THE EVOLUTION OF LIMESTONE AND DOLOMITE. 



II {Concluded) 



EDWARD STEIDTMANN 

 University of Wisconsin 



PART II. CALCIUM AND MAGNESIUM IN THE PRODUCTS OF 



METAMORPHISM 



The products of metamorphism of the rocks may be classed 

 as solids and solutes, or residuals and losses. Residuals, the mate- 

 rials which remain in situ after a rock has suffered chemical change ; 

 losses, the materials which are dissolved, transported in solution, 

 and redeposited elsewhere. A study of the fate of calcium and 

 magnesium in rocks subjected to metamorphism shows that there 

 is nearly always a marked tendency for a greater percentage loss 

 of calcium than of magnesium. Magnesium tends to remain 

 with the residuals to a greater degree than calcium. It will be 

 shown that in the movement and redisposition of the residuals 

 and losses of rock alteration, and in the reworking of these products 

 by the same processes, again and again throughout geologic time, 

 lies the history of a progressive enrichment of the lands in calcium ( 

 and their progressive depletion in magnesium. The evidence for 

 the selective splitting off of calcium from the parent rocks in 

 response to metamorphic processes and the accumulation of 

 magnesium in the residuals follows. 



Materials lost by the weathering of acid igneous rocks. — The 

 weathering 1 of acid igneous rocks results in the loss of lime, mag- 

 nesia, soda, potassa, and silica. The percentage loss of the various 

 constituents approximately follows the descending order in which 

 they are named. For purposes of comparison alumina may 

 be regarded as constant. The ratio of calcium to magnesium lost 

 in the weathering of an acid igneous rock can only be given in 

 terms of tendencies. In Table V the figures for the ratio of cal- 



1 Edward Steidtmann, "A Graphic Comparison of the Alteration of Rocks by 

 Weathering with Their Alteration by Hot Solutions," Economic Geology, III, 381-409. 



39 2 



