72 SEA AND LAND 



is ofeneralh' a sinsfle breach in this wall throucrh which the tide 

 gains ingress, and through which the lantl waters pass into 

 the sea. If the outflowing streams of fresh water are small, 

 as is generally the case, it is only necessary to place a dam at 

 this exit, provided with gates which open at low tide to allow 

 the fresh water to escape, and close as soon as the tide begins 

 to flow in. In this way the surface of the soil once over- 

 flowed at each tide is kept permanently from three to six 

 feet above the level of the water in the river channel. It 

 requires some time and care to take out the saline matter 

 froni the marsh, but within a few years the soil will produce 

 luxuriant and varied crops. Its fertility is greater than that 

 of the prairies, and from its depth and the variety of its 

 natural constituents it seems almost inexhaustible. An 

 excellent example of the possibilities of this interesting means 

 of winning land from the sea is afforded by the Green Harbor 

 lands of Marshfield, Mass., where a district of about fifteen 

 hundred acres was freed from the sea by a dam costing a few 

 thousand dollars. 



Where the tidal fall and rise is less than five feet, it is 

 difficult to secure the necessar}' dryness for tillage by any 

 automatic system of drainage. In these districts recourse 

 will probably have to be had to pumping by means of wind- 

 mills, a method of drainage which has been found effective 

 and economical in the low countries about the mouth of the 

 Rhine. 



Before we close this imperfect story of the sea-beaches we 

 may well note the comnion fact that the sand of which the 

 bulk of their masses is composed is vastly more durable than 

 the seemingly more resisting pebbles. As we have seen, 

 pebbles wear out rapidly. Scarcely any, even the hardest, 



