6 Proceedings of the Boyal Physical Society. 



careful consideration of the evidence seems to me inevitably 

 to lead. 



Before the memorable voyage of the " Beagle/' the 

 generally-received opinion regarding the origin of the circular 

 coral-reefs or atolls of mid-ocean was, that they had grown 

 up on the rims of submerged volcanic craters. The enormous 

 size of some of the atolls — thirty miles in diameter — might 

 have been thought a sufficiently formidable objection to this 

 explanation. But it did not appear insuperable, even to so 

 cautious a philosopher as Lyell, who only noticed it to refer 

 his readers to the great dimensions reached by truncated 

 volcanic cones, which, he thought, might retain their forms 

 more easily under a deep sea than on land.^ 



An earlier and better theory, as Darwin admitted, had 

 been started by Chamisso, who supposed that the circular 

 form of an atoll was due to the fact that, as the more massive 

 kinds of coral thrive most vigorously in the play of the surf 

 they naturally keep to the outside of the reef, and raise that 

 portion to the surface first. But when Darwin's own views 

 were published, first in abstract before the Geological 

 Society in 1837, and subsequently more fully in his separate 

 volume on the structure and distribution of coral-reefs, in 

 1842, they were soon generally accepted, and were regarded 

 not only as affording a satisfactory explanation of the whole 

 phenomena, but as comprising one of the most impressive 

 generalisations with which Geology, fertile in such achieve- 

 ments, had yet astonished the world. 



The theory proposed by Darwin, now so famihar, con- 

 nected all the types of reef together as stages of one long 

 process, every step in which could be illustrated by actual 

 examples. At the one end stood the Fringing-reefs, some of 

 which might only lately have been started upon a recently- 

 upraised sea-bottom. Out of this stage, by continuous or 

 intermittent subsidence, came Barrier-reefs. Then, as the 

 depression went on, and the islands encircled by the barrier- 

 reefs disappeared, their sites were taken by Atolls. Lastly, 

 where the rate of subsidence was too rapid for the upward 

 growth of the corals, an atoll might become a submerged 



1 Principles of Geology, 4tli edit. (1835), vol, iii., p. 310. 



