Similar lakes or Maare occur in Auvergne, in Java, in the Canary Islands, in New Zealand, and in the volcanic districts of Italy. The beautiful lakes of Albano and Nemi, which have been so often sung by ancient and modern poets, belong to this class; but Fr. Hoffmann, a celebrated German geologist, ascribes the origin of the former to a landslip caused by the falling in of the roof of a vast subterranean cavern. [Illustration: STALACTITE CAVERN AT AGGTELEK, HUNGARY: THE CAVE OF BORODLA.] CHAPTER XII. ON CAVES IN GENERAL. Their various Forms—Natural Tunnels—The Ventanillas of Gualgayoc—Eimeo— Torgatten—Hole in the Mürtschenstock—The Trebich Cave—Grotto of Antiparos—Vast Dimensions of the Cave of Adelsberg and of the Mammoth Cave—Discovery of Baumann’s Cave—Limestone Caves—Causes of their Excavation—Stalactites and Stalagmites—Their Origin—Variety of Forms— Marine Caves—Shetland—Fingal’s Cave—The Azure Cave—Cave under Bonifacio—Grotta di Nettuno, near Syracuse—The Bufador of Papa Luna— Volcanic Caves—The Fossa della Palomba—Caves of San Miguel—The Surtshellir. The natural excavations which abound in many mountain chains, or on rocky shores washed by the stormy sea, are extremely various in their forms. Many are mere rents or crevices in the disruptured rocks; others wide vaults, not seldom of hall or dome-like dimensions, or long and narrow passages branching out in numerous ramifications. Not seldom the same cave alternately expands into spacious chambers, and then again contracts into narrow tunnels or galleries. The walls of many are smooth and nearly parallel; the sides of others are irregular and rugged. Many have narrow entrances and swell at greater depths into majestic proportions; while others open with wide portals, and gradually diminish in size as they penetrate into the rock. Sometimes an excavation pierces a mountain from side to side like a natural tunnel, so as to allow a passage to the light of day. Such, among others, are the numerous perforations or _windows_ (ventanillas) in the serrated bastions of the rich silver mountain Gualgayoc in the Peruvian Andes, or the opening through one of the high peaks of the romantic island of Eimeo which rises within sight of Tahiti out of the dark blue ocean. According to a popular tradition, this hole owes its origin to Oro, the powerful god of war, who, having one day quarrelled with the minor god of Eimeo, hurled his mighty spear at him over the sea. As even gods, when losing their temper, are apt to miss their aim, the puny delinquent escaped unhurt, while the dreadful lance flew like a thunderbolt through the mountain, leaving the perforation as a lasting memorial of its passage. In Europe we likewise meet with several remarkable instances of such natural tunnels. One of the most celebrated is the grotto of Torgatten in Norway, which perforates a huge rock, 400 feet above the level of the sea.