On Coral Reefs and Islands. : 187 
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 encireles 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 
tom.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. 
Jal 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 
cen 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 
as that of Sweden, are to be found in any geological treatise. 
, rae 
generally: be estimated from the slope of the shore. On this princi~ 
Ple it has been shown} that the thickness of the distant barrier, 
teef 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./ + This Journal, [2], vol. xi, p. 871. 
