AVERAGE RATIO OF CA TO MG IN DIFFERENT PERIODS 165 



Table IV. 



Period. 



Pre-Cambrian — 



a. From North America, except those in b 



b. From Ontario (Miller) 



c. Average of a and b 



d. Best general average 



Cambrian (including 17 of the Shenan- 

 doah limestone ) 



Ordovician 



Silurian ... 



All pre-Devonian 



Devonian 



Carboniferous 



Cretaceous 



Tertiary 



Quaternary and Recent 



Total 



i. 



Number of 

 analyses 

 averaged. 



28 

 33 

 61 

 49 



30 

 93 



208 



392 



106 



238 



77 



26 



26 



865 



Ratio of CaC0 3 

 to MrCO.. 



1.64 

 4.92 

 2.93 



2.58 



2.96 



2.72 

 2.09 



2.39 



1 



4.49 



1 



8.89 



1 



40.23 



1 



37.92 



1 



25.00 



1 



Ratio of 

 (a to Mg. 



2.30 : 1 

 6.89 : 1 

 4.10: 1 

 3.61 : 1 



4.14 

 3.81 

 2.93 



3.35 

 6.29 

 12.45 

 56.32 

 53.09 

 35.00 



It will be observed that the ratio of calcium to magnesium is fairly 

 constant for all the (392) pre-Devonian analyses, in which the average is 

 3.35 : l. 12 The ratio abruptly rises in the Devonian and increases rapidly 

 in the Carboniferous. The Cretaceous shows an apparent maximum, but 

 it is quite possible that a larger number of analyses of Tertiary and later 

 formations would give average ratios at least as high as that of the Cre- 

 taceous. 13 



The ratio for the -pre-Cambrian limestones (3.61:1 to 4.10:1), like 

 that of all the pre-Devonian, is significantly close to the ratio of calcium 

 to magnesium in the rivers now draining pre-Cambrian terranes, as may 

 be seen, for example, in the Ottawa river analyses made at the capital 

 (low-water stage, 3.82:1; high-water stage, 3.50:1; their average, 

 3.69:1). This comparison of itself suggests that during the pre- 

 Devonian time the river-borne magnesium and calcium were wholly pre- 

 cipitated after diffusing to the sea-bottom. In fact, the correspondence 

 must be regarded as giving powerful support to the hypothesis. 



The abrupt change in passing from the Silurian to the Devonian may, 



12 On account of the difficulty of finding enough analyses stated for the rocks of the 

 other continents, the comparison of the limestones has been largely confined to the 

 North American formations. An incomplete, preliminary study seems, however, to 

 show that there has been a parallel succession of chemical types among the limestones 

 of the other continents. 



13 Cf. C. R. Van Hise : Treatise on metamorphism, 1904, p. 801, and Chamberlin and 

 Salisbury : Geology, vol. 1, 1904, pp. 360, 404. 



