FORMATION OF CORAL REEFS. 153 
be situated in that area of subsidence, it will, 
of course, sink with the floor on which it rests, 
carrying down also the Coral wall to a greater 
depth in the sea. In such instances, if the rate 
f 
of subsidence be more rapid than the rate of 
growth in the Corals, the island and the. wall 
itself will disappear’ beneath the ocean. But 
whenever, on the contrary, the rate of increase 
in the wall is greater than that of subsidence in 
the island, while the latter gradually sinks below 
the surface, the former rises in proportion, and 
by the time it has completed its growth the cen- 
tral island has vanished, and there remains only 
a ring of Coral Reef, with here and there a 
break perhaps, at some spot where the more 
prosperous growth of the Corals has been checked. 
If, however, as sometimes happens, there is no 
such break, and the wall is perfectly uninter- 
rupted, the sheet of sea-water so enclosed may 
be changed to fresh water by the rains that are 
poured into it. Such a water-basin will remain 
7% 
