90 JOURNAL OF THE [June, 



of unstable pyrite ; also another, on which are mounted a 

 number of the crystals, yet unaltered, and from the same mar- 

 ble, which were obtained by dissolving away five pounds of the 

 stone. There was some doubt whether so unstable a form of 

 pyrites might not consist of marcasite only ; but these crystals 

 show the form of an ordinary modification of cubes of pyrite of 

 pale brass-yellow color and apparent purity, with no visible rea- 

 son for decay. In another slide are mounted the siliceous 

 and insoluble minerals associated with the pyrite in the same 

 marble ; viz., tourmaline, phlogopite, tremolite, quartz, and ru- 

 tile, all in such small quantities as to have escaped detection in 

 the chemical analyses of this rock, though easily recognized in 

 this way under the microscope. 



In searching for the cause of these great differences in the 

 same mineral, little use has hitherto been made of the microscope. 

 Berzelius alone has recorded a microscopical examination of an 

 efflorescent marcasite, simply stating that, " seen under the micro- 

 scope, it presented a mass seamed by little cracks filled with a 

 white and efflorescent salt, whose interstices appeared to consist 

 of white pyrite unattacked and more or less crystalline ;" also, 

 on dissolving out the efflorescent salt, he could distinguish no 

 sulphur in the residue. 



In connection with a chemical examination which has been 

 presented elsewhere (before the New York Academy of Sci- 

 ences), I have resorted to the microscope in the hope of getting 

 new light. I selected, as a material most promising of results, 

 a portion of a nodule of pyrite in a state of decomposition, from 

 Marsden's Diggings, Illinois. The conical specimen consisted 

 of a finely fibrous material, with fibres about eight centimetres 

 in length, and mostly 0.2 millimetre in thickness, radiating 

 from the apex of the cone (the centre of the original nodule), 

 becoming coarser toward their outward extremities, and there 

 terminating in a close aggregation of distorted cubes, 4 to 8 mm. 

 on a side. The inner material was of a pale yellowish-white 

 color, and exceedingly brilliant lustre, crossed by three or four 

 concentric lines of concretionary growth ; an easy cross-fracture 

 occurred at the 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, 



