PREFACE. 



It, would have been probably a more logical order of publication to have issued 

 the contents of this volume before the final conclusions and mapping contained in 

 volume iv. Still in the main this volume was so far along that it was warrantable 

 to make brief summaries of the main structural and petrological conclusions in the 

 preface of volume iv, with reference to the discussions contained in this. In the 

 course of the final revision of the manuscript of this volume but one point of 

 importance has been discovered in which it is necessary to depart from the views 

 presented in volume iv. That relates to the origin of the Mesabi iron-bearing rocks, 

 which will be found fully presented in Part III. 



In the final systematic discussion (Part III) of the facts embraced in Part II 

 (petrographic descriptions) it was found that in order to limit the size of the book 

 to reasonable bounds some considerations must be omitted and all must be abridged. 

 Hence, the most important and fundamental results only are given. These embrace 

 therefore only the discussion of the rock-forming minerals* and a synoptical treat- 

 ment of the rock groups. There remains material enough in our field-notes and in the 

 drawers containing our samples to carry forward researches into the geology and 

 genesis of the crystalline rocks through the space of another volume equal in size to 

 this. Such investigation would pertain to the nature and extent of the original 

 rocks that now carry the Mesabi iron ores. We have carried the research far enough 

 simply to reach the main conclusion. The application and scope of that result are 

 yet to be considered. Such additional research would also involve the "red rocks" 

 of northeastern Minnesota and would lead to the inquiry as to how much of that 

 group of rocks can be attributed to actual acid eruption and how much to alteration 

 of basic eruption in submarine conditions. It would also lead to an investigation of 

 the tectonic relation of such supposed altered basic rocks to the gabbro of the Cabo- 

 tian, and of the Cabotian to the iron-bearing rocks of the Mesabi Iron range. Such 

 investigation also would lead to the microscopical examination of the Animikie strata 

 with view to ascertain whether an igneous debris is not more widely disseminated 

 in them than has been supposed. There is also a large chapter yet in the future 

 relating to the visible effects of progressive metamorphism of clastic sediments. The 

 threshold only of this subject has been trod incidentally in the lines of this book. 



*It had been intended originally to include in the final report of the survey an annotated list of all the minerals of the state. 



