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



tion the silica is all or nearly all jasperized. In passing to 

 lower horizons less and less of it is jasperized, until in the 

 lower parts of the formation the siliceous bands are white. 

 In the cases in which the jasperization is incomplete, it usually 

 affects the outer parts of the bands and progresses inward. 

 Frequently all stages may be seen between completely jasper- 

 ized bands and those in which the jasperization has begun to 

 affect only an outer film of the layers. It sometimes happens 

 that above a band of paint-rock or soap-rock the silica is all jas- 

 perized, while below it is the white chert. It follows from the 

 foregoing that we have associated with many of the ore-bodies, 

 either banded ore and chert or banded ore and jasper. 



The remaining important variety of rock associated with 

 the ore is magnetite- actinolite schist. Where magnetite is the 

 prominent oxide of iron, actinolite or griinerite* is almost 

 always present. In this phase of material the iron is mainly 

 concentrated into layers, giving the rock a banded appearance 

 the same as with the ferruginous cherts and jaspers. 



Recent study has shown that iron carbonate is also an im- 

 portant constituent of the ore-bearing formation. Messrs. TV. 

 S. Bayley and W. N. Merriam have most frequently found 

 this material at places where the formation dips under a green- 

 stone. In one case iron carbonate occurs abundantly in the 

 deeper workings of a mine. Transitions are seen between the 

 unaltered carbonate and those phases of the formation in 

 which the carbonate is partly or wholly oxidized to limonite, 

 hematite or magnetite. 



It is believed that the cherty carbonate of iron, as first advo- 

 cated by Irving, is the original source of the various forms of 

 ferruginous rocks occurring in the ore-bearing formation. 

 However, it is not the purpose of the present paper to con- 

 sider in detail the processes by which the many kinds of rocks 

 were produced. This part of the subject for another district 

 has already been somewhat fully treated.f 



Associated formations. — In considering the character, posi- 

 tion and genesis of the ore-deposits, it is necessary to take into 

 account two associated formations : the overlying conglomerate 

 and quartzite of the Upper Marquette series, and the erup- 

 tives. 



The eruptives occur in bosses and in dikes. They were 

 originally diabases, but frequently have passed over into dio- 

 rites, and these again have further altered. In the northeast 

 part of the great ore-producing township of Marquette county, 

 T. 47 ~N., R. 27 W., is a large area of greenstone. This area 



* Notes on Michigan Minerals, A. C. Lane, H. F. Keller and P. F. Sharpless : 

 this Journal, III, xlii, 505-508, 1892. 



f Tenth Annual Report, U. S. Q. S., pp. 393-408. 



