160 CORAL AND ATOLLS 



surf, and more than this, in an environment where, ever and 

 again, loose fragments come hurtling along at the mercy of the 

 waves. These conditions completely eliminate any delicately 

 branching forms of coral, and only those which grow as rounded 

 knobs, as stout short branches or plates, or as encrusting 

 layers can hope to flourish. To live at all in that tumult of 

 waves and moving masses of rock, only those things which are 

 very hard and rounded, or are very soft and yielding, can exist 

 free, all else must hide in crevices or creep in burrows. Of 

 the very soft things which can stand these severe conditions 

 are many of the Alcyonacem, which live in great abundance in 

 depressions of the seaward edge of the barrier, and of the hard 

 and rounded things are those most typical of all the barrier 

 corals, the massive Pontes. 



An " outer edge " of the barrier has been spoken of, but 

 the use of this phrase by no means implies that here the 

 barrier ends. The submarine slope shelves away from the 

 land ; its line of retreat being the barrier flats, and on the 

 strength of the waves and the amount of the tide variation 

 will depend the slope of 'these flats. At dead low tide the 

 surf-line has receded from the land until it has travelled a 

 hundred yards or so, and has fallen by three or four feet from 

 its high-water mark. At this low tide surf-line therefore will 

 be the junction of that portion of the coral bottom which is 

 subjected to the action of the surf, and the portion which lies 

 undisturbed below. Storms, and the action of high waves, 

 have at intervals displaced large rocks, and have then been 

 powerless to trundle them up the flats ; and so the " outer 

 edge " of the barrier is in many places marked by the ex- 

 posure of large and irregular rock masses at low tide. It 

 must always be borne in mind in accounting for the disposition 

 of the different parts of the barrier, that there is a perpetual 

 force acting over a wide area, but the intensity of this force varies 

 at difl'erent points of its activity ; and its action is inconstant 

 throughout. At the seaward edge its lifting force is greatest, 

 for here the surge and swell of larger waves is felt ; it is here, 

 therefore, that the largest rock masses are moved. But again. 



