I/O NEW YORK STATE MUSEUM 



masses of such form and magnitude as to be difficult to reconcile 

 with a sedimentary origin, as is true in the present instance. Thus it 

 is entirely possible that pyrite deposits might be formed by both of 

 these processes, and be quite indistinguishable after metamorphism 

 sufficiently intense to involve complete recrystallization. 



That any considerable concentration of pyrite was effected by the 

 metamorphism is improbable. The process doubtless resulted in a 

 crystaHization of the minerals already present in the rocks with no 

 decided change of ultimate chemical composition. Original pyrite 

 would be recrystallized, and where the proper materials and condi- 

 tions existed, new pyrite would be formed, just as other minerals 

 were formed throughout the whole mass of rocks from materials 

 already present. All of this pyrite, both recrystallized and newly 

 formed, would bear to the other minerals of the metamorphic rocks 

 relations similar to those which these latter bear to each other. In 

 other words, the pyrite would appear as a perfectly normal and 

 essential constituent of the rock, taking part in its regular crystalline 

 texture. Some of the thin sections examined show this relation, 

 and doubtless if many sections of gneiss with scattered grains of 

 pyrite were examined the relation would be found to be quite 

 general. But from the descriptions of sections of the ores already 

 given, it is quite apparent that in these most of the pyrite bears a 

 very different relation to the metamorphic minerals, indicating its 

 deposition as being subsequent to, rather than simultaneous with, 

 their formation. In so far as this is the case it is evident that the ores 

 are neither original sediments nor premetamorphic replacements; 

 and there is much evidence justifying this conclusion. In particular, 

 the widespread chloritic alteration of the gneisses associated with 

 the ore deposits, the very close connection between this alteration 

 product and the pyrite, the late deposition of the latter mineral, so 

 evident in many cases, replacing the minerals produced by the general 

 metamorphism and often crystallizing in the chlorite secondary after 

 these, the frequent abundance of vein quartz as a gangue, and the 

 presence, in many cases, of pegmatite containing much pyrite replac- 

 ing the primary minerals, are all phenomena, any one of which 

 might perhaps be reconciled with the hypothesis of original sedi- 

 mentation or of premetamorphic replacement, but in mass weigh 

 heavily against them. Whatever the original source of the pyrite, 

 these phenomena point to its relatively late concentration, subsequent 

 to the period of extreme metamorphism. 



If this conclusion is justified it follows that the ore bodies were 



