ZINC ORES OF THE EDWARDS DISTRICT 1 3 



GENERAL DISCUSSION OF METAMORPHISM 



Metamorphism is a deep-seated process involving heat, pressure 

 and the presence of enough water to act as a medium of solution and 

 recrystallization though, as a rule, there is probably little free cir- 

 culation of water and, in consequence, little active transfer of 

 material from point to point. At least, this is the case when 

 metamorphism is of the type that results from the burial of rocks 

 to considerable depth, with consequent increase of temperature and 

 pressure, the latter often differential, with resultant rock fiowage. 

 Under these conditions, metamorphism is primarily a recrystalliza- 

 tion of material already present in the rock, with little change of bulk 

 chemical composition, though there may be, and usually is, a radical 

 change of mineral composition. The latter is particularly true in the 

 case of sediments which, having formed at the surface, are often 

 largely composed of minerals which are unstable at great depth 

 and, consequently, break down and yield their material to build up 

 new minerals stable under the new conditions. Even such stable 

 minerals as quartz and calcite, though surviving to form part of the 

 metamorphic rocks, undergo complete recrystallization, while, in 

 other cases, they may lose their identity, their constituents being 

 merged in other minerals. 



Another type of metamorphism results from the intrusion of 

 igneous magmas, with their accompanying hot gases and vapors and 

 great variety of mobile and potent chemical agents. Under these 

 conditions, great changes of chemical, as well as mineral, com- 

 position may be effected in the country rock, involving the removal 

 of original material and the substitution for it of new material on a 

 large scale. 



Metamorphism of the first type is apt to be much more widesprerd 

 than that of the second type, while either one may be superimposed 

 upon the other, resulting in extreme complexity. 



Of whatever type it may be, metamorphism is, as already implied, 

 merely a readjustment to new conditions. Every rock is an 

 aggregate of minerals and each mineral is stable only within certain 

 limits, both pliysical and chemical. When these limits are passed, 

 the mineral ceases to be stable and readjustment follows, by 

 recrystallization, paramorphism or complete chemical rearrangement, 

 with the formation of a rock of different mineral composition. The 

 limits of stability vary widely in different minerals, as does the 

 rate of readjustment. Often, readjustment lags greatly behind the 

 change of conditions so that a rock may retain, for many geological 



