir is a cavern of lava in Iceland, forty feet high, fifty broad, and one mile long. The caves of Agtelek in Hungary, and of Adelsberg in Carniola,—the latter noted for its transparent white pillars and brilliant stalactites,—are among the most remarkable in Europe. Adelsberg has an unusual interest for naturalists, because a strange reptile, called the proteus, half a lizard and half an eel, has its habitat there. It has an extremely elastic constitution, and an extraordinary adaptability, as may be inferred from the fact that it subsists equally well on land or in water, imbedded in rock or buried in mud, requiring neither air nor light, food nor drink, for the sustainment of its existence. What an excellent _littérateur_ the proteus would be as respects its limited necessities! If it happened to be an unappreciated genius, like most literary men, it need not feel any concern, for it could afford to wait until the world had come round to it, and the age had grown worthy of its thought. Fame, being a bubble, and therefore air, could not injure the nondescript creature, nor could the throwing of mud, as is the custom of journalists, mar it in the least. Much as it might be in (hot) water, it would not be troubled, and as to detraction and misrepresentation, its house would be (occasionally at least) built upon a rock, and would therefore stand firm. [Sidenote: THE CAVE OF GUACHARO.]