xxiv INTRODUCTION. 



patches of corals, the samples of the bottom, the slopes of the shoals or 

 ledges within the lagoon, all showing that solution is a prominent factor in 

 removing carbonate of lime from the interior of a lagoon. 



The Ellice, Gilbert, and Marshall Islands represent atolls in which the 

 movement of the material forming the land rim is the characteristic feature. 

 With the exception of a few atolls in the Gilberts where the old tertiary 

 ledge cropped out on the land rim we did not observe the structure of the 

 foundation upon which the material composing the land rim of the Ellice 

 or of the Marshall Islands rested. Wliile there is every indication that 

 the modern reef rock conglomerate or breccia which crops out on the islands 

 in every direction has been elevated a few feet, yet it has not been suffi- 

 ciently raised to expose the underlying rocks. 



The formation of the extensive Millepore and Porites flats described from 

 Taritari represents a stage of land growth somewhat more advanced than 

 we find it on some of the faros of the Maldives where the faces of the 

 slopes and surface of the flats are covered with thriving corals (generally 

 Pocillipores and Madrepores). The formation of the flats at the Maldives 

 I shall shortly have occasion to describe. I need here only call attention 

 to the existence at the Maldives of an earlier stage in the formation of 

 such flats as the Millepore and Porites flats of the Gilbert and Marshall 

 Islands. 



The atolls of the Ellice, Gilbert, and Marshall Islands are noted for their 

 extensive lagoon flats, for the formation of gaps, of bays, and of secondary 

 lagoons, where they take a greater development than in any other group of 

 coral reefs in the Pacific. There are only a few small coral islands among 

 these groups which have no lagoons ; they are more numerous among the 

 Marshall Islands. The shape of the atolls varies greatly in the Ellice, 

 Gilbert, and Marshall Islands, and in no groups is the outline less like the 

 mythical typical circular atoll ; this is especially marked in the Gilbert and 

 Marshall atolls. 



The land rims of these groups are narrow, but nowhere is the land rim 

 reduced to such an insignificant width as in the Marshall Islands. There are 

 no important islands in any of the lagoons ; they contain shoals, ledges, and 

 sand keys, but no well-wooded islands, as in the Paumotus and Fiji, except 



