FORMATION OF CORAL REEFS AND ISLANDS. 239 



I. BAKRIER AND FRINGING REEFS. 



The existence of harbors about coral-bound lands, and of 

 entrances through reefs, is largely attributable to the action of 

 tidal or local marine currents. The presence of fresh- water 

 streams has some effect toward the same end, but much less 

 than has been supposed. These causes are recognized by 

 Mr. Darwin in nearly the same manner as here: yet the 

 views presented may be taken as those of an independent wit- 

 ness, as they were written out before the publication of his 

 Avork. 



There are usually strong tidal currents through the reef 

 channels and openings. These currents are modified in char- 

 acter by the outline of the coast, and are strongest wherever 

 there are coves or bays to receive the advancing tides. The 

 harbor of Apia, on the north side of Upolu, affords a striking 

 illustration of this general principle. The coast at this place 



HARBOR OF APIA, TJPOLTJ. 



has an indentation 2,000 yards wide and nearly 1,000 deep, 

 as in the accompanying sketch, reduced from the chart by the 

 Expedition. The reef extends from either side, or cape, a mile 

 out to sea, leaving between an entrance for ships. The har- 

 bor averages ten feet in depth, and at the entrance is fifteen 

 feet. In this harbor there is a remarkable out-current alone 



o 



the bottom, which, during gales, is so strong at certain states 



