LAGOON FORMATION 101 



in the water, much of it being removed by the ebbing 

 of the tidal waters. 



By such methods as the above the inner boulders of 

 the boulder zone might be removed, while the zone 

 itself continues to extend outwards on to the reef flat 

 as the reef grows seawards. It is easy to imagine the 

 processes going farther, and hollowing out the inner 

 part of the reef to form a boat channel and finally a 

 lagoon. Should the reef start as a small mass in mid- 

 ocean it would pass through similar stages a flat in 

 the centre, then a shallow pool, and finally a lagoon. 

 As a barrier the reef should in some degree conform 

 to the contour of the land, but as an isolated reef it 

 should be more or less round in its early stages. Later, 

 both the barrier and the atoll would show the effect 

 of currents in carrying and depositing mud and other 

 material, so as to make the extension of the reef in 

 certain directions easier, destroying any regularity of 

 contour. From the first formation of any inner pool, 

 channels would form through the boulder zone to allow 

 the escape of the tidal water. The latter would 

 especially rush out over the lowest parts of the en- 

 circling reef. Being charged with much sediment in 

 suspension, it would kill any corals and other sedentary 

 organisms in its path, and further would prevent any 

 fresh corals from commencing to grow in its channel. 

 Where dead rock occurs boring organisms enter, but 

 where the surface is covered with living tissues they 

 cannot penetrate. This is the reason why the seaward 

 face of the reef, which is almost covered with growing 

 organisms, is scarcely affected by solution and abrasion 

 as compared with the lagoon. The rock under a 

 channel, being bare, will be bored and rotted like the 

 rocks in the lagoon, with the result that its depth may 

 increase more or less equally with that of the lagoon 



