For our present purpose, and as a worker out of Venetian history, Po, notwithstanding the far greater volume of his waters, is of less importance than the six other small streams which bear him company.  He, carrying down the fine alluvial soil of Lombardy, goes on lazily adding, foot by foot, to the depth of his delta, and mile by mile to its extent.  They, swiftly hurrying over their shorter course from mountain to sea, scatter indeed many fragments, detached from their native rocks, over the first meadows which they meet with in the plain, but carry some also far out to sea, and then, behind the bulwark which they thus have made, deposit the finer alluvial particles with which they, too, are laden.  Thus we get the two characteristic features of the ever-changing coast line, the Lido and the Laguna.  The Lido, founded upon the masses of rock, is a long, thin slip of the terra firma, which form a sort of advance guard of the land.

The Laguna, occupying the interval between the Lido and the true shore, is a wide expanse of waters, generally very few feet in depth, with a bottom of fine sand, and with a few channels of deeper water, the representatives of the forming rivers winding intricately among them.  In such a configuration of land and water the state of the tide makes a striking difference in the scene.  And unlike the rest of the Mediterranean, the Adriatic does possess a tide, small, it is true, in comparison with the great tides of ocean—­for the whole difference between high and low water at the flood is not more than six feet, and the average flow is said not to amount to more than two feet six inches—­but even this flux is sufficient to produce large tracts of sea which the reflux converts into square miles of oozy sand.

Here, between sea and land, upon this detritus of the rivers, settled the detritus of humanity.  The Gothic and the Lombard invasions contributed probably their share of fugitives, but fear of the Hunnish world-waster—­whose very name, according to some, was derived from one of the mighty rivers of Russia—­was the great “degrading” influence that carried down the fragments of Roman civilization and strewed them over the desolate lagoons.  The inhabitants of Aquileia, or at least the feeble remnants that escaped the sword of Attila, took refuge at Grado.  Concordia migrated to Caprularia (now Caorle).  The inhabitants of Altinum, abandoning their ruined villas, founded their new habitations upon seven islands at the mouth of the Piave, which, according to tradition, they named from the seven gates of their old city—­Torcellus, Maiurbius, Boreana, Ammiana, Constantiacum, and Anianum.  The representatives of some of these names, Torcello, Mazzorbo, Burano, are familiar sounds to the Venetian at the present day.