132 ON CORAL AND CORAL REEFS 



encircling reef. And it will be obvious that when the 

 rising of the sea has gone so far as completely to cover 

 the highest points of the island, the reef will have 

 passed into the condition of an atoll. 



But how is it possible that the relative level of the 

 land and sea should be altered to this extent ? Clearly, 

 only in one of two ways : either the sea must have risen 

 over those areas which are now covered by atolls and 

 encircling reefs ; or, the land upon which the sea rests 

 must have been depressed to a corresponding extent. 



If the sea has risen, its rise must have taken place 

 over the whole world simultaneously, and it must have 

 risen to the same height over all parts of the coral zone. 

 Grounds have been shown for the belief that the general 

 level of the sea may have been different at different 

 times ; it has been suggested, for example, that the ac- 

 cumulation of ice about the poles during one of the 

 cold periods of the earth's history necessarily implies 

 a diminution in the volume of the sea proportioned to 

 the amount of its water thus permanently locked up in 

 the Arctic and Antarctic ice-cellars ; while, in the warm 

 periods, the greater or less disappearance of the polar 

 ice-cap implies a corresponding addition of water to 

 the ocean. And no doubt this reasoning must be ad- 

 mitted to be sound in principle ; though it is very hard 

 to say what practical effect the additions and subtrac- 

 tions thus made have had on the level of the ocean; 

 inasmuch as such additions and subtractions might be 

 either intensified or nullified, by contemporaneous 

 changes in the level of the land. And no one has yet 

 shown that any such great melting of polar ice, and 

 consequent raising of the level of the water of the ocean, 

 has taken place since the existing atolls began to be 

 formed. 



