EVOLUTION OF LIMESTONE AND DOLOMITE 



403 



to the amount of calcium and magnesium in the sediments and to 

 the relative solubility of calcium- and magnesium-bearing minerals 

 in the sediments. 



In his discussion of "The Metamorphic Cycle," C. K. Leith 1 

 says: 



Averages of sections made from field observations give uniformly a lower 2 

 percentage of shales and higher of limestones. An average of twenty-one 

 sections from different parts of the United States shows thirty per cent of 

 limestone. If the difference of proportion determined by the chemical 3 and 

 field methods is a real one, as inspection of the data seems to indicate, the 

 significant questions are raised, (1) whether there may not be a concen- 

 tration of limestones on the continental areas, their complimentary shales 

 and muds being in the deep sea, (2) whether limestone may not be concen- 

 trated in the upper, observed part of the lithosphere, because of its known 

 inability to remain in the deep seated zones of high pressure and temperature. 



That the Paleozoic sediments of the Mississippi Valley show 

 a surprising concentration of limestones amounting to from 23.6 

 to 66.6 per cent of the sections averaged and a marked deficiency 

 of shales and sandstones is brought out in an admirable study 

 made by Miss F. W. Carter. 4 The results of this study are com- 

 piled in Table XIV. 



TABLE XIV 



Table Showing the Relative Proportions oe Limestones, Shales, and Sand- 

 stone in the Paleozoic of the Mississippi 



State 



Pennsylvania. . . . 



Alabama 



Ohio 



Michigan 



Indiana 



Wisconsin 



Minnesota 



Iowa 



Missouri 



Oklahoma 



Colorado — eastern 

 Colorado — central . 



1 C. K. Leith, "The Metamorphic Cycle," Jour. Geol., XV (1907), 304. 



2 Lower than the percentage gotten by distributing an average igneous rock into 

 average sediments. 



3 The chemical method of W. J. Mead {op. tit.). 



4 Unpublished thesis (19 10), University of Wisconsin. 



