70 SEA AND LAND 



afford numerous harbors, but for the reason that they lead to 

 the production of certain i^roups of marine marshes, which 

 niay be readily converted into \ery rich arable land. Some 

 of the most fertile of the dyked lands of Holland were origi- 

 nally of this nature ; and on the eastern shore of the United 

 States there is awaiting the thrift which shall win it to agricul- 

 ture an area far greater than that occupied by all the Nether- 

 lands. These marshes, the products of the sea-beaches, afford 

 such promise of good harvests in the generations to come, that 

 we ma)- devote a portion of this chapter on the history of the 

 shore to a brief account of their curious [phenomena. 



As soon as a space of tidal waters is protected by a bar- 

 rier-beach from the incursions of the sea-waves, its bottom is 

 occupied by various si)ecies of marine plants, of which the 

 well-known eel-grass is the most common. These form.s make 

 a network in which the currents of the tides, or those of the 

 rivers which may discharge into the embayed area from the 

 land, are deadened and lay down such sediments as they may 

 be bearing. A host of animals dwell in this vegetation, and 

 contribute to the deposits which rapidly shallow the water. 

 When by these accumulations the surface is brought to the 

 level of low^ tide, several kinds of. true grasses and other 

 fiovvering plants, as well as certain sea-weeds, continue the 

 construction of the sedimentary deposit until the surface is 

 brought to a level a little below high-tide mark. This growth 

 of the higher tidal marshes usually begins next the shore, 

 while that of the eel-grass tlats, which never rise above the 

 retreating tide, may take place anyw^here in the basin where 

 the currents are not strong enough to sw^eep the plants away. 

 Starting at the shore the grassed marshes extend gradually 

 outward until they leave only narrow channels for the en- 



