TARAWA. 251 



The spurs usually connect the central parts of the islands, showing that 

 the outer islands, at any rate, have increased in size uniformly on the two 

 sides of the original spit. 



Still farther north we find a series of three lines of islands ; the second 

 and the outer row are sometimes connected by a spit fully as well covered 

 with vegetation as the inner or the outer islands. The prolongation of the 

 three lines of islands is not always well marked ; they are somewhat 

 irregularly placed. Taken as a whole they form an enormous secondary 

 lagoon on the inner reef flat ; this at some points must be three fifths of 

 a mile in width. Farther north still, an islet of shingle and sand has been 

 formed directly across the gap of the two inner islets on the outer edge of 

 the reef flat and parallel with it. 



The extent of the secondaiy lagoons depends wholly upon the width 

 of the lagoon reef flat. AVhere it is narrow, the secondary lagoons appear 

 as small bays, facing either the sea or the lagoon, according to the local 

 conditions which have shut off gaps, more or less wide, either from the sea 

 or from the lagoon. Their depth depends upon the differences of level 

 found upon the lagoon reef flats, upon the depth of the gaps and the shal- 

 lower passes, as well as the distance to which the channels or sinks extend 

 lagoonward across the reef flat. The lagoon faces of an atoll as large as 

 Tarawa are raked by seas of considerable size, so that we often find, espe- 

 cially on the eastern and southern faces, bars and beaches of considerable 

 height thrown up in the interior of the lagoon, forming the boundaries of 

 secondary lagoons (Pis. 143, fig. 2 ; 144 ; 14.5, fig. 4). 



The width of the reef flats on the lagoon side of Tarawa is well seen on 

 PI. 143, fig. 1, and on PI. 142, fig. 2, where is exposed an extensive flat of 

 coral sand, of silt, and small fragments of corals and broken shells. This 

 flat forms a huge wide spur to the north of the inner line of islets flanking 

 the secondary lagoon. On PI. 143, fig. 2, is shown an extension to the east 

 of a high sand bar covered with vegetation, forming the northern boundary 

 of a long shallow sink or secondary lagoon. On PI. 143, fig. 1, we are 

 looking westward along the lagoon face of the land rim. Outliers of reef 

 rock and breccia and conglomerate crop up along the face of the lagoon ; 

 they have not as yet been disintegrated and ground into sand as they have 



