THE RIM OF THE REEF — DAWSON 371 



Second, the principal cementing agent hi the reefs is the nullipore. 



Third, calcareous algae have been found actively working at depths 

 of 600 feet. 



Fourth, the seaward margins of actively growing atoll reefs are 

 often covered by a pavement-like growth of Porolithon^ a species of 

 nullipore. 



Fifth, borings to depths of several thousand feet indicate the con- 

 sistent presence of great quantities of nullipore material throughout 

 the reefs. 



The most striking first impression of many atolls is of their 

 enormous size. Tens of thousands of servicemen have visited the 

 vast lagoons of Kwajalein or Eniwetok, 30 miles or more across, sur- 

 rounded by dots and lines of vegetation-clad islets. But these atolls 

 have not always been so large. They grew, and are still growing. 



One also finds small atolls, such as Palmyra in the Line Group, and 

 nearby reefs, such as Kingman, that barely break the surface of the 

 water. All these are related within the atoll phenomenon. To 

 miderstand an atolFs development, it must be considered as a kind of 

 dynamic, living organism. It is, really, a vastly complex association 

 of living things, all dwelling together in a marvelous balance, each 

 plant and animal contributing to the life and growth of the whole 

 atoll and to the delicate balance maintained between construction 

 and destruction. 



As a possible point of departure in the development of an atoll, 

 consider a reef such as Kingman, consisting of an association of coral 

 animals and coralline algae, growing on a slightly submerged sea 

 mount. Let us say that the reef is 5 acres in extent and barely breaks 

 the surface at low tide. Now, somewhat different optima for growth 

 and persistence exist among the corals and the nullipores with regard 

 to the surf that breaks over the reef. The rigid, branched, brittle 

 corals tend to break and fragment under severe wave action. But the 

 massive, reef-forming nullipores are unaffected by the pounding surf. 

 In fact, the stronger the surf the better they grow, for one of the 

 principal factors for their growth is the availability of oxygen for 

 their respiration during the dark hours of the night, when their photo- 

 synthetic pigments are inactive. 



Accordingly, on this 5-acre reef, the first organisms to grow above 

 the water surface, creating a condition of surf, will also create condi- 

 tions that tend to favor the growth of the nullipores and to reduce 

 the upward extension of the coral. Herein begins the formation of 

 an atoll from a reef, for the nullipores grow into the breaking surf. 

 As they do so, the intensity of the surf becomes greatest around the 

 margins of tlie reef. Here the nullipores grow upward and form a 

 rim slightly above sea level. This rim, which breaks the impact of 



