From all this you can gather why the existing Po flows far from the Alps and nearer the base of the Apennines. The Alpine streams in far distant days brought down relatively large floods of glacial mud; formed relatively large deltas in the old Lombard bay; filled up with relative rapidity their larger half of the basin. The Apennines, less lofty, and free from glaciers, sent down shorter and smaller torrents, laden with far less mud, and capable therefore of doing but little alluvial work for the filling in of the future Lombardy. So the river was pushed southward by the Alpine deposits of the northern streams, leaving the great plains of Cisalpine Gaul spread away to the north of it.
And this land-making action is ceaseless and continuous. About Venice, Chioggia, Maestra, Comacchio, the delta of the Po is still spreading seaward. In the course of ages--if nothing unforeseen occurs meanwhile to prevent it--the Alpine mud will have filled in the entire Adriatic; and the Ionian Isles will spring like isolated mountain ridges from the Adriatic plain, as the Euganean hills--those 'mountains Euganean' where Shelley 'stood listening to the pæan with which the legioned rocks did hail the sun's uprise majestical'--spring in our own time from the dead level of Lombardy. Once they in turn were the Euganean islands, and even now to the trained eye of the historical observer they stand up island-like from the vast green plain that spreads flat around them.
Perhaps it seems to you a rather large order to be asked to believe that Lombardy and Venetia are nothing more than an outspread sheet of deep Alpine mud. Well, there is nothing so good for incredulity, don't you know, as capping the climax. If a man will not swallow an inch of fact, the best remedy is to make him gulp down an ell of it. And, indeed, the Lombard plain is but an insignificant mud flat compared with the vast alluvial plains of Asiatic and American rivers. The alluvium of the Euphrates, of the Mississippi, of the Hoang Ho, of the Amazons would take in many Lombardies and half-a-dozen Venetias without noticing the addition. But I will insist upon only one example--the rivers of India, which have formed the gigantic deep mud flat of the Ganges and the Jumna, one of the very biggest on earth, and that because the Himalayas are the highest and newest mountain chain exposed to denudation. For, as we saw foreshadowed in the case of the Alps and Apennines, the bigger the mountains on which we can draw the greater the resulting mass of alluvium. The Rocky Mountains give rise to the Missouri (which is the real Mississippi); the Andes give rise to Amazons and the La Plata; the Himalayas give rise to the Ganges and the Indus. Great mountain, great river, great resulting mud sheet.