EVOLUTION OF LIMESTONE AND DOLOMITE 421 



The relative absorptive power of the crustal materials for 

 calcium and magnesium has not been adequately determined. 

 Certain it is that many shales, slates, muds, and soils have a higher 

 content of magnesium than of calcium. The probability that 

 the selective withdrawal of muds from the lands to the deep sea 

 has involved a selective loss of magnesium from the lands has been 

 pointed out on p. 414. 



The average calcium magnesium ratio of the solutions contributed 

 to the sea. — The Mississippi 1 at New Orleans, which may be regarded 

 as a mixture of the waters from the average Paleozoic terrane, 

 shows a ratio of calcium to magnesium of 3 .81 to 1. The average 

 calcium to magnesium ratio of 73 streams east of 100th meridian 

 of the United States observed daily at 94 stations for a period of 

 one year is about 4 to 1. In Sir John Murray's well-known com- 

 position of 19 streams of the world, the calcium to magnesium 

 ratio is 4.4 to 1. From Mellard Reade's 2 data, the ratio of cal- 

 cium to magnesium in the materials of chemical denudation is 

 8.25 to 1. A better based figure for the average composition of 

 the streams of the earth is that recently made by Clarke. 3 The 

 ratio of calcium to magnesium in Clarke's average as previously 

 cited is about 6 to 1. 



While the ratio of calcium to magnesium of the solutions con- 

 tributed to the sea is higher than it would be if the lands had the 

 composition of an average igneous rock, the largest streams do 

 not seem to show the high calcium to magnesium ratio that one 

 would expect from the amount of limestone on the continents. 

 However, the Mississippi is about the only large stream whose 

 composition is accurately determined. It may be that the large 

 amount of suspended material in rivers tends to lower the calcium 

 ratio, since the muds, particularly of humid climates, tend to be 

 high in magnesium. The arid nature of about one-fifth of the land 

 area is another factor which may cause a retention of calcium by 

 the land. If this hypothesis is correct, a compensating increase 

 in the calcium ratio of rivers will be contemporaneous with oceanic 

 expansion. 



1 R. B. Dole, W.S. Paper 236, U.S.G.S., 117. 



2 Mellard Reade, Chemical Denudation in Relation to Geologic Time (1879), 1-6 1. 



3 F. W. Clarke, Study of Chemical Denudation, Smithsonian Institution, Vol. 

 LVI (1910), No. 5, p. 8. 



