Tortugas Marine Organisms. 99 



Copper has been reported as a component of the surface mud of the 

 ocean bottom at other locaUties, and both zinc and copper have been 

 reported as being present in small amounts in many limestones and 

 dolomites. 



The calcareous mud of the Marquesas is limestone in the making, 

 and when the conditions under which it accumulates are considered 

 there can be very little doubt that the copper and zinc content is 

 derived from organisms which have concentrated these metals from 

 sea- water, and at death the metal content of their decaying tissues is 

 fixed as sulphides and becomes a part of the limestone or sedimentary 

 rock thus formed. 



Even though the percentage (0.016 per cent for copper and 0.009 

 per cent for zinc) is seemingly small, the actual weight of metal 

 present when calculated for large areas of limestone is considerable, 

 as each cubic meter of rock would contain 432 grams of copper and 

 243 grams of zinc — an amount quite sufficient to produce metallic 

 deposits of commercial value after secondary concentration by natural 

 agents. 



