AGASSIZ: FIJI ISLANDS AND CORAL EEEFS. 



Tuvutha. 



Plates 30, 88, 89. 



Tuvutha Island is triangular iu shape ; its northern face is about one 

 and three quarters miles long; the island is nearly three and a half 

 miles long, and trends in a northwesterly direction from the southern 

 point to the northern face (Plate 20). It is surrounded by a barrier 

 reef. The greatest width of the enclosed lagoon is about three quarters 

 of a mile on the eastern side ; it is somewhat naiTower off the western 

 face of the island. At two points on the northern side of the island the 

 outer reef becomes a fringing reef for a short distance. There are a 

 couple of boat passages into the lagoon through the outer reef flats, one 

 on each side. The lagoon varies in depth from two to nine fathoms. 

 With the exception of the central part of the eastern lagoon, it is 

 studded with coral patches and negro-heads. Tiie latter are especially 

 numerous off the northern flxce of the island between those points 

 ■where the outer reef becomes a fringing reef, and empounds a small 

 distinct lagoon. 



The central ridge, occupying the interior of the northern part of the 

 island, runs parallel to the west coast, and as far as I could judge con- 

 sists of coralliferous limestone elevated to a height of eight hundred feet. 

 The central ridge is surrounded by a more or less continuous ridge, form- 

 ing an outer rim and surrounding the inner depressed part of the island. 

 This ridge consists entirely of elevated limestones, forming along the 

 northern face a collar of steep bluffs (Plate 88). Other parts of the 

 ridge, especially on the west side of the island, have been greatly 

 denuded and eroded into rounded peaks and domes, while the southeast 

 point is marked for its steep bluffs and sharp ridges (Plate 89). Where 

 the bluffs rise from the shore of the lagoon, they are deeply undercut. 

 The outer ridge not being low and broken at any point, no interior 

 lagoon or sound has been formed with the exception of a small bight on 

 the southern part of the eastern face of the island (Plate 20). 



The central volcanic mass which has elevated Tuvutha as well as 

 Naiau has not broken through the elevated limestones, although it has 

 assumed the appearance of the wide round rim of an extinct crater. 

 But, as will be seen in the case of Naiau, the formation of the central 

 basin is not due to volcanic agencies, but to atmospheric causes. 



