INTRODUCTION. xxiu 



ous limestone to one cut into separate columns or wedges surrounding a 

 central sound or basin, or a basin more or less filled with islands and 

 islets of limestone to one where the outer limestone masses or the islands 

 are reduced to a minimum forming either an elevated central area encircled 

 by a barrier reef or by a belt of low islands. 



The volcanic islands surrounded by encircling reefs show a similar 

 gradation from islands with a flanking belt of fringing or barrier reefs 

 enclosing a narrow lagoon, to islands with an extensive lagoon enclosed by 

 a barrier reef belt, or even to an atoll with a volcanic island on the outer 

 reef flat. 



In the Fiji as in the Society Islands, the wider fringing reef flats often 

 pass gradually into barrier reefs with a narrow lagoon between the outer 

 edge of the reef platform and the shore line. The rotten condition of 

 the inner part of a wide fringing reef is most favorable to its removal 

 by solution or mechanically, and leads to the formation of narrow lagoons, 

 which may, as is the case in Tahiti, become wide and deep barrier reef 

 lagoons. The small number of islands and islets of the outer barrier reef 

 in Fiji atolls contrast strongly with the well-wooded islands and islets on 

 the encircling belt of the Society Islands. 



The disintegration of the masses of corals growing upon a reef is due to 

 the boring Echini, Mollusks, Annelids, Crustacea, and sponges which infect 

 the larger masses ; as these become weakened they are torn off by the 

 waves, and rapidly reduced to shingle. The smaller fragments are then still 

 further disintegrated by boring sponges and Algoe, and by attrition on the 

 wide reef platforms both of the sea face and lagoon side reduced first to 

 coarser, then to fine sand and impalpable silt which may be carried off in 

 suspension. 



In addition to the mechanical destruction constantly going on, chemical 

 action takes place and sea water carries off in solution large quantities of 

 lime from the sea face, where we can trace the extent of its action from the 

 undercut faces of cliffs, of masses of corals, and the rotten condition of 

 smaller fragments of corals. The same action takes place in the lagoon 

 even to a greater extent ; in every direction we can trace the effect of solu- 

 tion on the beach rock beaches, the conglomerate or breccia ledges, the 



