STRUCTURE OF CORAL ISLANDS. 179 



blocks of coral, with their usual rough angular features and 

 blackened surface. There is but little depth of coral soil, 

 although the land may appear buried in the richest foliage. 

 Tn fact, the soil is scarcely any thing but coral sand. It is 

 seldom discolored beyond four or five inches, and but little of 

 it to this extent ; there is no proper vegetable mould, but only 

 a mixture of darker particles with the white grains of coral 

 sand. It is often rather a coral gravel, and below a foot or 

 two, it is usually cemented together into a more or less com- 

 pact coral sand-rock. 



One singular feature of the shore platform, occasionally 

 observed, remains to be mentioned. Huge masses of reef- 

 rock are sometimes found upon it, some of which lie loose 

 upon the reef, while others are firmly imbedded in 'it below, 

 and so cemented to it as to appear to be actually a part of the 

 platform rock. Sketches of two of these masses are here given. 



1 2 



BLOCKS OF CORAL ROCK ON THE SHORE PLATFORM. 



Figure 1 represents a mass on the island of Waterland 

 (one of the Paumotus), six feet high and about five in diam- 

 eter ; it was solid with the reef-rock below, as though a part 

 of it, and, about two feet above its base, it had been so nearly 

 worn off by the waters as to have become irregularly top- 

 shaped. Another mass, similarly attached to the reef at base, 

 observed on Kawehe (Vincennes Island), was sdx feet high 

 above low-water level, and seven feet in its longest diameter 



