450 E. STEIDTMANN ORIGT^N OF DOl.OMITE 



The domiiiaiice ol pure limestones and doloniites l)roiight out by stains 

 shows that the margin between dolomite and limestone-bearing condi- 

 tions is a narrow one, and hence near the critical stage slight changes in 

 chemical conditions may completely change the nature of deposition. 



Stains showed that all of the several hundred dolomite samples studied 

 contained FeO. It was absent in all primary calcite grains tested. The 

 uniform presence of FeO in marine dolomites shows that this compound 

 is a primary constituent. Marine dolomites therefore were formed under 

 reducing conditions. In the shallow seas in which they formed reducing 

 conditions could be looked for only within the ooze where decay was 

 rampant or in the lower strata of sea-water, below the aerating effect of 

 wave action. 



