J. D. Dana, Esq., on Coral Reefs and Islands. 227 



elements and natural decay are producing gradual accumu- 

 lations of material, like those of vegetation. The history of 

 the growing reef has, consequently, its counterpart among 

 the ordinary occurrences of the land about us. 



The progress of the coral formation is like its commence- 

 ment. The same causes continue with similar results, and 

 the reader might easily supply the details from the facts 

 already presented. The production of debris will necessarily 

 continue 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 lodge on its surface. 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. After- 

 wards, 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, from six to ten 

 feet. The ocean is thus the architect, while the coral polyps 

 afi'ord 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, in 

 all cases, 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 the outer reef along the 

 line of breakers, which are formed by the gradual growth of 

 layer upon layer of incrusting Nullipores ; but in the Pacific, 

 such formations are of small extent.* 



Among the peculiarities of coral islands, the shore plat- 

 form appears to be one of the most singular. It will be 

 remembered that it lies but little above low tide level, and is 

 often 300 feet in width, with a nearly flat surface throughout. 



* Professor Agassiz has recently observed that deposits of this last kind 

 constitute in many places the reef rock about the Florida K^ys, 



