ORGANIC DEPOSITS 



943 



Percentages of Particles of Silt and Clay Size in bottom Samples from the 

 Reefs at Murray Island, Australia, and at Gocoanut Point, Andros Island, 

 Bahamas. 



Murray Island 



Per cent 



200 feet from shore 1.9 



600 feet from shore 2.8 



1,200 feet from shore 9 



1,600 feet from' shore 1.4 



Average 1 . 75 



Cocoanut Point 



Sample 



190. 

 191. 

 192. 

 193. 



Per cent 

 1.5 

 1.8 

 1.7 

 1.9 



Average 1 .725 



Remarks. — It should be noted that as fine and medium sand are predominant 

 in the Cocoanut Point samples, they average finer than the Murray Island 

 specimens. 



Percentage of MgCOo (hypothetical Gomhination) in Samples considered in 



preceding TaMe 



Murray Island 



Per cent 



200 feet from shore 5.52 



600 feet from shore 5 . 95 



1,200 feet from shore 5 . 76 



1,600 feet from shore 5.83 



Average 5.745 



Cocoanut Point 



Per cent 

 Composite of samples 190 



to 193 5.24 



Average 5.24 



Material such as that on and behind the coral reefs at Murray Island, 

 Australia, and Cocoanut Point, Bahamas, are predominantly composed 

 of the remains of madreporarian corals and calcareous algae; in some 

 places the remains of one, in other places the remains of the other, group 

 of organisms predominating. Grains derived from these two sources 

 form between 74 and 76 per cent of the material across the Murray Island 

 reef, while, roughly, from 20 to 23 per cent is due to mollusca and to 

 shoal-water, bottom-living foraminifera. There is very little pelagic ma- 

 terial, although it should be mentioned that coccolithophoridBe are in- 

 variably present at each locality in separates of silt and clay size. There 

 is an important difference between the Murray Island and Cocoanut Point 

 samples in that those from Murray Island contain no alcyonarian spicules, 

 while there are many such spicules in the material from Cocoanut Point. 



Deposits of the kind just described are dependent on definite, areally 

 limited, ecologic conditions; and, according to present information, they 

 cover relatively small areas. In other areas molluscan remains predomi- 

 nate; in others tests of foraminifera, while in others there are relatively 

 few organic remains and the material is mostly a chemical precipitate. 

 Large deposits of calcium carbonate formed by the secreting activities of 

 organisms, as well as tliose deposits formed by chemical precipitation, are 



