Decomposition of Iron Pyrites. 173 



No. 18. Miircasite. Galena, Illinois. A bright crust, cov- 

 ered by the rectangular, nearly square faces (basal pinacoid) 

 of rhombic pyramids, grayish white and brilliant on fresh frac- 

 ture. Decomposition: dull yellow tarnish. 



No. 19. Marcasite. Galena, Illinois. A hemispherical nod- 

 ule, made up of concentric crusts, very finely fibrous, grayish 

 white and brilliant on fracture. Decomposition : a dull bronze- 

 colored tarnish and abundant efflorescence of whits vitriol, chief- 

 ly ferric sulphate, with a little ferrous sulphate. 



No. 20. Marcasite. Galena, Illinois. A crust of radial fine- 

 ly bladed structure, grayish white and brilliant on fracture ; 

 upon grayish crystalline lim3stone. The outer surface is stud- 

 ded by projecting tooth-like aggregates of the usual modified 

 rhombic pyramids and striated twins. The rectangular terminal 

 faces (OP) are very narrow and even, linear, like chisel-edges. 

 Decomposition : beautiful iridescence, blue and orange, with 

 high lustre. 



No. 21. Marcasite. Littmitz, Bohemia. A crust like the 

 preceding, on quartz ; with fine, fibrous radial structure, and 

 grayish white and brilliant on fracture. Surface covered by 

 tooth-like quadrilateral domes of twinned rhombic pyramids, 

 with terminal faces (OP) linear. Decomposition : a rather dull 

 yellowish tarnish, partly iridescent, on the faces of the crystals. 



No. 22. Marcasite. Joplin, Missouri. Thin crusts, grayish 

 white and brilliant on fracture, lining cavities in a brecciated 

 limestone, in close association with modified cubes of pyrite, and 

 crystals of sphalerite and greenockite. Surfaces drusy with mi- 

 nute rhombic prisms of marcasite, somewhat modified, truncat- 

 ed pyramids, and striated twins. Decomposition : a rather dull 

 brownish yellow tinge — in part, a blue and yellow iridescent 

 tarnish, on the surface and on old fractures. 



Concretionary nodule. Galena, Illinois. A large nodule 

 of concretionary structure, consisting successively, (passing from 

 the centre outward), of two distinct layers of marcasite, a and b, 

 and a crust of pyrite, c, on the outside. 



No. 23. {a, Fibrous nucleus). Marcasite. A bright core of 

 exceedingly fine fibrous structure, marked across the fibration 

 by delicate concentric lines, often six to a millimeter, which are 

 the edges of shining drusy mammillary surfaces ; these surfaces 



