STRUCTURE OF CORAL REEFS. 



117 



There is al?o to be found about inner reefs, over large 

 areas, the soHd white limestone already described, showing 

 internally no evidence of its coral origin, and containing rarely 

 a shell or other imbedded fossil. It is a result of the consoli- 

 dation of the fine coral sand or mud that is made and accu- 

 mulated through the action of the light waves that work over 

 the inner reefs. It has been said that large regions of barren 

 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 distinction 

 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 water, and there are species peculiar to the different 

 regions. 



