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Show 236 DELTA OF TilE PO. f the lofu.e st n' d ges of the Apennines. F1r omI the someh o most po.m t of tl le G u lf of Trieste, where . t 1e sonzdo nort ern he south of Ravenna, there is an unmtcrrupte ent.e rs, dfo wrenc eton t t accessi.O ns of land ' more than one dh undred ser"Ii es . o length w1 n .c h ' WI' thin the last two thousadn ' year1s , ml es ~n 'f t to twenty miles in brea ltt, T 1e have mcrTe ash~dam ernomto ,pw_loa ve, Br·enta ' AdiO"e, and Po, besides o I sonzo, ag. f, . '.· contribute to the advance of the many other Jdn enor l ;ve~~winO" of the gulf. The Po and the coast-line, an to tbJe s la . I eod as entering by one common Ad. y now e cons1c er . 1ge rna l . f the Adige are connected with delta, for two brainc Jes 0 f the p ence of the great concentration 0 n consequ arms o . f these streams since the system of the flooded waters o neral the r:te of encroachment of embankmle ntd b~c~~etl!e Adriatic, especially at that point of the n1e w p an d APd l.i Te enter, I·s sa1" d to have been greatly. whe;·c. t ,~ o;~ria wa: a seaport in the time of Aug~st~s, and accc e.I ate ·. nt times O"iven its name to the gulf; It IS now had' m anCle 1" ' o 'l inland Ravenna was also a sea-about twenty Ita Ianbmi e~ It~lian miles from the main t d is now a out !OUr . por ' an b f, e the practice of embankment was Introsea. Yet even . e or advanced with rapidity on the duced, the alluvmm of the Po . t city originally built in the d . · £ Spina a very ancien ' A ria tiC; or ' f reat arm of the Po, was, district of Ravenna, at the mout~ o a g eleven Italian miles so early as the commencement o our era, distant from the sea *. f h Adriatic, between Dalmatia and The greatest depth o . t e t fathoms . but a large h h f the Po rs twenty- wo ' . t e mout s o ! d the Adriatic, opposite Vemce, part of the gulf of Trieste an h th where is less than twelve fathoms deep. Fal't~er to ~h: sgo~f deepens ~r d b the influx of great rivers, d it is less auecte Y . dred iniT the bottom, discovere considerably. Donati, after g o d and artly of rock, the new deposits to consist partly of mu t"p shells He c d f lcareous matter' en crus mg . d the latter JOrme o ca . . f testacea were groupe also ascertained, that partlculardspeCiesbo oming slowly incorpo- toCTether m. cer t am. places ' au were ec. . t t s t Oliv1., aI s o, rared with the mud, or calcareous preclpl a e . * See Brocchi on tho van.o ns wr "l ters . on th1is . subj3e9ct . Conch. Foss. Subap., yol. i., p. 118. t Ibid., vo . I., P· . DELTA OF 'l'HE PO. 237 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 by the prevailing current*. 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, precisely similar to those of the Subapennine hills, and containing many of the same species of shells. 'I' he 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 there 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 main land, 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. Notwithstanding the present shallowness of the Adriatic, it is highly probable that its original depth was very great ; for if all the low alluvial tracts were taken away from its borders and replaced by sea, the high land would terminate in that abrupt manner which generally indicates, in the Mediterranean, a great depth of water near the shore, except in those spots where sediment imported by rivers and currents has diminished the depth. Many parts of the Mediterranean arc now ascet·tained to be above two thousand feet deep, close to the shore, as between Nice and Genoa; and even sometimes six thousand feet, as near Gibraltar. When, therefore, we find near Parma, an~ in other districts in the interior of the peninsula, beds of horizontal tertiary marl, attaining a thickness of about two vol*. iiS.,e ep . B94ro. cchi on the various writers on this subject. Conch. Foss. Subap., |
