this intervening space the corals are more or less smothered with silt from the outer reef and from the land, and are also deprived in large measure of the needful supply of food and oxygen by the vigorous growth of the corals on the outer rim.  The outer rim thus becomes a barrier reef and the inner belt of retarded growth is deepened by subsidence to a ship channel, s’s’ representing sea level at this time.  The final stage, where the island has been carried completely beneath the sea and overgrown by the contracting reef, whose outer ring now forms an atoll, is represented by s"s”.

In very many instances, however, atolls and barrier reefs may be explained without subsidence.  Thus a barrier reef may be formed by the seaward growth of a fringing reef upon the talus of its sea face.  In Figure 162 f is a fringing reef whose outer wall rises from about one hundred and fifty feet, the lower limit of the reef-building species.  At the foot of this submarine cliff a talus of fallen blocks t accumulates, and as it reaches the zone of coral growth becomes the foundation on which the reef is steadily extended seaward.  As the reef widens, the polyps of the circumference flourish, while those of the inner belt are retarded in their growth and at last perish.  The coral rock of the inner belt is now dissolved by sea water and scoured out by tidal currents until it gives place to a gradually deepening ship channel, while the outer margin is left as a barrier reef.

In much the same way atolls may be built on any shoal which lies within the zone of coral growth.  Such shoals may be produced when volcanic islands are leveled by waves and ocean currents, and when submarine plateaus, ridges, and peaks are built up by various organic agencies, such as molluscous and foraminiferal shell deposits.  The reef-building corals, whose eggs are drifted widely over the tropic seas by ocean currents, colonize such submarine foundations wherever the conditions are favorable for their growth.  As the reef approaches the surface the corals of the inner area are smothered by silt and starved, and their Submarine Volcanic Peak hard parts are dissolved and scoured away; while those of the circumference, with abundant food supply, nourish and build the ring of the atoll.  Atolls may be produced also by the backward drift of sand from either end of a crescentic coral reef or island, the spits uniting in the quiet water of the lee to inclose a lagoon.  In the Maldive Archipelago all gradations between crescent-shaped islets and complete atoll rings have been observed.

In a number of instances where coral reefs have been raised by movements of the earth’s crust, the reef formation is found to be a thin veneer built upon a foundation of other deposits.  Thus Christmas Island, in the Indian Ocean, is a volcanic pile rising eleven hundred feet above sea level and fifteen thousand five hundred feet above the bottom of the sea.  The summit is a plateau surrounded by a rim of hills of reef formation, which represent the ring of islets of an ancient atoll.  Beneath the reef are thick beds of limestone, composed largely of the remains of foraminifers, which cover the lavas and fragraental materials of the old submarine volcano.