ORIGIN OF CORAL REEFS 



affords a striking illustration of this general principle. 

 The coast at this place has an indentation 2000 yards 

 wide and nearly 1000 deep. The reef extends from 

 either side, or cape, a mile out to sea, leaving between 

 an entrance for ships. The harbor averages ten feet in 

 depth, and at the entrance is fifteen feet. In this 

 harbor there is a remarkable out-current along the bot- 

 tom, which, during gales, is so strong at certain states of 

 the tide that a ship at anchor, although a wind may be 

 blowing directly in the harbor, will often ride with a slack 

 cable ; and in more moderate weather the vessel may tail 

 out against the wind. Thus when no current but one in- 

 ward is perceived at the surface, there is an undercurrent 

 acting against the keel and bottom of the vessel, which 

 is of sufficient strength to counteract the influence of the 

 winds on the rigging and hull. The cause of such a cur- 

 rent is obvious. The sea is constantly pouring water over 

 the reefs into the harbor, and the tides are periodically 

 adding to the accumulation ; the indented shores form a 

 narrowing space where these waters tend to pile up ; es- 

 cape consequently takes place along the bottom by the 

 harbor entrance, this being the only means of exit. There 

 are many such cases about all the islands. In a group 

 like the Feejees, where a number of the islands are 

 large and the reef very extensive, the currents are still 

 more remarkable, and they change in direction with the 

 tides. 



' The results from marine currents are often increased 

 by waters from the island streams ; for the coves, where 

 harbors are most likely to be found, are also the em- 

 bouchures of valleys and the streamlets they contain. 

 The fresh waters poured in add to the amount of water, 

 and increase the rapidity of the out-current. At Apia, 

 Upolu, there is a stream thirty yards wide; and many 

 other similar instances might be mentioned. These 

 waters from the land bring down also much detritus, 

 especially during freshets, a,nd the depositions aid those 

 from marine currents in keeping the bottom clear of 

 growing coral. These are the principal means by which 

 fresh-water streams contribute toward determining the 

 existence of harbors ; for little is due to their freshening 

 the salt waters of the sea. 



" The small influence of the last-mentioned cause 



217 



