152 FORMATION OF COKAL REEFS. 



have a variety of outlines : they may be straight, 

 circular, semicircular, or oblong, according to the 

 form of the coast along which the little Reef- 

 Builders establish themselves ; and their height 

 is, of course, determined by the depth of the 

 bottom on which they rest. If they settle about 

 an island on all sides of which the conditions for 

 their growth are equally favorable, they will raise 

 a wall all round it, thus encircling it with a ring 

 of Coral growth. The Athols in the Pacific 

 Ocean, those circular islands enclosing sometimes 

 a fresh-water lake in mid-ocean, are Coral walls 

 of this kind, that have formed a ring around a 

 central island. 



This is easily understood, if we remember that 

 the bottom of the Pacific Ocean is by no means 

 a stable foundation for such a structure. On 

 the contrary, over a certain area, already sur- 

 veyed with some accuracy by Professor Dana, 

 during the United States Exploring Expedition, 

 it is subsiding ; and if an island upon which 

 the Reef-Builders have established themselves 



