308 THE VERMILION IRON-BEARING DISTRICT. 



tlie conclusion is unavoidable that the gabbro is vei-}^ much younger than 

 the Ogishke conglomerate and Knife Lake slates. 



Relations to basic dikes. — Cutting through the Ogislike conglomerate and 

 Knife Lake slates at various places, basic dikes of the character of dolerites 

 have been observed. These dikes are of exactly the same nature as those 

 which are found cutting through the gabbro which represents the youngest 

 member of the series in the Vermilion district, excluding, of course, as was 

 done in the statement at the beginning of this monograph, the glacial 

 deposits. Since the dikes in the conglomerate are lithologically the same 

 as those cutting the gabbro, they are assumed to be of the same age, 

 although direct relationship between the dikes in the conglomerate and slates 

 and those in the gabbro have never been observed. 



AGE OF THE LOWER HURONIAN SEDIMENTS. 



The descriptions given in the above paragraphs of the relations 

 existing between the conglomerates and slates of the Lower Huronian and 

 the adjacent formations throughout the district enable us to make with 

 confidence the following summary statement concerning the relative strati- 

 graphic position of these sediments in the Vermilion district. Since they 

 lie unconformably above the rocks of Archean age, they must of necessity 

 be younger than the Archean rocks. They are older than the Snowbank, 

 Cacaquabic, and Giants Range granites, which cut and metamorphose 

 theiB, and older than some basic dikes which are intrusive in them. The 

 chief interest, however, is in their relationship to the Animikie sediments, 

 which are very generally recognized as being of Upper Huronian age. 

 The sediments here classed as Lower Huronian are unmistakably of an 

 older period of formation than these Animikie sediments, since these 

 Animikie sediments rest unconformably upon them, as is shown by the 

 relations observed in the adjacent Mesabi district. Hence the three 

 conformable formations, the Ogishke conglomerate, the Agawa formation, 

 and the Knife Lake slates form one series of rocks of Lower Huronian age. 

 In the eastern part of the Vermilion district these sediments bear the same 

 relations to the adjacent formations as in the western part. They lie at 

 the base of the Algonkian sediments, and rest unconformably upon the 

 Archean rocks, and are correlated with the Lower Huronian series of the 

 rest of the Lake Superior region. 



