FORMATION OF CORAL REEFS AND ISLANDS. 241 



of an opening through a reef may require, therefore, no other 

 explanation ; and it is obvious that harbors may generally be 

 expected to exist wherever the character of the coast is such 

 as to produce currents and give a fixed direction to them. 



The currents, about the reef grounds west of the large 

 Feejee Islands, aid in distributing the debris both of the land 

 and the reefs. In some parts, the currents eddy and deposit 

 their detritus ; in others they sweep the bottom clean. Thus, 

 under these varying conditions, there may be growing corals 

 over the bottom in some places and not in others ; and the 

 reefs may be distributed in patches, when without such an 

 influence we might expect a general continuity of coral reef 

 over the whole reef-grounds. 



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 embouchures of val- 

 leys 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 men- 

 tioned. These waters from the land bring down also much 

 detritus, especially during freshets, and 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 — the one 

 most commonly appealed to — will be obvious, when we con- 

 sider the size of the streams of the Pacific islands, and the 

 fact that fresh water is lighter than salt, and therefore, in- 

 stead of sinking, flows on over its surface. The deepest rivers 



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