2 14 SEA AND LAND 



wide oceans and break upon their shores with calamitous 

 effects. 



With this preliminary statement as to the nature and origin 

 of the classes of energy which operate on a coast-line we 

 shall now proceed to consider somewhat in detail the effects 

 produced by these causes in increasing or diminishing the 

 value of our havens. First among them we have to note the 

 effect of the sediment brought into harbors by the rivers. 



The waste of the land which is brought down to the shore 

 by the rivers is conveyed in two different forms : in part as 

 completely dissolved sediment, which does not discolor the 

 water ; and in part as materials which are visible to the eye 

 as mud, rendering the water turbid, or grains of sand or small 

 pebbles rolled on the bottom, or at most carried a little dis- 

 tance above it. As long as the water of the river is held 

 within distinct banks, the energy of its flow to a great extent 

 prevents the mud from sinking to the bottom and the sand 

 and pebbles from coming to rest, so that the channel may 

 remain substantially unchanged as regards its position or the 

 depth of the stream. At the outlet, however, where the river's 

 tide is discharged into an open basin, the current is soon 

 arrested, and gravitation is free to act on all the mud it con- 

 veys. Only the completely dissolved materials, such as salt, 

 lime and various other substances, are free to journey onward 

 for indefinite distances in the ocean currents. 



Where the river discharges into a lake of fresh water the 

 suspended materials, at least the finer portions, find their wa)' 

 to the bottom much more slowly than where they enter a salt- 

 water basin, because the saline matter of the sea water has a 

 peculiar effect in hastening the precipitation of mud. In an\' 

 case, however, it rarely occurs that the river mud journeys 



