THE TROPICAL PACIFIC 377 



barrier reefs may be seen, at the Cambridge Museum, 

 in the beautiful model of the Island of Bora Bora, by- 

 Mr. G. C. Curtis, who was sent by Agassiz to the island, 

 and passed some time there making notes, measurements, 

 drawings, and soundings. 



Tetiaroa and Motu Iti, two atolls of the Society group, 

 are from their position probably volcanic, and represent 

 the final denudation of such an island as Bora Bora. The 

 first stage in this process is well exemplified in Maupiti, 

 consisting of a smaller and less lofty central island, and 

 a proportionally more extensive barrier reef flat. 



Like the Fijis, the Tonga group is partly volcanic and 

 partly elevated Tertiary coralliferous limestones, here 

 developed on a scale far beyond those of the Fijis. 

 Agassiz was able, from the examination of the Tonga 

 Islands, to satisfy himself that recent corals had played 

 no part in the formation either of the masses of land or 

 the plateau of the Tonga Ridge, where they are a mere 

 thin living shell, or crust, growing at their characteris- 

 tic depths, upon either limestone or volcanic platforms, 

 the formation of which has been independent of the 

 growth of recent corals. 



The existence of the archipelago of Truk in the Carol- 

 ines, one of the volcanic formations of the group, he 

 was convinced could be accounted for by the same agen- 

 cies instrumental in creating similar formations in the 

 Fijis. He concluded that some of the atolls in the Carol- 

 ines had probably volcanic and others limestone found- 

 ations, but in none of them did he find the bases exposed. 



Nor in the atolls of the Ellice, Gilbert, or Marshall 

 Islands, was Agassiz able to observe the character of the 

 underlying base which forms the foundations of the 

 land areas of these groups. In this respect these archi- 



