504 SUBSIDENCE OF CORAL-REEFS chap. 



will go on vigorously growing upwards ; but as the island 

 sinks, the water will gain inch by inch on the shore — the 

 separate mountains first forming separate islands within one 

 great reef — and finally, the last and highest pinnacle disappear- 

 ing. The instant this takes place, a perfect atoll is formed : 

 I have said, remove the high land from within an encircling 

 barrier-reef, and an atoll is left, and the land has been removed. 

 We can now perceive how it comes that atolls, having sprung 

 from encircling barrier-reefs, resemble them in general size, 

 form, in the manner in which they are grouped together, and 

 in their arrangement in single or double lines ; for they may 

 be called rude outline charts of the sunken islands over which 

 they stand. We can further see how it arises that the atolls 

 in the Pacific and Indian Oceans extend in lines parallel to the 

 generally prevailing strike of the high islands and great coast- 

 lines of those oceans. I venture, therefore, to affirm, that on 

 the theory of the upward growth of the corals during the 

 sinking of the land,^ all the leading features in those wonderful 

 structures, the lagoon -islands or atolls, which have so long 

 excited the attention of voyagers, as well as in the no less 

 wonderful barrier -reefs, whether encircling small islands or 

 stretching for hundreds of miles along the shores of a continent, 

 are simply explained. 



It may be asked whether I can offer any direct evidence 

 of the subsidence of barrier-reefs or atolls ; but it must be 

 borne in mind how difficult it must ever be to detect a move- 

 ment, the tendency of which is to hide under water the part 

 affected. Nevertheless, at Keeling atoll I observed on all sides 

 of the lagoon old cocoa-nut trees undermined and falling ; 

 and in one place the foundation-posts of a shed, which the 

 inhabitants asserted had stood seven years before just above 

 high-water mark, but now was daily washed by every tide ; on 

 inquiry I found that three earthquakes, one of them severe, 

 had been felt here during the last ten years. At Vanikoro 



' It has been highly satisfactoiy to me to find the followmg passage in a 

 pamphlet by Mr. Couthouy, one of the naturalists in the great Antarctic Expedition 

 of the United States : "Having personally examined a large number of coral-islands, 

 and resided eight months among the volcanic class having shore and partially 

 encircling reefs, I may be permitted to state that my own observations have 

 impressed a conviction of the correctness of the theory of Mr. Darwin." The 

 naturalists, however, of this expedition differ with me on some points respecting 

 coral formations, 



