116 0. B. Van Hise — Iron Ores of Michigan. 



Akt. XT. — The Iron Ores of the Marquette District of 

 Michigan ; by C. E. Yan Hise. 



Before considering the subject of this article, it is well to 

 recall the general views held by the late Prof. Irving as to the 

 Lake Superior ore-bearing formations, and those of the writer 

 as to the position and genesis of the ores of the Penokee dis- 

 trict. Prof. Irving maintained that the ores, jaspers and 

 associated rocks are derived directly or indirectly by the altera- 

 tion and silicification of an original lean iron-bearing carbo- 

 nate.* A detailed study of the Penokee-Gogebic district led 

 the writer to the following conclusions :f First, the original 

 rock of the iron-bearing formation was a lean cherty carbonate 

 of iron, magnesium and calcium ; second, the various phases 

 of rock now found in this formation, such as chert, jasper, 

 magnetite-actinolite schists and ore-bodies, are all produced 

 from this original carbonate by various alterations, the pro- 

 cesses involved in which are described in detail ; third, the 

 ore deposits occur in bodies which all reach the surface of the 

 earth, which have a longitudinal pitch and are roughly tri- 

 angular in cross-section. One of the lower sides of each 

 triangle is bounded by an impervious dike-rock, the other by 

 an impervious slate-formation ; fourth, the ores in their present 

 positions are concentrates produced by downward percolating 

 waters which carried iron carbonate to the apices of the 

 troughs, where it was precipitated by oxygen brought by 

 waters coming more directly from the surface. At the same 

 time silica was removed. 



At the close of the description of the Penokee ores, atten- 

 tion was called to the fact that in certain respects the occur- 

 rence of the Marquette ores are analogous ; but at that time 

 the study of the latter had not extended far enough to give 

 anything more than a general statement that it was probable 

 that the same principles of concentration are applicable to the 

 ore-deposits of both regions. Since that time a systematic 

 investigation of the entire Marquette district has been under- 

 taken, and while the study of the iron-bearing formation is far 

 from complete, certain results have been reached, to set forth 

 which is the object of the present paper. 



* Origin of the Ferruginous Schists and Iron Ores of the Lake Superior region, 

 R. D. Irving: this Journal, III, xxxii, pp. 255-272, 1886. 



f The Iron Ores of the Penokee-Gogebic Series of Michigan and Wisconsin, C. 

 R. Van Hise: this Journal, III. xxxvii, pp. 32-48, 1889: The Penokee Iron-bear- 

 ing Series of Michigan and Wisconsin, by R. D. Irving and C. R. Van Hise : 

 Terith Annual Report of the Director of the U. S. Geological Survey, 1888-89, pp. 

 341-507. 



