The analyses of numerous limestone samples showed that there is a general increase in the average Ca/Mg ratio in going up the geologic column with superimposed periodic fluctuations. A general decline in abundance of dolomites in going up the geologic column was previously shown by R. A. Daly (1910). The writer, however, failed to establish a simple relationship between the Ca/Mg ratio and the age of carbonate rocks. The cyclic occurrence of dolomitic and calcitic limestones, observed by the writer, suggests that the formation of dolomites was occurring in cycles with a gradual accumulation of magnesium in sea water until favorable conditions were established for the formation of dolomites.

Inasmuch as high CO₂ pressure of the atmosphere during pre-Cambrian and Paleozoic time is advanced by Russian geologists as a possible explanation for the abundance of dolomites, an attempt was made to precipitate dolomite directly out of sea water at high CO₂ pressure. This attempt was successful and indicated that some pre-Cambrian and Paleozoic dolomites may have been formed by direct chemical precipitation out of sea water.