There are three kinds of coral reef-namely, fringing reefs, barrier reefs, and atolls.
"Geology"
James Geikie
The upper surface of atolls and barrier reefs often peers at separate points above the level of the sea, so as to form low-lying islets.
"Geology"
James Geikie
Strangely enough they have nearly all explained that coral-polypes aggregate themselves in the forms of atolls and barrier-reefs by a mysterious "instinct," mediocrity's only term for screening its ignorance, and which is also given as the cause for their secreting lime.
"The Human Side of Animals"
Royal Dixon