178 ARE SOILS ENRICHED, IMPOVERISHED, pt. nr. 



continues to advance, the rise will increase. In this 

 light, seas as well as rivers may be considered as filling 

 valleys as well as excavating them. 



The steeper the slope and the more rapid the 

 stream, the straighter. It is in the alluvial part that 

 the river winds, and shifts its course. But ' erosion of 

 banks ' and ' acting on lines ' must not be claimed from 

 these changes ; for, besides that the river fills up the 

 old course which it has left, in the change it only takes 

 what it had before deposited, and had brought from a 

 distance. 



The Adriatic is filling up from its tributary valleys, 

 and 100,000 years may see it an alluvial plain, the Po 

 running through it, and falling into the Mediterranean. 

 Lyell ('Principles,' page 272) says that the delta ad- 

 vances ' a mile in a hundred years.' But, page 207, 

 he says, ' It is calculated that the mean rate of advance 

 of the delta of the Po on the Adriatic between the 

 years 12? 0 and 1600 ' (before the embankments) ' was 

 25 yards or metres a year ' (this is at the rate of a mile 

 and a half nearly, that is, a mile and 740 yards, in a 

 hundred years), ' whereas the mean annual gain from 

 1600 to 1804 ' (after embanking began) ' was 70 



below ; or it may return to tlie river by lateral hach channels cut 

 tbrougli the alluvial banks down to tbe low-water mark ; or 

 more than one of these causes may act in some places, or all of 

 them. All such difference of circumstances, and all accidental 

 alteration of such difference of circumstances, will cause infinite 

 variation in the direction and degree of the gradients in large 

 alluvial valleys. 



