Page:The Journal of geology (IA journalofgeology21894univers).pdf/319

Van Hise,^[1] 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 carbonates, and called by him cherty iron carbonate. This disseminated 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 precipitated; 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 carbonates, 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 materials like silica, which are very resistant to surface influences, may, under proper conditions, be replaced on a large scale.