270 SEDIMENT OF THE PO. 



it to carry most of its sediment to tlie Adriatic, It lias, there- 

 fore, raised neither its own channel nor its alluvial shores, as it 

 would have done if it had remained unconiined. But, as the 

 surface of the water in floods is above the general level of the 

 plains through which it flows, the Po can, at that period, receive 

 no contributions of earth from the washing of the fields of Lom- 

 bard j, and there is no doubt that a large -proportion of the sedi- 

 ment it now deposits at its mouth descended from the Alps in 

 the form of rock, though reduced by the grinding action of the 

 waters, in its passage seaward, to the condition of fine sand, and 

 often of silt. 



We know little of the history of the Po, or of the geography 

 of the coast near the point where it enters the Adriatic, at any 

 period more than twenty centuries before our own. Still less 

 can we say how much of the plains of Lombardy had been formed 

 by its action, combined with other causes, before man accelerated 

 its levelling operations by felling the first woods on the mountains 

 whence its waters are derived. But we know that since the Ro- 

 man conquest of Northern Italy, its deposits have amounted to a 

 quantity which, if recemented into rock, recombined into gravel, 

 common earth, and vegetable mould, and restored to the situa- 

 tions where eruption or upheaval originally placed or vegetation 

 deposited it, would fill up hundreds of deep ravines in the Alps 

 and Apennines, change the plan and profile of their chains, and 

 give their southern and northern faces respectively a geographical 

 aspect very different from that they now present. Ravenna, forty 

 miles south of the principal mouth of the Po, was built hke Yen- 

 ice, in a lagoon, and the Adriatic still washed its walls at the com- 

 mencement of the Christian era. The mud of the Po has filled 

 up the lagoon, and Ravenna is now four miles from the sea. The 

 town of Adria, which hes between the Po and the Adige, at the 

 distance of some four or five miles from each, was once a harbor 

 famous enough to have given its name to the Adriatic Sea, and it 

 was still accessible to large vessels, if not by the open sea at least 

 by lagoons, in the time of Augustus. The combined action of 

 the two rivers has so advanced the coast-line that Adria is now 

 more than fourteen miles inland, and, in other places, the deposits 

 made within the same period by these and other neighboring 

 streams have a width of twenty miles. 



