Islands and Coral-reefs of the Fiji Group. 239 



originally have been formed bv subsidence, as other massive 

 deposits have been, yet may also have been formed by the 

 grr.dual pushing out to seaward of the outer edge of a reef; 

 the reef increasing both in height (depth) and in width by 

 the constant pushing out of the mass of debris and of blocks 

 detached from the outer edge, forming a talus upon which 

 corals may grow whenever the talus has reached the depth 

 at which they thrive. I am inclined to think that the careful 

 study of such a shore-reef will alone give us a correct idea of 

 the manner in which such thick masses of coralline limestone 

 may have been formed. 



There is still another phase in the formation of atolls wdiich 

 has received but little attention. 1 refer to the formation of 

 atolls as the result of the denudation and erosion of volcanic 

 summits or of extinct craters. There are in the Fiji two 

 extinct craters which are most interesting ; one of these is the 

 small extinct crater of Thombia on theRinggold Islands, The 

 highest point of its rim, the exterior circumference of which 

 is about 2 miles, is nearly 600 feet, and it is continuous with 

 the exception of a small part of its easterir edge, about a fifth 

 of a mile, across which reaches a fringing reef, the extension 

 of the fringing reef surrounding the island. This reef closes 

 the entrance into the crater, which is about half a mile across 

 at the level of the sea and has a depth of 24 fathoms. The 

 other extinct crater is that of the island of Totoya, an isolated 

 peak in the southern part of the group. It is about 6 miles 

 in outer diameter, with an inner basin of 3 miles, and a depth 

 of 34 fathoms. The higliest point of the rim is 1200 feet, 

 and at two points it is low, forming in one case a narrow 

 isthmus separating the crater from the outer lagoon. The 

 horns of the open rim are connected by a fringing reef-flat ou 

 which thunders the Pacific swell, piling up the water into the 

 gieat basin of the crater. This water finds its way out 

 through an opening called the " Gullet," which, though 

 narrow, forms an excellent passage to the anchorage inside of 

 the crater, Totoya 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 fornr a chain of small 

 islands, ■\ve should have, as soon as corals had covered the flats 

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

 atolls of nearly ciicular form — the one, that of Thombia, 



