20 NEW YORK STATE MUSEUM 



small quantity as to be negligible, appear to be intercrystallized with 

 the anhydrous silicates, diopside, tremolite etc., as if formed at the 

 same time. But under the microscope, it is seen that the relations 

 of the minerals can not be so simply expressed and it is evident 

 that the sulphides belong to a later period of mineral formation. 

 Furthermore, it is evident that there is a distinct order of succession 

 in the formation of the sulphides, pyrite being the oldest, blende next 

 and galena youngest, although there is no reason for believing that 

 there is any great age difference. On the contrary, the sulphides, 

 aside from minor amounts to be referred to later, may be safely 

 ascribed to one period of mineralization. 



ORDER OF SULPHIDE DEPOSITION 



The relative ages of the sulphides appear in the fact that, when 

 in contact with one another, blende is always molded upon pyrite. 

 This might be explained by the very evident stronger tendency of 

 pyrite to develop crystal form, were it not for the added facts that 

 blende often entirely surrounds pyrite and sometimes fills cracks 

 in it. In a very few cases the relation of blende and pyrite suggests 

 the possibility that the former has replaced the latter, as described 

 by Teas,^ but in no instance seen by the writer was this explanation 

 capable of proof. 



Galena occurs as scattered aggregates, often of skeletal form, 

 growing upon the other sulphides and evidently younger than either. 

 It is, indeed, probably more widely separated from them in age than 

 they are from each other, and may be, in part, a secondary 

 concentration. 



As already indicated, pyrite often shows a marked tendency 

 toward crystal form. On the other hand, many grains are quite 

 irregular. Blende invariably occurs in grains and masses, with no 

 suggestion of crystal form. 



As to both minerals, however, it is evident that, instead of 

 crystallizing simultaneously with the anhydrous silicates and con- 

 temporaneous calcite, they have been formed subsequently, space 

 for them being afforded not by open cavities but by replacement. 

 In other words, calcite and silicates have been dissolved and pyrite 

 and blende deposited in their place pari passu. The sulphides eat 

 their way into- the older minerals, from margins and cracks, spread- 

 ing in all directions, and gradually change a diopside-tremolite 



^ Teas, L. P., The Relation of Sphalerite to other Sulphides in Ores : 

 Trans. Am. Inst. Min. Eng., Feb. 1918, p. 1921. 



