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 reef 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 Ngele Levu 
Island are the extensive reef flats on the north and south side of the 
deeper lagoon. 
Figures 9 and 11, 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- 
posing 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 broken 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- 
