ZINC ORES OF THE EDWARDS DISTRICT 23 



pentine may be younger than the main part of the mineral. As a 

 matter of fact, there is evidence that, to some extent at least, the 

 serpentine is analogous to both the blende and the calcite in that it 

 formed either through a long period or at two distinct times, 

 probably the former. 



At first sight, it would seem that the structural relations between 

 the various minerals, as shown by the microscope, would determine 

 their relative ages, but the value of this criterion is destroyed by the 

 alterations and replacements that have occurred. 



As previously stated, there are two types of serpentine, 

 pseudomorphous and replacement, and it is the latter that veins the 

 sulphides. On the other hand, blende may occasionally be found 

 veining the pseudomorphous serpentine. This might be taken as 

 evidence that the sulphides are younger than the pseudomorphous 

 serpentine while older than the replacement serpentine. But this 

 relation can not be proved until it is shown that the blende was 

 actually deposited in cracks in serpentine and not in cracks in 

 diopside, which was subsequently altered to serpentine. Moreover, 

 most, if not all, of this blende, veining serpentine, is of the younger 

 type. 



One of the most striking phenomena shown by the microscope is 

 the molding of the sulphides upon serpentine. This may be seen 

 in nearly every section where the two minerals are in contact (plate 

 6, figure 2; plate 7, figure i; plate 10, figure i). It is obvious 

 that the sulphides were deposited around an older mineral which 

 controlled their form. At present, the controlling mineral is 

 commonly serpentine, but it is not evident whether the sulphides 

 were deposited around serpentine itself or around diopside which 

 has, subsequently, altered to serpentine. While, in most of these 

 cases, as just stated, the form-controlling mineral is entirely serpen- 

 tine, there often still remains a residual core of unaltered diopside, 

 pointing clearly to the origin of the serpentine (plate 10, figure 2). 

 These cases make particularly clear the difficulty of determining 

 whether the serpentinization preceded or followed the deposition of 

 the sulphides, as the actual structural relations of the minerals would 

 be the same in either event. 



The frequent presence of veins of serpentine passing, from the 

 central mass of serpentine, out through the surrounding sulphides 

 seems, at first glance, to point definitely to serpentinization after 

 deposition of sulphides. But on closer examination it becomes 

 evident that the serpentine of these veins is often not continuous 



