CHAPTER VI. 

 THE INTRUSIVES. 



Under this general head there is here included an extremely varied 

 assortment of rocks exhibiting in common intrusive relations to sedimentary 

 and igneous rocks. This division is here used merely because it simplifies 

 the classification of the rocks of the district, and the term "intrusives" is not 

 to be interpreted as synonymous with the "dike rocks" (ganggesteine) of 

 some authors, a petrographical division which, in the opinion of the writer, 

 is not warranted. 



These intrusive rocks differ very materially in field occuiTence, petro- 

 graphically, and in point of age from the igneous rocks thus far described. 

 In age much younger than the volcanics, they still bear a close resemblance 

 to some of them; indeed, some forms are identical in character. Massive 

 granular rocks are the common forms. Porphyritic varieties are very 

 subordinate. 



The rocks are all considered as intrasives into either the Lower or the 

 Upper Huronian. In most cases the intrusive relations may be said to be 

 rather inferred than demonstrated, for the direct contacts have been observed 

 in very few cases. However, where isolated sets of knobs of eruptiA^e rocks 

 are found in areas, the greater portion of which are underlain by sedimen- 

 taries, the natural inference is that they penetrate these sedimentaries. 

 Where isolated sets of knobs are composed of the same kind of rock or 

 show variations of the same type, they may be presumed to be connected. 

 For the most part the dikes and bosses are too small to admit of indication 

 upon the accompanying map. Wherever their size has warranted it, they 

 have been represented, as in the case of the acid intrusives between the Paint 

 and Michigamme rivers, and of the basic intrusives north of Crystal Falls. 



