1 66 REVIEWS 



and Whitney represent reasonably close approximations to the facts as then known, 

 and that, notwithstanding their many imperfections, they have been of service at the 

 time of publication. 



J. E. Spurr. " The Original Source of the Lake Superior Iron Ores." 



American Geologist, Vol. XXIX (1902), pp. 335-49. 



Spurr discusses the origin of the pre-Cambrian iron ores of the Lake Superior 

 regions. He repeats his conclusion that the Mesabi ores have resulted from the altera- 

 tion of a green ferrous silicate of the class of glauconite, and further states that his 

 conclusion in reference to the Mesabi iron formation may be " probably applied to 

 most of the other Lake Superior iron ores." 



Comment. — -This paper is practically a reply to a brief abstract published in the 

 Engineering and Mining Journal oi an informal talk given by the writer before the 

 Geological Society of Washington. While fully agreeing with Mr. Spurr's major 

 conclusion that the Mesabi ores have resulted from the alteration of ferrous silicate 

 granules, the writer has emphasized certain facts which seem to prevent the applica- 

 tion of the name "glauconite" to this silicate.^ 



As to the statement that conclusions applicable to the Mesabi ores apply to most 

 of the other Lake Superior iron ores, presumably this is based on certain similarities 

 in granules and concretionary forms to be observed in the iron-bearing rocks of the 

 Mesabi and Gogebic districts. This similarity the writer has discussed elsewhere,^ 

 and believes it will afford no support for Dr. Spurr's somewhat sweeping statement. 



C. K. Leith. "A Comparison of the Origin and Development of the Iron 

 Ores of the Mesabi and Gogebic Iron Ranges." Proceedings of the Lake 

 Superior Mining bistitute, Vol. VII (igo2), pp. 75-81. 



Leith compares the origin and development of the Gogebic and Mesabi iron 

 ores. The ores of the two districts occur in the same geological horizon; they result 

 from the alteration, under weathering conditions, of a ferrous compound of iron, 

 through the agency of percolating waters, and are localized in channels of vigorous 

 circulation of water. But the differences in the development of the ores of the two 

 districts are important. The original ferrous compound of iron is mainly iron silicate 

 in the Mesabi district, and iron carbonate in the Gogebic district, although both 

 substances appear in each district. The localization of the ores in the Gogebic dis- 

 trict during their concentration has been within clear-cut pitching troughs with defi- 

 nite shapes, while in the Mesabi district the very gentle folding of the iron formation, 

 its fracturing, and the absence of intrusives combine to make the channels of vigor- 

 ous flow within .the iron formation most devious, resulting in the curious and exceed- 

 ingly irregular shapes now to be observed in the Mesabi ore deposits. 



The original ferrous silicate from which the ores develop in the Mesabi district is 

 in minute homogenous granules, the form of which remains even after the substance 

 is changed. Associated with these granules are undoubted concretions of iron oxide 

 and chert with concentric structure. In the Gogebic district there appear numerous 



'C. K. Leith, "The Mesabi Iron-Bearing District of Minnesota," Monograph 

 XLIII, U. S. Geological Survey, 1903. 



^ C. K. Leith, " A Comparison of the Origin and Development of the Iron Ores 

 of the Mesabi and Gogebic Iron Ranges." Proceedings of the Lake Superior Mining 

 Institute, 1902, pp. 75-81. (See summary below.) 



