BARRIER REEFS.—ATOLLS. 27 
Barrier Reefs.—This name is given to reefs formed 
as above, but distant from the shore, and separated from 
it by deep water, as the reef, one thousand miles long and 
thirty miles from shore, on the Australian coast. 
Fringing Reefs.—These are formed near the shore, 
generally in smooth water, having no great depth between 
them and the adjacent land. 
Atolls.—When a reef reaches the surface, the waves 
from the side of the prevailing winds grind up the dead 
Fic. 26.—Atoll in the Pacific Ocean. 
coral, and wash it over in the form of mud and sand, 
where it settles in smooth water. Here branching corals, 
that give shelter to innumerable animals, flourish, as well 
as lime-secreting algz,* all continually growing and being 
ground up by fishes and worms, and disintegrated by the 
solvent action of the carbonic acid in the water, until 
finally a shallow lagoon or flat is formed, wholly or partly 
* This is one of the most important factors of lagoon and key 
making. The keys of Tortugas, especially Sand Key, are made up 
almost entirely of these limy plates, as well as broken shells. 
