122 Agassiz — Islands and Coral Reefs of the Fiji Group. 



Such large volcanic centers with extensive craters of con- 

 siderable depth are not unknown. We can reconstruct condi- 

 tions from Totoya which would give us an atoll open to the 

 west, with a few islands on the outer rim and a greatest depth 

 of 250 fathoms inside the lagoon. Again Haleakala in the 

 Sandwich islands has a crater with a depth of nearly 250 

 fathoms, while many small volcanic peaks, some fully 1200 

 feet in height, rise from its bottom. The diameter of Halea- 

 kala is fully as great as that of any of the atolls in Fiji. So 

 that the great depth of the lagoons of some of the atolls can 

 no longer be considered as a proof of the theory of subsidence. 



Fulanga is also interesting as illustrating the formation of 

 an atoll by the same causes which have produced the Sounds 

 in the Bermudas. In the case of Fulanga we have a volcanic 

 summit which has raised the elevated coral reef forming its 

 rim to a height of probably 200 feet. This coral rim has been 

 broken through and the action of the sea has gradually 

 hollowed out in the interior a circular sound resembling a 

 crater, which is due solely to the disintegration of the inner 

 parts of the elevated reef. In the interior and outer edge of 

 the Sound of Fulanga many parts of this reef still exist as 

 small mushroom-shaped islands or small rounded or conical hills. 

 It is also probable that some of the elevated reef-flats forming 

 the rims of atolls owe their origin to causes similar to those 

 which have shaped the crater-like Sound of Fulanga. This 

 has perhaps been the case with such atolls as INgele Levu, the 

 Oneata, Ongea and Yangasa clusters and others. 



The great variety of causes which have been active in shap- 

 ing the present physiognomy of the reefs and atolls of Fiji 

 shows the impossibility of assigning any one factor, like sub- 

 sidence for instance, as is done by Dana and Darwin, as the 

 single cause for the formation of the many different kinds of 

 atolls and barrier-reef islands to be found in the Fiji group. 

 The formation of the great barrier reef of the southern shores 

 of Yiti Levu is due to causes very similar to those which have 

 given to the northern coast of Cuba between Kuevitas and 

 Matanzas its present physiognomy. Along those parts of the 

 island where denudation and erosion proceed rapidly owing to 

 the soft character of the shore rocks, very extensive flats have 

 been formed as those south of Ovalau. When the reef-barrier 

 flats have been eroded from a harder base, like volcanic rocks, 

 the flats are less prominent and somewhat more extensive when 

 the old elevated coral reef formed the shore hills ; or the reef- 

 flats may disappear altogether when the harder volcanic rocks 

 have been only little affected by erosion or denudation. From 

 the nature of the negro-heads scattered upon the reef-flats it is 

 generally a simple matter to ascertain the character of the 

 base of the reef-flats of an atoll or of a barrier reef. 



