l60 NEW YORK STATE MUSEUM 



THE MINES AT PYRITES 



At Pyfites, formerly called High Falls, pyrite was mined on a 

 fairly large scale several years ago, but when visited by the writer 

 the mines were idle and evidently had been so for some time. The 

 locality is however of much interest and, standing alone, would be 

 apt to lead to views quite unlike those presented in the sequel as to 

 the origin of the ore bodies. Extensive exposures are afforded by 

 the deep gorge of the Grasse river, at the lower end of which the 

 pyrite is shown better than at any other locality visited. At this 

 point a fine section is shown, the rusty gneiss being very strongly 

 developed, dipping northwest at a high angle and striking north- 

 east to north. A marked lens of pyrite about eight feet thick occurs 

 in the gneiss, and the latter is itself permeated with pyrite. This 

 lens with somewhat varying thickness extends to the top of a 

 precipitous bank, but in the floor of the gorge pinches out abruptly 

 along the strike. Veinlets of pyrite cut all the rocks including the 

 ore lens itself, showing a certain amount of circulation late in the 

 history of the formation, probably subsequent to all pronounced 

 mechanical disturbance. 



The rusty gneiss shows its usual contortions and abrupt changes 

 of strike and dip, is strongly banded and foliated, and in every 

 way appears to be sedimentary or, if igneous, is of extreme an- 

 tiquity and has suffered most intense metamorphism with resultant 

 obliteration of all original characters. 



Immediately underlying the gneiss, with apparently conformable 

 contact, is a rather massive, coarse, micaceous, nearly black rock, 

 which is shown by exposures farther up stream to be a phase of 

 the coarse gabbro so strikingly developed in the gorge above. While 

 this rock is unquestionably later than the rusty gneiss, the contact 

 between the two gives no suggestion of being an eruptive one, a fact 

 difficult to explain (unless faulting be appealed to) but quite fre- 

 quent in contacts between Grenville and later intrusive rocks 

 throughout the region. The only thing suggesting the relations of 

 the two rocks is the abnormal character of the gabbro at the con- 

 tact as compared with exposures farther up stream, more in the 

 heart of the intrusions. There is a decided contrast between the two 

 rocks, but the character of that at the contact, though rather finer, 

 is. not such as to point clearly to contact modification. Thus, with- 

 out the evidence afforded by other exposures, neither the nature 



