Sect. II. ATOLLS. 37 



Chagos group, with Mourileu atoll * in the Caroline 

 Archipelago, and with the barrier reef (Plate I. fig. 8) 

 of the Grambier Islands, where Captain Beechey was 

 first led to observe the peculiarity in question. At 

 Peros Banhos the submerged 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 occurs where there is a 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 Archi- 

 pelago), 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 

 Archipelago, both these modifications of the reef concur ; 

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

 fathoms of 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 



1 Frederic 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. 



