238 SEA AND LAND 



The way in which the growth of these marshes is effected 

 varies somewhat, so far as details are concerned, according to 

 the pecuHarities of their site. The carbonaceous matter which 

 forms a large part of the fibrous layer is taken from the atmo- 

 sphere and by the process of decay gradually returns to it. 

 The permanent accumulations of the marsh are contributed in 

 part by the mud which is stirred into the sea water by the 

 incoming tide as it rubs upon the bottom and is then distributed 

 among the grass stems where it settles during the time when 

 the motion of the water is arrested, as it is for an hour or two 

 when the level is highest. This mud is effectively confined 

 amid the vegetation and adds to the mass of the marsh shelf. 

 Another element of increase is found in the development of 

 shell-bearing animals, principally mollusks and crustaceans, 

 which find a suitable site for their development amid the dense 

 herbaee. At their death these creatures leave their shells to 

 be built into the growing deposit. 



Marine marshes of the nature above described can in most 

 cases readily be won to agriculture and afford lands of extra- 

 ordinary fertilit)'. Unfortunately, however, the means whereby 

 they may be so brought under tillage are necessarily prejudicial 

 to the conditions of the harbors where they lie. In their natural 

 state they afford only light crops of poor hay. To fit them for 

 other use it is necessary to bar out the sea waters, an action 

 which necessarily diminishes the energy and consequent scour- 

 ing power of the tidal streams. A good instance of this effect is 

 shown by an experiment made ten years ago on a tract of about 

 fifteen hundred acres of marsh in the county of Plymouth, 

 Massachusetts. On the seaward border of this area there was 

 a small harbor which was useful to fishing vessels and other 

 craft drawine not more than six feet of water. When, how- 



