158 NEW YORK STATE MUSEUM 



Another specimen from the foot wall is a pyritiferous pegmatite 

 quite similar to that at the Farr mine. It is a coarse aggregate of 

 quartz, feldspar, green from alteration, and pyrite. In thin section it 

 bears a strong resemblance to the preceding rock, but is coarser. The 

 feldspar appears in all stages of alteration to the chloritic aggre- 

 gate, and the pyrite occurs in this rather than in the quartz. At 

 several points there is a pronounced skeletal growth' (see figure 

 23) of pyrite in the chlorite, and here the latter mineral is some- 

 what fibrous, the fibers being perpendicular to the pyrite margins and 

 meeting along a straight line in the middle of the narrow veinlike 

 spaces. The conclusion seems unavoidable that this part of the 

 chlorite has been deposited after the pyrite, or has undergone some 

 rearrangement with reference to it. The latter is more probable. 

 At another point, chlorite fills a narrow tortuous space between pyrite 

 and quartz and between two adjacent grains of pyrite. Again, the 

 fibrous chlorite has its fibers in clusters radiating from points along 

 the margins of sharply banded pyrite crystals. Elsewhere the 

 chlorite is clearly molded upon quartz. All these relations point to 

 the mobility of the first-named mineral. 



In general the microscopic examination of these sections 

 strengthens the impression gained from other thin sections and in 

 the field, that there is a close connection between the formation of 

 pyrite and of chlorite. Many exceptions occur where pyrite is asso- 

 ciated with fresh quartz and silicates but the ores proper all show 

 the chloritic alteration. Practically the same thing may be said 

 of pyrite and graphite, the latter being usually abundant but some- 

 times absent. 



Sections of the gneisses adjacent to the mines shed little light 

 upon their origin. They are aggregates of hornblende, plagioclase, 

 a little pyroxene and some quartz and while, as previously stated, 

 they are thought to be igneous rocks related to the gabbros, their 

 true nature is open to question and they may be metamorphosed 

 sediments. This latter view is favored by Newland.^ 



The material of one of the small pyritiferous bands in the dark 

 hornblende gneiss, south of the Anna shaft, is of interest when com- 

 pared with the large ore bodies. The hand specimen shows a rather 

 coarse aggregate of hornblende, an amorphous black material and 

 pyrite, the latter often in good crystals. The thin section shows 

 some hornblende, greatly decomposed feldspar, apatite and an 



^ Newland, D. H. The Mining and Quarry Industry of New York. N.Y. 

 State Mus. Bui. 120, 1908, p. 51. 



