188 CORAL AND ATOLLS 



spot. To any one who has for long watched these minor — yet 

 striking — changes, it must be evident how fallacious are those 

 arguments concerning the age of atolls, or the manner of atoll 

 formation, which are based on the measurement of the quan- 

 tities of sand piled up, or denuded, in any one point in the 

 whole circle of more than twenty miles. Where in the island 

 ring the sea runs strongly through the gaps and into the 

 lagoon, spits are formed wherever, by some interference, the 

 current slackens ; in this way the crescent shape of the islands 

 is brought about, and the great sand flats of the southern 

 portion of the lagoon are accumulated. 



Drift sand is the main factor in causing the shallowing of 

 the lagoon, and drift sand it may be safely said will, lacking 

 some general upheaval, be the agent which will ultimately win 

 the lagoon to the dry land ; but it is the most inconstant and 

 capricious agent, and no man may say that so many feet will 

 be laid down this year, and so many next, and that therefore 

 in a certain meaningless number of years the lagoon will cease 

 to exist. The very presence of the sand in the lagoon 

 introduces a complication into the question, for sand is one of 

 the most potent causes of coral death ; the shallowing of the 

 lagoon by the accumulation of coral growth must therefore be 

 arrested when the shallowing by sand deposit sets in. It is 

 under these circumstance that the very slight building in- 

 fluences of LithothamnionidcG come into play, for they do not 

 appear to be so sensitive to the influences of silt as are the 

 corals. In certain places at the southern side of the atoll, in 

 the border-line region of barrier flat and lagoon flat, the 

 growth of these calcareous algse occupies a fairly large area, 

 which is free of living coral in any quantity : but apart from 

 arresting the drift sand in their meshes they do not have 

 any influence on the rate of building. It must not be 

 imagined that the great southern part of the lagoon is by any 

 means devoid of living coral colonies, for dotted about at 

 intervals are some of the most luxuriant of all the coral 

 growths. It has been said that the distribution of the living 

 colonies on the barrier and the barrier flats was a distribution 



