Subsidence Explains the Details of Coral Reefs

Not only are the grand features in the structure of barrier reefs and atolls, and their likeness to each other in form, size, and other characteristics, explained by the theory of subsidence—a theory that we are independently forced to accept in the very areas in question because bases must be found for the corals within the requisite depth—but many details of structure and exceptional cases can also be simply explained in this way. I will give only a few instances. In barrier reefs it has long been remarked with surprise that the passages through the reef exactly face valleys in the included land, even in cases where the reef is separated from the land by a lagoon channel so wide, and so much deeper than the actual passage itself, that it seems hardly possible that the very small quantity of water or sediment brought down could injure the corals on the reef. Now, every reef of the fringing class is breached by a narrow gateway in front of the smallest rivulet, even one that is dry during the greater part of the year, for the mud, sand, or gravel occasionally washed down kills the corals on which it is deposited. Consequently, when an island fringed in this way subsides, most of the narrow gateways will probably be closed by the outward and upward growth of the corals, but any that are not closed (and some must always be kept open by the sediment and impure water flowing out of the lagoon channel) will still continue to face exactly the upper parts of those valleys at whose mouths the original basal fringing reef was breached.

We can easily see how an island fronted on only one side, or on one side with one end or both ends encircled by barrier reefs, might after long-continued subsidence be converted into a single wall-like reef, into an atoll with a great straight spur projecting from it, or into two or three atolls tied together by straight reefs—all of which exceptional cases actually occur. Since the reef-building corals require food, are preyed on by other animals, are killed by sediment, cannot adhere to a loose bottom, and may easily be carried down to a depth from which they cannot spring up again, we need feel no surprise that the reefs of both atolls and barriers become imperfect in parts. The great barrier of New Caledonia is imperfect and broken in many parts in this way; hence, after long subsidence, this great reef would produce not one great atoll 400 miles long but a chain or archipelago of atolls of very nearly the same dimensions as those in the Maldive archipelago. Moreover, once an atoll has been breached on opposite sides, it is extremely improbable, because the oceanic and tidal currents are likely to pass straight through the breaches, that the corals, especially during continued subsidence, would ever be able to unite the rim again; if they did not, one atoll would be divided into two or more as the whole sank downward. In the Maldive archipelago there are distinct atolls so related to each other in position, and separated by channels either unfathomable or very deep (the channel between Ross and Ari atolls is 150 fathoms deep, and that between the north and south Nilandhe atolls is 200 fathoms deep), that it is impossible to look at a map of them without believing that they were once more intimately related. In this same archipelago, Maalhosmadulu atoll is divided by a bifurcating channel from 100 to 132 fathoms deep in such a way that it is scarcely possible to say whether it ought strictly to be called three separate atolls or one great atoll not yet finally divided.