258 CORALS AND CORAL ISLANDS. 



ated, the barrier reef includes a large sea, and the island it en- 

 closes is but a rocky peak within this sea. 



Can we account for this diversity in the position of bar- 

 rier reefs, and in their extent as compared with the enclosed 

 land ? There is evidently one way, in accordance with Mr. 

 Darwin's theory, in which these features might have been pro- 

 duced. If, for example, such an island as Angau were very 

 gradually to subside, from some subterranean cause, two results 

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

 coral reef, ever in constant upward increase, as has been ex- 

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

 subsidence were not beyond a certain rate. This subsidence 

 might go on till the last mountain peak remained alone above 

 the waters. Should we not then have a Nanuku ? Suppose 

 the subsidence not to have proceeded to this extent, but to leave 

 still a single ridge and a few isolated summits 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 en- 

 circling a mere point of rock might be explained even to 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 lev- 

 el over vast areas of the earth's surface have taken place is fully 

 proved, and accounts of some of them which are now in pro- 

 gress, as that of Sweden and that of Greenland, are to be 

 found in any geological treatise. 



But it admits of direct demonstration that such a subsi- 

 dence has actually taken place. It has been stated that the 

 depth of the reef at different distances from the shore it encir- 

 cles may generally be estimated from the slope of the shore. 

 On this principle it has been shown on a former page (p. 156) 

 that the thickness of the distant barrier reef cannot be less in 



