38 ATOLLS. Ch. I. 



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 denned 

 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, 

 is quite insignificant. Lastly, in several parts of the 

 Pacific and Indian Oceans there are banks, lying at 

 greater depths than in the cases just mentioned, of the 

 same form and size with the neighbouring atolls, but 

 with their atoll-like structure wholly obliterated. It 

 appears from the survey of Freycinet, that there are 

 banks of this kind in the Caroline Archipelago, and, as 

 is reported, in the Low Archipelago. When we discuss 

 the origin of the different classes of coral formations, 

 we shall see that the submerged state of the whole of 

 some atoll-formed reefs, and of portions of others 

 generally but not invariably on the leeward side, and 



