38 On Coral Reefs and Islands. 
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 : 4 
determining their configuration and the 
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 it 
action. Coral harbors are therefore nearly as secure from any 
new obstructions as those of our continents. The growing of @ 
reef in an adjoining part of the coast may in some instances dir — 
minish or alter the currents, and thus prepare the way for more 
important changes in the harbors; but such effects need seldom — 
e 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. e 
When channels have a bottom of growing coral, they form. 
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. pe 
Again, when the channels are more than twenty fathoms 12 
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, id ee ay 
