Agassiz — Islands and Coral Reefs of the Fiji Group. 121 



has not only a fringing reef but also a barrier reef somewhat 

 triangular in shape, surrounding the island. It is evident that 

 the barrier reef has been formed upon the denuded and eroded 

 spurs of the island which once extended seaward from the 

 outer rim of the volcano. 



Supposing now that the erosion of both Thombia and 

 Totoya had continued long enough to reduce the rim of these 

 volcanoes to the level of the sea or to form a chain of small 

 islands, we should have, as soon as corals had covered the flats 

 thus formed, which indicate the former existence of the rim, 

 atolls of nearly circular form. The one, that of Thombia, 

 being quite small with a circumference of 2 miles and a 

 depth of 24 fathoms without patches in the central lagoon ; the 

 other being much larger, more than 25 miles in circumference, 

 having a depth of 34 fathoms inside the lagoon. The lagoon 

 of the Totoya atoll would be dotted with patches some of 

 which formed parts of the rim, others being the remains of 

 eroded spurs extending towards the center of the extinct 

 crater. 



There are in Fiji a number of small atolls from one to three 

 or more miles in circumference the formation of which, it 

 seems to me, can only be satisfactorily explained on the theory 

 that they have been formed upon the eroded summits or 

 rims of extinct craters, the rim of the volcano having been 

 eroded 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 Lailai, the Adolphus reef, Bell 

 reef, Williamson, Pitman and the Horseshoe reefs and Thakau 

 Lakaleka. Of course it is possible that some of these atolls 

 may have been formed from the erosion and denudation of 

 isolated peaks or ridges. It is also possible that some of the 

 larger atolls in which are enclosed volcanic islands, like Yanua 

 Mbalavu, Komo, Motha, Lakemba, Mbenga, the Ringgold 

 islands and others, may represent parts of the rim or ridges 

 and spurs of volcanic peaks and extinct craters which have 

 disappeared by erosion and have left the outer flats upon 

 which the barrier-reef corals have grown ; while the deeper 

 valleys and gorges of these now eroded volcanic islands repre- 

 sent the undulations in depths of the lagoons. The depths 

 inside the lagoons vary greatly : in the case of Yanua Mbalavu 

 we And 72 fathoms on parts of the eastern slope of the lagoon. 

 These great depths, far beyond any at which corals can grow, 

 represent the elevated gorges and slopes of the volcanic peaks 

 which probably once extended over the whole area enclosed by 

 the outer reef, during the elevation of w r hich the reef which 

 covered a part of the same area was lifted to its present or 

 even to a greater height. 



