Aprils 1836. theory of lagoon islands. 559 



the amount of movement very great^ which would be neces- 

 sary to change a weil-characterized encircHng 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 

 encircling 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 between 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 



