AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 107 



" tlie rare case of coral forming on the lip of a volcanic crater, one part 

 of whicli alone, perhaps tlie plug, has resisted the action of the sea, 

 which has worn the rest of it down to the limits of wave action." 

 The foregoing figure is a view of the rock (trachytic) rising to a height 

 of sixty-two feet on the rim of the atoll. Admiral Wharton calls atten- 

 tion to the great depth of this lagoon, perhaps fifty fathoms, as a depth 

 not improbable if it be the crater of an extinct volcano. It would be 

 most interesting to have Clipperton carefully examined and mapped. 

 In the mean time, from its analogy with Totoya and Thombia, it seems 

 to me that Admiral Wharton's explanation is the only possible one. 

 We may perhaps add that the old rim may have also been subject to 

 atmospheric denudation and erosion, in addition to being blown away 

 in part during some eruption. 



There are in the Fijis a number of small atolls from one to three or 

 more miles in circumfei'ence, the formation of which can also be satisfac- 

 torily explained on the theory that they have been formed upon the 

 eroded summits of extinct craters, reducing the rim of the volcano either 

 to a continuous flat or to flats separated by deeper passages, as in the 

 case of the low parts of the rim of Totoya, forming entrances into the 

 enclosed lagoons. Such atolls are Motua Levu, INEotua lai lai, the Adol- 

 phns Eeef (Plate 18), Bell Reef, Pitman Reef, Williamson Reef (Plate 

 19), Horseshoe Eeef (Plate 14), and Thakau Lekaleka (Plate 27), 

 although it is possible that some of these atolls may have been formed 

 from the submarine erosion and denudation of volcanic peaks or of 

 elevated limestone masses. It is also possible that some of the larger 

 groups in which volcanic islands are found, like Yanua Mbalavu (Plate 

 19), Konio, Mothe (Plate 22),Lakemba (Plate 21), and Mbengha (Plate 8), 

 may represent parts of the rims of extinct craters, the bulk of the vol- 

 canic peaks having disappeared from erosion, and left the outer flats 

 upon which the barrier reefs have grown, while the deeper valleys and 

 gorges of these volcanic islands represent the undulations of the lagoons, 

 which vary greatly in depth, reaching in the case of the Yanua Mbalavu 

 (Plate 19) 72 fathoms in parts of the eastern slope of the lagoon. These 

 great depths, far beyond any at which corals can grow, represent the 

 elevated gorges of the slopes of the volcanic peak which probably once 

 extended over the whole area enclosed by the outer reef, during the 

 elevation of which the reef which once covered a part of the same area 

 was lifted to its present or even a greater height. 



Such large volcanic centres with extensive craters of considerable 

 depths are not unknown.. Haleakala (Plate 71) in the Sandwich 



