ORIGIN OF CHANNELS WITHIN BARRIERS. 89 



zation of facts. The explorations of the Expedition afford strik- 

 ing illustrations of his views, and elucidate some points which 

 were still deemed obscure, establishing the theory on a firm basis 

 of evidence, and exhibiting its complete correspondence with 

 observation. 



A. Channels within barriers. — We may turn again to the 

 chart of the Feejee Group, and glance successively at the isl- 

 ands Goro, Angau, Nairai, Lakemba, Argo Reef, Exploring Isles, 

 and Nanuku.* In Goro, the reef closely encircles the land upon 

 whose submarine shores it was built up. In the island next 

 mentioned, the reef has the same character, but is more distant 

 from the shores, forming what has been termed a barrier reef; 

 the name implying a difference in position, but none in mode of 

 formation. In the last of the islands enumerated, the barrier 

 reef includes a large sea, and the island it encloses is but a rocky 

 peak within this sea. 



Can we account for this diversity in the position of barrier 

 reefs, and in their extent as compared with the enclosed land? 

 There is evidently one way in which these features might have 

 been produced. If, for example, such an island as Angau were 

 very gradually to subside, from some subterranean cause, two re- 

 sults would take place : — the land would slowly disappear, while 

 the coral reef, which is ever in constant increase, as has been ex- 

 plained, might retain itself at the surface, if the rapidity of sub- 

 sidence was not beyond a certain rate. This subsidence might 

 go on till the last mountain peak remained alone above the wa- 

 ters. Should we not then have a Nanuku? Suppose the subsi- 

 dence not to have proceeded quite as far as this, it might leave 

 only a single ridge and a few isolated summits peering above the 

 waves. Would not its condition in this case be that of the Ex- 

 ploring Isles? On such a supposition, reefs of large size encir- 

 cling a mere point of rock might be explained in every feature. 

 The subsidence of Goro, on the same principle, would produce 

 an Angau, or, carried further, a Nanuku. 



It may here be remarked, that the fact that changes of level 

 in the earth's surface have taken place over vast areas, is fully 

 proved, and accounts of some of them which are now in progress, 

 as that of Sweden, are to be found in any geological treatise. 



But it admits of direct demonstration that such a subsidence 

 has actually taken place. It has been stated that the depth of 

 the reef at different distances from the shore it encircles may 

 generally be estimated from the slope of the shore. On this princi- 

 ple it has been shownf that the thickness of the distant barrier 

 reef cannot be less in some instances than a thousand feet ; and 



* This chart will be found in copies of the Narrative of the U. S. Exploring 

 Expedition, by Capt. Wilkes, U. S. N. f See page 19. 



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