1 28 THE VERMILION IRON-BEARING DISTRICT. 



Huronian, and Upper Hurouian. The genesis of the ore deposits is then 

 discussed. 



The general process of ore formation for the Lake Superioi- region as 

 a whole is the same as that already described in the monographs on the 

 Penokee and Marquette districts. The iron is leached from an older forma- 

 tion — in case of the Archean from igneous rocks, the greenstones — and is 

 then deposited as a sedimentary formation, in the form of a cherty carbonate 

 or some iron compound, which, however, becomes this cherty carbonate. 

 These formations are folded and intruded by igneous rocks, and pitching 

 troughs with relatively impervious sides and bottom are formed. Meteoric 

 waters carry downward in solution iron derived from the iron-bearing for- 

 mation, and this is precipitated as an oxide in the structural basins in the 

 formation and in the spaces left by removal of the silica. As result of this 

 replacement, enrichment of the iron-bearing formation occurs at favorable 

 places, and the ore deposits are formed. 



Sections are devoted to a consideration of the influence of topography 

 and denudation on ore de^josition, and to a discussion of the time and 

 depth of concentration of the ore deposits. 



In Chapter II the individual districts are taken up. The section devoted 

 to the Vermilion district is written by Van Hise and Clements, and in this 

 is given the first detailed statement made by the United States Geological 

 Survey on this district. A preliminary map accompanies the description. 

 Since this section on the Vermilion district is in fact a ver)^ brief abstract 

 of the present monograph, it will not be reviewed in detail. It may be well 

 to mention the fact that the Archean is here made to include certain sedi- 

 mentary rocks In the last chapter, the third, there is a comparison and 

 summary of all the districts. The Vermilion district is again briefly con- 

 sidered, and some suggestions are off'ered in regard to exploration in it. 



