THE STRUCTURE OF THE ISLANDS 188 



the other and more likely causes have not been excluded. 

 In Pulu Tikiis fresh water cannot be obtamed in wells, and 

 Pulu Tikus, being the island that has retained most of its 

 original features, shows in this as in the case of other pheno- 

 mena the half- formed product that more easily tells its origin. 

 At high tide the water rises, the fish-pond fills up, the 

 Kajjetings holes show water a few inches below the surface, 

 and puddles form in the more low-lying parts of the island. 

 At low tide a visit to the barrier will show this water running 

 out of the land again, welling up in holes and trickling down 

 the beach as the sponge empties. If, on this island, a hole 

 be dug as far from the sea as possible, the water that is met 

 a foot or so below the surface will be found to be either quite 

 fresh, brackish, or wholly salt ; and it depends entirely on the 

 amount of recent rain which of these three conditions is 

 found. After a wet period, fresh water will be found any- 

 where towards the centre of the island, and it will rise and fall 

 in the hole as the tide goes through its cycles : but when the 

 dry weather sets in the fresh water becomes more and more 

 brackish, and in the course of some days becomes entirely 

 salt again. The reason for this is easily seen, for since the 

 land is so little raised above the ocean-level, the water which 

 the accumulated rains produce must lie as a subsoil water very 

 near to the surface, and, as it is lighter than the sea water, it 

 rises and falls as a surface layer with the movements of the tide. 



Fresh water will remain separate as a surface layer even in 

 the open sea for a considerable time. After a tropical down- 

 pour in Singapore roadstead the surface specific gravity was 

 lowered from 1026° to 1021°, and on two occasions in this 

 atoll so continuous was the rain that large numbers of fish died 

 from the freshness of the water, for the rain stood for a height 

 of several inches on the surface of the salt waters of the lagoon. 



The diffusion which takes place slowly in the sea takes 

 place still more slowly when the rain water, after percolating 

 through the porous debris of which the island is composed, 

 lies in the depressions of the compact breccia layer that forms 

 the basis of the islands ; and in an island so porous as is Pulu 



