404 



EDWARD STEIDTMANN 



The composition of the limestones of the Paleozoic of the 

 Mississippi Valley averages that of magnesian limestones, with a 

 lime percentage higher than that of a normal dolomite. 



The unconformities in the Paleozoic of the Mississippi Valley 

 represent the removal or lack of deposition of both limestones and 

 elastics, mostly elastics, as follows from the compilation below, 

 also made by Miss Carter (Table XV). Both sedimentation 

 and erosion seem to have worked hand in hand toward the con- 

 centration of limestones. 



TABLE XV 



Table of Unconformities in the Paleozoic of the Mississippi Valley 



It seems to follow that the sediments of the Mississippi Valley 

 were either derived from sediments already high in limestones, 

 or else the complementary muds have been carried elsewhere, to 

 the margins of the continents perhaps. But even granted that 

 they were derived from sediments high in limestones, it is diffi- 

 cult to escape from the conclusion that ultimately redistribution 

 was selective. Concentration of limestones on the lands began 

 somewhere at some time. A selective withdrawal of muds from 

 the continents began . somehow, for the Paleozoic sediments of 

 the Mississippi Valley show a proportion of limestones far in excess 

 of the proportions gotten by redistributing either rhyolite or 

 basalt, the two dominant magmatic differentiates. 



A remarkable preponderance of limestone is also evident from 



