12 Proceedmgs of the Royal Physical Society. 



heightened from this cause, will, in course of time, be 

 brought up to a depth at which sponges, hydroids, deep-sea 

 corals, annelids, alcyonarians, mollusks, polyzoa, echinoderms, 

 and other organisms can flourish abundantly. When this 

 has taken place, the upward growth of the calcareous forma- 

 tion will be accelerated by the accumulation of the remains 

 of this abundant fauna as it lives and dies on the bottom. 

 At last, the zone of reef-building corals will be reached, and 

 thereafter a growth of coral rock will bring the sea-floor up 

 to the level of low-water. That coral-reefs, undistinguishable 

 from barrier-reefs and even atolls, might be formed upon 

 banks of sediment in a deep sea, was admitted by Darwin.^ 

 But the assumption of so many submerged banks as this 

 explanation would require, seemed to him so improbable that 

 he dismissed it from further consideration. He was not 

 aware, however, of the enormous abundance of minute cal- 

 careous organisms in the surface-waters, and of the comparative 

 rapidity with which their remains might be accumulated on 

 the sea-bottom. 



Eeef-builders, starting on a submarine bank, whether pre- 

 pared for them by erosion, by subsidence, or by the upward 

 growth of organic deposits, would form reefs that must 

 necessarily tend to assume the atoll form. The central 

 portion of the colony or clump of coral will gradually be 

 placed at a disadvantage, as compared with the peripheral 

 parts of the mass, in being further removed from the food 

 supply, and will consequently dwindle and die. In propor- 

 tion as the reef approaches the sea-level, these central parts 

 are brought into increasingly uncongenial conditions, until at 

 last an outer ring of vigorous growing coral-reef encircles an 

 inside lagoon overlying the central stunted and dead portions. 

 The possibility of such a sequence of events was likewise 

 recognised by Darwin. " If a bank, either of rock or of 

 hardened sediment," he says, " lay a few fathoms submerged, 

 the simple growth of the coral, without the aid of subsidence, 

 would produce a structure scarcely to be distinguished from 

 a true atoll." - 



1 Coral Islands, 2d edit., p. 118. 



2 Op, ciL, p. 134. 



