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 
