C. R. Van Hise — Iron Ores of Michigan. 117 



In much of the field work upon which this article is based, 

 I was in company with Prof. Raphael Pumpelly. The com- 

 mon conclusions reached at this time are consequent upon our 

 joint observations. To Mr. James R. Thompson, Mining 

 Engineer of the Lake Superior mine, I am indebted for de- 

 tailed observations upon the forms and relations, and for plats 

 of many of the ore-deposits of the Ishpeming-Negaunee area. 

 From these plats several of the figures are taken. 



Two ore-bearing formations. — The ore-deposits of the Mar- 

 quette district occur in two formations. It has been recently 

 recognized that these formations belong to different series, 

 separated by a great unconformity, and therefore that one of 

 the two is much older than the other.* The superior of these 

 is here called the Upper Marquette series, the inferior the 

 Lower Marquette series. The known ore-deposits in the upper 

 series are much less important than those in the lower, and 

 unless expressly stated the Lower Marquette series is always 

 referred to.f 



Character of Lower Marquette overbearing formation. — The 

 ore-deposits of the Lower Marquette series all occur in or are 

 associated with a single formation, known as the ore-bearing 

 formation. 



The non-fragmental character of the quartz and partly indi- 

 vidualized silica associated with the ore-bearing formations of 

 the Lake Superior region has been insisted upon in papers 

 already published. The part of the Marquette iron-bearing 

 formation containing the majority of the ore-bodies consists 

 normally of bands of nearly pure silica, alternating with 

 bands composed chiefly of oxides of iron, although frequently 

 bearing more or less silica. The alternating layers are gene- 

 rally not more than an inch in thickness and are more fre- 

 quently in the neighborhood of half an inch. A single layer, 

 if followed for some distance, is usually found to gradually 

 narrow and die out or to have a rounded oval termination. 

 Sometimes the belts of quartz are so short as to be no more 

 than tolerably long ovals. When the quartz is free from oxide 

 of iron it has a white color and is usually called chert, although 

 the microscope shows that the silica is wholly individualized. 

 When the exteriors of the quartz granules are stained with 

 hematite, or particles of hematite are included within them, 

 giving the siliceous bands a red color, the material is called 

 jasper. It is a general rule that near the top of the forma- 



* An Attempt to harmonize some apparently conflicting Views of Lake Su- 

 perior Stratigraphy, C. R. Van Hise: this Journal, III, xli, pp. 117-137, 1891. 



f For the equivalents of the Upper and Lower Marquette series in other parts 

 of the Lake Superior region, see paper cited. 



Am. Jour. Sci.— Third Series, Vol. XLIII, No. 254.— February, 1892. 



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