290 THE OCEAN. 
BouaBoua. 
island of Bolabola, however, is surrounded by a ring 
of land almost unbroken, on which are growing 
groves of cocoa-nuts; the reef being wholly elevated 
above the sea. 
The openings in the reefs in the larger islands 
are almost invariably placed opposite the mouth 
of a river. One can readily understand, that a 
current of fresh water would be detrimental to the 
health of a polype formed for living in the sea, 
and therefore the openings here might have been 
expected. But this effect is increased by the sedi- 
