THE STRUCTURE OF THE BARRIER 155 



rush over into the lagoon, exactly as it does to-day where 

 the gaps in the dry land ring let the ocean and the lagoon 

 communicate. 



The Breccia Platform is, therefore, to be seen in its simplest 

 form in the gaps in the atoll ring, in the places where no 

 islands exist, and here it is usually called " Barrier " in its 

 whole extent. In such places as this it may be easily examined 

 at low tide, for, on a calm day, one can walk from one island 

 to the next through the water which washes the rock flat. At 

 the seaward edge of the flat is the line of surf, and the 

 contrast between the blue rollers on the one side and the surf- 

 washed rocks on the other is very marked ; it is this contrast 

 which gives the impression that the barrier edge is like the side 

 of a house against which waves beat. Standing on the barrier, 

 as near to the breakers as is possible, an appearance is pre- 

 sented as though at a line a few yards from you the deep 

 ocean ended, and its blue rollers hit on a sharp upstanding 

 edge of rock that caught their feet and scattered them in spin- 

 drift among the rocks on which you are standing. But this 

 appearance is really a delusion, and on a very calm day, even 

 from the level of the barrier, discoloured patches of bottom 

 can be seen through and reflected in the rollers before they 

 break ; and from such trivial heights as it is possible to attain 

 on a coral island, these patches are seen to stretch a long way 

 to seaward of the breaker line. Moreover with the rise and 

 fall of the tide this surf-line moves, coming nearer to and 

 retreating further from the land; and no matter where the 

 line be at the moment, the appearance presented is always the 

 same — a sheer sharp edge on which deep ocean breaks. 



The falling away of the rock ledge into the ocean is not in 

 reality such a sudden fall as has been imagined, and its falling 

 to the lagoon is very much more gradual. The barrier is then 

 a wide rock flat, falling to the ocean at its outer edge, shelving 

 to the lagoon at its inner edge, and extending almost entirely 

 round the summit of the ridge. 



There can be no doubt left in the mind of any one who 

 watches the great ocean waves broken at the edge, and swirl 



