230 CORALS AND CORAL ISLANDb. 



surface. The layer of dead coral rock which makes the body 

 of the reef, has its border of growing corals, and is thus un- 

 dergoing extension at its margin, both through the increase in 

 the corals, and the debris dropped among them. 



But besides the small fragments, larger masses will be 

 thrown on the reefs by the more violent waves, and commence 

 to raise them above the sea. The clinker fields of coral by 

 this means produced, constitute the first step in the formation 

 of dry land. Afterward, by further contributions of the 

 coarse and fine coral material, the islets are completed, and 

 raised as far out of the water as the waves can reach — 'that is, 

 about ten feet, with a tide of three feet; and sixteen to 

 eighteen feet with a tide of six or seven. 



The Ocean is thus the architect, while the coral polyps af- 

 ford the material for the structure; and, when all is ready, it 

 sows the land with seed brought from distant shores, covering 

 it with verdure and flowers. 



The growth of the reefs and islands around high lands is 

 the same as here described for the atoll. The reef-rock is 

 mainly a result of accumulations of coral and shell debris. 

 There are reefs where the corals retain the position of growth, 

 as has been described on a former page. But with these the 

 debris comes in to fill up the intervening spaces or cavities, and 

 make a compact bed for consolidation. There are other parts, 

 especially portions of the outer reef along the line of break- 

 ers, which are formed by the gradual growth of layer upon 

 layer of incrusting Nullipores ; but such formations are of 

 small extent, and only add to the results from other sources. 



II. ORIGIN OF THE SHORE PLATFORM. 



Among the peculiarities of coral islands, the shore plat- 

 form appears to be one of the most singular, and its origin 



