188 Decomposition of Iron Pyrites. 



the surface, with spots of brownish j^ellow iron-oclire ; some 

 surfaces are deeply penetrated by gangue-matter and little 

 pockets of limonite-ochre. 



No. 45. Limonite, after pyrite. Brockville, Canada. Sharp 

 polished octahedra, with highly composite faces, and with solid 

 angles modified by the pyritohedron ; in hepatic I'eddish brown 

 limonite, pseudomorphous after pyrite ; imbedded in a fine 

 grayish limestone. 



No. 46 Pyrite. Roxbury, Massachusetts. Strongly striated 

 cubes, with curved faces produced by oscillation with the pyrito- 

 hedron, pale brass-yellow and splendent on fracture ; in groups 

 associated with ochreous siderite. Decomposition : a slight tar- 

 nish on all faces, and many little spots and films of red iron- 

 oxide. 



No. 47. Pyrite. Duluth, Minnesota. Flattened, sharply de- 

 fined cubes, with occasional octahedral planes upon their solid 

 angles ; rather pale brass-yellow and splendent on fracture. 

 The weathered surfaces of the cubes are generally found, after 

 digestion in acid, to be fretted or pitted with octahedral pianos. 

 Decomposition : a slightly iridescent tarnish, and generally a 

 sliining film of brownish black iron-oxide. 



No. 48. Pyrite. Thunder Bay, Lake Superior, British Amer- 

 ica. Yellowish sharply defined cubes, with their solid angles 

 commonly modified by planes of the octahedron, and with faces 

 crossed by short lines of aggregation or cubic cleavage ; implant- 

 ed on white quartz -crystals. On fracture, very pale bi'ass-yellow 

 and brilliant. Decomposition : dull brownish yellow tarnish, 

 feebly ii'idescent, on all faces, 



No. 49. Pyrite. Silver Cliffs, Colorado. Dull striated pyrito- 

 hedra, very joale yellowish white and sjilendent on fracture, with 

 faces sometimes striated, especially on the sides implanted in 

 the matrix ; attached to a finely granular, cellular, blackish ma- 

 trix of pyrite, w^itli little brownish gray coatings. Decomposi- 

 tion : a general dull blackish gray tarnish, in part yellow. 



No. 50. Pyrite. St. Lawrence County, New York. Sharp- 

 ly defined bright cubes wdth finely striated faces, rarely slightly 

 curved, pale brass-yellow and splendent on fracture: imbedded 

 in an ash-gray argyllite, with ochreous films over its lamination- 

 seams. Decomposition : a reddish orange to orange-yellow tor- 



