Lake Bolsena discharges its surplus waters through the Marta into the Mediterranean. The fine Lake of Bracciano, to the south of it, gives rise to the Arrone. It, too, appears to be a basin formed by a subsidence of the ground or erosion, and not a crater. The Lake of Vico, on the other hand, clearly occupies an ancient volcano, though its rampart has been gutted towards the east. Close to the lake, and within the encircling rampart, rises Monte Venere, a perfect cone, the gentle slopes of which are luxuriantly wooded. Formerly the lake surrounded this cone, but the breach through which its emissary escapes to the Tiber having gradually been deepened, the waters of the lake subsided. Tradition says that an ancient city lies at its bottom. On crossing the Tiber we reach the beautiful volcanic group of Albano, within the great crater of which may still be traced the remains of several secondary craters, some of them occupied by lakes. The principal one of these, Monte Cavo (2,790 feet), rises in the very centre of the exterior rampart. Tradition points it out as one of Hannibal’s camps. The exterior slopes of the mountain consist of pozzuolana, small stones, and ashes, through which the torrents have dug out furrows in divergent directions. The diversity of these {261} volcanic products enables us to trace the phases of activity of this Roman Vesuvius, which was active at a much more recent epoch than the volcanoes farther north, and sent its streams of lava to the very gates of Rome. [Illustration: Fig. 91.—VOLCANOES OF LATIUM. From the Austrian Staff Map. Scale 1 : 294,000.]