EVOLUTION OF LIMESTONE AND DOLOMITE 



405 



the following averages made from sections in the interior of China, 

 described by Blackwelder in Researches in China. 



The continent of Europe shows a similar dominance of lime- 

 stones over clastics. In the southern province of sedimentation, 

 the record is nearly continuous from the Cambrian to the Pliocene, 

 and presents a proportion of limestones far in excess of the ratio 

 gotten by distributing an average igneous rock into the sediments. 



Another peculiarity of the sediments on the continental interiors 

 is that they are generally less disturbed and less anamorphosed 

 than the sediments on the margins of the continents. The mar- 

 ginal distribution of mountain ranges and volcanoes harmonizes 

 with this generalization. The fact that the sediments of the 

 continental interiors are generally less anamorphosed than those 

 of the margins is significant in regard to their chemical denudation. 

 Anamorphism tends to cause the decomposition of carbonates 

 and the development of complex silicates, but the silicates are 

 less easily dissolved, hence the relatively small amount of ana- 

 morphism of these sediments increases their importance as sources 

 of calcium and magnesium in river waters. 



Taking the lime and magnesia contents of Clarke's average 

 sediments merely as objects of illustration, the following table 

 suggests how important the concentration of limestone on the 

 continents may be in changing the ratio of calcium and magne- 

 sium in the river waters from what it would be if the lands had 

 the composition either of an average igneous rock, rhyolite or 

 basalt. 



