376 PHYSICAL GEOLOGY 



to weathering and were enriched (i) by the removal of the impurities 

 by solution ; (2) by the replacement of the impurities by iron oxides 

 as the former were dissolved out; or (3) by concentration, as a 

 result partly of the removal of impurities and partly of replacement. 

 So wide and deep are some of these Lake Superior iron deposits 

 that they are excavated by steam shovels. The excavation in the 

 Mesabi region, taking into account both the removal of the ore and 

 of the glacial drift which overlies it, is far more extensive than the 

 work conducted at the Panama Canal. In most of the deposits 

 underground mining methods are employed. 



REFERENCES FOR ORE DEPOSITS 



Lindgren, W., — Mineral Deposits. 



Ries, H., — Economic Geology. 



U. S. Geol. Survey Bulletins, Professional Papers, and Monographs. 



