FORMA TION OF CORAL REEFS AND ISLANDS. 205 



" Through the channels and among the inner reefs of the 

 Austrahan 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 destroyed 

 by depositions of earth or sand, and require, for most species, 

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

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

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

 explanation ; and it is obvious that harbours 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 harbours 

 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 detri:us, 

 especially during freshets, and the depositions aid those from 



