CORAL-REEFS. 53 



contingent on the marginal channels or breaches being 

 wide ; and, consequently, on the whole interior of the 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 marginal 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 lagoons of all the atolls in the southern part of the 

 Archipelago are from ten to twenty fathoms deeper than 

 those in the northern part. This is well exemplified in the 

 case of Addoo, the southernmost atoll in the group, for 

 although only 9 miles in its longest diameter, it has a 

 depth of 39 fathoms, whereas all the other small atolls 

 have comparatively shallow lagoons ; I can assign no 

 adequate cause for this difference in depth. In the central 

 and deepest part of the lagoons, the bottom consists, as 

 I am informed by Capt. Moresby, of stiff clay (probably a 

 calcareous mud) ; nearer the border it consists of sand, and 

 in the channels through the reef, of hard sand-banks, 

 sandstone, conglomerate rubble, and a little live coral. 

 Close outside the reef and the line joining its detached 

 portions (where intersected by many channels), the bottom 

 is sandy, and it slopes abruptly into unfathomable depths. 

 In most lagoons the depth is considerably greater in the 

 centre than in the channels ; but in Tilla-dou-Matte, where 

 the marginal ring-formed reefs stand far apart, the same 



