160 abstracts: geology 



(10) In some places before, and in other places after, concentration the 

 iron-bearing formations have been extensively modified by deformation 

 or by igneous contact effects such as tend to prevent further concentra- 

 tion. (11) The sequence of events in the formation of the ores may be 

 outlined for each district and for the region as a whole. (12) The de- 

 velopment of the ores in general represents a partial metamorphic cycle. 



The iron-bearing formations are bedded and locally cross-bedded and 

 contain recognizable sedimentary material, such as iron carbonate, 

 greenalite, shale, sand, and conglomerate. The orignial constituents of 

 the iron-bearing formations were dominantly cherty iron carbonate and 

 iron silicate (greenalite) , with minor quantities of hematite and magnet- 

 ite and with varying amounts of mechanical sediments. 



The close association of iron-bearing sediments with contemporaneous 

 basic eruptive rocks in the Lake Superior region and in other parts of 

 the world, the richness of these eruptive rocks in iron salts, and the prob- 

 able derivation of the upper Huronian slates associated with the iron- 

 bearing formations from the eruptions make it likely that these iron-rich 

 eruptive rocks were the principal source of the iron in the iron-bearing 

 sediments. As to the manner in which the iron was transferred from 

 the eruptive rocks to the place of sedimentation, there are several 

 hypotheses: (1) It may have been transferred in hot solutions migrating 

 from the eruptive material during its solidification and carrying iron 

 salts from the interior of the magma; (2) so far as the lavas were subaeri- 

 ally extruded, iron may have been transferred by the action of meteoric 

 waters, either hot or cold, working upon the crystallized iron minerals 

 of the rock ; (3) the iron may have been transferred by direct reaction of 

 the hot magma with sea water, in which the iron-bearing sediments were 

 deposited. 



The essentially new features of the monograph are : (1) a much more 

 detailed and quantitative study of the secondary concentration of the 

 iron ores than has before been attempted; (2) the establishment of gen- 

 etic relationship between the primary iron-bearing beds and contempo- 

 raneous volcanism; (3) the interpretation of structure on a broad scale 

 as indicating the Lake Superior region to be essentially an old shore line 

 of heavy deposition, mountain-making, and volcanism; (4) the correla- 

 tio nof the iron and copper deposits of the Lake Superior region, as well 

 as the nickel, cobalt, and silver deposits in Ontario to the east, as all 

 related to volcanism, along an axis of folding parallel to the Lake Superior 

 syncline, all in the region of heavy shore line deposition in pre-Cambrian 

 times. C. K. L. 



