There were other matters, too, of very really important, but altogether material interest, to be debated between the Duke and the father of the faithful. All the rich cities, with their fertile territories, which are situated in the lower part of the great valley of the Po, have suffered from the earliest historic times, and to the present day still suffer from very great difficulty in getting rid of the waters brought down to them from the higher country by the Po itself, and sundry other rivers. As usual in the compensatory processes of nature, the same causes have produced their prosperity, and the circumstances which tend to diminish it. The whole of the wonderfully fertile region in question is alluvial soil of extremely recent formation. Very numerous and exceedingly curious discoveries, made at different times within the last two centuries at various points, extending over the whole district from Modena to the Adriatic, prove that the country has been at former times inhabited, when the surface of the soil was from four to ten or more feet lower than it is at the present day. On the left bank of the Brenta, near Fusina, a mosaic pavement, and other Roman remains, were found _in situ_ at a level now two feet and a half below that of the mean height of the Adriatic. And a great variety of similar facts have been placed on record,[65] which satisfactorily prove that the level of the sea has been raised, _pari passu_, with that of the adjoining low lands. The result of the process thus always going on is, that an exceedingly small margin of difference between the level of the land and that of the sea remains to render possible the discharge of the waters from the mouths of the different rivers. The fertilising materials, which are brought down from the Apennines in vast quantities, not only tend to raise the general level of the district, but also, and with much greater rapidity, to raise the beds of the rivers, and obstruct their course. From a very early period, this evil has been met by the inhabitants by constructing lofty banks along the courses of their rivers. The more the bed of the stream became raised in the process of time, the higher these moles had to be built to follow it, till some of the rivers now traverse large districts of country in a perfectly artificial canal, constructed upon, instead of in, the face of the soil. Near Ferrara itself, the surface of the Po is now higher, it is said, than the highest building in the city. In such a state of things, it may easily be imagined how important a public concern was, and still is, the maintenance and regulation of these vast artificial banks, and how vital a question the management of the waters of those secondary streams, which contribute their volume to the mass that threatens the lower country.