40 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



island with a ridge having a height of about 1,100 feet, a western island 

 with a height of about 800 feet, and a northwestern island with a sum- 

 mit of the same height. These islands might thus be reduced to three 

 separate ridges, giving no indication that they had formed part of the 

 rim of the crater of an extinct volcano. 



The denudation and erosion could be carried still farther, leaving only- 

 islets, the summits of the higher peaks, to indicate the former position 

 of the rim, the islets being joined by coi'al patches connecting their 

 extremities, much as the present opening between the horns of the rim 

 of the crater is closed by the outer reef. AVe may still further imagine 

 it to be so far cut down as to form reef flats upon which coral would 

 grow, thus forming a nearly circular atoll with a depth of 35 fathoms, 

 — an atoll with the formation of which subsidence has had nothiua' to 

 do. But this is not an imaginary atoll I am reconstructing. A nmnber 

 of such atolls are found in Fiji, the formation of which can be satisfactorily 

 explained on the theory that the I'ing of coral patches represents the 

 rim of an extinct volcano which has been cut away to below low water 

 mark. Such atolls in the Fijis are probably Thakau Momo, Thakau 

 Lasemarawa, Thakau Lekaleka, Motaa Levu, Motua lai lai. Pitman 

 and Williamson Eeefs, and perhaps others. 



The example of Thombia, one of the iLinggold Islands, in which there 

 is only a distant outer reef, would also indicate the possibility of the rim 

 of the crater of a small volcanic peak cut down to the surface and forming 

 the circular flats upon which corals might grow. In the case of Thom- 

 bia such a condition would result in forming a diminutive atoll not more 

 than a third of a mile in diameter, enclosed within an encircling barrier 

 reef. 



We might also consider the " Boilers," the diminutive " Serpuline 

 atolls" inside of the lagoon of the outer reef off' the south shore of the 

 main island of the Bermudas,-^ as a series of such interior atolls, though 

 the mode of origin is very different from that of subordinate atolls, 

 formed, as I have suggested, upon the rim of an extinct crater like 

 Thombia. In either case, the explanation of the formation of such 

 interior or subordinate atolls is radically different from that given by 

 Darwin ^ for their growth in the Maldive atolls, an explanation also 

 accepted by Dana. 



It is becoming more and more apparent that each locality must be 



1 Bull. Mus. Comp. Zoiil., XXVI. No. 2, 1895, Plates XXII.-XXVI., p. 253. 



2 Darwin's Coral Reefs, 3d ed., p. 44. 



