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 Mathnata, 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 debris 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 zoophytes is also greater in the stiller 

 waters, and there are species peculiar to the different regions. 



