l886.] NEW-YORK MICROSCOPICAL SOCIETY. 89 



alternating brown and white lines, evidently the edges of alter- 

 nating films of red iron oxide, and white gypsum, of which these 

 crusts are made up. 



The chemist will understand that, as fast as the iron sulphate 

 has been formed by slow oxidation of this pyrite, the iron oxide 

 has been at once precipitated by calcium carbonate, derived 

 from the surrounding calcite-gangue ; the gypsum resulting has 

 been mostly washed away, except in the case of occasional films 

 thus protected by envelopment. 



We are here met by questions of great practical importance. 

 Is it safe to roof a house with slates full of cubes of golden-yel- 

 low pyrites ? Practical roofers and scientific observers agree 

 that there is no danger of decay and discoloration with the py- 

 rites of some slate quarries. What makes this difference, and 

 how can we determine whether to accept or reject enormous 

 bodies of pyritiferous slates which are found ready for quarry- 

 ing ? Again, when coal is brought out from the mines of Penn- 

 sylvania, it is piled up ready for shipment ; but these piles must 

 often remain for months exposed to the weather, until the mar- 

 ket requires their delivery. Why is it that the product of some 

 collieries soon crumbles to a powdered condition, with great in- 

 jury to its market value, while that from other localities resists 

 decomposition and crumbling ? This variation has been sus- 

 pected to have connection with some conjectured peculiarity in 

 the little bright grains of pyrites, abundantly scattered through 

 most varieties of coal. But what is its real cause, and how can 

 such disastrous results be anticipated, and therefore prevented? 



Again, most residents of New York City have noticed the 

 offensive and unequal yellow discoloration which has attacked 

 the once pure white marble of our Court House building, a stone 

 which was brought from a quarry in West Stockbridge, Massa- 

 chusetts. This stone is full of decaying particles of pyrites, and 

 yet the marbles from other beds in the same State, and in Ver- 

 mont, contain forms of pyrites which appear to be perfectly sta- 

 ble in character. How can a builder distinguish between the 

 two kinds of the same mineral, pyrite, whether stable or unstable, 

 and decide whether to accept or reject a stone ? In illustration 

 of this last subject, I present a slide of the white marble of Lee, 

 Massachusetts (from a block exposed to the weather for a few 

 months), with a band of discoloration around a decaying particle 



