EVOLUTION OF LIMESTONE AND DOLOMITE 325 



various hypotheses have been offered for its explanation. Probably 

 the majority of geologists hold that it is due to a secondary altera- 

 tion of limestone to dolomite, which is roughly proportional to 

 time. Daly 1 has suggested that it is due to a change in the nature 

 of the life processes which effect the precipitation of carbonates 

 in the sea. Still another alternative hypothesis is herewith offered, 

 that the percentage of dolomite developed in the sea has declined 

 with time, and that this was controlled by a progressive increase in 

 the ratio of calcium to magnesium contributed from the lands. 



Absolute proof for any one of these hypotheses is impossible 

 from the very nature of the problem. For instance, all of them are 

 based on the theory of uniformitarianism, a theory deep rooted 

 in geologic evidence, but a theory nevertheless. All the hypotheses 

 offered involve the question of whether dolomite develops pre- 

 dominantly in the sea, regardless of the specific processes of for- 

 mation, or whether it is predominantly a secondary product from 

 limestones, after their emergence from the sea. If its origin is 

 mainly in the sea, which of the factors controlling deposition in 

 the sea has changed so as to cause a decline in its deposition with 

 time? Was it temperature, pressure, life processes, the chemical 

 composition of the sea, or some combination of these factors? 

 To these no final answer can be made. The available evidence 

 for each view can be sifted. Conclusions may be drawn, but the 

 probability may be freely recognized that in the future some of 

 the conclusions may become invalidated. 



PART I. THE EVIDENCE ON THE ORIGIN OF DOLOMITE 



That the predominance of dolomite in the ancient sediments is due 

 in a larger measure to primary conditions of deposition in the sea than 

 to the metamorphism of limestones after their emergence from the 

 sea seems to follow from a comparison of the evidence on the origin 

 of dolomite in the sea with that on the origin of dolomite by the 

 metamorphism of limestone after its emergence from the sea. 



A. THE EVIDENCE FOR THE ORIGIN OF DOLOMITE IN THE SEA 



Inter stratification of limestones, magnesian limestones, and dolo- 

 mites the result of primary conditions of sedimentation, or of the 

 1 Ibid. 



