PRE-CAMBRIC LIMESTONES 



333 



monia. was removed from the mixture in the first precipitate. From 

 the experiments cited, it appears that hydrous carbonate of mag- 

 nesium can be precipitated by ammonium carbonate emitted from 

 decaying animal remains, the precipitation being much slower than 

 in the case of calcium carbonate, and being retarded by the pres- 

 ence of calcium salts in the solution. If the pre-Cambric ocean was 

 nearly limeless, as argued by Daly, the proportion of precipitated 

 magnesium carbonate would be high, even possibly approaching the 

 ratio of true dolomite. The general proportion of dolomites to 

 limestones in the various formations of the earth's crust is shown 

 in the following table. (Daly-12: i6j.) 



Table sJiozving proportion of dolomites to limestones in the 

 geological series. 



Period 



Pre-Camhric: 



a. From North America except those in b . 



b. From Ontario (Miller) 



c. Average of a and b 



d. Best general average 



Cambric (including 17 of the Shenandoah 



limestone) 



Ordovicic 



Siluric 



All pre-Devonic 



Devonic 



Carbonic (including Mississippic) 



Cretacic 



Tertiary 



Quaternary and Recent 



Total 



No. of 

 analyses 

 averaged 



28 



33 

 61 



49 



30 



93 



208 



392 

 106 



238 



77 

 26 

 26 



865 



Ratio of 



CaCOs to 



MgCOa 



2.39 



4-49 



8.89 



40.23 



2>7-92 



25.00 



Ratio of 

 Ca to Mg 



This table shows that the ratio of calcium to magnesium is 

 fairly constant for all pre-Devonic rocks (shown by 392 analyses 

 chiefly from North America), the average being 3.35:1. In the 

 Devonic the ratio rises abruptly and increases rapidly in the Car- 

 bonic. The Cretacic shows an apparent maximum, but might be 



