278 SEDIMENT OF THE PO. 
the fields of Lombardy, and there is no doubt that a large pro- 
portion of the sediment 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 Roman 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 situations where eruption or upheaval origi- 
nally 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 north- 
ern faces respectively a geographical aspect very different from 
that they now present. Ravenna, forty miles south of the prin- 
cipal mouth of the Po, was built like Venice, in a lagoon, and 
the Adriatic still washed its walls at the commencement 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 lies between the Po and the Adige, at the dis- 
tance 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 ac- 
tion 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. 
What proportion of the earth with which they are charged 
these rivers have borne out into deep water, during the last two 
