WHY THE ROCK IS A DOLOMITE 623 



grained dolomites characteristic of the pre-Cambrian, Cambrian, and 

 Ordovician formations in both eastern and northwestern North America. 

 His comment is that "the evidence shows that the magnesian content of 

 the staple pre-Devonian limestone is original, in the sense that the mag- 

 nesium carbonate was precipitated from sea-water. In many, if not all, 

 cases the dolomite crystals may have been formed at or near the surface of 

 the ancient calcareous muds by the interaction of the magnesian salts of 

 sea-water with the more easily precipitated calcium carbonate." 



The subject has also been reviewed by Steidtmann, 25 who concludes that 

 the formation of dolomite in this manner is not only possible, but that an 

 increase in the proportion of magnesium to calcium in the sea-water may 

 be sufficient to bring about dolomitization. 



A consideration of the subject up to this point naturally tempts one to 

 go farther and seek a solution for related questions. If the Bighorn dolo- 

 mite was formed in the way suggested, under what conditions was the 

 change brought about ? An answer to this would help us to understand 

 why dolomite was laid down in some parts of what is now the United 

 States at approximately the same time that pure limestone was being de- 

 posited in others, and would likewise explain many additional cases, such 

 as the apparent equivalence of the Niagara dolomite of Wisconsin and the 

 Louisville limestone of Kentucky. Experimental work has shown that 

 strong solutions of magnesium salts are deleterious to the growth of ani- 

 mals and plants, but inasmuch as the Bighorn Sea evidently contained 

 abundant living organisms — many of them, like the corals, very delicately 

 adjusted to their environment — it seems unlikely that the magnesium con- 

 tent could have been more than two or three times as great as in the 

 present ocean. Another fact of possible suggestiveness is that the rivers 

 of hot arid regions normally carry a much larger proportion (two to four 

 times as much) of magnesium, as compared with calcium, than do the 

 rivers of cool moist climates. It. is also a fact, shown by many laboratory 

 experiments, that the alteration of calcite to dolomite takes place more 

 readily in warm than in cold solutions, and at 100° centigrade the reac- 

 tion requires hours rather than the centuries which are available for 

 geologic processes. Interesting as these questions are, however, it is not 

 very profitable to discuss them until more is known about the chemical 

 reactions in sea-water of varying temperatures, pressures, salinities, and 

 composition. 



25 Edward Steidtmann; Toe evolution of limestone and dolomite. Journal of QeolQgJ 

 vol. 19, 19X1, p. 337. 



