I/O IV CORAL-REEF HARBORS ARE CONSTRUCTED 205 



comes to the surface of the water, the more effectively it 

 debars the polyps on the interior of the reef from the visi- 

 tation of the nourishing streams flowing inward from the 

 sea. The result is that when a reef has attained to about 

 the level of low^ tide, which is the limit of its growth, there 

 is generally an unfdled channel occupied by a lagoon-like 

 expanse of waters on its inside, the basin communicating 

 with the sea by various passages or breaks in the line of 

 the reef. 



Where such a coral reef is formed against the shore of a 

 mainland or island, the reef usually extends in a direction 

 generally parallel to the coast for the distance through which 

 the ocean currents exercise their effect on the development 

 and growth of the polyps. Thus along the coast of Florida 

 the Gulf Stream has favored the formation of these reefs, 

 from the shoals at the southern and western extremity of the 

 peninsula to a point a score or two of miles north of Cape 

 Florida. As far north as Biscayne Passage at the above- 

 mentioned cape, the reef, except for the occasional open chan- 

 nels, rises to near low-tide mark. North of that point, owing 

 perhaps to the coolness of the water, as well as to the south- 

 ward movement of sands above described, the reef is stunted, 

 its summit does not rise to the level of the sea, and the whole 

 structure gradually fades away. 



As soon as the outer part of a coral reef has risen to near 

 the surface of the water, the sea in times of storm breaks upon 

 the barrier, so that the lagoon between the rim of the reef 

 and the mainland may afford excellent shelter for ships. If 

 the coast be long and straight the enclosed channel may be 

 open to the rake of the winds in certain directions, but in gen- 

 eral at points betw^een the site of the channels through the 



