AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 123 
not able to visit it, but, as will be seen, it closely resembles the Horse- 
shoe Reef (Thakau Momo, Plate 23°, Fig. 6), which we examined. All 
these islands are atolls with an outer reef flat and a lagoon of moderate 
depth, varying from ten to about twenty fathoms as the greatest depth, 
with entrances into the lagoon, and with only rocks and patches and no 
islands on the outer reef flats. 
Thakau Vau, Thakau Lasermarawa (Plate 20), and Thakau Nawa en- 
close impounded water. South of Thakau Levu (Plate 22) are Thakau 
Thikondua, Thakau Reivareiva, Thakau Nasokesoke, and Thakau Teteika, 
representing probably as well as Wilkes Reef north of Namuka, and the 
two reefs Tavanuku i wai and Tavanuku i vanua and Frost Reef, the 
summits of small peaks or of crests, now covered only by heads on which 
corals have found a footing. In the case of Tavunasithi (Plate 22), Nuku 
Songea, Yaroua, Maafu Rock, and the Nukutolu Islets west of Yathata, 
the summits are still visible. Naiabo Islet is the remnant of a small 
island now surrounded by a very narrow outer reef flat enclosing a lagoon. 
We did not visit the Nukutolu Islets (Plate 19), which seem to be the 
summits of a former narrow ridge. Frost Reef, to the west of Mango 
(Plate 19), is a flat circular reef of about a mile in diameter, with a rock 
at its northern edge. 
Neither did we examine the following islands and reefs to the south 
of the Exploring Isles (Plate 19): Malevuvu Reef, an atoll nearly three 
miles long by half that in width, and a lagoon with thirteen fathoms 
greatest depth, accessible to boats on the west side; Katavanga, a small 
island, consisting, according to Captain Cocks, of elevated coralliferous 
limestone rising to a height of 180 feet, situated in the western part of 
the elliptical lagoon encircled by a reef flat widest at the eastern face, 
and over three miles in greatest diameter, with a greatest depth of thir- 
teen fathoms, and an opening for small vessels on the northern side of 
the lagoon; and Vekai, an elevated limestone rock nearly thirty feet high 
on the inner edge of a circular reef about two miles in diameter, with a 
boat passage on the northwest side leading into a lagoon with a greatest 
depth of eighteen fathoms. The Malima Reef resembles Kimbombo Reef. 
The islets are near the centre of a lagoon about two miles in diameter. 
It would not be difficult in most cases to determine how these smaller 
reefs have been formed. They are either in part volcanic and in part 
elevated coralliferous limestone, or wholly volcanic, or elevated limestone 
alone. The result would in either case be the same. The volcanic 
islets disintegrating more slowly than the tertiary limestone, it is prob 
able that the reefs, which show no signs of volcanic rocks, have a sul- 
