CORAL-REEFS. 



45 



Mourileu atoll, 1 in the Caroline Archipelago, and with the 

 barrier-reef (Plate II., Fig. 5) of the Gambier Islands. 

 I allude to the latter reef, although belonging to another 

 class, because Captain Beechey was first led by it to observe 

 the peculiarity in the question. At Peros Banhos the sub- 

 merged part is nine miles in length, and lies at an average 

 depth of about five fathoms ; its surface is nearly level, and 

 consists of hard stone, with a thin covering of loose sand. 

 There is scarcely any living coral on it, even on the outer 

 margin, as I have been particularly assured by Captain 

 Moresby ; it is, in fact, a wall of dead coral-rock, having 

 the same width and transverse section with the reef in its 

 ordinary state, of which it is a continuous portion. The 

 living and perfect parts terminate abruptly, and abut on the 

 submerged portions, in the same manner as on the sides of 

 an ordinary passage through the reef. The reef to leeward 

 in other cases is nearly or quite obliterated, and one side of 

 the lagoon is left open ; for instance, at Oulleay (Caroline 

 Archipelago), where a crescent-formed reef is fronted by an 

 irregular bank, on which the other half of the annular reef 

 probably once stood. At Namonouito, in the same Archi- 

 pelago, both these modifications of the reef concur ; it 

 consists of a great flat bank, with from 20 to 25 fathoms 

 water on it; for a length of more than 40 miles on its 

 southern side it is open and without any reef, whilst on the 

 other sides it is bounded by a reef, in parts rising to the 

 surface and perfectly characterised, in parts lying some 

 fathoms submerged. In the Chagos group there are 

 annular reefs, entirely submerged, which have the same 

 structure as the submerged and defined portions just 



1 Frederick Lutke's Voyage autour du Monde, vol. ii. p. 291. 

 See also his account of Namonouito, at pp. 97 and 105, and the chart 

 of Oulleay in the Atlas. 



