622 E. BLACKWELDER ORIGIN OF BIGHORN DOLOMITE OF WYOMING 



before actual lithification took place, in consequence of chemical reactions 

 in the basal la}^er of the sea-water in which deposition was proceeding. 

 According to Pfaff, 23 the precipitation of dolomite may be brought about 

 by the presence of either ammonium carbonate, ammonium sulphide, or 

 hydrogen sulphide. These are all common products of the decay of or- 

 ganic matter and are likely to be present in notable quantities in the sedi- 

 ments accumulating on the ocean floor wherever the tissues of marine 

 plants and animals are in process of decay. It appears, however, that in 

 Pfaff J s experiments the solutions were considerably concentrated through 

 evaporation. I have found no record of experiments of this kind carried 

 out in such a way as to reproduce closely the conditions on the bottoms of 

 shallow, warm, clear seas. 



If it is true that the reactions are chemically feasible in the seas of 

 today or at least were so in the epicontinental seas of Ordovician time, 

 with their somewhat different conditions of temperature, salinity, or bio- 

 chemical processes, then the Irypothesis tends to explain many peculiari- 

 ties of the Bighorn and similar dolomites. The recrystallization of the 

 mass, which is an essential effect of the postulated process, explains the 

 uniform and moderately coarse-grained texture of these rocks. It like- 

 wise accounts for the general lack of caverns, veins, and pores. The hy- 

 pothesis also affords a ready explanation of the fact that the corals, crinoid 

 stems, and other identifiable animal structures, which originally consisted 

 of either calcite or aragonite, are here fully changed into crystalline dolo- 

 mite. The vague outlines of the fossil shells and of the problematical 

 •algae may also be accounted for by the process of crystallization, which 

 would give little heed to such boundaries in a chemically homogeneous 

 mass. If, as assumed, this process of converting lime carbonate into dolo- 

 mite during crystallization took place in the loose sediment lying undis- 

 turbed on the ocean floor, there seems to be no reason why it should have 

 affected a layer more than a few inches in depth at any one time. If this 

 view is correct, it helps to explain the altenation of dolomites and lime- 

 stones in many formations and the fact that the beds underlying the Big- 

 horn formation are pure limestone rather than dolomite. 



To this hypothesis of the dolomitizing of lime carbonate deposits in the 

 course of their deposition I find no positive objection, and as it apparently 

 explains most of the observed facts, it seems to me the most promising of 

 the suggestions which have been made. The idea is, of course, not at all 

 new. Daly 24 has recently reached the same conclusion for the dense fine- 



23 Recorded by F. W. Clarke : Data of Geochemistry. TJ. S. Geological Survey, Bull. 

 491, 1911, p. 538. 



24 R. A. Daly : The first calcareeous fossils and the evolution of the limestones. Bull. 

 Geol. Soc. Am., vol. 20, 1909, p. 170. 



