Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

April, 1836. theory of lagoon islands. 559 the amount of movement very great, which would be neces- sary to change a well-characterized encircling reef, into as characteristic a lagoon island. It will at once be evident that a coral reef, closely skirt- ing the shore of a continent, would, in like manner after each subsidence, rise to the surface ; the water, however, always encroaching on the land. Would not a barrier reef necessarily be produced, similar to the one extending parallel to the coast of Australia ? It is indeed but uncoiling one of those reefs which encircle at a distance so many islands. Thus the three great classes of reef, lagoon, encircling, and barrier, are connected by one theory. It will perhaps be remarked, if this be true, there ought to exist every intermediate form between a closely-encircled and a lagoon island. Such forms actually occur in various parts of the ocean : we have one, two, or more islands encircled in one reef; and of these some are of small proportional size to the area enclosed by the coral formation ; so that a series of charts might be given, showing a gradation of character between the two classes. In New Caledonia, where the double line of reef projects 140 miles beyond the island, we may imagine we see this change in progress. At the northern extremity, reefs occur, some of which are of the encircUng kind, and others almost with the character of true lagoon islands. The line of reef which fronts the whole west coast of this great island, has by some been called a barrier. It is four hundred miles long ; and may be said thus to form a link iDetween an ordinary encircling reef and the great Australian barrier. I should perhaps have entered before into the consider- ation of one apparent difficulty in the origin of lagoon islands. It may be said, granting the theory of subsidence, a mere circular disc of coral would be formed, and not a cup-shaped mass. In the first place, even in reefs closely fringing the land (as before remarked), the corals do not grow on the shore itself, but leave a shallow channel. Secondly, the strong and vigorous species which