262 



li. HILLIS-COLnsrVAUX 



both the amounts of Halimeda segments shed and the types of currents 

 may have seasonal components. At the Enewetak sites I visited, the 

 commonest species were Tnacrophysa, opuntia, cylindracea, gigas, 

 distorta, copiosa and lacunalis f. lata. These, then, are the species, about 

 half the total number found on the atoll reef, which would contribute 

 most of the Halimeda sediments (Hillis-Colinvaux, 1977), but it is likely 

 that two or three from this short list produce the bulk of the Enewetak 

 sediments. Tydemania expeditionis, the only other calcareous green alga 

 present at Enewetak in sizeable populations (Gilmartin, 1960, 1966; 





 DEPTH r- 



15-55 



% Grain Composition 

 10 20 30 



40 



157 



209 



249 



295 



308 



Fig. 86. Composition of sand-sized sediment from the fore-reef slope ( — 15 m to — 55 m) 

 and island slope ( — 123 m to —308 m) of Discovery Bay, Jamaica. Graph bars 

 from top to bottoin represent Halimeda, calcareous red algae, corals, molluscs, 

 foraminifera and "other". (Data from Moore et al., 1976.) 



Hillis-Colinvaux, 1977), also contributes to the sediments but not in an 

 easily recognizable form. 



In Jamaica most of the Halimeda sediments derived from the 

 shallower parts of the reef, that is, from the back-reef, reef crest and 

 fore-reef to about — 55 m, are dammed by sill reefs at the base of the 

 fore-reef slope (Goreau and Goreau, 1973; Goreau and Land, 1974; 

 Moore et al., 1976). The contribution of the genus to identifiable sedi- 

 ments of the island slope (depth approximately — 122 m to — 305 m) is 

 at least as great (Fig. 86). In the suite of samples available for analysis 



