246 CORALS AND CORAL ISLANDS. 



obstruction from reefs. Any growing patches within them 

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

 gradually extend and contract their limits ; yet only at an ex- 

 tremely slow rate. Notwithstanding such changes, the chan- 

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



// 



"WHIPPET HARBOR, VITI LEVU. 



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 diminish or alter the 

 currents, and thus prepare the way for more important chan- 

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

 results from them would be appreciable only after long periods, 

 since, even in the most favorable 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 no cause sufficient to prevent its growth, the reef will 

 continue to rise slowly toward the surface. 



