2G2 THE PENOKEE lEON-BEAEING SERIES. 



not seen, but the exposures approacli to withiu a few feet of eacli other, aud the rela- 

 tive attitudes of the two formations are such as to leave no question whatever with 

 regard to the unconformitj-. Not only is this shown by the vertical ixtsition of the 

 schists as contrasted with the tiat inclinations of the slaty series, but also by the way 

 in which the latter beds, to the north of the two lakes mentioned, tit into the sinuosi- 

 ties of outline of tlie older formations. The entire contrast as to lithological charac- 

 ters between the two sets of rocks furnishes further proof. . . . So far as it is 

 developed along the national boundary line the lowest layers of tlie Animikie series 

 in sight are those on Gunflint lake [see PI. xxxvii, this Monograph]. The highest 

 layers are those in the vicinity of Grand Portage bay, tlie whole succession between 

 these points being some thousands of feet in thickness. The iron-bearing horizon at 

 the base of this succession is lithologically identical with that of the Penokee series 

 of northern Wisconsin and Michigan, while the black slates, graywackes, etc., which 

 succeed the iron-bearing horizon, are in turn the counterparts of those which form 

 the middle and upper portions of the Penokee series. The interstratified gabbros of 

 the Animikie are wanting, luiwever, or are relatively rare in the Penokee region. 



The iron formation of the above described area of the Animikie series 



contains all the phases fonnd in the Penokee district, with the exception 



that no extensive ore deposits have yet been developed. There is, further, 



a much more intimate association of the less altered and most crystalline 



forms than in the Penokee series. The relations of the original cherty 



carbonate with the various phases of the actinolitic slates are as intimate 



* 



as are the relations of the unaltered rocks with the ferruginous cherts in 



the Penokee series. 



The rherfi/ iron carbonates (PI. xxv). — The cherty ii"on carbonates are 



found extensively exposed in various localities. They are known as far 



east as lake Superioi*, on the ridge which is just back of port Arthur. 



They are also largely found on the Kaininistiquia river, l)ut the best known 



and most characteristic exposures are at and northeast of Gunflint lake, T. 



65 N., R. 2 and 3 W., Minnesota. At times these rocks are very nearly pure 



carbonate, and contain from 35 per cent to 45 per cent of metallic iron 



(for analyses see p. 192). These beds of comparatively pure carbonate 



are, however, not usually of any considerable thickness. They alternate 



with layers which contain a large percentage of calcium and magnesium 



carbonates and with belts (»f chert of greater or less purity. Sometimes 



the transition from the thick bands of carbonate to those of chert is al)rupt; 



