200 THE VERMILION IRON-BEARING DISTRICT. 



younger sedimeutaiy series contains in it fragments of the underlying, 

 older series, tlie iron formation. The details concerning the relations of 

 these two formations are given under the discussion of the later sediments. 



RELATIONS TO BASIC ERUPTIVES. 



In several places the iron-bearing formation is found to have been 

 cut by basic eruptives. Thus, for example, in sec. 27, T. 62 N., R. 15 W., 

 the jasper is cut by dikes of greenstone which must be younger than the 

 jasper, and very probably belong with the Lower Hiironian basic intrusives. 

 Again, south of Ely, in sees. 3 and 4, T. 62 N., R. 12 W., basic intrusives 

 are found to cut across the iron formation. 



AGE. 



From the preceding paragraphs it will have been learned that the 

 Soudan formation is in general younger than the Ely greenstone, the oldest 

 rock of the district, but on the whole so intimately associated with it that 

 the two must be considered as belonging to the same great period of the 

 earth's history, the Archean. 



THICKNESS. 



It is impossible to make any reliable estimate of the thickness of the 

 iron formation, and this for many reasons. In the first place, the exposures 

 of the formation are so isolated and the formation itself throughout is of 

 such uniform character that it is impossible to recognize the same horizons 

 in it in different parts of the district. No definite basement has been found 

 from which to begin an estimate of the thickness. The very close folding 

 to which the rocks of the district have been subjected adds to the complica- 

 tions. The thickness of the Soudan formation, as inferred from its surficial 

 extent rather than from any definite measurements, is presumed to reach 

 several hundred feet 



INTERESTING LOCALITIES. 



On the bare hills just north of the nortliernmost houses of the town of 

 Tower there are a number of exposures that show the relations between 

 the iron formation and the associated rocks. For instance, the southern- 

 most exposures on these hills are conglomerates made up of pebbles of 

 jasper, slate, and chert. These materials have been derived from the iron- 



