256 



L. HILLIS-COLINVAtrX 



Table XXIV. Average Skeletal, Composition of Sand -size Components of 

 Some Lagoonal Sediments" 



"Adapted from Milliman (1974). Figures are percentage skeletal components in total 



sediment. 

 Tr., trace. 



processes in the growth of reefs of which we maynot be fully aware. Each 

 lowering of sea-level by eustacy would be expected to eliminate the 

 lagoonal (largely Halimeda) contribution to the atoll mass in circum- 

 stances where at least the horizontal component of reef growth may 

 continue, even if attenuated. Each eustatic lowering also affords an 

 opportunity for the exposed reef ridge to be weathered and deposited 

 on the Halimeda matrix of the old lagoon floors. Thus a history of 

 eustacy will alternately favour the deposition of Halimeda and non- 

 Halimeda sediments into the atoll mass. Added to this time-variant 

 function must be the effects of different rates in the growth of the reef 

 ridges themselves. 



Coral matrix is formed by the vertical growth of the reef ridge which 

 accompanies changes in relative sea-level associated with both eustacy 

 and isostacy, by the horizontal growth of the reef ridge and by the 



