TARAWA. 249 



which have led to the formation of these secondary lagoons (PL 141, fio;. 2). 

 The inner island is in reality a bar thrown up on the inner or lagoon side of 

 the reef flat at right angles to the outer reef flat, and a dam ' has been 

 formed connecting the inner and outer islands ; these bars or dams are edged 

 with beach rock or shingle (PI. 141, figs. 2-4). The areas thus shut oif, 

 finally become rectangular enclosures, entirely separated from the lagoon 

 (PI. 141, fig. 2), forming the peculiar secondary lagoons the structure of 

 which has so greatly puzzled many observers. The material for the islets 

 bounding the secondary lagoon is derived from the breaking up of the con- 

 glomerate beach rock, or breccia ledge cropping out everywhere on the 

 platform on which tlie land rim has been thrown up. The inner reef flat 

 is wide (Pis. 141, fig. 2 ; 142, 143), and extends far into the lagoon. We 

 were unable to ascertain how far north the flats extend. 



The coral shingle found above the beach rock conglomerate consists of 

 fragments of the beach rock itself, more or less disintegrated. The flats 

 between the islands and islets are covered with fine sand, almost silt, and on 

 the lagoon side they are covered with masses of broken land shells. The 

 natives use these secondary lagoons, or parts of them, as fish ponds 

 (Pis. 144 ; 145, fig. 4), the islets acting as barriers, which they connect by 

 artificial dams to enclose limited areas of the lagoons. Immediately behind 

 the principal island, which has formed the dam of the secondary lagoon, 

 are three successive bars on the lagoon side, forming long spits parallel to 

 one another, and coming out as spurs from the main land rim. These, 

 when dammed off at their lagoon extremity, will in their turn form a third 

 and even a fourth line of lagoons in the same way as the secondary lagoon 

 we have just described has been formed. 



The wide beach flat slopes most gradually to the Nullipore and Pocillipore 

 edge, which on this flat reef platform apparently indicates merely the low- 

 water line. There are no great cuts in the outer edge of the Nullipoi'e 

 knolls as in other atolls, except an occasional gully of three to four feet in 

 depth, not more than a few feet in length, flanked by knolls rising from 

 one to two feet above the general level. On the slope of the reef flat scat- 

 tered over its whole face, fine corals are growing ; the principal heads 

 consist of Porites, of pink Madrepores, and of Millepores ; the flat is also 



