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 naiTow, edged on the outer side by masses of negi'o-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 comjDosed 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 cpiite 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- 

 liferoiis limestone. 



The reefs which encircle Mbengha, Komo, and Budd Eeef, 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 Kom.o and of Budd Pteef, 

 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 



