We know sufficient of the bottom of the Lake of Como to enable us to judge of the manner in which it is becoming gradually filled up with alluvium. The mud deposited in its northern portion has filled up all the original inequalities of the soil, and even in the centre of the lake, and in its south-eastern arm, the bottom is almost a perfect level. In the Como arm, however, which receives no tributary river of any importance, the bottom is still full of inequalities. These differences amply prove to us the geological agency of the rivers, which must terminate in the lake being converted into a bottom-land, with a river flowing through its centre. The third of our diagrams (Fig. 62) shows that the greatest depth now hardly exceeds 1,300 feet, whilst, if we may judge from the slopes of the hills which bound it, the depth in former times cannot have been less than 2,300 feet. The Sebino, or Lake of Iseo, and the lakelet of Idro, which are fed by the glacier streams of the Adamello, exhibit the same features as the lakes farther to the west. The Benaco, or Lake of Garda, however, the most extensive of these Alpine lakes, is very stable as regards its outline and the configuration of its bottom, a fact sufficiently explained by the small size of its tributary streams as compared with its vast area. The old Alpine lakes of the Venetian Alps have disappeared long ago, and there remain only a few ponds, filling cavities in the dolomitic rocks and peat bogs, to indicate their ancient sites.[64] {201} [Illustration: Fig. 63.—VILLA SERBELLONI, ON THE PENINSULA OF BELLAGIO, LAKE OF COMO.] These lacustrine basins, like all other reservoirs of the same kind, regulate the outflow of the torrents which empty into them. During the freshets they store up the superabundant waters, and only part with them in the dry season, and upon their difference of level in different seasons depend the oscillations of the emissary rivers which issue from them. In the case of the Lake of Garda, which drains but a small area in proportion to its size, this difference is small, and throughout the year the pellucid waters of the Mincio flow tranquilly beneath the blackened ramparts of Peschiera. Such is not the case as regards either the Lago Maggiore or the Lake of Como, for the volume of water discharged into them is so considerable that their level in summer and winter varies to the extent of several yards, and corresponding differences may be observed in the rivers issuing from them. Lake Como rises no less than 12 feet, and increases 70 square miles in area, whilst the Lago Maggiore sometimes rises 22 feet, and {202} increases to the extent of one-fifth. The volume of the Ticino, when at its highest, almost equals the average volume of the Nile, and if it were not for the regulating influence of the lake from which it issues, it would alternately convert the plains of Lombardy into a sheet of water and leave them an arid tract of land.[65]