ORIGIN OF THE BARRIER REEF. 2G1 



There are two peaks in the lagoon, precisely as above ; and 

 although we have no soundings of the waters in and about it, 

 nor sketches of peaks, facts observed elsewhere authorize in 

 everj' essential point the transverse section here given, which 

 resembles closely, as is apparent, the preceding. The section 

 is made through the line b 6, V b\ of the map. It is unneces- 

 sary to add other illustrations. They may be made out from 



MAP, AND IDEAL SECTION, OP AIVA ISLAND. 



any of the eastern groups of the Feejees, the Gambier Group 

 of the Paumotus, or Hogoleu in the Carolines. Wallis's 

 Island is another example of islets of rock in a large lagoon 

 enclosed by a distant barrier. 



It has been asked, why the interior channels do not become 

 filled by coral reef, as the island sinks, and thus a plane of 

 coral result, instead of a narrow belt ; and this has been urged 

 against the theory. But it is a sufficient reply to such an argu- 

 ment to state the fact that the subsidence admits of no doubt, 

 and that the islands, referred to as exemplifications of it, pre- 

 sent this very peculiarity. It should be received, therefore, 

 as a consequence of it, instead of an objection to the view, for 

 it is the most common feature with all islands that have broad 

 reef-grounds, or in other words, that show evidence of subsi- 

 dence during the growth of the reefs. Broad channels, and 



