If we descend the Italian slope of the Alps, we pass gradually from the more ancient to the most recent geological formation, until we finally reach the alluvial plain. Metamorphic rocks, _verrucano_, dolomites, and other rocks overlie the granites, the gneiss, and the schists of the more elevated mountain masses. These are succeeded by beds of Triassic and Jurassic age. Lower still we meet with {193} terraces and hills composed of tertiary marls, clays, and conglomerates. Monte Bolca, so famous amongst geologists on account of its fossils, belongs to this formation.[62] The whole of the plain of Lombardy and Piemont, with the exception of the isolated hillocks rising in it, and a few marine deposits near its margin, consists of débris brought down by the rivers. The depth of this accumulation is not yet known, for hitherto no borings have pierced it; but if we suppose the slopes of the Alps and the Apennines to continue uniformly, it would amount to no less than 4,130 feet. The two diagrams (Fig. 53) are intended to illustrate this feature. In the upper of these the heights are exaggerated tenfold; in the lower both the horizontal and the vertical scales are the same. A glance at this diagram reveals the astounding fact that the volume of this débris almost equals that of the existing mountain systems. [Illustration: Fig. 54.—SLOPE OF THE VALLEY OF THE PO. The vertical scale is ten times larger than the horizontal.] The vast plain stretching from the Adriatic to the foot of the Monte Rosa and the Viso may boast of its peninsulas, its islands, and even its archipelagos, as if it were a sea. The tertiary hills of Northern Monferrato, to the east of Turin, attain a height of 1,600 to 2,000 feet, and the valley of the Tanaro completely separates them from the Ligurian Alps and the Apennines. Even at the very foot of the Alps, as at Cavour and elsewhere, isolated granitic or porphyritic pyramids and domes rise in the midst of the plain sloping down towards the Po.[63] The hump-backed Bosco Montello, to the south of the Piave, is another isolated hill; and on the banks of the Po may be seen a hillock of pebbles and marine sands, abounding in fossils, which bears the village of San Colombano and its vineyards. Several volcanic peaks, surrounded by cretaceous formations, rise in the midst of the plains to the east of the Lake of Garda. The craters of the Berici, near Vicenza, and of the Euganean Hills, near Padua, have not vomited {194} flames within the historical epoch, but the hot and the gas springs which issue from clefts in the trachytic and basaltic rocks prove sufficiently that volcanic forces are not yet quite extinct in that part of Italy. Earthquakes occur frequently in the neighbouring Alps, and particularly near Belluno and Bassano. [Illustration: Fig. 55.—MUD VOLCANOES AND HOT SPRINGS OF THE NORTHERN APENNINES. Scale 1 : 1,160,000.]