120 Agassiz — Islands and Coral Reef s of the Fiji Group. 



coral reefs, either barrier, or atolls, of universal application. 

 Each district must be examined by itself, — at least such has been 

 my experience in' Florida, in the Bermudas, the Bahamas, in 

 Cuba and the West India Islands and the Sandwich Islands. 

 The results of this trip show plainly that the theory of 

 Darwin and Dana, of the formation of atolls and of barrier 

 reefs by subsidence, is not applicable to the Fiji islands, not- 

 withstanding the boring at Funafuti. In all the localities I 

 have visited the coral reefs form but a thin crust upon the 

 underlying base (it is not more than 50 to 60 feet thick in 

 Florida), and the shape and slope of this base is in no way due 

 to the growth of the corals living upon it. 



This still leaves open the question of the formation of such 

 thick masses of coral-reef rock, which though they may origi- 

 nally have been formed by subsidence, as other massive deposits 

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

 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 which 

 has received but little attention. I 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 the Ringgold islands. The 

 highest point of its rim, the exterior circumference of which 

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

 the exception of a small part of its eastern 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 six miles 

 in outer diameter with an inner basin of three miles and a 

 depth of 34 fathoms. The highest 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 on 

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

 great 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 



