Decomposition of Iroji Pyrites. 171 



pyrjiinids, with the terminal face (OP) rounded and finely stri- 

 ated. Decomposition : a bright greenish gray or bronze tar- 

 nish, spotted with copper-red or iridescent films. Minute yel- 

 lowisli and reddish white pcirticlcs of efflorescent vitriol, chiefly 

 fen-ic sulphate. 



No. 9. Marcasite. Galena, Illinois. Thin drusy crusts, on 

 cubes of g;ilenite, adhering to sphalerite. Sharp tooth-like 

 rhombic pyramids, like those of Nos. 7 and 8, with many 

 twinned forms ; terminal faces (OP) sometimes nearly square, 

 sometimes linear and barely visible. Grayish white and brilliant 

 on fracture. Decomposition : rather dull brownish yellow tar- 

 nish over the surface. 



No. 10. Marcasite. Champion, Jefferson County, New York. 

 A coarsely fibrous crust with bunches of radial structure, upon 

 a granular nucleus made up of grains of radial structure. 

 Grayish white and brilliant on fracture. Over the surface of 

 the crust the radiating bladed fibres terminate in the apices of 

 tooth-like pyramids, whose terminal faces (OP) are unusually 

 broad and nearly square ; some twinned forms also occur. 

 Specimen, as received, mislabelled ''pyrite." Decomposition: 

 yellow iridescent tarnish on some fibres, and reddish brown 

 iron-ochre spread over tlie surface of the crust. 



No. 11. Marcasite. Folkestone, England. Spear-headed, 

 deeply striated twinned ci-ystals and groups, imbedded iu ash- 

 gray chalk. Grayish white and brilliant on fracture, and very 

 hard. Decomposition : a bright yellow or blue iridescent film, 

 or a dull bronze-colored tarnisli, passing into a soft crust of 

 orange-colored iron-ochre. 



No. 12. Marcasite. Joplin, Missouri. A thick crust of long 

 rhombic prisms, composite and platy and marked, by strong stri- 

 ation, producing a fibrous appearance. All the faces are curved. 

 Grayish white and brilliant on fresh fracture. Associated with 

 sphalerite and quartz. Decompositio7i : the fresh fracture soon 

 colored yellowish ; the surface of the crystals rendered beautiful 

 and brilliant by a bronze-yellow iridescent tarnish. 



No. 13. Marcasite. Central Park, New York City. A 

 compact shining crust, grayish white and brilliant on fresh frac- 

 ture and imperfectly fibrous, adhering to biotitic gneiss. The 

 botryoidal surface resembles those of Nos. 2 and 5, being covered 



