288 ''ALBATROSS " TROPICAL PACIFIC EXPEDITION. 



by the breakers over the summit of the steep shingle beacli, at least eight 

 to ten feet above high-water mark. From the base of the shingle beach 

 extends a wide outer reef flat consisting of beach rock conglomerate, with 

 occasional coral heads, planed down to a gradual slope from the base of 

 the beach to the outer edge of the reef flat. 



At the northeastern extremity of the island a shingle bar has been 

 thrown across the outer sea flat. On this an island has been formed, and 

 a secondary lagoon cut of directly across the wide gap which separates 

 Imrodj from the island to the east. The formation of the Island and of 

 the secondary lagoon on the outer reef flat, within less than seven years, 

 shows the extent to which modifications may take place in the arrange- 

 ment of the material constituting the outer land rim of the Marshall 

 Islands. In the course of a few years charts indicating the position of 

 the islands and islets of the land rim may become rapidly obsolete, as 

 well as the soundings on the lagoon face, where sand is blown over the 

 narrow land rim, or whei-e gaps between adjoining islands have been closed, 

 changes which rapidly alter the general appearance of the outer land rim. 

 The condition of the sea face reef flat of Imrodj shows that the island 

 flanking it on the lagoon side is made up of material thrown up on its 

 inner slope, derived from the growing corals on the sea face and of the 

 older beach rock and coral conglomerate material once elevated a few 

 feet above high-water murk. 



The efficiency of a wide outer reef flat in protecting from disintegration 

 islands forming a land rim«is well seen at the Marshall Islands. AVhere the 

 outer reef flat platform is narrow, the huge breakers and rollers strike the 

 beach slope at high tide (Pis. 161, fig. 1; 165, fig. 1), and either carry its 

 material out to sea again or break through the beach and widen the outer 

 reef flat at the expense of the land rim. Where there is a wide outer reef 

 flat platform there are at high tide sometimes five or six successive lines 

 of breakers on the flat (PI. 167, fig. 2), the first of which strikes the outer 

 sea face of the shingle or sand beach with a very moderate force ; while 

 over a narrow platform a single line of breakers thunders directly upon 

 the beach and distributes an inunense amount of loose material at each 

 high tide. The pools on the lagoon side of wide outer reef flats are 



