STRUCTURE OF CORAL ISLANDS. 29 



ens, as to resemble the clinkers of Manna Loa ; moreover, they 

 ring like metal under the hammer. Such regions may be trav- 

 elled over by leaping along from block to block, with the risk of 

 falling into the many recesses among the huge masses. On 

 breaking an edge from the black masses, the usual white color of 

 coral is at once apparent. Some of the blocks, measuring five 

 or six feet in each of their dimensions, were found to be portions 

 of individual corals, while others have the usual conglomerate 

 character of the reef-rock. 



In the next stage, coral sand has found lodgment among the 

 blocks; and though so scantily supplied as hardly to be detected 

 without close attention, some seeds have taken root, and vines, 

 purslane, and a few shrubs begin to grow, relieving the scene, by 

 their green leaves, of much of its desolate aspect. 



Both of these stages are illustrated 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 discolored by 

 vegetable or animal decomposition. There is but little depth of 

 coral soil, although the land may appear buried in the richest 

 foliage : and scattered among the trees, stand, still uncovered, 

 many of the larger blocks of coral, with their usual rough angu- 

 lar features and blackened surface. The soil is seldom discol- 

 ored beyond four or five inches, and but little of it to this depth ; 

 there is no proper vegetable mould, but a simple 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 to- 

 gether into a more or less compact coral 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 platform rock. 

 Sketches of some of these masses are here given. 



1. 2. 



Figure 1 represents a mass on the island of Waterland, (one of 

 the Paumotus,) six feet high, and about five in diameter ; 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-shape. Figure 2 is 



