DecoDiposilion of Iron Pyrites. 135 



its dei)osits always suggest igneous action, })resent the ap})cur- 

 ancc of veins, contain no water, and are stable on exposure to 

 tlic atmosphere. 



b. Gra}' cubical i)yrito, with brownish-black powder, contain- 

 ing both clay and water, commonly occurring in sedimentary 

 deposits, and easily altered to ferric sulphate. 



c. White marcasite, gray to yellow or greenish-yellow color ; 

 lighter than pyrite, with dark greenish-gray powder, and sp. gr. 

 =4.7. Its masses are connected always Avith aqueous action, 

 occur in deposits caused by chemical double decomposition, and 

 ai"e extremely subject to eflflorescence of copperas. In explana- 

 tion of these differences in weathering, it is stated : ''We must 

 yet remark that, in the specimens of yellow pyrite, water of 

 combination or moisture is almost never found, as in the white 

 ])y rites. Some authors think that the facility with which these 

 pyrites generally efltioresce in the air is connected with the fact 

 that these minerals contain protosulphide of iron, That may 

 be ; but I think that it can be also admitted (after my present 

 analyses) that the molecular state of the substance, or, still more, 

 the enclosure of a clay, easily attacked either by atmospheric 

 agents or by water, may well be a ready cause of their altera- 

 tion ; and by this very fact it is natural to conceive, that sedi- 

 mentary or chemical deposits (in the interior of formations) 

 ought to readily assume this kind of constitution, which would 

 be, so much the more, a common element of their spontaneous 

 destruction." On this suggestion of the influence of clay, it 

 must be noted that the material used in all the painstaking re- 

 searches of these analysts was unsuitable for the solution of 

 the delicate problem involved in the peculiar decomposition of 

 marcasite. Their analyses appear to have been made, not on 

 pure crystals, so far as stated, but on nodules or massive and 

 granular forms of the two minerals. In all, the amount of sil- 

 ica, etc., reaches 4 to 15 per cent.; the specific gravity figures 

 are correspondingly very low, 4.17 to 4.81 ; and the amount of 

 water appears to have but partial connection with that of clay, 

 since the proportion of alumina is small. 



8. Uneven condition of the surface. Like nearly all the later 

 authors who have written on the subject, since the time of Ber- 



