424 THE VERMILION IRON-BEARING DISTRICT. 



RELATIONS TO ADJACENT FORMATIONS AND AGE. 



These basic rocks wherever found are clearly intrusive in the rocks 

 which surround them. Hence, in all cases the rocks are younger than those 

 in which they occur. These dikes cut all the rocks of the district from the 

 Archean up to and including the Keweenawan gabbro, but have not thus 

 far been found cutting the granite dikes which were found in the gabbro in 

 the Vermilion district. However, in the Keweenawan area south of the 

 Vermilion district similar basalt dikes have been seen cutting through the 

 syenite and granite massives referred to above, and it is strongly probable 

 that these dolerite dikes are of the same age. Since, then, these intrusives 

 cut all other rocks of the district they must be the youngest of the pre- 

 Cambrian rocks which are represented. 



METAMORPHIC EFFECTS. 



In several instances where the contacts between the Rove slates and 

 the dikes were exposed, observation showed that the slates were indurated 

 in the vicinity of the dikes, this induration diminishing as the distance 

 from the dikes increased. Special observations would be required to deter- 

 mine accurately the character of the metamorphism which produces the 

 induration. These examinations were not warranted by the amount of time 

 available for the study of the district, consequently the details of this change 

 must be left for future studies. In all probability this change consists in 

 the recrystallization of the quartz and feldspar and the production of biotite, 

 as is found to be the case in the contacts of the Logan sills and dikes upon 

 the same slates. 



