lUO THE CRYSTAL FALLS lEON-BEAKING DISTRICT. 



TIk' only apparent exception is that part of the great mass in T. 43 N., R. 

 31 ^y., which extends north and south along the Michigamrae River; but. 

 this is really not an exception, since the folds of the Mansfield slates here 

 run in the same direction. 



SECTION I.— UNRELATED INTRUSIVES. 

 CLASSIFICATION. 



Under the above heading are included intrusive rocks which occur in 

 such isolated outcrops that no definite relations can be shown to exist 

 between them and other igneous rocks of similar or related characters. 



The intrusives described under this heading comprise rocks of acid, 

 basic, and ultrabasic composition, represented respectively by the granites, 

 the dolerites and basalts, and the picrite-porphyries. 



ACID INTRUSIVES. 



The acid intrusive rocks may be divided into ordinary biotite-granite 

 (granitite) with a micropegmatitic variety, and muscovite-biotite-granite 

 and rhyolite-porphyry. 



GEOGRAPHICAL DISTRIBUTION AND EXPOSURES OF GRANITES. 



The biotite-granite proper does not occur in the district in large quan- 

 tity. It is found in dikes penetrating the Upper Huronian rocks in sees. 15 

 and 22, T. 42 N., R. 31 W., and in dikes, cutting diorite intrusives in the 

 Upper Huronian, in sec. 22, T. 42 N., R. 31 W. 



The micropegmatitic variety of the biotite-granite is confined to an 

 area underlain by the Lower Huronian rocks. It occurs at N. 750, W. 740, 

 sec. 17, T. 43 N., R. 31 W., and N. 1750, W. 1580, sec. 29, T. 43 N., R. 31 

 W., in small quantities in isolated dikes, cutting the dolerites which penetrate 

 the Lower Huronian series. 



Owing to their small size, none of the above-mentioned exposures are 

 represented on the maps. 



The muscovite-biotite-granite forms large, bold, isolated knobs in sees. 

 19, 20, 29, and 30, T. 42 N., R. 31 W., between the Paint and Michigamme 

 rivers. These knobs are so closely related petrographically that they are 

 presumed to represent a large boss, and they are therefore represented as a 

 unit on the geological map of the district, PI. III. Other smaller areas of 

 granite occur and are shown on the detail map, PI. XVIII. 



i 



