CAUSES MODIFYING THE FORMS AND GROWTH OF REEFS. 



83 



WHIPPEY HARBOR, VITI LEBU. 



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 harbors. A remarkable in- 

 stance of the latter is exemplified in the 

 annexed chart of Whippey harbor, Viti 

 Lebu, reduced from the chart of the 

 Expedition to the scale of half an inch 

 to the mile. 



The existence of harbors should there- 

 fore 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 sub- 

 ject will be further illustrated in the 

 following pages. 



The permanency of coral harbors fol- 

 lows directly from the facts above pre- 

 sented. They are secure against any 

 immediate obstruction from reefs. Any 

 growing patches within them may still 

 grow, and the margins of the enclosing reefs may gradually extend 

 and contract their limits ; yet only at an extremely slow rate. Not- 

 withstanding such changes, the channels will remain open, and 

 large anchorage grounds clear, as long as the currents continue in 

 action. Coral harbors are therefore nearly as secure from any 

 new obstructions as those of our continents. The growing of a 

 reef in an adjoining part of the coast may in some instances di- 

 minish or alter the currents, and thus prepare the way for more 

 important changes in the harbors ; but such effects need seldom 

 be feared, and results from them would be appreciable only after 

 long periods, since the growth of reefs is very slow in the most 

 favorable circumstances. 



When channels have a bottom of growing coral, they form an 

 exception to the above remark ; for as the coral is acted upon by 

 no cause sufficient to prevent its growth, the reef will continue 

 to rise slowly towards the surface. 



Again, when the channels are more than twenty fathoms in 

 depth, they have an additional security beyond that from currents, 

 in the fact that corals will not grow at such a depth. The only 

 possible way in which such channels could close, without first 

 filling up by means of shore material, would be by the extension 

 of the reef from either side, till they bridge over the bottom be- 

 low. But such an event is not likely to happen in any but very 

 narrow channels. 



