26 NEW YORK STATE MUSEUM 



In general also thin bands of limestone and of quartzite are inter- 

 bedded with them. It therefore becomes necessary to map the 

 whole complex as Grenville schist in the majority of cases. It is 

 only in exceptional instances that any one of these rocks occurs in 

 sufficient bulk and sufficiently free from admixture with the other 

 varieties to warrant separate mapping. Within the Gouverneur 

 quadrangle are two belts of hard, mica gneiss, usually garneti- 

 ferous, which have received separate mapping. One of these belts 

 enters the quadrangle near its southwest corner, and has been 

 traced for 12 miles when it is apparently cut out by granite. 

 Another and still broader belt occurs in the southeast part of the 

 quadrangle, and only a small part of its mass lies within the 

 quadrangle's limits. 



There is considerable of the rusty, pyritous gneiss, which is such 

 a characteristic Grenville rock, within the quadrangle but in such 

 thin beds that separate mapping is not practicable. One such belt 

 wedges up within the limestone just west of Gouverneur and thence 

 runs continuously clear across the quadrangle and beyond. A num- 

 ber of openings for pyrite have been made along it and the Stella 

 mine at Hermon, just without the quadrangle's limits, is still 

 active. But the rusty gneiss is simply a thin, uppermost bed to a 

 belt of hard, fine-grained gneiss and coarser mica gneiss inter- 

 bedded in the limestone, too thin to separate from them in mapping. 

 Another thin band of the rusty gneiss overlies the limestone belt 

 which comes into the quadrangle on the west side of Sylvia lake, 

 lying between the limestone and the gabbro which borders it on the 

 west and north. 



The Grenville rocks across the quadrangle have a general north- 

 east strike and northwest dip. The mapping clearly brings out the 

 fact, however, that the structure is not that of a simple monocline 

 but consists of a series of closely compressed and overturned folds ; 

 in other words, that it is isoclinal. Southeast dips occur here and 

 there, but only locally. The dips are in places very flat, elsewhere 

 so steep that they are nearly or quite vertical. At one stage in 

 the work it was hoped that one limb of a fold would prove to have 

 a steeper dip than the other limb, and that this might give aid in 

 working out the structure; but no such relation could be demon- 

 strated. Many of the folds, probably all of them, have a decided 

 pitch, but in the vast majority of cases it can not be determined 

 whether the pitch is to the northeast or the southwest; in other 

 . words, whether the structure is anticlinal or synclinal. 



