972 THE GEOLOGY OF MINNESOTA. 



[Petrology of the crystalline rocks. 



of metamorphism have crossed them. Epochs of intense pressure and of crushing 

 have passed over them. These epochs of revolution have been the birth-dates of 

 new minerals. Metamorphism has been carried to complete fusion and the resultant 

 molten rock has solidified as from the molten interior of the earth. But when these 

 revolutions had subsided and the crust had resumed its normal quiet the new min- 

 erals and mineral enlargements took on the profound quiet and permanence which 

 have lasted to the present. The changing temperatures of the atmosphere and of 

 the superficial portion of the rocky crust have no perceptible effect below a few feet, 

 and since-the Glacial epoch their effect at the immediate surface is very insignificant 

 under ordinary conditions. 



Therefore, as a general rule, the Archean minerals are permanent. The only 

 evidence of shearing and crushing pressure that is widely observable consists in the 

 greater or less epochal metamorphism and in the local complete fusion which they 

 have suffered. It will appear that many rocks that have been regarded crushed and 

 sheared crystalline rocks, are more probably sub-crystalline pressed and sheared 

 clastic debris. Permanence rather than change is the normal condition of the Archean 

 crystalline rocks. 



(b) THE PETROLOGY OF THE CRYSTALLINE ROCKS. 



The most important petrological conclusions drawn from the examination of 

 Minnesota crystalline rocks, detailed in the chapter devoted to microscopical 

 descriptions, are three in number, viz.: 



1. The origin of the Archean granite, etc. 



2. The origin of the Taconic gabbro, etc. 



3. ' The nature of the greensand that produced the iron ores of the Mesabi 

 Iron range. 



Nos. 1 and 2 have been substantially presented in Part I of this volume, and at 

 this place it will be necessary only to supplement that discussion by some further 

 facts and collateral observations. No. 3 is a result reached later, and has taken final 

 shape only in the course of the review and revision incident to the publication of the 

 volume, though attended by the examination of some fresh material. 



1. The origin of the Archean granites, etc. 



Under the term granite are here embraced all the igneous rocks of the Archean, 

 including dioryte and excepting such gabbros and diabases as are plainly of later 

 date than the Archean, and all other igneous rocks that are not of too basic a com- 

 position to be included under the term granite. Quartz-porphyry is also excepted 

 for special treatment. 



In the preliminary discussion of this subject, included in Part I of this volume, 

 devoted to " Structural Geology," it is shown that all the structural phenomena of 



