1 146 A TREATISE ON METAMORPHISM. 



I know of no clearer illustration of the phenomenon of crustification 

 described by Posepny, and indeed of the general principles of ore deposition 

 by underground waters, than that furnished by the Upper Mississippi Valley 

 district. The larger parts of the crustified sulphide products are believed to 

 be the work of ascending waters. In many cases it appears that there were 

 two or more cycles of precipitation, so far at least as the sphalerite and galena 

 are concerned, but the first cycle was by far the more important. As 

 explained below, it is thought to be probable that the galena in octahedral 

 crystals (No. 6 of the general succession), and in many cases the sphalerite 

 (No. 4) and the niarcasite (No. 5), are due to a second concentration. 



As to the manner of transportation of the metals which in the Missis- 

 sippi Valley were precipitated as sulphides at the first concentration, it is 

 believed that the lead and zinc travelled mainly as oxidized salts, and 

 probably largely as sulphates. All the evidence goes to show that in this 

 region the concentration took place under physical conditions substantially 

 the same as those which exist at the present time. There is no proof 

 that the solutions went to any great depth, or that unusual temperatures 

 prevailed. Nor is there any evidence that abundant alkaline sulphides and 

 carbonates of the alkalies were present. Therefore, it can not be supposed 

 that the sulphides were transported as such. Probably they were originally 

 disseminated through the limestones as sulphides, were oxidized to sulphates 

 and transported in that form, or as carbonates. 



The first precipitation of the ascending salts is believed to have 

 been accomplished through the agency, direct or indirect, of organic matter. 

 There is abundant organic material in the shales associated with the ores, 

 and a less amount in the limestones. While the precipitation of the 

 sulphides disseminated through the shales may have been largely accom- 

 plished by the direct reaction of the solid carbonaceous material, the more 

 common precipitation within the openings was probably accomplished 

 through the reducing solutions which were produced by the contact of the 

 water with the organic compounds. Waters which have been in contact 

 with such compounds, which also bear sulphides, are very likely to contain 

 hydrogen sulphide. This substance is a direct precipitant of the sulphates 

 as sulphides, but it is probable that dissolved organic compounds also act 

 as reducing agents at the point of precipitation in the crevices. 



