Van llise — Iron Ores of the Penokee- Gogebic Series. 33 



ure as a whole before considering the subject of this paper. 

 The series of rocks runs across the country in an approx- 

 imately east and west direction, the general trend being 

 somewhat north of east. It is a simple series, which has been 

 tilted to the north at an angle of from 60° to 80°. It contains 

 no subordinate folds, and thus the succession of belts is 

 easily made out ; one following above the other in perfect 

 conformity. The series rests upon a complex of granite, 

 gneiss, and various green schists. It is overlain by the 

 eruptives of the Keweenaw Series. In our new mapping it 

 will be divided into four belts (see fig. 1), the principle of 

 division being a fragmental or non-fragmental character. At 

 the base of the series is a cherty limestone-member which, in 

 one place, is as much as 300 feet thick, and which varies from 

 this to disappearance. The second member is a feldspathic 

 quartz-slate. On the average it is from 300 to 400 feet thick, 

 and is composed of green, red and brown fragmental slates 

 which contain a good deal of clayey matter. The upper part 

 of this fragmental member is a pure vitreous quartzite, the 

 induration of which has been due to the enlargement of the 

 quartz-grains originally deposited as a sandstone.* The 

 third member of the series is a belt of non-fragmental sedi- 

 ments, about 800 feet thick, which is known as the iron- 

 bearing member from the fact that all the known ore-bodies' 

 and heavily ferruginous rocks occur within it. The upper- 

 most member of the series is a thick layer of greywackes, 

 grey wacke- slates, and mica-schists and slates. This member is 

 several times as thick as the three lower combined, but in its 

 essential fragmental character it is to be considered as a unit 

 in the series. 



The origin of the ferruginous rocks and ores of the iron- 

 bearing member has been considered in a general way in a 

 paper by Professor Irving already referred to. The funda- 

 mental conclusion of that paper has been borne out by our 

 later investigations, i. e. that the original rock of the iron- 

 bearing formation is a cherty iron carbonate, f from which the 

 various phases of rock and the ore found in it have been pro- 

 duced by a complex series of alterations. 



The principal phases of rock in that part of the forma- 

 tion in which the ore-deposits occur, aside from the cherty 

 carbonate, are heavily ferruginous regularly banded slates, 

 brecciated and concretionary ferruginous cherts, and the ore- 



* Bull. TJ. P. Geol. Survey, No. 8. Upon the Secondary Enlargement of Min- 

 eral Fragments in. Certain Rocks; by It. D. Irving and C. R. Van Hise. 



f The origin of this cherty iron carbonate is a question of great interest, but one 

 which it would take an article of some length to discuss, so this rock is taken as 

 a starting point. 



Am. Jocr. Sci— Third Series, Vol. XXXVII, No. 217.— Jan., 1889. 

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