PREFACE. XXV 



of which it is impossible to uniquely define any single group. The extremes and 

 the mean are easily apprehended, and that is about all the classification that can be 

 given. The history and the multiplicity of petrographical nomenclature verify this 

 conclusion. 



7. The igneous rocks of the Archean are not derived one from the other by 

 any process of differentiation of magma. 



8. The rocks of the Archean are not unstable, but fixed. Their Archean com- 

 position and characters have come down to the present without showing, normally, 

 the least alteration. 



9. There have been epochs of intense metamorphism, of folding, crushing and 

 fusion, but these were local as to time and place and their effects were wrought out 

 in Archean time with the probable exception that in Taconic time similar revolu- 

 tions produced similar effects on the Archean rocks adjacent. 



10. The weather effects, which are superficial and have been i-emoved by glaci- 

 ation, the accidental location of oxidizable sulphides or carbonates so as to intensify 

 locally the changes in the adjacent rocks, and the rare instances of post- Archean 

 fracture and mountain-forming, are the abnormal conditions that may be appealed 

 to to justify the idea that the crystalline rocks are as changeable as an organic body! 

 But these conditions are exceptions to the normal state of the Archean rocks, and 

 can hardly be said to establish a great principle which contravenes the general 

 history and the verdict of the normal condition of the great mass of those rocks. 



11. The Archean in Minnesota was fully crystalline and brought to vertical 

 attitude before the deposition of the Taconic, and there was certainly a long time 

 interval between the Archean and the Taconic not represented in Minnesota by any 

 rocks, during which in other parts of the country there may have been formed other 

 rocks, both fragmental and igneous. 



12. As the Archean igneous rocks are derivable from Archean earlier rocks by 

 metamorphism and fusion, so later igneous rocks may have been produced by 

 similar alteration of later clastic rocks, and these would introduce a great many 

 additional varieties and peculiarities into petrographical nomenclature. 



13. But, whenever the basic original crust has been reached by such action, 

 the resultant rock has been a diabase, whose constancy of composition vouches for 

 the constancy and universality of its source. 



Diagrammatic scheme of the Archean in Minnesota. 



For the purpose of conforming to a conventional practice, and in order to make 

 somewhat clearer the composition and order of the Archean and Taconic in Minnesota, 

 as described in this report, the following structural scheme is given. Such schemes 

 are liable to convey error by their exactness of definition and the meagerness of 

 data they embody. Nothing is more plain to the practical geologist than that 



