RANGIROA. 49 



lagoon flat, the highest point usually being on the lagoon side of 

 the flat. 



The formation of sand bars and islets at right angles to the trend of the 

 outer reef flats is a most characteristic feature not only of the Paumotus but 

 of other atolls in the Pacific. These bars or islets are mainly the results of 

 two causes : the rushing of the water into the lagoon, and the wash of the 

 trades along the lagoon face of the land rims. The accumulations of coral 

 shingle and boulders derived from the decomposition of old ledge and 

 modern reef conglomerate act as jetties across the face of the reef flats. 

 The material worn from the flats, both on the sea face and on the lagoon 

 .side, is thus deposited on the flanks and extremities of the jetties. The 

 sand bars and islets thus formed often jut as long spits far into the 

 lagoon, and the beaches flanking the spits form a series of loops connected 

 at the base or leaving gaps for the passage of the sea in and out of the 

 lagoon. The material for these sand bars and islets is evidently derived from 

 the disintegration of the lagoon reef flat and is thrown uji by the waves as 

 sand to form these bars and islets ; they gradually increase in size and be- 

 come at first covered with scanty shrubs and finally with cocoanut trees. 

 That seems to me to have been the history of the formation of the land belt 

 on the lee side of Rangiroa. Local conditions of winds and tides and cur- 

 rents, as well as position in the outer belt of the atoll, all tend to make 

 certain points centres of accumulation and of rapid building up of islands 

 and islets, while at other places there is maintained a fair equilibrium be- 

 tween denudation and erosion and building up, and at other points all pos- 

 sible combinations of these two states. These conditions have, of course, 

 commenced to act on each atoll at somewhat different times, and have re- 

 sulted in modifications which will be noted in the description of the various 

 atolls. The changes all date back to the time when the original land flat, 

 or low ridge of elevated tertiary coralliferous limestone, which occupied the 

 area of the atoll began to be cut down to the level of the sea, and formed 

 finally on the sea face of the atoll the great shore platform which charac- 

 terizes the Paumotu atolls. The cutting down of the summits of the ridges 

 of the land rim. and of the islands or islets or bars in the lagoon to the same 

 level, supplied material for filling up in part the adjoining depths of the 



