SYNOPSIS 251 



tions any plants so well adjusted to the work of winning the 

 ground from the sea as are those which dwell along the sea- 

 board. This lack of variety in the water-loving vegetation, 

 tocrether with the relative absence of acquatic animal life and 

 the non-existence of the tides, makes these fresh water accu- 

 mulations of comparatively little interest except that which is 

 due to the fact that they injure or destroy a large part of the 

 shallow harbors of our navigable lakes. We shall therefore 

 dismiss them without further consideration and close our 

 general account of the organic influences which affect harbors 

 with a brief resume of the facts, as follows, viz.: 



First, the land plants which have become reconciled to 

 living in salt water are the principal organic agents in effect- 

 ing injurious changes in harbors. Their influence on the 

 whole seems greater than exercised by any other single agent. 

 Second, the greater part of this work is done by the plants 

 belonging to the kindred of the grasses. 



Third, the efficiency of the work is due in large measure to 

 the cooperative effect of the tides, which nourishes the vegeta- 

 tion and the animal life which dwells amongst it. 



Fourth, the effect of the tide-water grasses is to diminish 

 the area of harbors and usually to deepen the creeks which 

 remain. The eff"ect of mangrove growth is to occlude the 

 harbor by altogether cutting out the storage of water at high 

 tide. 



Fifth, the effect of animal life other than that associated 

 with the marine grasses is purely destructive to the interests 

 of harbors, serving to shoal the water throughout the basin. 



Sixth, all the organic deposits next the land — coral reefs, 

 mano-rove and erass marshes — are to be reloaded as belonging 

 in one great group of coastal accumulations, of which the 



