PLATE LXXVII. 



DUNKA RIVER PLATE OF THE MESABI IRON RANGE, 1899. N. H. WINCHELL. 



The northwestern half of this area is occupied by the Archean, consisting of a 

 light-colored granite, which varies, however, at the northwest end of Birch lake to 

 a darker rock which is dioritic. This Archean also includes some gneiss, especially 

 along the northern side of Birch lake, and some mica schist, the last appearing like 

 a breccia cemented by granite. The granite sometimes is coarsely crystalline, with 

 porphyritic Carlsbaded orthoclases. 



The iron ore horizon presents some interesting variations. The horizon of the 

 quartzyte, with respect to the taconyte, is not constantly the same, but the two are 

 sometimes interbedded. This interbedded quartzyte is finely fragmental, and its 

 grains have been enlarged by secondary silica. In this quartzyte are also grains of 

 feldspar, actinolite, iron ore and epidote, but these are rare, feldspar being the most 

 frequent of these accessories. There is a more coarse and granular quartzyte which 

 verges to a conglomerate whose nature depends on the adjacent Archean. This 

 forms the bottom of the Animikie. The coarser part of this conglomeratic quartzyte 

 is rather thin, and when the fine-grained portion (above) is well developed the 

 coarser quartzyte appears to be not more than twenty feet. This fine-grained 

 quartzyte is more in evidence along the Mesabi range westward, in form of boulders, 







than the coarser part near the base. These boulders are quite numerous between 

 Mountain Iron and McKinley, and, when weathered, at a distance they appear to be 

 limestone masses. Not only does the coarser quartzyte appear sparsely in scattered, 

 rounded grains in the bottom of the finer, but the fine cementing silica of the finer 

 rock is found to pass downward into the coarser rock, making secondary deposition 

 of quartz on the clastic grains. 



Observations made within the area of this plate seem to demonstrate the sepa- 

 rateness of the Animikie from the Upper Keewatin and the ferruginous taconitic 

 nature of the base of the Animikie where it lies on the Archean, whether on the 

 granite or on the Keewatin. 



The gabbro, in its northern margin, goes diagonally across this area, and comes 

 into contact, from southwest to northeast, successively with the different parts of 

 the Animikie and then with different parts of the Archean. In the region south 

 of Birch lake, in the north part of T. 60-12, and on the north side of Birch lake, there 

 is much doubt about the age of the iron-bearing rock. The composition and struc- 

 ture of the ore, as well as the nature of the associated minerals, are different from 

 the same further west. There is reason to believe that the ore associated with olivine 



