AGASSIZ: FIJI ISLANDS AND CORAL EEEFS. 43 



fathoms and an average depth of over twenty, except to the east of 

 Lauthala, where the outer reef joins the fringing reef which skirts 

 Lauthala and Ngamia. There are a number of coral patches all through 

 the lagoon, but they are most abundant along the inner ed<>-e of the 

 southern line of the outer reef. 



Ngamia and Lauthala are both volcanic ; the former rises to a height 

 of 1,000 feet. Both are indented with deep bays. 



Along the shore of Thurston Point we found a conglomerate in course 

 of formation, composed of rounded pebbles of lava cemented together 

 with broken fraguients of coral. 



ELEVATED ISLANDS COMPOSED OF CORALLIFEROUS 



LIMESTONES. 



Ngele Levu. 



Plates 17, 17*, Figs. 5-18, and Plates 95-99. 



Ngele Levu is an elongated pear-shaped atoll, somewhat constricted 

 at a few points (Plate 17). Its length is fourteen miles and its greatest 

 breadth seven. The lagoon is enclosed by a continuous outer reef, 

 varying in widtli from a quarter of a mile to over a mile. The western 

 face of the outer reef is broken into patches leaving excellent deep 

 passages for ships. The depth in the lagoon is quite uniform, the bot- 

 tom being very level, varying in depth from about five to nine fathoms 

 at the eastern extremity, and sloping very gradually to fifteen or sixteen 

 at the westei'u entrance. The bottom is composed of coralline algas, 

 broken shells, and coral sand, as well as masses of dead corals dei'ived 

 from the disintegration of the former elevated coralliferous limestone 

 which once covered the whole area of the lagoon. The lagoon is free 

 from coral patches except at the eastern end, which is studded with 

 heads of old coral and patches of living coral. These heads also form 

 a belt of considerable width along the inner side of the outer reef flats. 



The reef flats are made up of elevated coralliferous rock which has 

 been planed oflf to the level of the sea, and scooped out below it to 

 form the laooon. At the southern of the western entrances there is a 

 small sand key, and at the northeast end there are three islands, Ngele 

 Levu, Tai ni Mbeka (Plate 96), and Taulalia (Plate 95). The outer 

 faces of these islands form the sea face of the outer reef, there being no 

 outside reef flats. These islands are entirely composed of elevated 



