150 NEW YORK STATE MUSEUM 



gates, it nearly always has its margins bounded by crystal faces 

 (figure 8). There are some delicate aggregates of small crystals 

 clustered in such a way as to indicate that they have crystallized 

 within the rock after all mechanical disturbances. This rock 

 is a perfectly typical rusty gneiss of the Grenville, and at the 

 same time, when compared with the preceding rock, it is clearly the 

 first stage in the conversion of a similar rock into ore. 



Next, a lean ore shows in the section all the mica changed to 

 chloritic material. There is a little rutile, but no graphite. The 

 pyrite is in large areas, bounded by crystal outlines not of single 

 crystals, but of many crystals in a single mass as shown in figure 9. 

 Feldspar is practically absent, and the abundant quartz is largely, if 

 not wholly, a secondary infiltration. 



Another section of lean ore shows large single, but somewhat 

 incomplete crystals of pyrite with considerable feldspar altering to 

 sericite, and much green chlorite. The larger crystals of pyrite 

 partly or wholly inclose grains of the other minerals, clearly indicat- 

 ing an incomplete replacement of the latter by the pyrite (figure 10). 

 Another section differs from this only in the abundance of graphite, 

 resembling in this respect many specimens from other localities, 

 while the Hendricks material, on the whole, is rather poor in this 

 constituent. Another hand specimen shows much vein quartz and 

 rather perfect crystals of pyrite. In thin sections the latter is seen 

 to be crushed and the cracks infiltrated with calcite. 



As bearing upon the question of genesis of the pyrite deposits, the 

 most striking phenomena exhibited at this locality are the close 

 association of the ore body with crystalline limestone, the gradation 

 of ore into rusty gneiss and ordinary fine laminated gneiss, the 

 irregular distribution of graphite in the ore and often its entire 

 absence, the crystalline character of the pyrite apparently assumed 

 late in the history of the rock and but little controlled by other 

 minerals present, the alteration of mica to a chloritic material, which 

 seems to grow also at the expense of other minerals, and finally the 

 presence of considerable vein quartz. The small amount of calcite 

 appears to be of minor importance, but on the other hand it is pos- 

 sible that the injection of pegmatite in the gneiss of the adjacent 

 ridge is a vital factor and its presence must be kept in mind in fram- 

 ing hypotheses to account for the ore bodies. The crushing of pyrite 

 shown in one section simply serves to accentuate the extreme rarity 

 of this phemonenon throughout the region, for while there has 

 doubtless been considerable disturbance subsequent to the formation 



