IKTEODUCTION. xxvii 



Elevated coralliferous limestone islands, cut down nearly to the level of 

 the sea by atmospheric agencies, like Niau in the Paumotus, with a shallow 

 sink (lagoon) connecting with the sea only through the porous mass of the 

 land rim, or Rangiroa, and the majority of the islands in the Paumotus 

 and some of the Gilbert Islands (Apamama, Tapeteuea), with a lagoon 

 enclosed by a land rim composed of disconnected islands and islets form- 

 ing passes and gaps communicating with the sea ; islands all noted for the 

 great development of buttresses of modern reef rock and of tertiary age 

 on the reef platforms, the remnants of a higher land mass now denuded 

 to the level of the sea. Similar outliers in the lagoon form shoals, islands, 

 and islets. 



Extensive elevated limestone masses, like those of the Tonga Archi- 

 pelago with volcanic outbursts of limited extent that have pushed through 

 the limestone masses. 



Atolls with disconnected limestone and volcanic islands, the remnants of 

 islands partly volcanic and partly limestone, like Guam and Eua, only on 

 a smaller scale. In these the volcanic outbursts as well as the lime- 

 stone masses have been denuded and eroded, and formed the groups of 

 Vanua Mbalavu, Lakemba, Naitamba, Mothe, and the like. 



Low atolls, like those of the EUice, Marshall, and the Gilbert Islands 

 in part, where the land rim is reduced to a minimum and best developed 

 on the weather side, and where the material composing it is subject to 

 constant transport, the material being derived from the corals growing on 

 the sea slope of the reef fiat platforms and from the disintegration of the 

 slightly elevated modern I'eef rock conglomerate or breccia. Atolls all 

 pharacterized by the great changes taking place in the extent of the 

 islands of the land rim, owing to the formation of sand bars, shoals, flats, 

 bays, the closing of gaps, the falling of lagoons by sand blown in from 

 the sea face, the throwing up of extensive dams on the lagoon flats to form 

 secondary lagoons, the existence of reef platform lagoons; the islands on the 

 land rims of these atolls being flanked with coral sand, shingle or boulder 

 beaches on the sea face, with sand beaches or beach rock flats on the lagoon 

 faces. The islets, shoals, and islands in the lagoons are either patches of 

 elevated modern reef rock or knolls of growing corals, many of them with 



