AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 93 



occupying the area of Vanua Mbalavu Lagoon, caused by the uplifting 

 of the ridge forming the crest of Vanua Mbalavu, which has a short 

 western slope and a long comparatively easy eastern slope, with the 

 resulting deep valley leading to the American Passage, and perhaps a 

 secondary elevation caused b}^ the rising of Alunia Island. 



The depth of the western lagoon varies between 10 and 20 fathoms, 

 that of the western part of the eastern lagoon from 6 to 30, while the 

 eastern part of the lagoon slopes gradually from 20 or 25 fathoms toward 

 the 100 fathom tongue in the eastern face of the outer reef. On sound- 

 ing one mile north from the edge of the outer reef, off Blackswan Point, 

 we found, at 203 fathoms, hard bottom. The character of the bottom 

 of the lagoon varies greatly according to its proximity to shores or 

 heads 1:>r patches composed of volcanic rocks or of elevated coralliferous 

 limestone, or of a combination of both. 



ISTowhere in Fiji have we found the fringing corals growing in such 

 profusion as in the Ngillangillah Channel. AVhile steaming along the 

 northern face of the reef, close to the outer edge, we had a good oppor- 

 tunity to note the appearance of the outer reef flat, of a yellowish green 

 color covered with patches of violet, indicating the areas incrusted by 

 living corals. Being on the lee side, there was scarcely a ripple along 

 the outer edge, which was in sharp contrast to the very dark blue of 

 the water covering the outer slope of the reef, while on the inner side 

 there was a more or less wide light greenish belt bordering the reef flat 

 and passing through light blue into the darker blue of the narrow con- 

 tinuation of the Ngillangillah Passage, itself dotted with greenish or 

 yellowish patches of coral all along the reef flat, and on many of the 

 inner patches were scattered negro-heads, consisting of elevated coral- 

 liferous limestone. 



Mango. 



Plates 19, 33*, Fig. 3, and Plates 85-87. 



Manso is a circular island, the southeastern and southern faces of 

 which consist of a rim of volcanic rocks, through which rises to a 

 height of 670 feet the central elevated ridge of the island (Plato 19). 

 On the northern and northeastern faces an ancient coralliferous lime- 

 stone has been elevated to a height of from 200 to 300 feet or more. 

 That part of the elevated limestone which lies across the break of the rim 

 of the island has been eroded and denuded to form a small lagoon or 

 diminutive sound (Plate 85). The entrance is flanked by low conical 

 or dome-shaped mounds and patches deeply undercut, some of which are 



