LOW LIFE IN THE SEA 



69 



one hundred miles in diameter. The ring is usually from 

 half a mile to one mile in width, with openings on the lee- 

 ward side, through which vessels may pass. Seeds of plants 



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An Atoll. 



are brought to these reefs by the waves and by birds, and 

 there springs up a vegetation which covers the barren rocks 

 and converts the reefs into lands suitable for human habita- 

 tion. A coral-reef below the surface of the water, extend- 

 ing from the Caroline Islands toward Australia, is said to 

 be continuous for five thousand miles, rivaling in extent 

 the great mountain-ranges of the world. 



9. This bit of animal life, which is but little more than 

 a speck of animated jelly, is thus seen to be a necessary link 

 in the chain of causes which are slowly but surely changing 

 the whole surface of the earth. The rains fall upon the 

 land and form streams, which channel their way to the 

 sea. The water, percolating through the soil, slowly dis- 

 solves the lime of the rocks, the solution forming one of 

 the constituents of sea-water. Day by day the mountains 



