426 



DELTA OF THE PO. 



[Ch. XVIII. 



a much, greater depression anterior to the historical period 

 have also been brought to light by an Artesian well, bored at 



more tha 

 ,e modern 



The auger passed chiefly through beds of sand and clay, but 



them 



the excavation, it pierced beds of turf, or accumulations of 



similar 



erne 



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.* 



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 



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. f 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 by the prevailing current. J It is probable, there- 

 fore, that the finer sediment of all the rivers at the head of 

 the Adriatic may be intermingled by the influence of the cur- 

 rent ; 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 

 sand and mud, for it carries no pebbles farther than the spot 

 where it joins the Trebia, west of Piacenza. At the northern 

 borders of the Gulf of Trieste, the Isonzo, Tagliamento, and 

 many other streams, are forming immense beds of sand and 



me i 



* Arcliiac, Histoire des Progres de la 



t Brocclii, Condi. Foss. Subap. vol. l 



Geol. 1848, vol. ii. p. 232. 



p. 39. 



{ Ibid. vol. ii. p. 94. 







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