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 by the rising of Munia 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 or patches composed of volcanic rocks or of elevated coralliferous 
limestone, or of a combination of both. 
Nowhere in Fiji have we found the fringing corals growing in such 
profusion as in the Ngillangillah Channel. While 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, 22°, Fig. 3, and Plates 85-87. 
Mango 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 (Plate 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 conicai 
or dome-shaped mounds and patches deeply undercut, some of which are 
