378 THE VERMILION IRON-BEARING DISTRICT. 



and of such enormous economic value in the Mesabi range. Although 

 stratigraphically the same as the Biwabik, the rocks at the eastern end of 

 the Vermilion district, constituting the Gunflint formation, have been in 

 general much more metamorphosed than the Biwabik, and while showing 

 their derivation from rocks similar to those constituting the Biwabik, they 

 are now petrographically very different from them. 



The rocks of the Biwabik formation have been described by the Min- 

 nesota Greological Survey, especially by N. H. and H. V. Winchell " and 

 J. E. Spurr,^ and a later and more accurate description has been given by 

 Leith." To these articles the reader is referred for details. 



The following brief summary made by Leith, while preparing the 

 report on the Mesabi district, describes the petrographic character of the 

 rocks typically developed in that district and may aid in interpreting 

 the petrography of the Gunflint iron-bearing rocks: 



The Mesabi iron formation rocks are mainly ferruginous chert, but contain 

 also iron ore, small quantities of iron and calcium carbonates, thin seams of slate and 

 paint rock, and, finallj', certain peculiar green rocks containing minute dark-green 

 granules resembling an indurated greensand. The ferruginous chert and the iron 

 ores have been shown to develop mainl}^ from the alteration of the last-named rock. 

 The original green granules under the microscope are seen to lie in a matrix of chert 

 with a variety of textures. They have round, oval, crescent shaped, gourd shaped, 

 or more irregular forms; their color vai'ies from a bright green through a shade of 

 yellowish green to dark brown; under crossed nicols a fine aggregate polarization 

 appears, so fine that the substance appears practically isotropic. The green granules 

 were supposed by Spurr to be true glauconite, but later work bj' the United States 

 Geological Survey'' shows the substance to be essentially a hydrated ferrous silicate 

 lacking potash, and quite different in composition from glauconite. Moreover, instead 

 of being entirely organic, as supposed by Spurr, the substance of the green granules 

 is supposed to be the result of chemical developement in a manner analogous to the 

 development of the iron carbonates described by Van Hise for the other districts of 

 the Lake Superior region.'' The shapes, however, maj'^ be due to filling, replacement, 

 or accretion about minute organic liodies, which are probably commensurate in 

 variety both with those depositing glauconite and with those giving the granule 



«Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 6, 1891, pp. 113-146. 

 6 Geol. and Nat. Hist. Survey of Minnesojta, Bull. No. 10, 1894, p. 259. 



«The Mesabi iron-bearing district of Minnesota, by C. K. Leith: Mon. U. S. Geol. Survey 

 Vol. XLIII, 1903, pp. 101-159. 

 rf Ibid., p. 108. 

 e Twenty-first Ann. Rept. U. S. .Geol. Survey, Pt. Ill, 1901, pp. 326-328. 



