THE RIM OF THE REEF — DAWSON 373 



and coral fragments to the abyssal sea floor. Whenever the reef ridge 

 dissolves down to a level that permits the sea waves to pour in over 

 it, the increased supply of oxygen for nighttime respiration encour- 

 ages the growth of the pavement algae by which it is built up again. 



The seaward advance of the nullipore ridge, whose height above sea 

 level is controlled by desiccation, leaves behind it a reef flat that 

 stretches between the ridge and the central lagoon. Storms cast up 

 boulders and debris from the reef margin and the talus slope onto 

 this reef flat. 



Until such a pile of sand and rubble appears on the reef flat, all 

 rain water is dissipated into the open sea. With sufficient material 

 to confine it, however, the fresh rain water stands in the sand at sea 

 level, floating on the salt water below it, and mixing only very slowly. 

 As a pool of fresh water forms in the sand, seeds (carried to the islet 

 by wind and sea) sprout and penetrate to the water supply, and thus 

 the island vegetation comes into being. Sea birds come, and as they 

 feed on the fish of the reefs and lagoon, and nest in the vegetation, 

 they convert animal life of the sea into fertilizer for the land. The 

 birds thrive, trees and shrubs grow, and gradually humus forms on 

 what at first was only barren sand and rock. 



In all this atoll growth, the calcareous algae play an important 

 role, spreading their crusts over the rubble and the sand, cementing 

 and binding the reef margin, and advancing — ever so slowly, but 

 steadily — into the waves. 



