356 THE BOTTOM OF THE SEA. 



branches fill on either side the ocean depths — build- 

 ing up two immense slopes, separated by a narrow 

 ravine. Ultimately the two banks will become con- 

 nected, the deep water between them being gradually 

 filled up. 



The rapid formation of the delta of the Po, and 

 the slight depth of the Adriatic, into which sea this 

 river pours its waters, are well-known facts. But 

 other causes have contributed very considerably to 

 this effect ; these are the embankment of the rivers 

 and the clearing of the forests. 



The clearing of the soil and the destruction of 

 forests tend to augment the quantity of water which, 

 in the rainy seasons, flows down into the river- 

 courses. Thus man himself helps to increase the 

 quantity of material that streams and rivers carry 

 into the sea. 



Embankments produce similar effects by aug- 

 menting the rapidity of the current, which, in times 

 of flood, carries sediment much farther than when 

 allowed to spread at its pleasure over vast plains, 

 where it deposits great quantities of mud. 



The embankments of the Nile, of the Po, and of 

 the Mississippi, show how the growth of a delta may 

 be accelerated by narrowing the channel of the 

 river. 



The great labour of embanking the Po, and effect- 



