232 THE VERMILION IRON-BEARING DISTRICT. 



described above. The only explanation as to the origin of the ore which 

 appears to conform at all to the facts is that the ore is the result of a 

 process of replacement, and that the original rock was a banded rock, 

 either essentially the same as the present banded jasper or, as seems 

 more likely, the same kind of rock as that from which the jasper itself 

 has been derived by replacement. The presumed original nature of this 

 rock has already been discussed (p. 191) and the conclusion reached that 

 it was a cherty iron carbonate essentially similar to that described from 

 the various iron-bearing districts of Michigan, and especially from the 

 Penokee-Grogebic district of Wisconsin and Michigan. 



An explanation of the ore as a chemical deposit" contemporaneous with 

 the deposition of the remaining portions of the iron formation is, as Smyth 

 and Finlay have already stated, altogether incompatible with the occurrence 

 described above. 



In the case of every known body jasper forms at least one boundary in some 

 part of it, under such circumstances that the bands, if continued, would run into the 

 ore. This fact, taken in connection with the tortuous form of many of the bodies, 

 seems to us quite inexpUcable on any theory of contemporaneous deposition of jasper 

 and rich ore. For such a theory would involve the extraordinary assumption that 

 the conditions of sedimentation or chemical precipitation were so i-adically different 

 on opposite sides of an imaginary vertical plane in ocean water as to permit the con- 

 temporaneous deposition or precipitation of nearly pure silica on one side and nearly 

 pure ferric oxide on the other, and that such differences in conditions persisted long 

 enough to permit the accumulation, in some cases, of 100 feet or more of material.* 



The theory that the ore is primarily the result of replacement by iron 

 oxide of various substances," notably iron, calcium and magnesium carbon- 

 ates, and silica, and of accumulation of the replacement products in places 

 especially suitable, the location of these places being due to geologic 

 structure, is in direct accord with the facts which have been observed in the 

 district, and which have already been discussed in considerable detail. 



The time of the accumulation of the ore can be fixed approximately. 

 It was subsequent to the folding which produced the synclinal troughs 

 in which the ores are now formed. This folding was of course also 



"The iron ores of Minnesota, by N. H. and H. V. Winchell: Geol. and Nat. Hist. Survey of Min- 

 nesota, Bull. No. 6, 1891, pp. 103-112; N. H. Winchell, Geol. and Nat. Hist. Survey of Minnesota, 

 Final Kept., Vol. IV, 1899, p. 547. 



^ Smyth and Finlay, op. cit., pp. 643-644. 



cMon. U. S. Geol. Survey Vol. XXVIII, 1897, pp. 400-40.5. 



