STRUCTURE OF CORAL REEFS. 147 
sands or mud occur among the patches of growing corals, and 
these would give origin to this compact limestone. 
The formation of the inner reefs goes on at a less rapid 
rate than that of the outer, because the process depends on the 
growth of the corals with comparatively little aid from the 
action of the waves. Moreover, as is explained more par- 
ticularly in another place, impure or fresh waters and cur- 
rents often operate to destroy the living corals or retard their 
progress. 
Owing to the last mentioned cause, the inner reefs are not 
usually joined directly to the beach. They stand off a little, 
separated by an interval of shallow water. At Mathuata, in 
the Feejees, however, the reef extends quite up; and it is 
the more remarkable as the coast is flat, the site of a Feejee 
village, and a mile or two back stands a high bluff. On an 
island off this part of Vanua Lebu there is another exam- 
ple of this fact, and many more might be cited. In such 
cases, however, there is evidence that the shores upon which 
the corals grew were bare rocks, instead of moving beach- 
sands. 
From these descriptions it appears that the main distinc- 
tion between the inner and outer reefs consists in the less frag- 
mentary character of the rock in the former case, the less fre- 
quent accumulations of débris on their upper surface, and the 
more varied features and slopes of the margin. Moreover, 
the Nullipores, which seem to flourish best in the breakers, 
are here but sparingly met with. 
The variety of coral zodphytes is also greater in the stiller 
waters, when these have great breadth, and there are species 
peculiar to the different regions. 
