STRUCTURAL GEOLOGY. 45 



Pokcgama quartzyte.] 



phase occurs at the bottom of the Animikie further west, in the region where the 

 chief economic development of iron ore has taken place,* and at the falls of 

 Prairie river. 



Pokegama quartzyte. The basal conglomerate of the Animikie soon graduates 

 upward into quartzyte which is well developed at Pokegama falls on the Mississippi 

 river, causing the falls. From its favorable and conspicuous exposure at this place 

 it has been named Pokegama quartzyte. The thickness of this quartzyte is not 

 known to exceed fifty feet, and it is sometimes less than twenty-five. It varies from 

 an ordinary quartzyte whose rounded grains are cemented by secondary silica into 

 an interlocking firm plexus of quartz, to a very fine-grained one which approaches 

 flint in its texture. The finer portions prevail toward the east, and at Gunflint lake 

 a genuine flint is seen interstratified near the bottom of the Animikie. This flinty 

 character is quite prominent at the base of the Animikie still further northeast, but 

 as a quartzyte this member is lacking at Gunflint lake. There is, however, at 

 Gunflint lake, some fragments! quartz in the bottom of the iron-bearing member. 



The iron-bearing member exhibits a somewhat irregular manner of succession 

 after the quartzyte, and sometimes has an alternation with it in subordinate strata. 

 In such case the quartzyte is very fine grained, and grades into the quartzyte which 

 runs through the iron-bearing member, becoming crypto-crystalline and non- 

 f ragmen tal. The ore is sparsely disseminated in isolated grains throughout these 

 alternating bands.f In the same manner the upper limit of the iron-bearing member 

 is rather indefinite. The thickness of the iron-bearing member is several hundred 

 feet, and sometimes reaches nearly 1,000 feet. The ore is usually hematite on the 

 western Mesabi range and magnetite on the eastern. The original nature of the 

 iron-bearing member is believed to have been a glauconitic sandrock, but at the 

 present time it consists almost wholly of silica and hematite, in secondary condition 

 constituting the iron ores of the Mesabi iron range. The alteration of the original 

 glauconite is not entirely completed. Hence, by the examination of an extended 

 series of specimens, the steps of the alteration, its causes and its final products, can 

 be traced.:}: The great economic importance of this member of the Animikie has 

 warranted a more extended presentation in a special chapter on the iron ores of the 

 state (vol. iv). 



Limestone occurs just above the iron-bearing member. It is impure and usually 

 dark colored, and can with difficulty be distinguished from the overlying black slates. 

 It is but few feet in thickness. It extends, apparently, the whole length of the 

 Mesabi range, but has been identified in place only at the extreme ends, viz.: sec. 7. 



*J. K. SPDBB. Bulletin x, p. 5. 



t J. K. SPUBR. Bulletin x, p. 229. 



tin Bulletin x of the surrey this subject has been fully discussed and illustrated by J. B. SPUBB. 



