88 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



ISLANDS PARTLY VOLCANIC, AND PARTLY COMPOSED OF 

 ELEVATED LIMESTONES. 



Kimbombo. 



Plates 19, 61. 



The Kimbombo Islets (Plate 61) are a group of three small islands, 

 one, the highest, of volcanic structure ; the other two, the northern 

 islets, are composed of elevated limestone, situated in the axis of a tri- 

 angularly shaped lagoon, with twelve fathoms of water. The reef flat 

 surrounding the lagoon is continuous, except at the northwestern 

 point, where thei'e is a passage into the lagoon (Plate 19). In ad- 

 dition to the islands there are a number of coral patches and iso- 

 lated rocks along the inner edge of the reef flat and in the west- 

 ern part of the lagoon. The atoll is nearly five miles in its greatest 

 length, and the southern face is about three miles long. As we did not 

 enter this lagoon, I could not satisfy myself regarding the structure of 

 all the outlying rocks on the outer edge of the submarine platform, 

 some of which were evidently volcanic, others of elevated limestone. 

 This is an interesting group, showing how varied the structure of an 

 atoll may be, and yet the record on the chart be most deceptive, the 

 substructure of the Kimbombos being volcanic at one extremity, and 

 consisting of elevated limestone at the other. Half way from Kim- 

 bombo to Ngillangillah Island rises the small peak of Trigger Rock to 

 within four fathoms of the surface ; to the east of the group lies Bell 

 Reef, to the north Williamson and Dibble's Reefs, and Lookout Reef still 

 farther to the east, all separated by deep channels and rising abruptly 

 from deep water. The substructure of these reefs is of course unknown, 

 and may be either volcanic or limestone elevated or eroded to within a few 

 fathoms of the surface. 



Exploring Isles. 



Plates 19, 19% Figs. 1-3, and Plates 73-76. 



We entered the^itoll of Vanua Mbalavu, or the Exploring Isles, as it 

 was called by the United States Exploring Expedition in 18-tO, through 

 the Is'gillangillah Passage. The reef enclosing the islands is triangular 

 in shape ; its greatest length is over 22 miles along the southeastern 

 face ; the breadth from the southeastern to the northwestern horn is 20 

 miles. The outer reef flats are comparatively narrow, except in a few 

 places where, near the islands of Munia, Malatta, and Susui, they pass 



