Coral reefs may be grouped in three classes,—­fringing reefs, barrier reefs, and atolls.

Fringing reefs.  These take their name from the fact that they are attached as narrow fringes to the shore.  An example is the reef which forms a selvage about a mile wide along the northeastern coast of Cuba.  The outer margin, indicated by the line of white surf, where the corals are in vigorous growth, rises from about forty feet of water.  Between this and the shore lies a stretch of shoal across which one can wade at low water, composed of coral sand with here and there a clump of growing coral.

Barrier reefs.  Reefs separated from the shore by a ship channel of quiet water, often several miles in width and sometimes as much as three hundred feet in depth, are known as barrier reefs.  The seaward face rises abruptly from water too deep for coral growth.  Low islands are cast up by the waves upon the reef, and inlets give place for the ebb and flow of the tides.  Along the west coast of the island of New Caledonia a barrier reef extends for four hundred miles, and for a length of many leagues seldom approaches within eight miles of the shore.

Atolls.  These are ring-shaped or irregular coral islands, or island-studded reefs, inclosing a central lagoon.  The narrow zone of land, like the rim of a great bowl sunken to the water’s edge, rises hardly more than twenty feet at most above the sea, and is covered with a forest of trees such as the cocoanut, whose seeds can be drifted to it uninjured from long distances.  The white beach of coral sand leads down to the growing reef, on whose outer margin the surf is constantly breaking.  The sea face of the reef falls off abruptly, often to depths of thousands of feet, while the lagoon varies in depth from a few feet to one hundred and fifty or two hundred, and exceptionally measures as much as three hundred and fifty feet.

Theories of coral reefs.  Fringing reefs require no explanation, since the depth of water about them is not greater than that at which coral can grow; but barrier reefs and atolls, which may rise from depths too great for coral growth demand a theory of their origin.

Darwin’s theory holds that barrier reefs and atolls are formed from fringing reefs by subsidence.  The rate of sinking cannot be greater than that of the upbuilding of the reef, since otherwise the corals would be carried below their depth and drowned.  The process is illustrated in Figure 161, where v represents a volcanic island in mid ocean undergoing slow depression, and ss the sea level before the sinking began, when the island was surrounded by a fringing reef.  As the island slowly sinks, the reef builds up with equal pace.  It rears its seaward face more steep than the island slope, and thus the intervening space between the sinking, narrowing land and the outer margin of the reef constantly widens.  In