184 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



face, are not the sources from which the material for the great dunes of 

 the Bahama Islands is derived. 



How far formations similar to those of the Bahamas occur in districts 

 where there are coral limestones of considerable- thickness, and where 

 their original structure can no longer be detected, is an open question. 

 There are groups of islands in the coral reef districts of the Pacific which 

 from their resemblance to the Bahamas it is interesting to compare with 

 them. I refer specially to the Maldives, to the Louisiade Archipelago, 

 and to the Keeling Islands. The explanation which Darwin gives of 

 the disseverment of the larger Maldive atolls, that the smaller ones have 

 been formed during subsidence by a process of which traces can be de- 

 tected in the northern part of the group, does not seem natural. The 

 sunken coral reefs of which we have surveys show no such method of 

 formation, while, on the contrary, banks of which the depth is some- 

 what varied, forming more or less irregular knobs or folds, could give a 

 suitable substructure for the growth of irregularly shaped atollons such 

 as constitute the Maldive Archipelago and the Mahlos Mahdoo atolls. 

 We may compare with these atolls the circular reefs of the Mosquito and 

 Yucatan Banks, which have formed wherever the conditions of depth 

 and currents and configuration of the bottom have been favorable, in the 

 midst of other irregularly shaped patches of corals, many of which are of 

 considerable extent. The similarity of the eastern and western ranges 

 of land of the Great Bahama Bank to the distribution of the atollons on 

 the opposite sides of the Maldive atoll group has been suggested by Pro- 

 fessor Dana ; but while this is marked, the structure of the islands in one 

 case and of the atolls in the other is, as I have shown for the Bahamas, 

 very different. The similarity of the Bahamas to the Louisiade Archi- 

 pelago suggested by Dana does not seem to me so well established. In 

 the latter we have high islands of older rocks, and upon the edge of the 

 plateau surrounding them barrier reefs have developed ; while in the 

 former we have a fringe of islands of seolian origin edged by a barrier reef. 



In the account which Dana gives of the breaking up of the Maldive 

 group into an archipelago of atolls, and of the Louisiade Archipelago, 

 there is nothing clearer than that subsidence has taken place so as to 

 bury all the high land of the archipelago, but the connection between 

 the subsidence and the growth of the atolls and barrier reefs of these 

 archipelagos is not so apparent That there are submerged banks, such 

 as the Great Chagos Bank, the Macclesfield and the Tizard Banks 

 described by Captains Wharton and Aldrich, seems to me to show the 

 impossibility of the synchronism of the rate of subsidence of a district 



