104 bulletin: museum of comparative zoology. 



posed by him, and I know of no small atoll in which any such platform 

 has been observed. 



Neither Dana nor Lendenfeld seem to take into account the mass of 

 water which is poured into a lagoon, even if it has no boat passage, over 

 the reef flat at low tide, to say nothing of the period during which the 

 reef remains covered between low and high water times. 



A large atoll, like that of Ngele Levu, if it has been formed by 

 subsidence, should be of greater depth. This has already been noticed 

 by Admiral Wharton.^ Ngele Levu is thirty-three miles round, grad- 

 ually increasing in depth towards the western extremity from four to 

 sixteen fathoms. It rises gradually from a ridge with a depth of about 

 145 fathoms, and from a plateau of less than 400 fathoms in depth. 

 There is no filling up of that lagoon ; it is well scoured, and a strong 

 current is constantly deepening the entrance and outlets at the western 

 end, to say nothing of the mass of water which finds its way out over 

 the reef flat. The lagoon of an atoll is often referred to by writers on 

 coral reefs as a sort of stagnant pool, which must of necessity be grad- 

 ually filling. Sixch is certainly not the case in the atolls with which I 

 am acquainted, and it is the exception. If any one will take the trouble 

 to examine the hydrographic charts of the coral reef regions, both in the 

 Pacific and the Atlantic, they will find it to be the exception when the 

 atoll of a lagoon is really impounded, or that of a barrier reef is shut 

 out from a most active circulation cai'ried on by the breakers rolling 

 over the rim of the one and the bai*rier of the other into the enclosed 

 lagoon. In Fiji, the only atolls which are enclosed and surrounded by 

 a reef allowing no circulation or access to the sea are small, and play 

 no part in the physiognomy of the reefs of the group, and I know of 

 no barrier reef in Fiji the lagoon of which is not well threshed by 

 the breakers. 



An examination of the charts accompanying this report will show on 

 the rim of the larger atolls, both to windward and leeward, moderate 

 depths, from one to three fiithoms or less at innumerable places, forming 

 a regular sieve of passages through wliich the breakers force their way. 

 An examination of the charts will show the same results as regai'ds the 

 circulation across the barrier reefs of Fiji. Finally, we should re- 

 member that at high tide even fringing reefs are washed in every part 

 and scoured, and that, while at low tide a certain amount of stagnation 

 in pools may occur, it is only temporarj^, and not tlie natural state of 

 things. 



1 Nature, May 22, 1890, p. 81. 



