A similar volcanic zone extends along the northern slope of the Apennines, which bound the valley of the Po on the south. Hydrogen gas escapes from fissures in the rocks to the south of Modena and Bologna, and is utilised in several instances in the manufacture of lime, and for other purposes. These gas springs of Pietra Mala, Porretta, and Barigazzo were known by the ancients and during the Middle Ages as “fiery springs,” and they illuminated the path of the traveller overtaken by the night. Lower down the slope, almost on the verge of the plains, we meet with a line of mud volcanoes, or _bombi_, the most famous of which are those of Sassuolo, near Modena. The largest of these, that of Mirano, has no less than forty craters. The ancient gulf of the sea, now converted into a plain, is thus skirted by volcanic cones, mud volcanoes, hot springs, and deposits of sulphur. As high up as Piemont, and notably at Acqui, we meet with hot springs, attesting that volcanic activity is not yet altogether extinct. [Illustration: La Dent blanche, 14,319 ft.; Château des Dames, 11,998 ft.; Mt. Cervin, 14,705 ft.; Mischabel Hōrner, 14,942 ft.; Breithorn, 13,680 ft.; Monte Rosa (Dufour Spitze, 15,217 ft.). THE PENNINE ALPS, AS SEEN FROM THE BECCA DI NONA (PIC CARREL), 10,380 FEET.] The valleys of the Alps and the plains extending along their foot were filled, in a former geological epoch, with huge glaciers, descending from what was anciently the immense glacial region of Central Europe. There is not a valley between that of the Tanaro in the west, and that of the Isonzo descending from the mountains of Carinthia, but contains accumulations of débris carried down by the {195} glaciers, and now covered with vegetation. Most of these ancient glaciers exceeded those of the Monte Rosa and the Finsteraarhorn in extent, and several of them rivalled the existing glaciers of the Himalaya. If we would gain a notion of what the Alps were like during this glacial epoch, we must go to Greenland or to the Antarctic regions. [Illustration: Fig. 56.—THE ANCIENT GLACIERS OF THE ALPS. Scale 1 : 4,800,000.]