ACTION OF ORGANIC LIFE IN HARBORS 229 



of the stream is insigrnificant, and the only effect exercised 

 by the oscillations is found in a somewhat wider distribution 

 of the river detritus after it is discharged into the sea. The 

 tendency of the tides in such a case seems to be on the 

 whole to obliterate the channels which the stream would 

 make across the submerged apron of the delta. The general 

 tendency of this class of movements is to withdraw from the 

 shore and scatter over the sea bottom all the fine grained 

 detrital materials which are brought into the sea by the streams 

 or contributed to it by the beating of the waves on the coast- 

 line. The effect of this long continued action is indicated by 

 the formation on the borders of the continents and some of 

 the greater islands of a broad, submerged platform, which 

 has received the name of the continental shelf. 



The effect of organic life on harbors is generally very 

 o-reat. Save where, as in the case of coral reefs, the growth 

 of particular species tends to the institution of barriers which 

 may afford roadsteads or harbors, the influence is almost 

 wholly detrimental, for it leads to the accumulation of sedi- 

 ments and the consequent shoaling of the water of the basin 

 and along the ocean shore to the diminution in the amount 

 of the tide entering each inlet. Along the margins of our 

 o-reat lakes the harborages are less affected than is the case 

 with the havens of the sea-board, for the reason that fresh 

 water sustains a much less abundant life than does the brine 

 of the oceans. For convenience of presentation, we shall 

 divide the account of this organic work so as to consider, 

 first, the action which is accomplished by plants, and, next, 

 that which is brought about by animals. It must be said, 

 however, that in most cases the work of these two groups 

 of oro-anisms is, to a great extent, intermingled. 



