EVOLUTION OF LIMESTONE AND DOLOMITE 427 



SUMMARY 



The problem under discussion is, Why does the dolomite con- 

 tent of the geologic column decrease with time? Is it due to a 

 secondary alteration of limestone after emergence Irom the sea, 

 roughly proportional to time, or is it due to a gradual decline in 

 the primary development of dolomite in the sea? If the latter, 

 what factors controlling the deposition of dolomite have changed 

 during geologic time, temperature, pressure, life processes, or the 

 chemical composition of the sea ? 



The conclusions reached are: dolomite develops predominantly 

 in the sea, therefore the decrease in the dolomite content of the 

 sediments in going up the geologic column is mainly due to a 

 decrease in the proportion of dolomite developed in the sea with 

 time. 



The factors of deposition whose progressive change has probably 

 controlled the decline of dolomite development in the sea are life 

 processes and the chemical composition of the sea. There is no 

 definite evidence for a change in the nature of the life processes 

 in their relation to dolomite deposition. There is evidence for a 

 change in the chemical Composition of the sea; namely, the fact 

 that the present ratio of calcium to magnesium of the streams is 

 probably more than twice that of streams draining crystalline 

 terranes, comparable in composition to the primitive lands. Accept- 

 ing uniformitarianism, it follows that the present streams have a 

 much higher ratio of calcium to magnesium than the primitive 

 streams. It has been indicated that solutions high in magnesium 

 and low in calcium are more favorable to the development of 

 dolomite than those which are low in magnesium and high in 

 calcium. It is therefore highly probable that the chemistry of the 

 primitive sea was more favorable to the deposition of dolomite 

 than the present ocean. 



The increase in the proportion of calcium to magnesium in the 

 streams is believed to be due to selective processes whose effects 

 have been cumulative with time. Rock alterations tend to result 

 in a higher percentage loss of calcium than of magnesium, the 

 materials lost being largely transported in solution to the sea. 



