94 CORALS AND CORAL ISLANDS. 



Honden Island (Paumotus), all others having disappeared, 

 owing to its imperfect connection with the sea. Upon the 

 reefs inclosing the harbour of Revva (Viti Lebu), where a 

 large river, three hundred yards wide, empties, which during 

 freshets enables vessels at anchor two and a half miles off 

 its mouth to dip up fresh water alongside, there is a single 

 porous species of Madrepora {M. cribripord), growing here and 

 there in patches over a surface of dead coral rock or sand. 

 In similar places about other regions, species of Porites are 

 most common. In many instances, the living Porites were 

 seen standing six inches above low tide, where they were 

 exposed to sunshine and to rains ; and associated with them in 

 such exposed situations there were usually great numbers of 

 Alcyonia and Xeniae. The Sideringe endure well exposure to 

 the air. 



The exposure of six inches above low tide, where the 

 tide is six feet, as in the Feejees, is of much shorter dura- 

 tion than in the Paumotus, where the tide is less than half 

 this amount; and consequently the height of growing coral, 

 as compared with low- tide level, varies with the height of 

 the tides. 



Porites also occur in the impure waters adjoining the shores ; 

 and the massive species in such places commonly spread out 

 into flat disks, the top having died from the deposition of 

 sediment upon it. 



The effects of sediment on growing zoophytes are strongly 

 marked, and may be often perceived when a mingling of fresh 

 water alone produces little influence. We have mentioned 

 that the Porites are reduced to flattened masses by the lodg- 

 ment of sediment. The same takes place with the hemispheres 

 of Astraea ; and it is not uncommon that in this way large 

 areas at top are deprived of life. The other portions still live 

 unaffected by the injury thus sustained. Even the Fungiag, 

 which are broad simple species, are occasionally destroyed 

 over a part of the disk through the same cause, and yet the 

 rest remains alive. It is natural, therefore, that wherever streams 

 or currents are moving or transporting sediment, there no corals 



