304 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



The outer reef flat slopes gradually on the lagoon face ; we found sixteen 

 fathotns more than half a mile from the inner beach. No corals are found 

 on this gentle slope, but Algse grow in great abundance on the coralline 

 sand. In some places in the lagoon we found a few patches of corals, 

 mainly Madrepores, Porites, Pocillipores, and Millepores, but they are 

 comparatively few in number and moderately developed. 



The beach rock cropping up on the lagoon face (Pis. 171, fig. 3; 172, 

 fig. 2) is a coarse conglomerate composed of fragments of recent corals ; 

 these when ground have formed the high steep beach of coarse coral sand 

 mixed with small fragments of broken corals. On the lagoon face of the 

 outer beach the slope passes rapidly to the level of the bottom of the sink 

 which extends nearly two thirds across the land rim of the island. At the 

 point where we made the section, the land rim is nearly one third of a mile 

 wide, and the outer shingle beach must be about twelve feet in height. 

 The numerous water holes and sinks are surrounded with Pandanus, Poukas, 

 a few Pisonias and large hardwood trees (PI. 173), and on the outer beach 

 is the usual belt of broad-leaved bushes and scrub vegetation (PI. 172, 

 fig. 2). The beach conglomerate extends nearly half-way across the land 

 rim ; we then come upon masses of heads of corals until we reach the sea 

 beach ; its slope is covered with fragments of shingle of all sizes, varying 

 from moderately large heads to sand. This shingle has encroached far into 

 the vegetation of the land rim (PI. 172, fig. 2), and here and there we found 

 an isolated tree or a bush standing out from the shingle almost within reach 

 of the ordinary tides. The island of Rongelappelap has been built up both 

 from the sea and from the lagoon face ; the sand from the lagoon side and 

 the small shingle being blown eastward, while the disintegration of the 

 beach rock which once covered the outer reef platform has supplied the 

 material for the outer, high steep shingle beach (PI. 172, fig. 1); some of 

 the material of course has also been derived from the destruction of the 

 masses of coral heads and of Nullipores growing on the outer reef flat. Sand 

 dunes exist in the wide and shallow gaps to the north of Rongelappelap. 



To the prevalence of the northeast trades, which in the Marshalls blow 

 with greater strength than the southeast trades do in the Southern Pacific, 

 we must look for the forces which move the great mass of material con- 



