SUPERFICIAL ALTERATION OF ORE DEPOSITS. 303 



Van Hise,' in 1889, showed that the iron deposits of the Lake 

 Superior region are concentrations of iron formerly disseminated 

 in a siliceous rock containing carbonate of iron and other carbon- 

 ates, and called by him cherty iron carbonate. This dissemin- 

 ated iron was taken into solution by surface waters, carried down 

 until its passage was obstructed or impeded by less pervious 

 rocks, often dikes, and there precipitated by meeting with other 

 solutions of a different nature. These other solutions contained 

 oxygen, while the iron-bearing solutions had been largely robbed 

 of their oxygen and had been freed from silica by the large 

 amount of carbonic acid they contained. When, therefore, the 

 two solutions met, the iron in solution was oxidized and pre- 

 cipitated ; while the silica, in the spot where this precipitation 

 occurred, was, on account of the dilution of the carbonated 

 waters with the other waters, and through the agency of alkaline 

 carbonates, dissolved and carried off, thus gradually increasing 

 the amount of iron and removing the silica. By this theory, the 

 iron is largely a replacement of the silica of the cherty iron car- 

 bonates, and has been derived from the parts of the strata 

 exposed to superficial influences. The deposits are, therefore, 

 of only superficial extent, though they may reach over 1,000 

 feet below the surface, yet when they pass below the action of 

 surface influences, the iron has not been concentrated and they 

 are of too low grade to be mined for iron ore. The methods of 

 local concentration proposed by Professor Van Hise for these 

 Lake Superior iron deposits, are equally applicable to certain 

 other iron deposits, and are a most valuable addition to our 

 knowledge of chemical geology. They also bring out in a most 

 prominent manner, the fact that even rocks composed of mate- 

 rials like silica, which are very resistant to surface influences, may, 

 under proper conditions, be replaced on a large scale. 



^ C. R. Van Hise, The Iron Ores of the Penokee-Gogebic Series in Michigan and 

 Wisconsin, Amer. Jour. Sci., 3d series, Vol. 37, 1889, pp. 32-47 ; The Iron Ores of 

 the Lake Superior Region, Trans. Wisconsin Acad. Sci., Vol. VIIL, 1891. For a 

 fuller discussion by Van Hise on this subject see United States Geol. Survey, Tenth 

 Annual Report, 1888-89, Pt. I., pp. 409-422 ; Monog. U. S. Geol. Survey, No. XIX., 

 1892. 



