42 Mr Maclaren oa Coral Islands and Reefs, as 



-71 T S.2 



Second Stage — The Barrier reef. 



a b a — The island having subsided 200 feet, is now more than half sub- 

 merged ; but its double summit is still visible. 



S 2 — The surface of the sea in its second position. 



The fringing reef now raised to the level of S 2, forms r r, a " Barrier 

 reef." 



The small gutter which divided the reef from the island, is enlarged to 

 the wider and deeper cavity n n, and forms a " lagoon channel." 



Third Stage— The Atoll. 



a h a — The island having subdivided other 200 feet, is now completely 

 submerged. 



S 3— The sea in its third position. 



The barrier reef having 200 feet added to its height, now rises to r r. 



A broad lagoon n, now occupies the place of the island, and the reef 

 becomes an " Atoll." 



Mr Darwin endeavoured to collect some positive evidence 

 of subsidence in the islands, but it is not very satisfactory. 

 Geology, however, renders it certain that some portions of the 

 earth's surface have sunk to a lower level. The subsidence 

 assumed, therefore, involves no inconsistency ; and it enables 

 us to account for the otherwise puzzling fact, that though corals 

 do not live at a greater depth than 200 feet, yet numerous 

 reefs are found 1000 feet or more in depth, the basis of which, 

 as the steepness of their sides attest, can scarcely consist of 

 any thing else than coral. 



It explains also the appearance of the atolls in groups. 

 Suppose a tropical island, like Ireland in size, to sink under 

 the waves by slow stages. The hills being of diflPerent heights, 

 the corals would begin their work on those first submerged — 

 that is, the lowest — and new reefs would be founded succes- 

 sively on the higher ones as they descended, one after another, 



