During the glacial period the sea, in which have been deposited the chalk and other rocks composing the Sub-Apennines, was the scene of volcanic action on a grand scale. The matter ejected was heaped up in a line of volcanic cones, running in a direction nearly parallel with the Apennines and the coast of the Mediterranean. These cones are joined to each other by thick layers of tufa, which cover the whole of the plain as far as the foot of the calcareous mountains, and extend for a distance of nearly 120 miles, from Monte Amiata, in Tuscany, to the mountains of Albano, being interrupted only by the alluvial valley of the Tiber. Ponzi and other geologists are of opinion that this tufa was ejected from submarine volcanoes, carried away by the currents, and equally distributed over the depressions of the sea-bottom. No fossils have been discovered in it hitherto, which is accounted for by the presence of icebergs, which prevented a development of animal life. This volcanic region is remarkable on account of its numerous lakes. The largest of these, that of Bolsena, was formerly looked upon as an ancient crater. This crater would have exceeded by far the largest volcanic vents met with in the Andes or in Java, for it has a circumference of twenty-five miles, and covers an area of forty-four square miles. Modern geologists, however, look upon this crateriform lake as a basin of erosion, and though it occupies the centre of a plateau formed of ashes, scoriæ, and lava, these do not form a steep edge towards the lake, as in the case of veritable craters in the same district. One of the most remarkable of these latter is that of Latera, to the west of the lake, in the centre of which rises a cone of eruption, the Monte Spignano, which has a diameter of nearly five miles. The district of the Bolsena is likewise remarkable on account of its vertical precipices of tufa and lava. Its picturesque towns and villages are perched upon {260} bold promontories looking down on the valleys. The old town of Bagnorea occupies the extremity of an immense mole, and is joined to the new town by a giddy path, bounded by steep precipices, which timid travellers do not care to venture upon. Orvieto stands on an isolated rock resembling a fortress. Pittigliano is surrounded by precipices: by cutting away a few yards of the narrow isthmus which joins it to the rest of the plateau, access to it would be impossible to all but birds. In the Middle Ages, when nobles and towns were continually at war, the capture of one of these eyries was looked upon as a grand achievement. [Illustration: Fig. 90.—THE LAKE OF BOLSENA. Scale 1 : 457,000.]