l8o NEW YORK STATE MUSEUM 



deposits there are several essential features which closely resemble 

 phenomena shown in the wall rocks of fissure veins, where they have 

 suffered alteration as a result of the attack of the vein-forming 

 solutions, derived generally from magmatic sources. Without put- 

 ting too much stress upon the similarity of phenomena, it is regarded 

 as giving additional support to the hypothesis here advanced. 



But while presenting this interpretation of the observed facts as 

 worthy of careful consideration, the writer wishes again to 

 accentuate the great complexity of the region as a whole and the 

 consequent obscurity of the particular problem in hand. To ascer- 

 tain all the factors entering into such a complex problem and to 

 give them their proper relative weight is a difficult task, and the 

 writer is by no means sure that he has succeeded in the attempt. 

 The evidences of relatively late circulation and crystallization of 

 pyrite and the associated chlorite and sericite have perhaps been 

 given too much prominence and should be regarded as minor 

 phenomena. The very agencies to which such potency has been 

 ascribed might as a result of this potency completely rearrange 

 pyrite already present in the rocks and thus destroy the evidence of 

 its early formation. This mobility of pyrite is a very serious diffi- 

 culty in the whole problem. 



In summing the matter up on the basis of present data, it appears 

 to the writer that four periods of pyrite formation are probable: 

 (i) A primary precipitation of pyrite contemporaneous with the 

 formation of the sediments; (2) a concentration of this pyrite by 

 circulating ground waters, with the addition, perhaps, of pyrite Of 

 deep-seated origin, before the period of metamorphism ; (3) but of 

 minor importance, a recrystallization of all pyrite, accompanied 

 perhaps by a certain amount of formation of new pyrite dur- 

 ing metamorphism ; (4) a further development and concentration 

 of pyrite by magmatic agencies, perhaps working in com- 

 bination with ground waters, as outlined above, following the 

 period of active igneous intrusion and metamorphism. The first of 

 these periods is suggested by the pyrite so generally disseminated 

 through Grenville rocks, although even this could of course, in view 

 of the widely disseminated intrusions, result from the operations 

 of the fourth period. The reasons for accepting the latter have been 

 given at length, while the second period is purely hypothetical, 

 being based merely on the marked tendency for disseminated pyrite 

 to be concentrated by .ground waters and the frequency with which 

 the mineral is brought up from deep-seated sources. Nevertheless, 

 though hypothetical, it is thought that this process is likely to have 



