48 "ALBATROSS" TROPICAL rAOIFIf EXI'EDITIOX. 



platform lagoon through the deep cuts which indent the outer edge of the 

 reef platform, and creates a current of considerable strength parallel with 

 the shore line. A few large masses of reef rock were scattered upon the 

 face of the outer reef platform. There was little animal life in the pools on 

 the reef flat. 



The base of the outer slope of the great wall varies in width from 100 to 

 200 feet ; the top, the height of which ranges from 10 to 14 feet, is per- 

 haps 50 feet wide, and the buttresses are well marked for 100 to 150 feet. 

 These buttresses of old reef rock could also be traced across the base of the 

 islands (PI. 14). 



Steaming to the westward of Avatoru Pass, we kept close to the land rim ; 

 this retained the general characters it possessed to the eastward, a succes- 

 sion of sand beaches and ledges and a wide shore platform. The land rim 

 consists of a long narrow island, which finally tapers off into a series of 

 islands placed upon a narrow outer reef, the horn of which is quite sharp at 

 the northwest point, and forms on the western extremity a long reef flat 

 covered with low islands and islets. The corrected sketch of the western 

 part of the island, given in PI. 204, is very different from that outlined by 

 Wilkes. After rounding the northeast corner of Rangiroa, we came upon 

 a series of gaps in the island very much wider than those we h.ad seen 

 between Avatoru and Tiputa Pass. The old ledge was exposed here and 

 there in the gaps between the islands and islets, as well as on the sea face of 

 the islands. The gaps were of varying width and depth. In some cases 

 the beach rock had been piled up in irregular low heaps so .as to leave no 

 access into the lagoon except to high seas. In others there were shallow 

 irregular channels allowing a more or less free access to the sea, while in 

 other cases the gaps were shallow channels with a depth of from one to three 

 feet at low tide, allowing the water to flow across the outer lagoon fiat. 



After we passed the northeast point of the atoll the outer islands became 

 smaller and separated by wider gaps, exposing to view the whole width of 

 the shore platform and of its extension into the lagoon flat. The islands 

 thus far were for the most part covered with shrubs and cocoanut trees, 

 but we now began to strike low sand bars and islets quite bare of vegeta- 

 tion. These bars extended at right angles to the shore line across the 



