mTRODUCTION. xvii 



the preponderance ; thus in adjoining atolls different results may take place, 

 according to the conditions which oljtain the mastery. The above factors 

 are at work in the Paurnotus upon plateaus of elevated coralliferous lime- 

 stone cut down towards the level of the sea, while in other groups where 

 the nature of the underlying plateau is not known, they are at work 

 upon a land rim of modern reef rock, or modern conglomerate or breccia. 



The bottom deposits on the deep parts of the ocean separating the 

 atolls or groups of atolls in the Paurnotus are truly oceanic in char- 

 acter. The many and wide passages dividing the islands are extensions 

 of the great red clay belt, covering the trough lying to the southeast 

 of the northern and central Paurnotus between the main belt of the 

 Paurnotus and the Gloucester Islands line, and a similar trough sepa- 

 rating the Paurnotus and Marquesas. There is, however, a marked differ- 

 ence ill the extension of this red clay into the volcanic bottom which 

 surrounds the Marquesas and its passage into the globigerina-ooze ; 

 Pteropod and finally coral sand ooze is developed round the Paumotu 

 Islands in shallower water. 



The Cook Archipelago consists of volcanic islands with encircling reefs, 

 of elevated coralliferous limestone islands and of low atolls, the under- 

 lying base of which is not known. The elevated islands have narrow reef 

 flat platforms formed by submarine erosion ; some of the elevated islands 

 are partly volcanic and partly limestone. 



On a small independent plateau between the Cook Islands and Tonga 

 rises the island of Nine, also composed of elevated coralliferous limestone. 



The elevated limestone islands of Tonga and Fiji are flanked by nar- 

 row reef platforms ; these are not continuous and occur only on parts of 

 the islands where the sloughing off of the terraces has formed a shore 

 platform which in its turn has been subject to submarine erosion. On this 

 platform corals have established themselves, usually as fringing reefs. 



The Tonga Archipelago consists of an extensive area of elevated coral- 

 liferous limestone, greatly denuded and eroded. At the two extremes of the 

 plateau land masses of considerable extent have been elevated ; the southern 

 mass, Tongatabu, is greatly indented at its northern face and forms the 

 southern face of an ill-defined lagoon. The northern land mass, Vavau, is cut 



b 



