THE MUD FLAT. 49 



or six thousand years. Put five or six thousand such blocks 

 together ; the aggregate would be a mountain range. 



There are seasons when the proud river climbs over its 

 bounds climbs over the artificial restraints which have been 

 imposed in the form of levees. Water and mud spread over 

 hundreds of plantations. Then, as in the overflowing torrent 

 of the Aar, the slackened motion of the water allows the fine 

 sediment to subside. Corn lands and cotton lands receive a 

 new contribution of fertilizing material. Such service the 

 Nile performs for Egyptian agriculture under the rule of the 

 Khedives, as during the reigns of the Pharaohs. Thus the 

 deltas of the great rivers are formed. Still the great prepon- 

 derance of river silt passes on to the outlets. Not only the 

 floating sediment, but a large amount of bottom mud, too 

 thick to float and too loose to lie unmoved. This the stream 

 pushes along into the sea year by year into deeper and deeper 

 water, as the shallower shore region becomes silted up. This 

 is the bar. By the annual extension of the bar, the delta 

 gradually protrudes a tongue of land into the sea. Look at 

 a map of the mouth of the Mississippi, or the Nile, or the 

 Ganges. Often the piled up bar-material so obstructs the exit 

 through the main channel, that the water sets back up the 

 stream during some flood, overflows its'banks, and seeks a new 

 route to the sea. This may be many times repeated. So 

 these great rivers acquire numerous outlets. Look at the 

 map again. The bar at the mouth of the Mississippi extends 

 three hundred and thirty-eight feet into the Gulf annually. 



Much of the Mississippi sediment, therefore, lies somewhat 

 permanently on the Gulf bottom, near the shore. Through 

 this Engineer Eads has staked out a channel, to which the 

 current of the Mississippi is confined after entering the Gulf, 

 until deep water is reached. Its velocity is thus preserved, 

 and its mud is carried beyond into the deeper basin. Before 

 this improvement, the water spread out fan-like, and slackened 

 its velocity to such an extent that the mud was deposited in a 

 region where the water was already so shallow that navigation 

 became seriously obstructed. 



5 



