ZINC ORES OF THE EDWARDS DISTRICT 21 



limestone into a zine ore (plate 7, figure 2). In this process, the 

 calcite is more readily attacked than the silicates and, often, sul- 

 phides are molded upon silicate grains, having replaced the calcite 

 by which the latter were originally surrounded (plate 8, figure i). 

 But the silicates themselves, in spite of their relative stability, do 

 not escape and may repeatedly be seen to have undergone replace- 

 ment (plate 8, figure 2, and plate 9, figure i). 



The pyrite, on account of its earlier development and stronger 

 tendency toward assuming crystal form, takes on a rather massive 

 and compact character, seldom appearing in veinlets or skeletal 

 crystals, although these latter do sometimes occur. The blende is, 

 also, often in fairly regular, equidimensional grains, but again occurs 

 in irregular patches and stringers, evidently introduced after the 

 crystallization of the rock as a whole, and eating into the other 

 minerals. 



During this process of sulphide deposition, the role of the calcite 

 is quite different from that of the primary silicates and requires 

 further consideration. The silicates mark a definite phase of meta- 

 morphism, their formation having been confined to a limited period 

 when temperature was at or near the maximum. The recrystall- 

 ization of calcite, on the other hand, doubtless began long before 

 the formation of the silicates and continued throughout the whole 

 subsequent period of mineral formation, as calcite fills veins in all 

 the other minerals. 



Thus, the age relation of calcite to the sulphides is not as simple 

 as is that of the anhydrous silicates, some of it being older than, 

 some contemporaneous with, and some younger than, the sulphides. 

 And, thus, while the sulphides show a clear molding upon or, less 

 often, replacement of, the silicates, they are often intercrystallized 

 with calcite, though again evidently replacing it. In the former 

 case, blende occurs in compact, equidimensional grains ; in the 

 latter, it has a more irregular and branching habit, whose replacing 

 relation is clear. This suggests the formation of the mineral during 

 a considerable period of time, involving a corresponding change of 

 physical conditions. 



One effect of this change of conditions, suggested by the data in 

 hand, is a progressive change of composition of the blende, as 

 indicated by color. The older blende, intercrystallized with cal- 

 cite, is very dark colored, while, as the mineral loses its compact 

 habit and takes on more and more a replacing character, it becomes 

 lighter colored. Exceptions occur, it is true, but, in general, the rule 



