38 
ATOLLS. 
Ch. I. 
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 hank, 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 
parts rising to the surface and perfectly characterised, 
in parts lying some fathoms submerged. In the Chagos 
group there are annular reefs entirely submerged, wdiich 
have the same structure as the submerged and defined 
portions just described. The Speaker’s Bank offers an 
excellent example of this structure ; its central expanse, 
which is about 22 fathoms deep, is 24 miles across ; the 
external rim is of the usual width of annular reefs, and 
is well-defined ; it lies between six and eight fathoms 
beneath the surface, and at the same depth there are 
scattered knolls in the lagoon. Captain Moresby believes 
that the rim consists of dead rock thinly covered with 
sand, and he is certain that this is the case with the 
external rim of the Great Chagos Bank, which is also 
essentially a submerged atoll. In both these cases, as in 
the submerged portion of the reef at Peros Banhos, Capt. 
Moresby feels sure that the quantity of living coral, 
even on the outer edge overhanging the deep-sea water, 
