334 EDWARD STEIDTMANN 



of equilibrium with sea water than the corresponding magnesium 

 salt. Aside, then, from the inherent difference between the 

 solubility of calcium carbonate and magnesium carbonate, the 

 chemistry of the sea apparently facilitates the solution of calcium 

 carbonate and retards that of magnesium carbonate. Assuming 

 that the efficiency of the organic precipitation of calcium carbonate 

 has been about the same throughout Zoic times, it seems obvious 

 that the concentration of magnesium carbonate in calcareous 

 deposits by marine leaching would have been even more effective 

 than now, if at some time in the past the rivers had contributed 

 relatively less lime and more magnesia than at present. If there 

 is any probability that such has actually been the case, that would 

 constitute one factor which would operate to cause an increase 

 in the magnesium content of limestone in descending the geologic 

 time scale. 



The origin of dolomite by the secondary replacement of calcium 

 by magnesium in the sea. — Coral rocks originally contain only a 

 small percentage of magnesium carbonate, usually less than i 

 per cent. In 1843, J- D. Dana 1 in a rock from the coral island 

 of Makatea, in the Pacific, reported magnesium carbonate to the 

 extent of 38.07 per cent. This approached the dolomite ratio 

 which requires 45 . 7 per cent, and it was suggested that the rock 

 had been dolomitized through secondary replacement, the mag- 

 nesium carbonate probably having been derived from the concen- 

 tration of shallow lagoons of sea water. Since Dana's time, many 

 other investigators have recorded similar enrichments of coral 

 reefs. In a coral reef from Porta do Mangue, Brazil, T. C. Branner 2 

 reports 6 . 95 per cent of magnesia, equivalent to 14.6 of carbonate, 

 while the corals themselves contained only 0.20 to o . 99 per cent 

 of magnesia. E. W. Skeats 3 reports analyses of dolomitic coral 

 rock of the Pacific in which the magnesium carbonate rises to a 

 maximum of 43.3 per cent. 



The borings from the atoll of Funafuti as discussed by J. W. 

 Judd 4 present a remarkable case of transformation from reef 



1 J. D. Dana, Corals and Coral Islands (3d ed.), 393. 



2 T. C. Branner, Bull. Mus. Comp. Zobl., XLIV (1904), 264. 



3 E. W. Skeats, Bull. Mus. Comp. Zobl., XLII (1902), 53-126. 



4 J. W. Judd, The Atoll of Funafuti, quoted from Bull. 330, U.S.G.S., 487. 



