1 62 NEW YORK STATE MUSEUM 



pyrite, Is exposed in the river bed with a width of about forty feet; 

 on the east (below) is the main body of gabbro, while at the west 

 (above) the same rock appears and continues to the west bank 

 of the river where gray gneiss of Grenville aspect comes in. The 

 accompanying diagram (figure 2y) makes these relations clear. 

 At the upper contact of pyrite and 

 gabbro, and in the former, is a con- 

 spicuous pegmatite containing tour- 

 malin, garnet in large and fine 



//// k/i/i/i//^ y\'imZii v^vyv\/v 1/ 



crystals, and much pyrite. At ^"^^- ^^ ^^s.. gneiss and ore ncavny 



.1 1 r ii • 1 1 shsided; gabbro cross hachured 



the upper end of the island a 



few rods distant, the gabbro is continuous where the pyrite forma- 

 tion should occur, the latter having been cut out by the intrusion 

 of the former. This explanation of the absence of the pyrite forma- 

 tion is justified by the fact that unbroken outcrops on both sides of 

 the island eliminate the possibility of an abrupt change of strike 

 and at the same time render faulting improbable. 



From the foregoing description it is evident that the general im- 

 pression given by Pyrites is very different from that afforded by the 

 other pyrite localities. Instead of extensive outcrops of Grenville, 

 with the conspicuous rusty gneisses, the great development of nearly 

 black gabbro is the impressive feature ; and in view of the general 

 association of sulfid concentrations with gabbro, such an explana- 

 tion is immediately suggested for the present case. Further sup- 

 port is given to this idea by the fact reported by Brinsmade^ that a 

 large body of pyrrhotite is associated with the pyrite in the mines. 



The occurrence of the sulfids at the margin of an extensive 

 intrusion of gabbro is certainly most suggestive of their concentra- 

 tion by magmatic differentiation while, assuming the hornblende 

 gneiss at Stella mines to be part of the gabbro, a closely related 

 method of origin might be inferred for the ore bodies of the latter 

 locality. While much may be said in favor of this view, the writer 

 is unable to accept it since, in spite of their intimate association with 

 the gabbro, the ores at Pyrites actually occur as elsewhere in typical 

 rusty gneisses, quite indistinguishable from those in which they 

 occur at localities where gabbro is absent. For example, the ex- 

 posures at the lower end of the Pyrites gorge shows rocks in every 

 way similar to those at the Cole, Farr, Styles and other similar 

 localities, while at the two shafts worked in recent years the ma- 

 terial on the dumps can not be distinguished from that at other 



1 Brinsmade, R. B. Pyrite Mining in St Lawrence County, New York. 

 Eng. and Min. Jour. Oct. 28, 1905, p. 770-71. 



