LEAD AND ZINC ORES. 1153 



found below this level. These sulphurets might consist mainly of marcasite 

 and pyrite, with subordinate amounts of sphalerite and galena. However, 

 even in this deep belt, concentration of galena and sphalerite may occur to 

 some extent, although it receives no contribution from the lead and zinc 

 salts from above ; for even after the salts of lead and zinc traveling- down- 

 ward from the belt of weathering are all precipitated, the waters may still 

 hold oxygen. This oxygen would, to the largest extent, act on the marca- 

 site, producing to some extent soluble salts which would be abstracted, and 

 thus reduce the quantity of this material, and relatively enrich the deposits 

 in lead and zinc, although not increasing the absolute amount of lead and 

 zinc present in a given vertical distance. So far as the zinc and lead salts 

 were oxidized by the oxygen-bearing water, these would react on the iron 

 sulphide again, and they would be precipitated according to the reactions 

 given above. 



The above paragraph can not be said to apply generally to the deposits 

 of the Mississippi Valley. These deposits are usually of very limited 

 vertical extent. Many of them are apparently cut off by impervious strata 

 within short distances of the surface, but drilling at the Granby area, Mis- 

 souri, has developed a large amount of pyrite at depth, and thus, so far 

 as information goes, this area appears to conform to the general rule. In 

 the upper Mississippi Valley area, while information is not very full, 

 apparently below the galena horizon the iron sulphide is rather more abun- 

 dant at high than at low horizons. As a consequence the zinc sulphides 

 become less impure with depth. This has been noted at Shullsburg, at 

 Platteville, at Benton, at Dubuque, and at Mineral Point. But this is in the 

 belt in which marcasite and sphalerite alternate in the crustified ores, and 

 both are apparently for the most part the products of ascending water. If 

 this vertical distribution be regarded as chiefly due to the primary concen- 

 tration by ascending water, the order is as it should be, for at first the zinc 

 sulphide should be thrown down to a greater extent than the iron sulphide. 



GENERAL STATEMENTS. 



It is believed that concentration by ascending waters largely explains 



the crustified sulphide deposits of zinc and lead of the upper Mississippi 



Valley, at least so far as their main masses are concerned, although, as has 



been explained, superimposed upon the sulphides of the first concentration 



mon xlvii — 04 73 



