SUPERFICIAL ALTERATION OF ORE DEPOSITS. 305 



At the celebrated Iron Mountain in Missouri, a large part 

 of the ore came from conglomerates composed largely of frag- 

 ments of iron ore, which had been weathered out of the pre-Cam- 

 brian rocks that had originally contained them. These conglom- 

 erates lie at the base of the Cambrian strata which overlie the 

 pre-Cambrian rocks, and even in the latter rocks, where exposed, 

 the original ore has been made much more easy to work by the 

 decay of the enclosing material and its conversion to clay. 



In the iron region of eastern Texas, the limonite ores are often 

 a result of the solution of iron from the superficial oxidation of 

 iron pyrites, iron carbonate and glauconite. Sometimes the 

 sequel of this process is the downward passage of the solution to 

 an underlying laminated clay, and the gradual replacement of 

 this bed, forming a hard limonite,' which still preserves the lam- 

 inated structure of the clay. 



In Mexico certain hematite deposits described by R. T. Hill^ 

 as occurring in Lower Cretaceous limestone at or near the con- 

 tact with intrusive masses of diorite, and sometimes even in the 

 diorite itself, may, as Hill suggests, be the result bf superficial 

 concentration from the limestone. 



Very large deposits of hematite also occur in Grant county, 

 New Mexico, at the contact of limestone and an eruptive. The 

 origin of this ore is as yet somewhat obscure, but is probably 

 due to a concentration after the original deposition of the iron. 

 The iron deposits in the lakes of Sweden and Norway are 

 most striking instances of a concentration of iron ore due to sur- 

 face influences and going on at the present time. The iron is 

 derived from the oxidation of the neighboring rocks, carried by 

 carbonated surface waters to the lakes, and there, by further oxi- 

 dation and hydration, precipitated as hydrous sesquioxide (lim- 

 onite). The iron ore is dredged up and used, but the processes 

 of nature gradually replace it, and, in the course of years, the 

 lakes again accumulate a considerable thickness of ore. 



'R. A. F. Penrose, Jr., Geological Survey of Texas, First Annual Report, 1890, 

 pp. 72-76, 79-81 ; also Bulletin Geological Society America, 1892, pp. 47-50. 

 = Amer. Jour. Sci., Vol. XLV., 1893, PP- 111-120. 



