240 CORALS AND CORAL ISLANDS. 



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 inward 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 rig- 

 ging and hull. The cause of such a current 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 : escape 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 reefs very extensive, the currents are still more 

 remarkable, and they change in direction with the tides. 

 "Through the channels and among the inner reefs of the 

 Australian reef-region," says Jukes, "they run sometimes 

 with an impetuous sweep in the same direction even for two 

 or three days together, especially after great storms have 

 driven large quantities of water into the space between the 

 outer edge and the land." 



A current of the kind here represented will carry out much 

 coral debris, and strew it along its course. The transported 

 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 must be wholly unfit for 

 the growth of coral, since most zoophytes are readily destroy- 

 ed by depositions of earth or sand, and require, for most spe- 

 cies, a firm basement. Or if the flow is very strong, it will 

 scour out the channels and so keep them open. The existence 



