152 FORMATION OF CORAL 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 
