EVOLUTION OF LIMESTONE AND DOLOMITE 395 



Materials lost by the weathering of limestones. — In the present 

 stage of earth evolution, the principal contribution of calcium 

 and magnesium from the weathering of the sediments probably 

 comes from limestones. A compilation of the calcium magnesium 

 ratio of several fresh and weathered limestones seems to indicate 

 that the percentage loss of calcium is somewhat higher than that 

 of magnesium and that the amount of calcium given off by the 

 weathering of limestone greatly exceeds magnesium (Table VII). 



Resume of weathering. — It follows from the facts stated that 

 the weathering of igneous rocks and sediments results in the loss 

 of more calcium than magnesium and that in general the percent- 

 age loss of calcium is greater than that of magnesium. 



Materials lost by dynamic- metamorphism. — The dynamic meta- 

 morphism of sediments as well as igneous rocks seems to bring 

 about certain definite chemical changes. It is difficult, however, 

 to measure these changes since it is uncertain whether any of the 

 elements are stable in any given case. All that can be done in 

 the problem of determining the relative stability of lime and mag- 

 nesium under dynamic condition is to compare the magnesium 

 and calcium ratios in the unaltered materials with the calcium 

 magnesium ratios in their metamorphosed equivalents, as shown 

 by Table IX. Secondly, to compare the importance of calcium 

 and magnesium in the minerals of the unaltered and metamor- 

 phosed rocks. Table IX appears to indicate that the ratios of 

 calcium to magnesium are lower in the metamorphosed phases 

 than in the unaltered; that is, the percentage of lime lost by 

 dynamic metamorphism appears to be higher than that of mag- 

 nesium. Concordant with this apparent chemical change is the 

 fact that the minerals which are developed under conditions of 

 dynamic metamorphism are predominantly magnesium and 

 potassium bearing, rather than calcium bearing, such as the micas, 

 chlorites, and amphiboles of Table VIII. It appears that mag- 

 nesium is better adapted to dynamic condition than calcium. 

 A selective removal of calcium from the zone of anamorphism to 

 the zone of katamorphism and ultimately to the ocean seems to 

 be a logical sequence. 



