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100.00 
As to one of the subordinate constituents of the rock, pyrite, I 
have considered its durability in another paper.’ The variety of 
the mineral here present usually possesses a pure bright-yellow 
color, high specific gravity (4.99), conchoidal fracture, and a 
dense texture under microscopic examination—all satisfactory 
indications of strong resistance to atmospheric oxidation. These 
evidences of stability were further confirmed by its condition on 
the weathered outcrops of the rock, on whose roughened ledges 
its grains often project side by side with those of quartz, or are even 
found loosened but undecomposed where the rock has decayed 
and fallen into sand. So, too, its grains still sparkle brilliantly 
on the surfaces of ancient masonry, exposed over a half century 
to the ozonized atmosphere of the Berkshire mountain region. 
Similar testimony was borne by the slow and gradual decompo- 
sition of the mineral, when its crystals were subjected to the 
severe laboratory test of exposure to the action of the corrosive 
vapors from fuming nitric acid or from bromine. ‘The mineral 
seems to have little to do with the disintegration of the rock, 
and, indeed, probably contributes to its rigidity, its great strength 
under compression, and the blue shade of its gray color. 
The stability of the pyrite in this dolomyte layer contrasts 
strongly with the unfortunate frailty of the common variety 
which occurs in the marbles of the underlying ‘‘ Stockbridge lime- 
stone” stratum. ‘Thus, in the marble at Lee, Mass., decompo- 
sition sets in after even a few months’ exposure, and produces a 
conspicuous and offensive brownish-red blotch, often a quarter 
or half-inch in diameter, around a particle of pyrite barely 
visible to the eye, or even long reddish streaks across the face of 
the marble where a few of these particles occur in a row. 
The slight brownish tint assumed by the surface of the dolo- 
myte after very long exposure to the weather seems to be due to 
the alteration of the colorless scales of magnesian mica into a 
brown color; while the pyrite generally retains its high lustre 
and bright-yellow color, rarely assuming a copper-red film of 
oxidation in closed seams occupied by moisture. 
This fact seems to corroborate the stratigraphical distinction 
between the matrices of the mineral in the two localities; and it 
1*¢On the Variation of Decomposition in the Iron Pyrites, and its 
Relation to Density.” Ann. N. Y. Acad. Sci., III., [V., 1886-88. 
