Deconfjjo^ifiofi of Iron Pyrites. 141 



of chemical composition has nothing to do with the differing 

 quickness of tendency to decomposition, but also hint the*possi- 

 bility of its connection with some subtle peculiarity of structure. 



1). — Microscopical Examination of Pyrites. 



In order to throw more light on this difficult cpiestion, I have 

 resoited to a microscopical examination of several kinds of py- 

 rites. The only microscopical examination on record, to my 

 knowledge, is that by Berzelius, on an efflorescent marcasite ; 

 he simply states that, "seen under the microscope, it presented 

 a mass seamed by little cracks, tilled with a Avhite and efflores- 

 cent salt, whose interstices appeared to consist of white pyrite, 

 unattacked and more or less ciystalliue "; also, on dissolving out 

 the efflorescent salt, he could detect no sulphur in the residue. 



Pyrite from Gah7ia, 111. I selected, as a material most 

 promising of results, a portion of the same nodule of pyrite in a 

 state of decomposition, which had been used in the chemical 

 analysis just mentioned. The conical specimen consisted of a 

 finely fibrous material, with fibres about eight centimeters in 

 length, and mostly 0.2 millimeter in thickness, radiating from 

 the apex of the cone, the center of the original nodule, becoming 

 coarser toward their outw^ard extremities, and there terminating 

 in a close aggiegation of distorted cubes, 4 to 8 mm. on a side. 

 The inner material was of a pale yellowish-white color and ex- 

 ceedingly brilliant lustre, crossed by three or four concentric 

 lines of concretionary growth ; an easy cvoss-fracture occurred 

 at the concentric lines, leaving a surface — across the ends of the 

 fibres — which appeared to the eye perfectly aphanitic in texture, 

 exceedingly brilliant, and slightly mammillary. At the outer 

 surface, the aggregated cubes were stained to a brownish-black, 

 variegated by delicate whitish efflorescence Avithin the interstices ; 

 this efflorescence was found to penetrate to a depth of about 1 to 

 2 centimeters below the surface, along the fibres, and even to 

 the very centre of the nodule, at a depth of 8 centimeters, along 

 certain widened fissures among the fibres. The material differed 

 little from that of similar fibrous nodules from Galena, 111., 

 Linden, Wis., etc, whose specific gravities are presented in the 

 table near the close of this paper. Various fragments of this ma- 

 terial were mounted for examination by reflected light ; and for 



