CORAL ISLANDS. 1 95 



utmost depth at which the coral polyps can construct reefs is between 

 twenty and thirty fathoms. Now, there are enormous areas in the 

 Indian Ocean in which every island is a coral formation raised to the 

 height to which the waves can throw up fragments and the winds pile 

 up sand ; and the only theory which seems to account for all the cir- 

 cumstances embraced, is that of the subsidence of vast regions in this 

 ocean. " As mountain after mountain and island after island slowly 

 sunk beneath the water,"' he says, "fresh bases would be successively 

 afforded for the growth of the corals. I venture to defy any one to 

 explain in any other manner how it is possible that numerous islands 

 should be distributed throughout vast areas, all the islands being low, 

 all built of coral absolutely requiring a foundation within a limited 

 depth below the surface." 



The Forites, according to Mr. Darwin, form the most elevated 

 deposits of those which are situated nearer the level of the water : 

 Millepora complatiata also enters into the formation of the upper 

 banks. Various other branched corals present themselves in great 

 numbers in the cavities left by the Forites and Millepora crossing 

 each other. It is difficult to identify living species when they live in 

 the deeper parts, but, according to Darwin, the lower parts of the reefs 

 are occupied by polyps ot the same species as in the upper parts ; at 

 the depth of eighteen fathoms and upwards, the bottom consists 

 alternately of sand and corals. The total breadth of the circular reef 

 or ring which constitutes the atoll of the Keeling or Cocos Island 

 varies from 200 to 500 yards in breadth. Some little parasitic isles 

 form themselves upon the reefs, at 200 or 300 yards from their 

 exterior edge, by the accumulation of the fragments thrown up here 

 during great storms. They rise from two to three yards above the 

 sea -level, and consist of dead shells, corals, and sea urchins, the 

 whole consolidated into hard and solid rock. 



The description of the Island of Cocos or Keeling is as follows : — 

 " The ring-formed reef of the lagoon island is surmounted, in the 

 greater part of its length, by linear islets. On the northern, or lee- 

 ward side, there is an opening through which vessels can pass to the 

 anchorage within. On entering, the scene was very curious, and 

 rather pretty ; its beauty, however, entirely depended on the brilliancy 

 of the surrounding colours. The shallow, clear, and still water of the 

 lagoon resting in its greater part on white sand, is, when illumined 

 by a vertical sun, of the most vivid green. This brilliant expanse, 

 several miles in width, is on all sides divided, either by a line of 

 snow-white breakers from the dark heaving waters of the ocean, or 

 from the blue vault of heaven by the strips of land crowned by the 



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