THE STRUCTURE OF THE BARRIER 157 



factor in atoll formation. Darwin described the Nullipora 

 bed in this atoll,* and Dr. Guppy also called attention to the 

 unusual thickness of its growth.f In describing the cha- 

 racters of the coral reefs of the Solomon Islands, Dr. Guppy t 

 came to the conclusion that Nidlvporm formed only thin layers 

 that were unimportant in barrier building, but three years 

 afterwards, when he came to describe the Cocos group, he 

 mentions the slabs two feet thick which are commonly to be 

 found broken from the surface of the outlying barrier boulders. 

 It would appear that in these islands the Nullijjorce play a 

 more than ordinary part, but as their presence is not universal 

 at all parts of the atoll ring, it is evidently not at all essential 

 to successful barrier building. On the other hand, the Litho- 

 thamnionidce are particularly insignificant in the atoll, and 

 they cannot possibly be of the importance that, in the Mal- 

 dives and Laccadives, Mr. Stanley Gardiner § supposed them 

 to be. They are found only in certain sites, far from the 

 barrier edge, and the total bulk of all their remains cannot 

 make a very great difference to the sum of the atoll's building 

 assets. 



From ocean to lagoon the character of the coral flat varies, 

 and the changes that are seen are natural and easily explained 

 ones ; they throw a considerable light on the formation of the 

 barrier, and of the whole structure of a coral island, or an atoll. 



In the first place, the various forms of corals have a fairly 

 defined site of election ; but for all that their distribution is 

 capricious, and is only roughly to be argued for any given 

 spot of barrier from the knowledge gained at any other 

 spot. From reading of accounts of the corals of atolls, the 

 impression is easily gained that a remarkable order in their 

 distribution exists ; the corals of the reef and those of the 



-s tt The structure and Formation of Coral Reefs," p. 24. 



t Scot. Geog. Maga., vol. v. No. 9, p. 460. 



I " Characters and Mode of Formation of the Coral Reefs of the 

 Solomon Islands," Proc. Roy. Soc. Edhiburgh, July 1886, p. 859. 



§ " Fauna and Geography of the Maldive and Laccadive Archipelagoes, 

 1902^," vol. i. part 2, p. 175. 



