THE STRUCTURE OF THE BARRIER 161 



its time of action is short, for only at dead low tide can it 

 exert its full force ; and so it comes about tliat large masses 

 are displaced and frequently reseated on the " outer edge " of 

 the barrier. It is over the intermediate part of the barrier 

 that the maximum force is felt, for the crashing surf has the 



Fig. 54. 



DRAAVINa OF A LIVING SPECIMEN OP AN AlCYONARIAN CoRAL 

 "WHICH IS ABUNDANT IN THE RoUGH WaTER OP THE BARRIER. 



Two Zooids are seen completely expanded and several others in the 

 passive stage between contraction and expansion. 



power to act during the rise and during the fall, and at all 

 times, except at the very lowest spring tides, is this inter- 

 mediate portion a battle-ground of surf and rock. Here, 

 therefore, are the flats of level cemented rock, free of loose 

 fragments, free of jagged edges, and free too of living corals; 

 boulders are washed across and tend to level everything, and 

 only the Nulliporce may flourish. It is here that the finer 

 particles are forced by the spin-drift, as by a sand-blast, into 

 every crack and rift of the shore-washed rocks ; this is the 

 factory of the breccia. At the inner or shoreward edge of 

 the barrier the wave action is powerful during high tide, and 

 intermittent during the rise and fall. It is here that the 

 smaller boulders come to rest ; this is the factory of the 

 debris which makes the island. It is easy then to see that on 

 the barrier, as it stretches from an island to the ocean, actual 

 living coral growth is not abundant. It is only in pools and 



