ZINC ORES OF THE EDWARDS DISTRICT 29 



tion, calcite was subject to repeated solution and recrystallization. 

 The whole assemblage of minerals is regarded as formed during 

 continuously changing conditions, starting with those of contact 

 metamorphism and, finally, arriving at those normal for the depth 

 involved, uninfluenced by outside agencies, and with no time inter- 

 val of magnitude sufficient to admit of radical change of conditions 

 and consequent pronounced discontinuity in the series. Excepting 

 one locality, where blende has undergone rearrangement and 

 redeposition by superficial waters in recent geological time, the 

 deposits were formed, essentially as they now exist, in Precambrian 

 time. 



If the evidence as to the relative ages of sulphides and serpentine 

 were interpreted as indicating the reverse of that accepted above, 

 making the sulphides younger than the serpentine, the sulphides 

 would be relatively low-temperature minerals as compared with the 

 conditions to which their formation has been ascribed. 



As will be seen later, this view is in harmony with that held by 

 several investigators with reference to certain analogous deposits 

 and there is much to be said in its support. Nevertheless, it is felt 

 that, while the evidence available does not amount to a demon- 

 stration, it does point clearly to the earlier date and higher tem- 

 perature for the formation of the sulphides. 



Obviously, the alternative views are variants of a single hypothesis 

 rather than fundamentally distinct. 



COMPARISON OF ORES WITH ZINC ORES OF OTHER 



REGIONS 



Thus far the zinc ores have been treated exclusively in their 

 relation to geological conditions prevailing in a rather limited area, 

 but it is desirable to consider them from the standpoint of similar 

 ores in other regions. 



Broadly speaking, there are two main genetic types of zinc ores ; 

 those associated wi.th sedimentary rocks only, chiefly limestones, 

 like the deposits of the Mississippi valley; and those associated with 

 igneous rocks, like the many zinc-bearing bodies of the Cordilleran 

 region. Naturally, with the latter type, sedimentary rocks, also, 

 are commonly present. 



Deposits of the first type are generally interpreted as formed 

 through the concentration, by circulating meteoric waters, of zinc 

 originally disseminated in the sediments, though some investigators 

 maintain that, even in these deposits, the zinc is of direct magmatic 



