Decomposition of Iron Pyrites. 201 



edges by the octahedron and pyritohedron ; attached to crystal- 

 lized quartz. Fresh fracture, tin-white, splendent, andnneven ; 

 its true color requires some care to distinguish, on account of 

 the abundant minute planes of yellow tarnish. Streak, brown- 

 ish black, perhaps slightly greenish. Under the microscope, 

 the faces of the cubes display a beautiful iridescence, some stri- 

 ation, and a few white films of adhering quartz which have part- 

 ly contributed to the low density obtained. Decomposition : a 

 slight iridescent, pale brass-yellow tarnish, with delicate crimson 

 spots. 



No. 110. Pyrite. Ticonderoga, New York. Glittering, 

 light brass-yellow, tiny sharp cubes, commonly with sharp octa- 

 hedral faces upon their solid angles, forming a drusy surface 

 upon thin flakes of yellow pyrite and quartz. Fracture, yellow- 

 ish white, in part grayish white, splendent, and conchoidal. 

 The selected grains, used for the determination of density, were 

 afterwards found under the microscope to show adhering films 

 of white quartz, which must have had an influence on the low 

 figure obtained. Decomposition : a bright orange-yellow tinge 

 on the surface of many crystals. 



No. 111. Marcasitic pyrite. Isle of Sheppey, near London, 

 England. Fragments of a disintegrated fossil plant, Niyadus 

 ellipticus, converted mainly into pyrite, with coarse lamination 

 and transverse fibrous texture, evidently produced by woody 

 structure ; from the London Clay. Very fine-grained, light 

 brass-yellow, compact flakes and thin laminae, yellowish white 

 and glittering on fracture, and marked by fine brownish black 

 lines, the edges of alternating films of carbonaceous matter. 

 Here and there tiny flattened geodes occur, lined by sharp 

 microscopic octahedra of pyrite, in a few cases showing slight 

 modifications of their solid angles. For the specific gravity de- 

 termination, the material was subjected to careful washing, 

 friction, and elutriation, to remove the soft and light carbon as 

 far as possible, and on this the figures in the table were ob- 

 tained ; however, some black films still remained enclosed. De- 

 comi^osition : a bronze-yellow tarnish upon all surfaces, and a 

 very abundant white efilorescence of short curved needles of vit- 

 riol, mainly ferrous sulphate, but with much ferric sulphate in 

 many crusts. 



