Plate XXXV
Figs. 76 and 77. Coral Cliffs, Zanzibar
| Fig. | 76. | Chuaka Bay. | Note undermining of fallen fragments |
| „ | 77. | Bawi Island. | Rock masses supported by narrow stalks |
Coral reefs are classified into three sets according to their relation with other land[55].
I. Fringing reefs, which, as the name implies, border the land, are continuous with it, and the seaward edge of which can be reached by wading.
II. Barrier reefs, which run parallel to the coast but separated from it by deep water navigable for coasting vessels larger than canoes.
III. Atolls, ring- or crescent-shaped reefs having no obvious relation to any land and typically found far out in the ocean, from the great depths of which they rise with steep slopes to, at most, a few feet above high tide level.
Fringing reefs we have already dealt with; the two agents described—growth and abrasion of coral—will account for all of them. Barriers and atolls are more puzzling. Why should the barrier form its line parallel to the coast, though at a distance from it, and the very existence of atolls is one of the most striking phenomena of Nature.