REPORT OF THE DIRECTOR I917 195 



silicate. This is shown by the marked concentric structure of the 

 spherules and their close agreement in optical properties with 

 ckamosite from well-known localities (compare plate 6, figure i 

 with plates i and 2) as well as by the analyses of the gray oolite, 

 indicating a nonpotassic ferrous silicate of a composition similar 

 to certain chlorites of the thuringite type, instead of a potassic 

 ferric silicate, like glauconite (see analyses 8 and 9). 



There can be little doubt that the ferrous iron of the Georgia 

 ores is, in reality, derived from essentially the same silicate as that 

 of the Clinton specimens. Unfortunately, the analyses of the 

 former are not sufficiently complete to admit of a calculation of 

 the silicate. Indeed, the determination of ferrous iron in Clinton 

 ores is very unusual, most analysts, with purely commercial ends 

 in view, determining merely the total iron and calculating all of 

 it as ferric. It is most desirable that more complete analyses should 

 be made, representing different localities and varying depths. 



The existing data, however, are distinctly adverse to the glauconite 

 hypothesis and leave the problem of the precise origin of the silicate 

 unsolved. 



In this respect the situation is somewhat analogous to that which 

 arose in connection with the Mesabi ores some years ago. Spurr 1 

 explained the ores as due to the alteration of extensive deposits of 

 ferrous silicate, occurring in round grains without concentric 

 structure, and of green color (plate 6, figure 2), which he classed 

 tentatively as glauconite and supposed to have been formed under 

 conditions similar to those under which glauconite is forming at the 

 present time. 



This gave, for the time being, a very complete and satisfactory 

 explanation of the Mesabi ores, but Leith 2 has since shown that 

 the green mineral, from which they are derived, is not in reality 

 glauconite but a nonpotassic ferroso-ferric silicate, which he calls 

 greenalite (analysis 7). 



As, in contrast with the case of glauconite, the geological con- 

 ditions under which greenalite is formed are determinable only 

 indirectly, and not by the study of deposits now forming, this 

 leaves the first concentration of the Mesabi ores in relative obscurity 

 as compared with Spurr 's hypothesis. 



1 Spurr, J. E., The Iron Ores of the Mesabi Range; Amer. Geol. XIII, p. 335- 

 45, 1894, and The Iron-Bearing Rocks of the Mesabi Range in Minnesota, Bui. 

 Geol. Nat. Hist. Survey of Minn., no. 10, 1894. 



2 Leith, C. K., The Mesabi Iron Bearing District of Minnesota, U. S. Geol. 

 Survey, Mon. XLIII, 1903. 



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