Iron Ore Ranges of Minnesota. 6~ 



exploration in unknown areas can be carried on. Without a correct 

 understanding of how the ore lies in the formation much money 

 may be wasted in fruitless search in barren rock. If the lay of the 

 ore in the rock is understood a few crucial drill holes would, in 

 numerous cases, be sufficient to settle the question. 



If I am not mistaken a casual comparison of the Cuyuna range 

 with the description now given of the other two ranges will be 

 enough to show whether the Cuyuna ore is in rocks of the Archean 

 or in rocks of the overlying Taconie. Let us ask a few questions, 

 f i rst : 



As to the ore, Is it a soft ore like the Mesabi, or a hard one 

 like that of the Vermilion range? If it is soft, is its softness due 

 to the crushing of an ore which was originally hard, like the Chand- 

 ler ore? What are its impurities — silica, phosphorus, manganese or 

 sulphur, and is it hydrated, so as to make it limonite? Does the 

 ore graduate in coarseness in one direction so as to become a con- 

 glomerate, and in the other so as to be a paint-rock, or red shale? 

 Is the ore granular, and does it grade into a rock such as that called 

 taconyte on the Mesabi range? 

 second — 



As to the rocks in which the ore lies 



Are the rocks nearly or quite vertical or nearly horizontal? Are 

 they crystalline or fragmental? Are they greenstone, or any of the 

 forms of greenstone schist? Are they associated with mica schist 

 or with granite? Are they black slate? And if of black slate do 

 they consist largely of volcanic debris? Is there a large amount 

 of jasper associated with the ore? 



In the light of our present knowledge of the Cuyuna range some 

 of these questions cannot be answered decisively, and some of them 

 are perhaps not sufficiently distinctive. That is, if answered the 

 answers might be equally applicable to the Archean or to the Ta- 

 conie. Such, for instance, is the question whether the ore is gen- 

 erally hydrated, so as to make it limonite. Indeed an answer to 

 that question, based on what we know at present of the two ranges, 

 might be entirely misleading, for we do not know from anything in 

 Minnesota, whether the ores of the Archean are ever limonitic, but 

 we do know that the Mesabi ore is sometimes limonitic to a marked 

 degree. Hence since the Cuyuna ore is markedly limonitic the ans- 

 wer would show an agreement, with the Mesabi, and tend to prove 

 that the Cuyuna ore is of the age of the Mesabi. If we go outside 

 ot Minnesota, however, we find that iron ore from the Archean, in 

 the Lake Superior region is sometimes largely limonitic, as in the 

 Michipicoten region where, at the Helen mine, the bulk of the ore 

 shipped is limonite instead of hematite. Hence the fact that the 

 Cuyuna ore is limonitic, might be indicative of either the Mesabi or 

 the Archean 



All of the other questions, however, carry with themselves, and 

 in their answers, more or less import touching the main problem 

 i. e. whether the Cuyuna range is of the Archean or the Taconie. 

 The following answers can be given to the foregoing questions: 



As to the Ore. 



The ore is hard, but less hard than the Vermilion ore. It car- 

 ries considerable phosphorus, and occasionally much manganese. In 



