AVORK OF IRVING AND OF SPURR 323 



works of uearl}- all those who have more recently discussed the iron ores 

 of Lake Superior for the United States Geological Survey. 



J. E. Spuhr's Investigation of the Mesabi Okks 



As to the primary condition and origin of the iron embraced in this 

 cherty carbonate, there was but little inquiry until the work of J. E. 

 Spurr for the Minnesota survey on the Mesabi Eange, who considered 

 it to have existed in a silicate form in a mineral which he thought was a 

 kind of glauconite, since named greenalite/- This green mineral was by 

 him considered the source of all the iron ores contained in the formation, 

 whether of ferric oxide or of cherty carbonate. He described what he 

 considered the process of formation of the banded jaspers and the banded 

 cherty carbonate. To this source, also, he referred the residuary clay- 

 like deposits of silicate of alumina, or kaolin, some of the beds of which 

 are from 70 feet to 80 feet in thickness. The agents that caused the 

 decomposition of the greenalite were supposed to be oxygen, carbonic 

 and other acids carried downward from the surface by atmospheric 

 waters. In speculating over the nature of the original rock of the iron- 

 bearing member, of which the greenalite appeared to be the only remain- 

 ing representative, Spurr at the outset assumed an "excessively basic 

 lava," but he abandoned that idea and adopted a sedimentary rock as 

 the foundation of his hypothesis. 



Sedimentation 



Sedimentation, followed by profound metasomatic alteration and 

 segregation, therefore, was the basis of the hypothesis argued by Mr. 

 Spurr. In order to account for the greensand he assumed that there 

 was, in the time of the Animikie, abundant organic life, thus supporting 

 the assumption of Irving. The rocky matrix which surrounded the origi- 

 nal glauconite grains he presumed was largely calcareous, a composition 

 which rendered it liable to easy and rapid removal by descending acidu- 

 lated waters. In order to account for the rounded forms visible in the 

 altered iron-bearing rock after the elements of the rock had been removed 

 by metasonuitosis, Spurr appealed to a vague combination, which he 

 called "granular brecciation, concretionary action, impregnation, and 

 other forces." Had Mr. Spurr adhered to the idea with which he started 

 out in his research, namely, "an excessively basic lava," he would have 

 been much nearer the truth than he was. But he must be credited with 

 an im])ortant chapter in I he history of this long investigation when he 



^ BuHetin no. x, Minnesota Geological Survey, 1804, p. 235. 



