74 



ON CORAL REEFS AND ISLANDS. 



the lagoon or channel beyond, while other portions lodge on its 

 surface. But besides the small fragments, larger masses will be 

 Jhrown 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. Afterwards, by farther contributions of the coarse and fine 

 coral material, the islets are completed, and raised as far out of 

 water as the waves can reach — that is, from six to ten feet. The 

 Ocean is thus the architect, while the coral polyps afford the ma- 

 terial 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 platform ap- 

 pears to be one of the most singular. It will be remembered that 

 it lies but little above low tide level, and is often three hundred 

 feet in width, with a nearly flat surface throughout. 



Though apparently so peculiar, the existence of this platform 

 is due to the simple action of the sea. and is a necessary result of 

 this action. Passing to New Holland, from the coral islands of 

 the tropics, we there found the same structure exemplified along 

 the sandstone shores of this semi-continent, where it is continued 

 for scores of miles. At the base of the sandstone cliff, in most 

 places one or more hundred feet in height, there is a layer of 

 sandstone rock, lying, like the shore platform of the coral island, 

 near low tide level, and from fifty to one hundred and fifty yards 

 in width. It is continuous with the bottom layer of the cliff: 

 the rocks which once covered it, have been removed by the sea. 

 Its outer edge is the surf-line of the shore. At low tide it is 

 mostly a naked flat of rock, while at high tide it is wholly under 

 water, and the sea reaches the cliff. New Zealand, at the Bay of 

 Islands, afforded us the same fact, again, in an argillaceous sand- 

 rock ; and there was no stratification in this case to favor the 

 production of a horizontal surface ; it was a direct result from the 



* Prof. Agassiz has recently observed that deposits of this last kind constitute in 

 many places the reef rock about the Florida Keys. 



