190 THE VERMILION IRON-BEARING DISTRICT. 



and H. V. Wincliell, who liaye had excellent opportunities for studying 

 the Minnesota deposits, have so construed them." A condition which 

 would admit of the precipitation from a sea of a rock as acid as a 

 chert, consisting essentially of pure silica, followed immediately by the 

 precipitation of rock as basic as the bands of pure iron ore, is so 

 anomalous as not to be tenable. The only explanation which seems most 

 nearly to meet and to answer the requirements of texture, structure, 

 and composition is that the rocks are now in a very different condition, 

 both chemically and physically, from what they were when originally 

 deposited. Their present condition may be interpreted as due to secondary 

 changes acting upon rocks of banded character. 



The exact character of these original rocks is a question of much 

 moment. As the source of the iron-bearing . rocks of the Mesabi range 

 Spurr has suggested an original glauconitic greensand, partly of foraminif- 

 eral origin. The green material, called by Spurr glauconite, has been 

 carefully studied by C. K. Leith, and has been found to be not glauconite 

 but a hydrous ferrous silicate without any potassium, and it has been called 

 "greenalite.'"" Microscopic study of the rocks from the Archean iron- 

 bearing formation of the Vemiilion district has shown no evidence of the 

 former existence of such foraminiferal rocks or glauconitic greensands ; nor, 

 indeed, has any rock been found which can be proved to be the original 

 rock from which the ores and associated rocks have been produced. How- 

 ever, that kind of rock which approaches nearest to the supposed original 

 rock is the cherty iron carbonate forming a part of the iron formation. 

 Thi« now appears to represent a stage in the process of metamorphism 

 between the cherts and jaspers and ores on the one hand, and the relatively 

 pure iron carbonate on the other. 



Tlie presence of tliis cherty iron carbonate in the Vermilion district, in 

 association with the other members of the iron-bearing formation, offers also 

 a striking analogy between this district and those on the south shore of Lake 

 Superior." In the various monographs upon the iron ranges in the United 

 States portion of the Lake Superior region, Professor Van Hise has presented 



" Geol. and Nat. Hist. Survey of Minnesota,, Final Kept., Vol. IV, 1899, p. 547; Geol. and Nat. 

 Hist. Survey of Minnesota, Bull. No. 6, 1891, pp. 105-111. 



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

 Vol. XLIII, 1903, p. 115. 



c Tenth Ann. Kept. U. S. Geol. Survey, pp. 396-397; Monographs U. S. Geol. Survey Vols. XIX, 

 XXVIII, and XXXVI. 



