AGASSIZ: FIJI ISLANDS AND COKAL REEFS. 125 



and southern faces, but the western and northern faces are bordei'ed by 

 disconnected reef patches (Plate 21), and towards the northeastern face 

 from the central part in from twenty fathoms the lagoon slopes gradu- 

 ally to the 100 fathom line. The western part of the lagoon is full of 

 separate patches and clusters of rocks and coral patches. Off the west 

 face lie two small banks, one of which is awash, the other with two 

 fathoms of least water and covered with heads. Off the east face are 

 three similar small banks, all of which probably represent the remnants 

 of isolated peaks or spurs of the former land covering the Argo Lagoon. 

 The Argo Eeef is separated from the Vanua Masi (Plates 21, 22) atoll 

 by a narrow channel with a depth of 115 fathoms. 



The island of Yanua Masi is not quite half a mile long, eighty feet 

 high, and is composed of elevated coralliferons limestone. Bacon Islet, 

 sixty feet in height, is stated to be of volcanic origin ; it lies on the 

 eastern face of the narrow outer reef flat facing the lagoon. This is 

 open on the southwest, where the lagoon is studded with heads and coral 

 patches. The greatest depth of the lagoon of Yanua Masi is twenty 

 fathoms, the average from twelve to sixteen. The southern part of the 

 lagoon of Argo Eeef has a depth of thirty-four fathoms, and an average 

 depth of from twenty to thirty. 



We may look upon the Argo Reef (including Vanua ]\Iasi) as vastly 

 more denuded and eroded than that of Vanua Mbalavu, whicli it re- 

 sembles in many respects. The islands which probably once covered 

 the whole area of the Argo Reefs have been disintegrated, and there 

 remain of them only the islets in Vanua Masi and the innumerable 

 heads and patches which stud the slope of the Argo Reefs. The slope 

 of the Argo Reefs corresponds to that of Vanua Mbalavu, and represents 

 the slope of the volcanic island which thrust up the elevated limestones 

 now eroded which once covered the great part of the Argo Reef Lagoon 

 as at Vanua ]Mbalavu. The great open stretch on the northern face of 

 the Argo Reefs represents a tongue of the ocean which encroached upon 

 the northern slope of the laiid and has left in the shallower parts only 

 heads and patches of corals, while in the deeper parts corals have not 

 obtained a footing. 



Thikombia. 



Plate 17. 



We did not visit Thikombia, the northernmost island of the group, 

 which I am informed is composed in part of elevated coralliferons lime- 

 stone rising to a height of four hundred feet. The island is a narrow 



