22 NEW YORK STATE MUSEUM 



seems to hold good. The extreme result of this tendency appears in 

 a distinctly younger, colorless blende, whch re-cements and borders 

 cracked grains of the older, dark-colored variety. 



So far as these observations go, they indicate that, during the 

 period of blende formation, there was a decrease in the supply of 

 iron (which had its maximum in the preceding period of pyrite 

 formation) or else that the changing physical conditions reduced the 

 capacity of iron for entering into the composition of blende. The 

 obvious change of conditions would be a decrease in temperature 

 and it is of interest to recall in this connection Berg's^ statement 

 that the blende formed by contact metamorphism is always the dark- 

 colored, ferruginous variety. 



The natural conclusion in the present instance is that the dark- 

 colored ferruginous blende, intercrystallized with calcite, as well as 

 nearly all the pyrite, represents conditions approximating those 

 involved in the formation of the typical contact silicates, which 

 gradually gave way to lower temperature conditions, with resultant 

 formation of lighter colored, less ferruginous blende, taking on 

 more and more, in its relation to calcite, the character of a replacing 

 mineral, its deposition being subsequent to the major part of the 

 calcite recrystallization. 



Doubtless, the actual space now occupied by the dark blende, as 

 well as the light, was originally occupied by calcite, but, under the 

 high temperature conditions prevailing when the dark blende was 

 deposited, the calcite was undergoing such general recrystallization 

 that the two minerals assumed the relation of contemporaneous 

 crystallization. 



RELATION OF SULPHIDES TO SECONDARY SILICATES 

 Of interest in itself and bearing directly upon the question just 

 considered as to the age relations between the sulphides and the 

 primary silicates and calcite, is the problem of the relative ages of 

 the sulphides and of the secondary silicates, serpentine and talc. 



With reference to this problem, one thing is clear : In many 

 sections, grains of blende and pyrite occur which have been broken 

 and the cracks subsequently filled by serpentine (plate 9, figure 2). 

 In these cases, it is obvious that the serpentine is younger than the 

 sulphides. But this evidence is not conclusive as to the general age 

 relation between the sulphides and serpentine, since this vein ser- 



^ Berg, Georg, Die Mikroskopische Untersuchung der Erzlagerstatten, 

 1915, p. 123. 



