30 THE GEOLOGY OF MINNESOTA. 



[Facts of field observation. 



clase. If it be legitimate to infer, as has been done by Hunt in support of hiscrenitic 

 hypothesis, that orthoclase is one of the products of lixiviation from diabase, it is 

 also reasonable to ascribe metallic copper to the same source. On the other hand, it 

 is quite easy to account for all these foreign substances by allowing the action of 

 heated solutions circulating with fumarole activity through the rock long after it 

 was solidified, during the process of cooling, or even at a much later date.* 



If the original magma had been of intermediate character, as supposed by 

 Iddings, such differentiation might, perhaps, have produced the contrasted magmas, 

 but that would contravene the first term of the major hypothesis. 



In the presence of this difficulty it is evident that some other source ought to 

 be sought for the granitic magma, and for the igneous gneisses. This is not far to 

 seek. Indeed, it is easily demonstrable from two different lines of evidence, viz., 

 (1) Facts of field observation, (2) Facts of petrographic significance. After a brief 

 statement of these two convergent lines of evidence, we shall consider the question 

 of, (3) The source of the characteristic elements of the alkaline magma. 



(T). Facts of field obsen-nfioii. Early in the study of the granitic rocks of Min- 

 nesota, the fact of the gradation of the Keewatin rocks such as graywackes, green- 

 wackes, argillytes and sericitic schists, into mica schists, was impressed upon all the 

 field geologists, and such gradation is affirmed in all the annual reports by all the assist- 

 ant geologists whenever there was occasion to consider that question. Such transi- 

 tions are found in nearly all cases where a considerable granitic area occurs, the crys- 

 talline condition gradually increasing toward the point of granitic intrusion. There 

 are also very extensive areas of mica schists distant from any known granitic intru- 

 sion, which led to the conclusion that the crystalline transformation was due to some 

 wider cause than mere contact of granite. Sometimes such mica schists are distinctly 

 from rock originally clastic, containing still numerous boulder forms. This wider 

 operating force has been supposed to be that which is expressed by the term i - egional 

 metamorphism, and was attributed, in ultimate terms, to a general elevation of the 

 temperature of crust throughout the region affected, either by the approach of large 

 masses of the molten interior toward the surface, or by the heat generated by dynamic 

 movements, or both. A local, temporary settling also might crystallize the frag- 

 mentals, by bringing them within the range of higher isotherms. Whatever the 

 special and local cause, there is no question that such transformation is very wide- 

 spread, and accounts for the greater part of the mica schists, and of the mica horn- 

 blende schists. 



Equally early in the investigation of the crystalline rocks it was noted that the 

 crystalline schists gradually pass into gneisses. At first a distinction was drawn 



*Compare : The origin of the Archean igneous rooks. N. H. WINCHELL, American Geologist, vol. xxii, pp. 299-310, 



