i a 
AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 107 
“‘the rare case of coral forming on the lip of a volcanic crater, one part 
of which alone, perhaps the plug, has resisted the action of the sea, 
which has worn the rest of it down to the limits of wave action.” 
The foregoing figure is a view of the rock (trachytic) rising to a height 
of sixty-two feet on the rim of the atoll. Admiral Wharton calls atten- 
tion to the great depth of this lagoon, perhaps fifty fathoms, as a depth 
not improbable if it be the crater of an extinct volcano. It would be 
most interesting to have Clipperton carefully examined and mapped. 
In the mean time, from its analogy with Totoya and Thombia, it seems 
to me that Admiral Wharton’s explanation is the only possible oue. 
We may perhaps add that the old rim may have also been subject to 
atmospheric denudation and erosion, in addition to being blown away 
in part during some eruption. 
There are in the Fijis a number of small atolls from one to three or 
more miles in circumference, the formation of which can also be satisfac- 
torily explained on the theory that they have been formed upon the 
eroded summits of extinct craters, reducing the rim of the volcano either 
to a continuous flat or to flats separated by deeper passages, as in the 
case of the low parts of the rim of Totoya, forming entrances into the 
enclosed lagoons. Such atolls are Motua Levu, Motua lai lai, the Adol- 
phus Reef (Plate 18), Bell Reef, Pitman Reef, Williamson Reef (Plate 
19), Horseshoe Reef (Plate 14), and Thakau Lekaleka (Plate 27), 
although it is possible that some of these atolls may have been formed 
from the submarine erosion and denudation of volcanic peaks or of 
elevated limestone masses. It is also possible that some of the larger 
groups in which volcanic islands are found, like Vanua Mbalavu (Plate 
19), Komo, Mothe (Plate 22), Lakemba (Plate 21), and Mbengha (Plate 8), 
may represent parts of the rims of extinct craters, the bulk of the vol- 
canic peaks having disappeared from erosion, and left the outer flats 
upon which the barrier reefs have grown, while the deeper valleys and 
gorges of these volcanic islands represent the undulations of the lagoons, 
which vary greatly in depth, reaching in the case of the Vanua Mbalavu 
(Plate 19) 72 fathoms in parts of the eastern slope of the lagoon. These 
great depths, far beyond any at which corals can grow, represent the 
elevated gorges of the slopes of the volcanic peak which probably once 
extended over the whole area enclosed by the outer reef, during the 
elevation of which the reef which once covered a part of the same area 
was lifted to its present or even a greater height. 
Such large volcanic centres with extensive craters of considerabl: 
depths are not unknown.. Haleakala (Plate 71) in the Sandwich 
