AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 51 
Tuvutha. 
Plates 20, 88, 89. 
Tuvutha Island is triangular in 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 narrower 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. The latter are especially 
numerous off the northern face 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 Jess 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 undereut. 
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 voleanic mass which has elevated Tuvuthd 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. 
