330 EDWARD STEIDTMANN 



Experimental evidence for the chemical precipitation of dolomite. — 

 The direct precipitation of dolomite from solution has apparently 

 not been done, experimentally. Sterry Hunt 1 precipitated cal- 

 cium carbonate and the hydrous carbonate of magnesium with 

 sodium carbonate and found that he could develop dolomite by 

 heating this mixture to 120 C.-130 C. Obviously this experiment 

 did not accomplish direct chemical precipitation of dolomite nor 

 is it applicable to the explanation of the majority of dolomites in 

 nature. 



Daly's hypothesis of the direct precipitation of dolomite in the 

 primitive ocean. — Daly 2 postulates that dolomite was precipitated 

 from a nearly limeless primitive ocean by ammonium carbonate 

 generated by the decay of organisms on the bottom of the ocean. 

 The hypothesis is based on several assumptions, namely: (a) 

 the scavenging system of the pre-Cambrian and the early Paleozoic 

 ocean was less perfect than at present; (b) the post-Huronian 

 uplift gave a tremendous impetus to the transportation of lime 

 to the sea, which in turn stimulated the development of lime- 

 secreting organisms; (c) the improvement of the scavenging 

 system and the development of lime-secreting organisms gradually 

 brought the balance in favor of the deposition of limestones by 

 organic secretions over direct chemical precipitation. The theory 

 is offered to explain a number of facts: (a) the predominance of 

 dolomite in the older formations; (b) characteristics such as grain, 

 fossil record, and field relations of certain dolomites indicative of 

 direct chemical precipitation; and (c) the apparent similarity 

 between the ratio of calcium to magnesium in the older dolomites 

 and the ratio of calcium to magnesium of streams on pre-Cambrian 

 crystalline terranes. This theory fits many facts remarkably well. 

 However, the premises on which it is based are open to objection. 



It is true that the decay of marine organisms generates 3 ammo- 

 nium carbonate and other precipitants. But organic decay also 

 generates carbonic acid, which tends to keep the carbonates of 



1 Sterry Hunt, Chem. and Geol. Essays (1895), 80. 



2 R. A. Daly, "First Calcareous Fossils and the Evolution of the Limestones," 

 Bull. Geol. Soc. of America, XX, 153-70. 



3 G. Steinmann, Ber. Naturforsch. Gesell. Freiburg, IV (1889), 288; Challenger, 

 Report on Deep Seas, 255. 



