500 CHARLES DARWIN 



corals 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 



A'A'. Outer edges of the barrier-reef at the level of the sea, with islets 

 on it. B'B'. The shores of the included island. CC. The lagoon-channel. 



A" A". Outer edges of the reef, now converted into an atoll. C'. The 

 lagoon of the new atoll. 



N. B. According to the true scale, the depths of the lagoon-channel and 

 lagoon are much exaggerated. 



one great reef and finally, the last and highest pinnacle 

 disappearing. 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, 18 all the leading features in those wonderful structures, 

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



18 It has been highly satisfactory to me to find the following passage in 

 a pamplet 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 expedi- 

 tion differ with me on some points respecting coral formations. 



