INTRODUCTION. • xxv 



close to the lagoon edge of the land rim where they form secondary lagoons. 

 Nauru and Paanopa to the west of the Gilbert Islands are elevated corallif- 

 erous limestone islands, rising to over 250 feet, similar to the elevated islands 

 of the Paumotus and Fiji. 



We noted in the Marshall Islands the large amount of sand blown into 

 the lagoons from the sea face through gaps. Much of this sand is held 

 in suspension and carried out through the passes on the lee side, while a 

 part of it forms spits and flats near the gaps and passes of the weather side. 

 A part of the sand goes to fill the lagoon ; a part is carried off and reduces 

 the width of the land rim on the sea face. 



The Carolines are divided into high volcanic islands surroimded by fring- 

 ing, barrier, and encircling reefs, low atolls with groups of islands reduced to 

 narrow encircling reef flats on which a few islands have been thrown up 

 on the weather face, the underlying foundation of which is not exposed and 

 atolls where the islands and islets of the surrounding reef flat platform 

 consist of elevated coralliferous limestone. In Ponapi the barrier reef flats 

 are wider than round any other volcanic island in the Pacific. 



The southern islands of the Ladrones, the last group of islands we exam- 

 ined, are partly volcanic and partly composed of elevated coralliferous 

 limestone, while the northern islands are volcanic and have no coral reefs. 

 Coral reefs take but a slight development either in Guam or Rota; the reef 

 flat platforms are narrow, irregularly scattered ; at Guam they take their 

 greatest development at San Luis d'Apra and along parts of the western 

 coast. At San Luis d'Apra a deep reef harbor has been eroded from the 

 elevated coralliferous limestone flat which once jutted out to the north 

 of Orote Point to Cabras Island. Guam is one of the largest of the 

 islands composed in part of coralliferous limestone and in part of volcanic 

 material. 



We may approximately classify the atolls, elevated islands, and volcanic 

 islands where reefs are found into the following categories : 



Large volcanic islands with barrier and fringing reefs, like Tahiti, Viti, 

 and Vanua Levu, the larger Samoan Islands, New Caledonia, the Solomon, 

 New Hebrides, and Sandwich Islands ; in these the land mass occupies a 

 large area as compared with that of the reefs. Viti Levu and Vanua Levu, 



