36 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
which it is only separated by a narrow boat passage full of negro-heads, 
especially numerous off the southeast point of the island. The lagoon 
enclosed by the outer reef flat is elliptical. There are two ship passages 
through the north face of the outer reef. The northern and eastern reef 
flats are narrow, edged on the outer side by masses of negro-heads ; while 
the western and southern reef flats are fully half a mile broad, and edged 
by an inner belt of heads, the central part of the lagoon is quite clear of 
them. The rocks were composed of a volcanic puddingstone (Plate 64), 
much like that of Mbengha, Levuka, and elsewhere. This disintegrates 
readily, is easily undercut, as we had ample proof in the undermining of 
the shore bluffs and the formation of so many negro-heads on the reef 
flats and off the spits of the island (Plate 63). The aspect of the islands 
of volcanic origin and of elevated limestone is quite different as seen 
from the sea. The mode of disintegration and erosion of the two kinds 
of material can at once be distinguished from the peculiar physiognomy 
of the rounded bluffs when composed of rocks of volcanic origin, or of 
the vertical shore bluffs deeply grooved and streaked with red earth, 
or eroded into domes or conical hills when composed of elevated coral- 
liferous limestone. 
The reefs which encircle Mbengha, Komo, and Budd Reef, indicate 
approximately the land area once probably occupied by those islands. 
The islands must have been of considerably greater height ; they have 
been reduced by denudation, and their area has been further diminished 
by extensive submarine erosion wearing away the ridges and spurs of the 
volcanic islands, and leaving submarine platforms of varying width, — 
dependent upon the nature of the material to be eroded, and the height 
of the land to be cut down, — upon the surface and outer edge of which 
corals established themselves. In the case of Komo and of Budd Reef, 
the islets which remain show the extent of the denudation and erosion. 
In the case of Mbengha the larger islands probably retain more of the 
character of the island which once covered Mbengha, representing its 
higher peaks, while the islets and rocks are all that remain of its lower 
ridges and slopes. 
Olorua. 
On our way into Komo we saw in the distance Olorua (Plate 22), a 
small island with a ridge having three prominent humps, probably of 
elevated limestone, rising to a height of 250 feet. The island is sur- 
rounded by a fringing reef extending to a point for more than a mile off 
the south face. A small lagoon full of heads separates the north shore 
