250 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



covered with huge masses of Nullipores. The conglomerate beach rock is 

 composed entirely of fragments of modern species of corals, covered with 

 encrusting Nullipores. In some places the conglomerate is changed into a 

 hard ringing limestone, while in others the fragments of corals have merely 

 been agglutinated, and remain soft and crumbling. 



Skirting along the east coast of Tarawa, about two miles from the 

 southern point, we came across a number of gaps (PI. 140, figs. 2, 3) formed 

 by islands and islets, separated by shallow passages, flanked on the east by a 

 reef platform of variable width. Through the gaps we could see the lines 

 of inner islands, immediately in the rear of those of the land rim, towards 

 which spits or dams from the outer islets were gradually reaching, forming- 

 more or less distinct secondary lagoons. About the centre of the east face 

 of the atoll we came upon a large secondary lagoon, forming a huge bay, 

 its outer edge flanked by the reef platform on which the sea was pounding. 

 The width of this secondary lagoon was at least 2000 feet, and its inner 

 dam consisted of a series of low beach rock ledge walls, parallel with the 

 land rim separating the secondary lagoon from the main lagoon. Some- 

 what north of this point the inner or secondary row of islands on the wide 

 outer reef platform became even more prominent, and finally we came upon 

 a part of the lagoon with three and four rows of islands and islets, all more 

 or less parallel with the outer reef platform (PI. 140, fig. 4) ; on the lagoon 

 face they form prominent spits extending at right angles to the line of the 

 outer reef flat. The wide reef flat on which the inner rows of islands were 

 thrown up extends far into the lagoon. The islands and islets are flanked 

 on the sea face by beach rock conglomerate. 



The outer and older of the islands and islets are covered with vegeta- 

 tion, while the inner islets are generally mere bare ledges of beach rock 

 conglomerate or breccia or sand spits. Somewhat farther north we came 

 upon another secondary lagoon where the distance fi'om the lagoon edge of 

 the outer row of islands to the second row is from 1000 to 1200 feet. 

 Here the inner islands are as well covered with vegetation as the outer 

 ones ; they are occasionally connected by low sand spurs and form secondary 

 bays, with passages between the extremities of the island leading in one 

 case into the lagoon, and in the other opening out on the outer reef flat. 



