es.[341]

Adria was a seaport in the time of Augustus, and had, in ancient times,
given its name to the gulf; it is now about twenty Italian miles inland.
Ravenna was also a seaport, and is now about four miles from the main
sea. Yet even before the practice of embankment was introduced, the
alluvium of the Po advanced with rapidity on the Adriatic; for Spina, a
very ancient city, originally built in the district of Ravenna, at the
mouth of a great arm of the Po, was, so early as the commencement of our
era, eleven miles distant from the sea.[342]

But although so many rivers are rapidly converting the Adriatic into
land, it appears, by the observations of M. Morlot, that since the time
of the Romans, there has been a general subsidence of the coast and bed
of this sea in the same region to the amount of five feet, so that the
advance of the new-made land has not been so fast as it would have been
had the level of the coast remained unaltered. The signs of a much
greater depression anterior to the historical period have also been
brought to light by an Artesian well, bored in 1847, to the depth of
more than 400 feet, which still failed to penetrate through the modern
fluviatile deposit. The auger passed chiefly through beds of sand and
clay, but at four several depths, one of them very near the bottom of
the excavation, it pierced beds of turf, or accumulations of vegetable
matter, precisely similar to those now formed superficially on the
extreme borders of the Adriatic. Hence we learn that a considerable area
of what was once land has sunk down 400 feet in the course of ages.[343]

The greatest depth of the Adriatic, between Dalmatia and the mouths of
the Po, is twenty-two fathoms; but a large part of the Gulf of Trieste
and the Adriatic, opposite Venice, is less than twelve fathoms deep.
Farther to the south, where it is less affected by the influx of great
rivers, the gulf deepens considerably. Donati, after dredging the
bottom, discovered the new deposits to consist partly of mud and partly
of rock, the rock being formed of calcareous matter, incrusting shells.
He also ascertained, that particular species of testacea were grouped
together in certain places, and were becoming slowly incorporated with
the mud or calcareous precipitates.[344] Olivi, also, found some
deposits of sand, and others of mud, extending half way across the gulf;
and he states that their distribution along the bottom was evidently
determined