AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 123 



not able to visit it, but, as wili be seen, it closely resembles the Horse- 

 shoe Reef (Thakau Momo, Plate 23% Fig. 6), which we examined. xVU 

 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 Laseraarawa (Plate 20), and Thakau Is"awa en- 

 close impounded water. South of Thakau Levu (Plate 22) are Thakau 

 Thikondua, Thakau lieivareiva, 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 Xukutolu Islets west of Yathata, 

 the summits are still visible. Xaiabo Islet is the remnant of a small 

 island now surrounded by a very narrow outer reef flat enclosing a lagoon. 



AYe 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 

 gTeatest 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, witli 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 cii'cular 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 sub- 



