142 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the central limestone hill is flanked by two nearly equal platforms of 

 submarine erosion which are scarcely more than i-eef flats. In Figures 

 8 and 10 are represented conditions in which, as in Vatu Leile, we have 

 a very shallow lagoon with reef flats both on the east and west, and the 

 remnant of the original island rising to a hundred feet at the western 

 face. In Figure 10 we have the eastern end of the Ngele Levu Lagoon, 

 with the islet of Taulalia on the north side of a shallow lagoon. Imme- 

 diately beyond Taulalia all trace of the former elevated island composed 

 of limestone disappears ; the lagoon becomes deeper as we pass to the 

 western entrances, and the only traces left of the original Xgele Levu 

 Island are the extensive reef flats on the north and south side of the 

 deeper lagoon. 



Figures 9 and 1 1, Ongea and Oneata, indicate the amount of erosion 

 and denudation which probably has taken place to reduce the islands as 

 originally elevated to their present condition. 



Figure 7 represents the manner in which the great central Sound of 

 Fulanga, with its narrow outer lagoons, has probably been formed by 

 the denudation and erosion of the central part of the limestones com- 

 jwsiug the island, aided by the solvent action both of the fresh water 

 finding its way through the central limestone mass, and that of the salt 

 sea after it had once gained access to the inner Sound of Fulanga. 



Figure 12 gives a hypothetical section across Vanua Mbalavu, show- 

 ing: the extension westward of the volcanic rocks which have raised the 

 island of Thikombia i lau, near the central part of the lagoon, composed 

 of elevated limestone. The elevated limestone to the westward of 

 Thikombia either having all been denuded and eroded or bi'oken up 

 into masses readily decomposed, the fragments of which still exist to the 

 northeast towards Ngillangillah and south towards Malatta Island. 



The sections which I have given (Plates 11% 17% 19% 20% 22% 22^) 

 plainly indicate the general flatness of the lagoons, with a slight 

 inclination in the direction of the flow of the water in the lagoon 

 toward the ship passages leading into the lagoon, and the outline of 

 the islands which have first been cut down by atmospheric agencies 

 show irregularities which disappear finally when they have come 

 within the scope of submarine erosion, resulting in such " sunken " 

 banks as the Penguin Bank (Plate 23% Figs. 7, 13). By "sunken" 

 we do not mean in any way to refer to subsidence as a factor in 

 producing such a bank. The mass of water which is poured into a 

 lagoon on the windward face of a reef, and transforms it into a gigantic 

 pothole, is something enormous. The breakers follow one another in- 



