THE STRUCTURE OF THE ISLANDS 181 



When an island is formed of the typical coral debris, consisting 

 of detached slabs of breccia, boulders and fragments, it is 

 always evident that its windward margin is its highest land, 

 and, on section, it is roughly wedge-shaped, the apex of the 

 wedge being towards the lagoon : — for the island has been 

 built by the waves from the ocean side, and its wave-beaten 

 shore is piled the highest. On the other hand, when an 

 island is made of sand it is moulded mostly by the wind, and 

 the effect of the wind is to drive the light sand farther and 

 farther towards the leeward side, so that the island rises from 

 the ocean shore to the lagoon beach — the apex of the wedge 

 being- towards the ocean. So marked is this effect of wind 

 formation that, in the case of Pulu Bras, the seaward shore 

 rises as a gradual sandy slope, and ends abruptly towards the 

 lagoon as a sand cliff twenty-seven feet high, which is the 

 leeward limit of the island. On all the islands this wind- 

 driven sand makes some covering for the coral basis, and most 

 often it is collected into characteristic ridges and mounds, 

 making some little break in their otherwise level surface. As 

 the sand is being built up all over the islands, even in those 

 parts where wave action has long ceased to have any influence, 

 it must have been one of the most considerable of the agents 

 in raising the land from its sea-made level. The action of the 

 wind- driven sand has doubtless, in a great measure, w*aned since 

 the islands have become thickly planted with coco palms, and 

 now is probably secondary in importance to the influence of 

 the vegetation itself. 



On the lagoon shore the sand is the only material from 

 which the island now builds, and the wind-driven particles are 

 forced back by the lapping waves of the lagoon, with the 

 result that a characteristic rise of sand marks the fall to the 

 inner shore, just as the boulder rise marks the seaward edge. 



The outcome of these various building forces is that the 

 atoll islands have not only a characteristic geographical shape, 

 but they also possess a characteristic land contour. The sea- 

 ward edge is piled the highest, and is the mountain range of 

 the island — in Pulu Tikus, on the lee of the atoll, reaching 



