Island and Reef Builders. 



agency of vital laws, conquering the great mechanical 

 power of the waves of an ocean, which neither the art 

 of man nor the inanimate works of Nature could suc- 

 cessfully resist/* 



The outer side of the atoll usually sinks to a depth of 

 some two hundred to three hundred fathoms, shelving 

 abruptly at an angle of forty-five degrees or more, and it 

 is on this outer edge only that the solid wall increases. 

 Here the coral animals thrive in the midst of the surf 

 occasioned by the breakers an ever-changing and highly 

 aerated body of water washing over their surface, bring- 

 ing to them an abundance of oxygen and food. During 

 violent storms huge masses are torn off by the force of 

 the waves, and driven on shore towards the lagoon ; 

 but scars left by the detachment of these masses serve 

 as places of attachment for the young of the neigh- 

 bouring corals, the successive generations of which, by 

 their rapid growth, not only repair the damage caused 

 by the storm, but increase the outer area of the reef. 



While these reef-building corals cannot exist in a 

 vigorous state of growth at a greater depth than twenty 

 to thirty fathoms, owing to their need of light, warmth, 

 and highly oxygenated water, their tissues are so deli- 

 cate that a brief exposure to the air and the sun's rays 

 kills them. Thus it is only by constant immersion in 

 clear, warm, highly aerated sea water, absolutely devoid 

 of muddy sediment, that they can thrive. As the living 

 reef-building corals cannot flourish beyond a depth of 

 thirty fathoms, it seems obvious that the atolls and 

 barrier reefs are resting upon some substratum which 

 could not possibly have been formed by reef-building 

 corals at the same relative position it' has now. What, 



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