16 "ALBATROSS" TEOPICAL PACIFIC EXPEDITION. 



sustain Dana's^ statement regarding the removal of material from the inside 

 of lagoons ; one need only look at all the islands and islet beaches on the 

 inside of the lagoon at Rangiroa, Tikahau, and Matahiva. The sea face 

 platform, Dana also says, is always greater than that on the lagoon side. 

 Certainly this is not the case in a great number of the atolls in the Pau- 

 motus. Society, Fiji, and Marshall Islands, where the surface area of the 

 lagoon flats far exceeds that of the sea face platform. 



It became very evident, after we had examined a number of atolls, that 

 the underlying ledge is the remnant of a bed of tertiary coralliferous lime- 

 stone, which at one time covered the greater part of the area of the lagoon, 

 portions of which may have been elevated to a considerable height. This 

 limestone was gradually denuded and eroded to the level of the sea. Pas- 

 sages were formed on its outside edge, allowing the sea access to the inner 

 parts of the lagoon limestone flat. This began to cut away the inner por- 

 tions of the elevated limestone, forming large sounds, as in the case of Fiji 

 atolls, and leaving finally on the weather side only a flat strip of great width, 

 which has gradually been further eroded on the lagoon side and also on the 

 sea face, to leave only a narrow strip of land varying in height; the material 

 for this land has come from the disintegration of the ledge of tertiary 

 limestone, both on the sea face and the lagoon side. There exist in the 

 lagoons a number of small islets which also consist of outliers of this 

 same tertiary limestone in process of disintegration and transformation to 

 coral sand islets and bars. 



The undei'lying ledge is not the remnant of a modern reef ; its character 

 is identical with that of the elevated limestones of Fiji, which are of Tertiary 

 age, and the rock is in every respect the same as that I observed on many 

 of the elevated islands of Fiji. The atoll of Rangiroa, for instance, is in a 

 stage of denudation and erosion very similar to that of Ngele Levu, in Fiji, 

 only in Ngele Levu the elevated limestone attains a height of about 60 

 feet, whereas at Rangiroa the coralliferous limestone ledge is from 12 to 

 14 feet high and about 40 to 50 feet wide at the top, forming on the sea 

 face of the islands and islets of the land rim a great stone-wall more or 

 less broken into distinct parts, a ledge consisting of elevated limestone as 



1 Loc. cit., p. 300. 



