92 ON CORAL REEFS AND ISLANDS. 



emplified, for the view is a good transverse section of either of 

 them, b'" b'" are sections of the distant enclosing barrier, and 

 c'" c.'", and other intermediate spots, the water within. 



The supposed similarity between these ideal sections and ex- 

 isting islands is fully sustained by actual comparison. Fig. 2 

 is a sketch of the island of Aiva in the Feejee Group. 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 every essential 

 point the transverse section given in figure 3, resembling closely, 

 as is apparent, that in figure 1. The section is made through 

 the line b 6, b' b', of figure 2. It is unnecessary to add other 

 illustrations. They may be made out from 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 inclosed 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 of Mr. Darwin. But it is a sufficient reply to such 

 an argument, to state the fact that the subsidence admits of no 

 doubt, and that the islands referred to as exemplifications of it, 

 present 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 subsidence 

 during the growth of the reefs. Broad channels, and even open 

 seas within, as in Nanuku and the Exploring Isles, are therefore 

 to be received as results of the subsidence, for which explana- 

 tions should be sought. 



These explanations are at hand, and accord so exactly with 

 facts ascertained, that the existence of inner passages becomes a 

 necessary feature of such islands. It has been shown that the 

 ocean acts an important part in reef-making; — that the outer 

 reefs exposed to its action and to its pure waters, grow more rap- 

 idly than those within which are under the influence of marine 

 and freshwater currents and transported detritus. It is obvious, 

 therefore, that the former may retain themselves at the surface, 

 when through a too rapid subsidence the inner patches would 

 disappear. Moreover, after the barrier is once begun it has grow- 

 ing corals on both its inner and outer margin, while a fringing 

 reef grows only on one margin. Again, the detritus of the 

 outer reefs is, to a great extent, thrown back upon itself by the 

 sea without and the currents within, while the inner reefs con- 

 tribute a large proportion of their material to the wide channels 

 between them. These channels, it is true, are filled in part from 

 the outer' reefs, but proportionally less from them than from the 



