134 SOME WONDERS OF THE SEA. 



Another well-known form of Coral Island is that which 

 forms a large circle. There is deep water close to the 

 edge both inside and outside, forming a natural harbour 

 so perfect that although a tempest may rage outside it, 

 there is smooth water within. This is also due to a sub- 

 marine mountain. 



In those regions volcanic action is a conspicuous 

 element, and volcanoes, both active and extinct, are 

 plentiful. When an extinct volcano rears its summit 

 tolerably near the surface of the sea, the Coral-makers 

 are sure to settle upon it sooner or later. As the Coral 

 must necessarily be founded on the edge of the crater, it 

 is evident that when it reaches the surface of the water 

 it must retain the circular form. The Coral, not being 

 able to extend itself upwards, in consequence being 

 almost invisible by day, and quite so at night, both 

 these types of Coral Islands would become exceedingly 

 dangerous, and, indeed, make navigation almost impos- 

 sible. But another provision steps in, and not only robs 

 them of their terrors, but converts them into havens of 

 rest and safety. 



Before very long, seaweeds accumulate, and are flung 

 by the storms upon the surface of the "reef, "as the Coral 

 mass is called. Mixed with the seaweed are quantities of 

 marine worms, molluscs, and other specimens of ocean 

 life. Being unable to exist out of the water, they die, 

 and by their decay form a fertile earth capable of afford- 

 ing nourishment to plants of a higher order. 



Floating co.coa-nuts, which have the power of drifting 



