FORMATION OF CORAL REEFS AND ISLANDS. 209 



witer currents, in preventing the growth of coral, but little is 

 due to the freshening influence of the streams of islands. 



But while observing that currents have so decided an in- 

 fluence on the condition of harbours, we should remember 

 another prevalent cause already remarked upon, which is 

 perhaps more wide in its effects than those just considered. I 

 refer to the features of the supporting land, or the character of 

 soundings off a coast. We need not repeat here the facts, 

 showing that many of the interruptions of reefs have thus 

 arisen. The wide break off Matavai may be of this kind. 

 The widening of the inner channel at Papieti, forming a space 

 for a harbour, may be another example of it ; for the reef here 

 extends to a greater distance from the shores, as if because the 

 waters shallowed outward more gradually off this part of the 

 coast. 



The same cause — the depth of soundings, on the principle 

 that corals do not grow where the depth much exceeds a 

 hundred feet — has more or less influence about all reefs in 

 determining their configuration and the outlines of harbours. 

 A remarkable instance of the latter is exemplified in the 

 annexed chart of Whippey harbour, Viti Levu, reduced from 

 the chart of the Wilkes Expedition to the scale of half an 

 inch to the mile. 



The existence of harbours should therefore be attributed, to 

 a great extent, to the configuration of the submarine land ; 

 while currents give aid in preventing the closing of channels, 

 and keeping open grounds for anchorage. This subject will be 

 further illustrated in the following pages. 



The permanency of coral harbours follows directly from the 

 facts above presented. They are secure against any immediate 

 obstruction from reefs. Any growing patches within them 

 may still grow, and the margins of the inclosing reef may 

 gradually extend and contract their limits ; yet only at an 

 extremely slow rate. Notwithstanding such changes, the chan 

 nels will remain open, and large anchorage grounds clear, as 

 long as the currents continue in action. Coral harbours are 

 therefore nearly as secure from any new obstructions as those 



p 



