xiAU. G7 



beach, Avliich will encroach on the land rim, and finally advance as dunes 

 do, and little by little fill the lagoon. 



It is ditlicult to say whether the lagoon ever has been the lagoon of an 

 atoll or has been formed as a sink. Both explanations are plausible. The 

 land rim, while solid and unbroken at present, may at one time have con- 

 sisted of several small islets surrounding a shallow sound, separated by low 

 gaps which have gradually been filled from the coral sand material supplied 

 from both the sea face and the lagoon side of the atoll, as we have seen it to 

 take place in other atolls in the Paumotus, or it may have been a 

 flat patch of elevated limestone the central part of which has been 

 gradually eroded or dissolved out into a sink by atmospheric agencies 

 assisted by the solvent action of the sea percolating through the land rim. 

 That the sea has had access to it, and that it has been shut off for a 

 long period of time, would seem to be proved by the presence of the 

 diminutive race of marine shells living in the lagoon, and the presence 

 of Lagena-like Foraminifera in the lagoon sand. The mud of the bottom 

 of the lagoon is very fine ooze, consisting of decomposed ledge rock sand. 

 Algae are growing in great abundance on the shore flat of the lagoon. 

 Disintegrated corals from the old ledge and fragments of it are found 

 along the lagoon beach. 



The shore platform forming the reef flat of Niau shows most plainly 

 (PI. 33) the manner in which the old land rim has been planed away by 

 submarine erosion, from the outer edge of the reef flat to its present 

 position, leaving on the edge of the land rim buttresses of the old 

 ledge running at right angles across the coral sand beach and lower 

 outliers of the same upon the reef platform, the space between some 

 of these buttresses and outliers being filled with modern reef rock. 

 The same process can be observed on the west shore, the southern 

 extremity of Makatea, the south shore of Rangiroa, and many other 

 atolls in the Pacific, but the changes which have taken place are 

 specially evident at Niau. 



On the southern shore of Niau (PI. 34, fig. 3) the land rim faces 

 upon deep water, and there the low cliffs have merely been under- 

 cut, as no platform could be cut on that face of the island. 



