Jh'cif/Hjxjsi/ion of' Iron Pjii-ilcs. \'Vi 



in which they were phicecl. OthcM's, like those from Roxbiirv. 

 Conn., and I'loni certain gold mines of Colorado, have i-emained 

 nnchanged, thongh for years lying in a room over the chemical 

 laboratories. Xo facts in chemical geology were more interest- 

 ing and mysterious than those connected with pyrites : such as its 

 close comjninionship with gold, the conditions of which have uot 

 been determined ; its nnchano-eableness in some cases, its de- 

 strnctibility in others. Sometimes its ci-ystals or concretions 

 are completely changed to limonite, with not the least change of 

 form or markings ; sometimes, by oxidation, it is converted into 

 snlphate of ii'on, which is washed away, leaving cubical cavities, 

 or a spongy mass of quartz ; and sometimes even the iron has 

 disappeared, leaving the cavities lined with sulphur. These dif- 

 ferences have not yet been satisfactorily explained, and they con- 

 stitute an inviting subject of investigation for the chemist and 

 mineralogist. Dr. Newberry had noticed that the pyrites so 

 common in coal, and ])yrite replacing wood, are particularly 

 pi'one to oxidation ; the concretions in clay are liable to it, and 

 the brilliant crystallizations in mineral-veins and in metamoi'- 

 l)hic slates, less so." 'J'he fact of the resistance of a foi'm of true 

 })yrite to decomposition has been known to many mineralogists, 

 and is thus strongly stated by some authors, usually without at- 

 tempt at ex]ilanation : " It does not decompose in a moist atmos- 

 l)here."' " The faces of its crystals are very brilliant, the action 

 of the air does not tarnish them."^ 



'• Pyi'ite is not altered in the air, and does not decompose in 

 collections.'" " The oxidation of pyrite is a very subordinate 

 ])henomenon on the Comstock. It is well known that various 

 occurrences of pyrite differ greatly in their behavior tow^ard oxi- 

 dizing agents. That found on the Comstock is for the most 

 part very stable, and often remains exposed for years with no 

 greater eifect than tarnishing."* A variety of pyrite commonly 

 abounds in most alum-slates, in regard to which T. Scheerer ' 

 remarks: ''Not only the marcasite, but also ordinary pyrite 



' Cours MiD. et G30I., A. de Selle. (1878), I, 472. 



•'• Traite Min., Dufrenoy, (1856), II, 540. 



=* Lectures on Min., T. Egleston, (1871). Pt. 11, 122. 



^ U. S. Geol. Survey, Geol. of Comstock Lode, pp. 882 and 231. 



' Fogg. Ann., (1888), XLY, 188. 



