STRUCTURE OF CORAL ISLANDS. 



145 



Both of these stages are ilhistrated on the greater part of 

 coral islands. 



In the last stage, the island stands six to ten feet out of 

 water. The surface consists of coral sand, more or less dis- 

 coloured by vegetable or animal decomposition. Scattered 

 among the trees stand, still uncovered, many of the larger 

 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. 

 In fact, the soil is scarcely anything but coral sand. It is 

 seldom discoloured 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 



BLOCKS OF CORAL ROCK ON THE SHORE PLATFORM. 



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

 pact coral sand-rock. 



One singular feature of the shore platform, occasionally ob- 

 served, 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 plat- 

 form rock. Sketches of two of these masses are given above. 



Figure i represents a mass on the island of Waterland 

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

 meter; 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- 



i, 



