24 "ALBATROSS" TEOPICAL PACIFIC EXPEDITION. 



the interior of the lagoon, and yet are described as closed because they have 

 no boat passages. I could mention, as instances of such lagoons, those of 

 the atolls of Takume, Hikueru, Anaa, etc., or the weather faces of many 

 atolls which may be said to be closed, yet into which a huge volume of 

 water is poured at every tide over low parts of the encircling reef flats. 

 . When Dana speaks of there being no entrance to a lagoon, it merely 

 means that no boats can pass the reef flats ; it does not mean that the 

 water in the lagoon is not constantly replaced from the outside sea rushing 

 over the low sea reef flats or denuded parts of the reef platform. Dana^ 

 says Margaret (Nukutipipi) has no entrance, but the reef flat is low and 

 submerged on the east side, allowing full access of the sea to the lagoon. 

 He also says^ Taiaro has no entrance; there is one. Having thus assumed 

 the non-existence of openings to many atolls, — Anu-Anurunga, Honden 

 (Puka-Puka), and others, — he bases upon their absence a number of theoreti- 

 cal considerations regarding the closing of lagoons,^ which naturally fell to 

 the ground now that the existence of passages has been proved. 



The large number of flat-topped or slightly dished elevated islands of 

 coralliferous limestone like Nine, Makatea, Henderson Island, Tongattibu, 

 Vavau, Guam, Vatu Vara, and others in Fiji and elsewhere, seem to indicate 

 a gradation between those which are nearly flat-topped, like Nine and Hen- 

 derson and Guam, to others like Makatea and Eua, which are dished or 

 gouged by valleys, and to such islands as are so deeply dished as to appear 

 like elevated atolls, as Mango, Kambara, Naiau, and others. 



As I have already called attention in my Report on the Fiji Reefs,'* 

 we have in the Lau group all the gradations between the so-called elevated 

 atolls and such atolls as Fulanga, Yangasa, and others, in which the uplifted 

 limestones are reduced to diminutive islets which represent all that remains 

 of an uplifted limestone plateau. Naiau (in Fiji) would soon pass into the 

 condition of a ring composed of limestone islets were the sea to find access 

 to its sink by breaking through the land rim. We might in that case have 

 on a small scale an atoll like Rangiroa, in which what is left of the original 

 elevated limestone plateau would be reduced to a low wall on the weather 



1 Loc. cit., p. 199. « Loc. cit., p. .'iOi. 



■i Loc. cit., p. 200. * A. Ag., Bull. M. C. Z., XXXTII. 1899, p. 43. 



