PLATE LXXIII. 



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



The granitic portion of the Archean here shows a variation. It is rather a dark 

 schist, or sometimes dioritic, containing hornblende. This schistose rock is supposed 

 to be derived from the Keewatin by metamorphism. Also, in sec. 11, T. 58-21, has 

 been found an area of greenstone, but its size is not known. It is probably much 

 larger than is represented on this plate. Overlying the Archean is the Animikie, 

 consisting here of three members, viz., quartzyte, taconyte and black slate. The 

 quartzyte member is sometimes very scant or wanting, allowing the iron-bearing 

 rocks to lie directly on the Archean. The iron-bearing rocks are very productive 

 in the central part of this area. Here ore is shipped from mines at Mahoning, 

 Hibbing and Pillsbury. 



Mr. J. E. Spurr, from samples collected in sec. 22, T. 58-20, and from other 

 localities, derived the conclusion, which has been verified by later examinations, 

 that the ore of the Mesabi Iron range is due to an alteration of a peculiar greensaud, 

 which he likened to foraminiferal glauconite. This sand was deposited originally 

 in the shallow water of the ocean along the Taconic shore line, not far from the 

 Archean land area, and it took the sedimentary bedding which is yet plainly visible 

 in much of the taconyte rock and in the ore itself. The changes in this greensand 

 gave rise chiefly to silica, oxide of iron, and small amounts of actinolite. When 

 atmospheric air, or carbonated waters, had free access to this sand, its iron some- 

 times took the form of siderite. 



The writer fully accepts this view of the origin of the Mesabi ore, but does 

 not believe that the greensand was of foraminiferal origin. The reasons for believ- 

 ing that the greensand was originally a volcanic glass-sand of a very basic nature 

 are too complicated and technical to be even summarized in any satisfactory manner 

 in this atlas. They are given in Part III of volume v of this report, much of the 

 microscopical evidence being in Part II of the same volume. N. H. w. 



