462 EDWARD STEIDTMANN 



The distribution of the iron ores bears no relation to the present 

 structural features of the formation nor to its present erosion 

 surface. 



The relation of the iron formation to the associated igneous 

 rocks is expressed by the following statements: The iron formation 

 is sedimentary; it conformably overlies the ellipsoidal greenstones, 

 in that no period of subaerial erosion intervened between the deposi- 

 tion of the iron formation and the greenstone; the iron formation 

 and the overlying volcanic breccia and rhyolite porphyry are also 

 conformable. The evidence of the sedimentary origin of the iron 

 formation consists in its banding, the iron carbonate content, and 

 the parallelism of the individual bands to each other and to the 

 basal plane. 



The evidence for conformity with the basal greenstone lies in the 

 parallelism of the banding of the iron formation with the plane of 

 contact with the greenstone, the absence of detrital material, and 

 the i)rol)able subaqueous origin of the ellipsoidal greenstones. The 

 conformity of the iron formation and the overlying acid igneous 

 rocks appears probable but is not supported by many lield observa- 

 tions, owing to the sparsity of exposed contacts. Wherever 

 observed, the contacts are sharp and show no detrital materials. 



From Held relations of the iron formation to igneous rocks in the 

 Woman River and other areas but mainly from the work of Van 

 Hise and Leith, Allen believes that the genesis of the iron formations 

 was related to igneous rather than sedimentary agencies and 

 processes. He interprets the physical history of this area to be 

 extrusion of basic greenstones upon a submerged floor, rapidly 

 followed by the precipitation of iron formation, contributed from 

 magmatic solutions, without the intervention of clastic sedimen- 

 tation. The precipitation of the iron formation was followed by 

 the extrusion of acid igneous rocks. The three rock types, how- 

 ever, basal greenstone, iron formations, and acid igneous rock, are 

 perhaps differentiates of one parent-magma. 



Adams' describes the occurrence of silver and calcite, in wall 

 rock adjacent to a Assure vein of the Cobalt district. 



' F. D. Adams, "Notes on the Occuitoiuc ol" the Ore liody at the City of Cobalt 

 Mine," Jour. Can. Mi'it. Inst., XIT (igoo), 414-17. 



