250 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 


WHIPPEY 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. 
Again, when the channels are over twenty-five fathoms 
in depth, they have an additional security beyond that from 
currents, in the fact that reef-making corals rarely grow at 
such a depth. The only possible way in which such channels 
could close, without first filling up by means of shore mate- 
rial, would be by the extension of the reefs from either side, 
