THE STRUCTURE OF THE BARRIER 167 



much more important one of the grinding movements ot the 

 water. This is one of the greatest forces with which we have 

 to deal in estimating the physical surroundings of the atoll, 

 for it is carried on without ceasing. Every wave moves some 

 fragment and triturates some particle, always tending to make 

 finer and finer the broken pieces of coral which are for ever 

 being removed from the coral beds beneath the surf-line. 

 The branches and boulders become chips and fragments, and 

 these in their turn are washed to and fro, becoming smaller 

 and smaller in the process, and a diminishing scale of coral 

 material is created, in which the first term is the growing 

 boulder and the last is the fine white mud. 



The great quantities of sand formed in a coral atoll may 

 be realised when the silt banks are watched in their growth, 

 and when it is remembered that not a tithe of the sand ever 

 reaches the lagoon, but lies on the submarine slopes of the 

 ocean bed for a distance of several miles all round the atoll. 



The structure of the barrier reef is then in reality a simple 

 one, it is a great platform built round the top of a large flat 

 plateau, and, as a rampart, it encircles the summit for a dis- 

 tance of some seventeen miles. 



