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AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 125 
and southern faces, but the western and northern faces are bordered 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 Reef is separated from the Vanua Masi (Plates 21, 22) atoll 
by a narrow channel with a depth of 115 fathoms. 
The island of Vanua Masi is not quite half a mile long, eighty feet 
high, and is composed of elevated coralliferous 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 Vanua Masi is twenty 
fathoms, the average from twelve to sixteen. The southern part of the 
lagoon of Argo Reef 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 Masi) as vastly 
more denuded and eroded than that of Vanua Mbalavu, which 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 land 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 coralliferous lime- 
stone rising to a height of four hundred feet. The island is a narrow 
