46 CORAL-REEFS. 



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 6 and 8 fathoms beneath the surface, and at the 

 same depth there are scattered knolls in the lagoon. Cap- 

 tain Moresby believes the rim consists of dead rock, thinly 

 covered with sand, and he is certain 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, Captain 

 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 struc- 

 ture 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 the 

 existence of more deeply submerged banks now possessing 

 little or no signs of their original atoll-like structure, are 

 probably the effects of a uniform cause, — namely, the death 

 of the coral, during the subsidence of the area, in which the 

 atolls or banks are situated. 



There is seldom, with the exception of the Maldiva atolls, 

 more than two or three channels, and generally only one 

 leading into the lagoon, of sufficient depth for a ship to 

 enter. In small atolls, there is usually not even one. 



