audits Bearing on the Coral Reef Problem. 199 



< 'In mical explanation of shallow water origin of dolomites. 



The author* has in a previous paper attempted to suggest 

 an explanation of the shallow water origin of dolomite based 

 upon the varying solubilities of calcium and magnesium carbon- 

 ates, respectively, in water containing C0 2 in solution. There 

 is considerable experimental evidence to show that tinder 

 atmospheric pressure calcium carbonate is more soluble than 

 magnesium carbonate in carbonated water. However, under 

 increased pressure the solvent action of carbon dioxide on the 

 magnesium and calcium carbonates is reversed. f 



Based upon this principle a process was for some time em- 

 ployed for the commercial formation of Epsom salts by the 

 extractidn of nearly pure magnesian salts from dolomite. This 

 consisted in subjecting the rock when finely ground and mixed 

 with water, to the action of C0 2 under a pressure of about four 

 atmosplieres. Under these conditions the magnesian carbonate 

 passed readily into solution while very little calcium carbonate 

 was taken up. 



As at atmospheric pressure calcium carbonate is more soluble 

 than magnesium carbonate in carbonated water, while under 

 four atmospheres pressure the reverse conditions obtain, it 

 should follow that at some pressure between 1 and 4 atmos- 

 pheres carbonated water should dissolve the two carbonates in 

 molecular proportions. At such a pressure, under conditions 

 of saturation, dolomite may be deposited, and such a pressure 

 may well afford the most favorable conditions for the replace- 

 ment of calcium carbonate by magnesium carbonate up to 

 45 - 65 per cent, the composition of dolomite. 



In the sea, especially where organic decay is going on round 

 coral reefs, carbon dioxide will be present and such a pressure 

 as indicated above will occur not far below the surface. 



The zone in the Funafuti bore between 15 and 30 feet below 

 the surface may very well represent this favorable zone since 

 in all three borings at about this level, while no dolomite crys- 

 tals have developed, MgC0 3 is present in the coral limestones 

 up to about 16 per cent. It seems probable therefore that 

 the limestones at this depth are now passing through an early 

 stage of dolomitization. 



Since it is notorious that all shallow water limestones do 

 not become dolomitized the question arises as to what circum- 

 stances may retard or facilitate the process. In the present 

 state of our knowledge it is only possible to suggest one or two 

 factors which may operate in nature. 



The activity of bacteria;}: has been suggested by some writers 

 as influencing the formation of dolomite. Time certainly is an 

 important factor. The process of replacement is probably slow 



* E. W. Skeats, Q. J. G. S., lxi, 135, 1905. 



f Dingl, Holjt. Jour., ccix, 467; abstract, Cliem. Soc. Jour., xii, 96. 



\ Walther, Geschichte der Erde und des Lebens, p. 90. 



