THE STRUCTURE OF THE ISLANDS 177 



lagoons ; and many forms of smaller Crustacea, sand-hoppers, 

 Avorms, &c., dwell on the flats and furnish food for the shore 

 bh'ds which frequent these places. 



The chief vegetation consists of bushes of " tea tree " 

 (FevijjJiis acidula, Forst., Kayu Burung) which grow on the 

 higher parts— islands during high water, and mounds when 

 the tide is out ; these and pioneer coco palms are studded all 

 over the white mud. Some coarse grasses {Lefturus repens, 

 Forst.) that bind the loose sand with their spreading roots, are 

 also agents, and apparently active ones, in helping to win these 

 lagoons to the island dry land. These lagoonlets differ of 

 course from the great lagoon of the atoll in that they have no 

 depth of water, and no growing coral ; but they are formed in 

 the same way as their larger representative and their fate — 

 that of slow silting with coral sand — foretells the history of 

 the lagoons of atolls in general. 



The approximation of the individual islands to a crescentic 

 form is due to a very obvious cause. At those tide-washed 

 gaps which separate island from island is the connection of the 

 outer ocean with the lagoon ; for here, at high tide, the rollers 

 sweep across the barrier flats. Every wave which breaks on the 

 barrier washes minute fragments of coral, and fine coral sand, 

 across the flats, and deposits its load of silt in the stiller 

 waters of the lagoons. It is but natural that the suspended 

 matter becomes first heaped up at the sides of the gap, for 

 here the current slackens first, and so the extremities of the 

 islands become the site of the deposition of sand-banks, Avhich 

 are formed as in-curved continuations of the land. Dr. Guppy 

 has laid much stress on this simple phenonenon as an agent in 

 land formation, and it is certainly an important one in the 

 shaping of the islands : in fact it is a continuation of the very 

 process which has, in the past, shaped the atoll itself. The 

 current that lays down the banks at the ends of adjacent 

 islands still continues to carry silt in its moving waters, and 

 this is ultimately dropped farther out in the lagoon, opposite 

 the gap. Sand-banks in the lagoon therefore, when far out, 

 tend to be opposite the interspaces of the island ring, whilst 



