vi.] ON COEAL AND CORAL EEEFS. 127 



aa the rising sea, covering more and more of the land, 

 would occupy a wider space between the edge of the 

 reef and what remained of the land. Thus the rising 

 sea would eventually convert a large island with a 

 fringing reef, into a small island surrounded by an en- 

 circling 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 dimi- 

 nution 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 dcubt this reasoning must be admitted 

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

 what practical effect the additions and subtractions thus 

 made have had on the level of the ocean ; inasmuch as 

 such additions and subtractions might be either inten- 

 sified or nullified, by contemporaneous changes in the 

 level of the land. And no one has yet shown that any 



