140 PROCEEDINGS OF SECTION C. 



12.— CORAL REEFS OF THE COOK AND SOCIETY ISLANDS. 

 By PROFESSOR P. MARSHALL, M.A., D.Sc, F.G.S. 



[Plates X., XL] 



An expedition to the Cook and Society Islands, undertaken as 

 a member of a Research Committee appointed by this Association 

 to investigate the distribution of alkaline eruptive rocks in the 

 Pacific, enabled me to make some observations on the coral reefs of 

 the islands of these groups. These are perhaps sufficiently impor- 

 tant to justify me in stating them in the form of a paper to be 

 communicated to this meeting of the Association. Before detailing 

 these observations it is as well to state briefly the theories that have 

 been advanced from time to time in order that it may be clearly seen 

 to what extent they are supported or opposed by the statements 

 here made. The following appears to me to be a brief but fair 

 statement of the different theories : — 



(1) Darwin:^ The theory of subsidence. Coral structures 

 are all supposed to have commenced as fringing reefs. Subsidence 

 of the land as rapid as the upward growth of the coral would cause 

 the fringing reefs to develop into a barrier. Still further subsidence 

 until the land completely disappeared would change the barrier into 

 an atoll. This view was strongly supported by Dana in his work.^ 



(2) Murray, Sir J. :^ "That when coral plantations grow up 

 from submarine banks they assume an atoll form owing to the more 

 abundant supply of food to the outer margin and the removal of 

 dead coral from the interior portion by currents and by the action 

 of carbonic acid gas dissolved in sea water. 



That barrier reefs have been built out from the shore on a 

 foundation of volcanic debris or on a talus of coral blocks, coral 

 sediment and pelagic shells, and the lagoon channel is formed in the 

 same way as a lagoon." 



Murray's theory is often known as the solution theory. 



(3) Agassiz, A. :* The Coral Reefs of the Tropical Pacific. This 

 eminent observer, who has spent more time than any other author 

 in investigating this subject, regards all coral reefs as arising on sub- 

 marine platforms, the surface of which is less than 100 fathoms 

 below the sea level. In nearly all cases these platforms are sup- 

 posed to be the result of the action of marine erosion on previously 

 existing land. The theory might almost be called the theory of 

 erosion. Thus on page 16 he says : " It became very evident after 

 we had examined a number of atolls that the underlying ledge is a 

 remnant of a bed of tertiary coralliferous limestone which at one time 

 covered the greater part of the area of the lagoon, portions of which 

 may have been elevated to a considerable height." 



(4) Stanley-Gardiner :& This author from his observations in the 

 Maldives further extended the action of marine erosion in preparing 



1 "Geological Observations on Coral Reefs," 1851. 



2 "Corals and Coral Islands," 1872. 



3 Proc. Rov. Soc. Edin., 1879 80 p. 517. 



4 Mem. Mus. Camp. Zool. Harv., Vol. XXVIII., 1903. 



5 Geographical Journal, 1802, XIX., p. 396. 



