Decowjjositioii (if Iroii Pyrites. 181 



tlirongh the series, will be understood from the following pre- 

 liminary descriptions. 



Concretionary nodule. Marsden's Diggings, Galena, Illinois. 

 A concretionary nodule, with finely fibrous structure, especially 

 at its centre, and indications of concentric arrangement. 



No. 1. (E'ibrous core). Marcasitic pyrite. A^ery finely fibrous, 

 pale brass-yellow and splendent. ISee analysis already given. 

 Decomposition: dull bronze-colored tarnish, along fissures reach- 

 ing from the exterior. 



No. 2. (Main columnar crust). Marcasitic pyrite. Coarse- 

 ly fibrous to columnar, pale brass-yellow and splendent. De- 

 composition : tarnished surfaces, and grayish white effiorescence 

 of vitriol in abundance, mainly ferrous sulphate, with a trace or 

 more of ferric sulphate. 



No. 3. (Outer crystallized surface). Marcasitic p3a'ite. At 

 the surface of the nodules, the columns of No. 2 end in a close 

 aggregation of cubes in immediate juxtaposition, each cube cap- 

 ped with a roof -like modification by thepyritohedron (pentagonal 

 dodecahedron). Deco7npositio7i : dark gray tarnish, with abun- 

 dant efflorescence of white vitriol in the interstices. 



No. 4. Pyrite. Cumberland, England. An aggregate of 

 bright yellow cubical grains, associated with galenite, fluorite, 

 and quartz. A strongly marked platy cleavage, passing into 

 curved scales. Many sharply defined cubes, with the solid angles 

 slightly modified by octahedral planes ; fracture, pale brass-yel- 

 low and splendent. Streak, brownish black. Decomposition ; 

 a beautiful and brilliant iridescent yellow tarnish. 



No. 5. Pyrite. Rio Marina, Elba. Brilliant, pale brass-yel- 

 low pyritohedrous with striated faces ; on a mixture of specular 

 iron and some granular pyrite. Decoynposition : trace of orange- 

 brown iron-ochre, in cavities of the surface between the 

 crystals. 



No. 6. Pyrite. Tevis district, Cochise County, Arizona 

 Territory. Large bright and yellowish grains, with some mi- 

 nute striated cubes, associated with galenite in grayish white 

 quartz. They are reported to contain 140 of gold and silver per 

 ton. On fracture, pale brass-yellow and splendent. Decomposi- 

 tion: yellow tarnish, bronze-colored on a few particles of pyrite, 

 lying in minute rusty cavities. 



