336 "ALBATEOSS" TROPICAL PACIFIC EXPEDITION. 



the composition of the shingle of the outer barrier reef flat at points where 

 the barrier reef is narrow and impinges upon the volcanic slope, that the 

 barrier reef flat has been formed by submarine erosion, that on its surface 

 have grown the reefs which supply the material for the sand and for the 

 sand beaches on the lagoon face of the reef and of the islands, while here 

 and there in the basins of the lagoon and in the channels a few coral 

 patches rise. Even on the outer sea face of the reef flat, the shingle is 

 partly mixed with volcanic rock showing that the outliers and outcrops of 

 the lava masses are only covered by a thin sheet of limestone. South 

 of Chabrol Harbor the beach rock conglomerate contains a number of 

 volcanic pebbles. 



The gi'eater part of the corals on the flats themselves have been 

 killed by the volcanic silt washed down the deep valleys of Kusaie, and 

 by the vegetable matter derived from the decay of the mangroves ; this 

 when held in suspension in sea water kills the corals in the immediate 

 vicinity on the mangrove belt.^ 



We entered the little reef harbor of Lottin through a passage which 

 reminds one of the lagoon openings of the barrier reefs of some of the 

 Society Islands (PI. 229). At the head of the harbor, the reef flats 

 become almost fringing reefs ; they are narrower than on the south 

 side of Kusaie, where the platform off the base of the gentle southern 

 slope of Kusaie is far wider than that of any barrier reef we have 

 visited thus far (PI. 229). The entrance to Lottin Harbor is on the 

 south side of Kusaie ; it is flanked on one side by two low islands, thrown 

 up on the outer reef flat, on the other by a ledge of coral boulders on the 

 spit forming the eastern entrance to Port Lottin (PI. 182, fig. 3). The 

 outer edge of the reef flat of the southern coast, from Cape Tupinier to 

 Port Lottin, consists of a series of reaches of beach rock and of large 

 coral boulders. Numerous deep passages, similar to the one leading into 

 Chabrol Harbor, cut through the outer edge of the reef platform, and 

 form a series of tortuous channels leading into the great expanse of the 

 shallow lagoon which separates the outer reef flat from the shore belt 



^ Corals are killed by overflows from the swamps of the Everglades along the southern extremity 

 of the coast of Florida. 



