132 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



lagoon flat being quite wide (Pis. 82, fig. 1 ; 83), there is considerable sweep 

 for the sea in this lagoon. 



An extensive ledge of beach rock and of recent conglomerate runs along 

 the inner face of the southwestern edge of the lagoon reef platform (PI. 82, 

 fig. 2). The reef flat on the sea face of the principal islands of the atoll is 

 narrow (Pis. 79, 80). 



The beaches on the south side are formed by a series of low dunes alter- 

 nating with masses of coral rubble, or shingle beaches with ledges of beach 

 rock at the base (Pis. 80, 81). On the reef flat there are sand bars and islets 

 with shingle bars or coral rubble dams. These are especially well developed 

 on the southwest reef, where a well-marked islet is thrown up well back on 

 the bank of the reef flat (PI. 83, fig. 1). 



We landed on the west side of the atoll of Hereheretue, on the outer edge 

 of a reef flat nearly three quarters of a mile wide (PI. 82, fig. 3). The boat 

 was run up on the reef flat in one of the many indentations gouged out be- 

 tween the NuUipore knolls. The reef flat consisted of old ledge, planed 

 down to about low-water mark, full of pot-holes of all sizes and shapes 

 (PI, 82, fig. 2). Between the outer edge of the flat and a sand bar perhaps 

 two feet high, separating the reef flat into two parts, there were numerous 

 pools left from two to three feet in depth, some of them quite extensive, 

 where many Holothurians and other Echinoderms, mollusks, and corals were 

 found congregated. Some of the corals, mainly Millepores, Pocillipores, and 

 Porites, were well-developed isolated heads of considerable size. Here and 

 there knolls of beach rock cropped out above the old ledge reef flat, and an 

 occasional horse or wall of old ledge. Close to the shore we came upon a 

 great undercut ledge of recent beach rock and conglomerate lying above the 

 old ledge flat. 



Where the reef flat of the atoll is comparatively narrow there is a well- 

 developed platform lagoon running parallel with the base of the beach. At 

 the horns of the atoll this lagoon often expands into one of considerable 

 width (PI. 79). 



The island where we landed has been thrown up diagonally across the ex- 

 tensive reef flat of the west side of the atoll, and, like other similar islands, 

 is edged on the lagoon side of the flat (Pis. 82, fig. 1 ; 83) with coral sand 



