80 ON CORAL REEFS AND ISLANDS. 



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 : escape consequently takes 

 place along the bottom by the harbor-entrance, this being the 

 only means of exit. This is a correct history of numerous cases 

 about all the islands. In a group like the Feejees, where many 

 of the islands are large, and the reefs very extensive, the currents 

 are still more remarkable, and they change in direction with the 

 tides. The general mode of action, however, is the same. 



A current of water of the kind here represented, will carry 

 out much coral debris, and strew it along its course. The trans- 

 ported material will vary in amount from time to time, according 

 to the force and direction of the current. It is therefore evident 

 that the ground over which it runs is wholly unfit for the growth 

 of coral, since zoophytes are readily destroyed by depositions of 

 earth or sand, and require a firm basement to commence growth. 

 The existence of an opening through a reef requires, therefore, 

 no other explanation j and it is obvious that harbors may gen- 

 erally 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 vary- 

 ing 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 should 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 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 fresh- 

 ets, and the depositions aid those from marine currents in keeping 

 the bottom clear of growing coral. These are the principal 

 means by which freshwater streams contribute towards determin- 

 ing the existence of harbors j 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 consider 

 the size of the streams of the Pacific islands, and the fact that 

 fresh water is lighter than salt, and therefore, instead of sinking, 

 flows on over its surface. The deepest rivers are seldom over 

 six feet, even at their mouths ; and three or four feet is a 



