398 



EDWARD STEIDTMANN 



TABLE XII 



Limestone Contact Metamorphism 



Materials lost from rocks by hot solutions. — The alteration of 

 rocks by hot solutions along fissures also shows a marked tendency 

 toward rapid removal of lime, and a much slower rate for the 

 removal of magnesium. In some cases calcite, epidote, and other 

 lime-bearing minerals develop in basic igneous rocks, but often 

 calcium is practically absent from the secondary minerals. The 

 compilation on page 399 has been made showing the calcium and 

 magnesium ratio of fresh and altered rocks adjacent to ore-bearing 

 fissures. 



Resume of calcium and magnesium in the products of metamor- 

 phism. — The data on metamorphism which have been presented 

 indicate that the percentage loss of calcium which rocks sustain 

 through metamorphic processes tends to be higher than that of 

 magnesium. In fact, it could be shown that the percentage of 

 calcium lost tends to be higher than that of any other element. 

 Exceptions are noted, of course. In view of this tendency, Salis- 

 bury's 1 estimate that the disintegration of 55,000,000 cubic miles 

 of average igneous rock would yield the common salt of the sea, 

 while the disruption of three or more times as much rock would 

 be required to yield the limestones, is suggestive. 



1 R. D. Salisbury, "The Mineral Matter of the Sea," Jour. Geol., XIII, 476-77- 



