CORALS AND CORAL ISLANDS. 



of our continents. The growing of a reef in an adjoining 

 part of the coast, may in some instances diminish or alter the 

 currents, and thus prepare the way for more important changes 

 in the harbour ; but such effects need seldom be feared, and 

 results from them would be appreciable only after long periods, 

 since, even in the most favourable circumstances, the growth 

 of reefs is very slow. 



When channels have a bottom of growing coral, they form 



an exception to the above remark ; 



.;'; ->., for since the coral is acted upon by 



,\ "'-Vx no cause sufficient to prevent its 



i ; >., growth, the reef will continue to 



• •■ '\ rise slowly toward the surface. 



\ Again, when the channels are 



\\ \ more than twenty fathoms in depth, 



,\ ij they have an additional security 



'• '■ beyond that from currents, in the 



\\ \\ fact that corals will not grow at 



// ;•; such a depth. The only possible 



i \ li way in which such channels could 



,// \\ close, without first filling up by 



means of shore material, would be 



by the extension of the reefs from 



either side, till they bridge over the 



bottom belov/. But such an event 



is not likely to happen in any but 



narrow channels. 



In recapitulation, the existence of 

 passages through reefs, and the character of the coral har- 

 bours, may be attributed to the following causes : — 



1. The configuration and character of the submarine land ; 

 — corals not growing where the depth exceeds certain limits, 

 or where there is no firm rocky basement for the plan.tation. 



2. The direction and force of marine currents, with their 

 transported detritus ; — these currents having their course 

 largely modified, if not determined, as in other regions, by the 

 features of the land, the form of the sea-bottom and the posi- 



WHIPPEY HARBOUR, VITI LKVU 



