ATOLL AND REEF FORMATION 223 



1904). In these publications, at least foui" distinct methods 

 of atoll formation are postulated, and these are briefly stated 

 as follows : 



Method No. 1. — An elevation of the ocean floor becomes 

 covered with sediment ; it reaches the depth at which reef 

 corals flourish, and a reef is developed upon it. " Boring 

 organisms enter on its central part and cause the rock to 

 decay. Sand-feeders follow and triturate up the fragments, 

 throwing a constant stream of fine mud into suspension in the 

 water, to be removed by tidal and other currents. Assisted 

 by the solution of coral in sea water, a lagoon is formed in the 

 centre of the reef, and passages are cut later from it along the 

 lines by which its muddy water escapes." 



Method No. 2. — High land rising from the ocean is first 

 present, this high land becomes disintegrated and broken down, 

 and finally, by the action of currents, reduced to a level 

 plateau some depth below the surface of the sea. On this 

 plateau Mr. Gardiner pictures an atoll to be very readily 

 formed. "At depths below 50 fathoms it is obvious that from 

 the first the organisms on the periphery of the bank so 

 formed would grow more rapidly, and so an atoll as such 

 would directly arise. The whole action might proceed with 

 extreme rapidity." 



Method No. 3. — A high oceanic island is pictured with a 

 fringing reef around its shores, denudation and solution remove 

 the island land, and the reef stands out from the shore as a 

 barrier reef. " Subsequently the edge of the terrace grows 

 outwards and its inner part is removed as in (1), forming a 

 barrier reef. Eventually the original island, owing to similar 

 causes, disappears, leaving an atoll. This method is of quite 

 wide occurrence in areas where elevated coral reefs are found." 

 The stages of progress in this third method are, therefore, 

 those pictured by Darwin, but their manner of development is 

 widely different. 



Method No. 4 grants that subsidence alone, or subsi- 

 dence plus the agencies brought into methods Nos. 1 and 2, 

 may cause atoll formation. 



