5. 











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D 





k 



there. 



roni 



bich is 



rivers 

 1 sand, 



also 

 it )n- 



nee ^r 



le over 



; Capt. 



DfflSti 



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 id to 



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 lue, i» 



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15 



I 



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11:. 



a= 







Ch. XVIIL] 



DELTAS OF INLAND SEAS. 



423 



Limnea and Planorbis, are the same ; and in regard to other 



The changes 



must 



as I shall endeavour more fully to explain when speaking of 

 the imbedding of plants and animals in recent deposits. 



DELTAS ON INLAND SEAS. 



Having thus briefly considered some of the lacustrine 

 deltas now in progress, we may next turn our attention to 

 those of inland seas. 



Deltas of the Po and Adige. — The Po affords an instructive 

 example of the manner in which a great river bears down to 

 the sea the matter poured into it by a multitude of tributaries 

 descending from lofty chains of mountains. 

 Gradually effected in the great plain of Northern Italy, since 

 the time of the Roman republic, are considerable. Extensive 

 lakes and marshes have been gradually filled up, as those 

 near Placentia, Parma, and Cremona, and many have been 

 drained naturally by the deepening of the beds of rivers. 

 Deserted river-courses are not unfrequent, as that of the 

 Serio Morto, which formerly fell into the Adda, in Lombardy. 

 The Po also itself has often deviated from its course, having, 

 after the year 1390, deserted part of the territory of Cremona, 

 and invaded that of Parma ; its old channel being still re- 

 cognisable, and bearing the name of Po Morto. There is 

 also an old channel of the Po in the territory of Parma, 

 called Po Yecchio, which was abandoned in the twelfth cen- 

 tury, when a great number of towns were destroyed. 



To check these and similar aberrations, a general system 

 of embankment has been adopted ; and the Po, Adige, and 

 almost all their tributaries, are now confined between high 

 artificial banks. The increased velocity acquired by streams 

 thus closed in, enables them to convey a much larger portion 

 of foreign matter to the sea ; and, consequently, the deltas of 

 the Po and Adige have gained far more rapidly on the 

 Adriatic since the practice of embankment became almost 

 universal. But, although more sediment is borne to the sea, 

 part of the sand and mud, which in the natural state of 

 things would be spread out by annual inundations over the 

 plain, now subsides in the bottom of the river-channels ; and 



