12 THE GEOLOGY OP MINNESOTA. 



[Stratigraphic structure. 



existing north of Moose lake, already referred to, found in this conglomerate, rather 

 than with the ore of the Animikie. In a similar manner magnetite belonging in the 

 Lower Keewatin is produced in the green schists. This is found at Garden lake, on 

 the north side of Long lake and near Tower. A belt of magnetite also occurs in the 

 green schists at a couple of miles southwest from Ely. In each case the accom- 

 panying gangue rock is quartz in a finely-granular but compact state. It is only in 

 contact with the gabbro that this ore is associated, so far as known, with the ferro- 

 magnesian minerals .mentioned at Disappointment lake. 



It should be stated here that owing to the sameness of the result o_f the gabbro contacting on these 

 different ore horizons, whether of the Animikie, the Upper Keewatin or the Lower Keewatin, it is impossible to 

 distinguish them on lithological grounds. The structural environments and the nature of the associated rocks 

 must be taken into account. There is a series of magnetite deposits extending from the vicinity of Iron lake 

 (southwest from Birch lake) eastward as far as to the vicinity of Gunflint lake, which fall into this doubtful 

 category. They occur on the north and south sides of Birch lake, the north side of Thomas lake, the north side 

 of Eraser lake, N. W. J-4 sec. 20, T. 61-6 W., the south side of Gabimichigama lake, along the south side of the 

 stream in the northern part of sec. 35, T. 65-5 W., at Paulson's lake (Gunflint Lake Iron company), and 

 especially about a mile north of the Gunflint Lake Iron company's working, in section 28. The Animikie of the 

 region of Gunflint lake extends unquestionably about two miles west from that lake, at which point it seems 

 to swing southward, the strike of its lowest strata lying in the low, broad valley, which seems to be the northern 

 continuation of the Cross River valley at the point where that river turns eastward to join Gunflint lake, 

 Westward from that vicinity the iron ores above enumerated present ambiguous characters, and there are good 

 reasons for considering some of them metamorphic conditions of lodes of jaspilyte that really belong in the 

 Keewatin. The chief objection to their Animikie age is their intimate association with a rock (muscovadyte) 

 which at Disappointment lake is found to be the product of metamorphism by the gabbro in contact with the 

 Keewatin. These ores, however, in the chapters in volume iv, and on the geological plates, as in all the annual 

 reports, have been represented as of Animikie age. They are as a class the ores which have sometimes been 

 denominated " olivinitic iron ores."* Their accidental occurrence in the presumed line of strike of the base 

 of the Animikie, which here also nearly coincides with the northern limit of the gabbro, was the initial cause of 

 their being grouped with the Animikie. The absence of the black slate of the Animikie in any of its modifica- 

 tions, from the area extending from near Iron lake to within a few miles of Gunflint lake, has been attributed to 

 the overwhelming volume of the gabbro, but it is more than possible that the Animikie never extended so far 

 north, and that all the modified rocks that have been described in this belt, frequently called muscovadyte; as 

 well as the iron ores mentioned, are parts of the Keewatin, chiefly of the greenstone phases. This remark is 

 perhaps to be extended to include the crystalline limestone or breccia of flint embraced in limestone (No. 312), 

 which has been described at Gunflint lake, but not the iron ores nor the horizontal flint seen on the north side of 

 Gunflint lake. 



STRATIGRAPHIC STRUCTURE OF THE ARCHEAN FRAGMENTALS. 



The structure of the Archean is made out by the occurrence of conglomerates 

 which indicate stratigraphic breaks in the succession, inasmuch as they contain frag- 

 ments from the older and not from the later strata. These conglomerates also strike 

 across the structure features of the older series, and frequently dip in marked non- 

 conformity with them. In addition to non-conformities, it has been observed that 

 some of the granitic intrusions are earlier than others, and hence serve as guides in 

 separating the Archean into grand divisions. It has not been found possible as yet 

 to employ thus the ferro-magnesian or basic dikes and other forms of intrusion of 

 the Archean of later date than the basal greenstones, but it is probable that structural 

 and petrographic differences could be discovered to serve such a purpose if a suffi- 

 ciently extended study and comparison of them should be undertaken. 



* Bulletin vi, p. 117. 



