1^6 



CORALS AND CORAL fSLAXDS. 



4<» 10 ratJivnjs. 



to 20 r"} 



atoll being freely exposed to the waters of the open sea. 

 When the channels are narrow, or few in number, although 

 the lagoon be of great size and depth (as in Suadiva), there 

 are no ring-formed reefs; where the channels are somewhat 



broader, the margmal portions 

 of reef, and especially those 

 close to the larger channels, 

 are ring-formed, but the central 

 ones are not so : where they 

 are broadest, almost every reef 

 throughout the atoll is more 

 or less perfectly ring-formed. 

 Although their presence is thus 

 contingent on the openness of 

 the marginal channels, the theory 

 of their formation, as we shall 

 hereafter see, is included in that 

 of the parent atolls, of which 

 they form the separate portions."' 

 The G?'€at Chagos Bank. — 

 This bank lies about ten degrees 

 south of the Maldives, and is 

 ninety miles long and seventy in 

 its greatest breadth. It is a part 

 of the Chagos Group, in which 

 there are some true atolls, some 

 bare atoll-reefs, and others, like 

 the Great Chagos Bank, that are 

 quite submerged, or nearly so. 

 Its rim is mostly from four to 

 ten fathoms under water. 



Mr. Darwin confirms the opi- 

 nion of Captain Moresby, that 

 this bank has the character of a 

 lagoon reef, resembling one of the Maldives; and he states, 

 on the evidence of extensive soundings, that, if raised to the 

 surface, it would actually become a coral island, vrith a lagoon 



