GROWTH OF DELTAS. 369 



time new deposits are built up against the assaults 

 of the waves, as we have seen to be the case with 

 the Mississippi. It results that the delta of former 

 times is no longer that of to-day, and that the navi- 

 gable channels are always in course of change. 



The channels of the Nile do not extend more than 

 about four yards in a year. This is partly owing 

 to the system of canals established by the Egyptian 

 priests, partly to the current which flows along the 

 coast, and which carries a large part of the suspended 

 matter towards the east, and occasionally breaks 

 down its banks. 



In the time of Augustus the sea washed against 

 the walls of Adria (a city near the mouth of the Po) ; 

 the shore is now eight leagues distant, in consequence 

 of the growth of the delta of the Po. The increase 

 from the twelfth to the sixteenth century was at the 

 rate of about twenty-seven yards annually ; it has 

 augmented since then, and is now at the rate of 

 about seventy-five yards. 



The diiltsL of the Rhone advances, perhaps, fifty- 

 five yards each year, and that of the Mississippi about 

 380 yards. The immense delta of the Ganges, situ- 

 ated at the head of a gulf, must grow rapidly ; 

 but, as the spot is very unhealthy, it has never been 

 inhabited, and there are no data which would enable 

 us to form a ju Igment on the subject. 



2 B 



