by the prevailing current.[345] It is probable, therefore,
that the finer sediment of all the rivers at the head of the Adriatic
may be intermingled by the influence of the current; and all the central
parts of the gulf may be considered as slowly filling up with horizontal
deposits, similar to those of the Subapennine hills, and containing many
of the same species of shells. The Po merely introduces at present fine
sand and mud, for it carries no pebbles farther than the spot where it
joins the Trebia, west of Piacenza. Near the northern borders of the
basin, the Isonzo, Tagliamento, and many other streams, are forming
immense beds of sand and some conglomerate; for here some high mountains
of Alpine limestone approach within a few miles of the sea.

In the time of the Romans, the hot-baths of Monfalcone were on one of
several islands of Alpine limestone, between which and the mainland, on
the north, was a channel of the sea, about a mile broad. This channel is
now converted into a grassy plain, which surrounds the islands on all
sides. Among the numerous changes on this coast, we find that the
present channel of the Isonzo is several miles to the west of its
ancient bed, in part of which, at Ronchi, the old Roman bridge which
crossed the Via Appia was lately found buried in fluviatile silt.

_Marine delta of the Rhone._--The lacustrine delta of the Rhone in
Switzerland has already been considered (p. 251), its contemporaneous
marine delta may now be described. Scarcely has the river passed out of
the Lake of Geneva before its pure waters are again filled with sand and
sediment by the impetuous Arve, descending from the highest Alps, and
bearing along in its current the granitic detritus annually brought down
by the glaciers of Mont Blanc. The Rhone afterwards receives vast
contributions of transported matter from the Alps of Dauphiny, and the
primary and volcanic mountains of Central France; and when at length it
enters the Mediterranean, it discolors the blue waters of that sea with
a whitish sediment, for the distance of between six and seven miles,
throughout which space the current of fresh water is perceptible.

Strabo's description of the delta is so inapplicable to its present
configuration, as to attest a complete alteration in the physical
features of the country since the Augustan age. It appears, however,
that the head of the delta, or the point at which it begins to ramify,
has remained unaltered since t