196 CORALS AND CORAL ISLANDS. 



to go on : a part will be swept by the waves, across the patch 

 of reef, into the lagoon or channel beyond, while other portions 

 will fill up the spaces among the corals along its margin, or be 

 thrown beyond the margin and lodge on its surface. The layer 

 of dead coral rock which makes the body of the reef, has its 

 border of growing corals, and is thus undergoing 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 dryland. 

 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 

 afford the material for the structure ; and, when all is ready, 

 it sows the land with seed brought from distant shores, cove- 

 ring 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 breakers, 

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

 incrusting NuUipores ; 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 sho7'e platform 

 appears to be one of the most singular, and its origin has not 



