The Sub-Apennine hills to the south of the Arno are of rounded contours, of a gloomy grey colour, and devoid of all verdure. Whilst the Apennines consist exclusively of Jurassic and cretaceous rocks, the Sub-Apennines are of tertiary formation, their sandstones, clays, marls, and pudding-stones being pierced here and there by serpentine. Well-defined ranges can hardly be said to exist. Southern Tuscany, indeed, may be described as a table-land intersected by rivers in all directions, surmounted by irregular groups of hills, and pierced by “sinks,” which swallow up some of the rivers. The cavities of the Ingolla form one of these sinks, in which several rivulets lose themselves, to reappear lower down as the source of the Elsa Viva, one of the principal tributaries of the Arno. The most elevated hills of this Sub-Apennine region form the water-parting between the Arno, the Cecina, and the Ombrone, and in the Poggio di Montieri, a mountain abounding in copper, they attain an elevation of 3,323 feet. The Labbro (3,815 feet), Cetona (3,650 feet), and Monte Amiata (5,450 feet), to the south of the Ombrone valley, rise to a greater height, but geologically they belong already to Central Italy. The Cetona is a Jurassic outlier surrounded by recent formations. Monte Amiata, a trachytic cone, is the most elevated volcano of continental Italy. It no longer vomits lava, but numerous hot springs and solfataras prove that the volcanic forces are not yet quite extinct. The Radicofani (2,950 feet) is likewise an extinct volcano, whose lava resembles petrified froth, and can be cut with a hatchet. Subterranean agencies must indeed be very active in Tuscany, for metalliferous {242} veins ramify in all directions, and the number of mineral springs of every description is larger than in any other part of Italy. Amongst these springs there are several of world-wide reputation, as, for instance, those of Monte Catini, of San Giuliano, and of the Bagni di Lucca. The brine springs of Tuscany are very productive; but the most curious, and at the same time most useful, springs of all are the famous _lagoni_, in a side valley of the Cecina, and at the northern foot of the Poggio di Montieri. From a distance dense clouds of white vapour are seen rolling over the plain, and the bubbling noise made by gases escaping through the ponds, or _lagoni_, is heard. These ponds contain various salts, silica, and boracic acid, which is of great value in the manufacture of china and glass, and yields a considerable revenue to Tuscany. Nowhere else in Europe, except, perhaps, in the crater of the Eolian Vulcano, is boracic acid met with in sufficient quantities to repay the labour of extracting it. In Tuscany, however, there are several other localities where it might be won with advantage, as, for instance, near Massa Maritima, to the south of the Montieri.