116 THE VERMILION IRON-BEARING DISTRICT. 



occur at or near the surface. The dip of this formation varies from an 

 average of 46° to 50° on the west to less than 1.5° on the east, and the 

 thickness varies from 650 feet or less on the west to 900 feet on the east. 



(3) The black slate is essentially a fine-grained, black, more or less 

 siliceous, apparently carbonaceous slate. 



(4) The graywacke-slate member is composed of black to gray slates 

 and fine graywackes, with some flinty slates; the upper part shows coarser 

 detrital material, and tlie highest beds seen are fine-grained quartzites and 

 quartz-slates. This member is well exposed on the south shore of Loon 

 Lake. 



Associated with all of the strata of the Animikie are diabase sills, and 

 bounding the Animikie rocks on the south is the Great gabbro mass. These 

 are igneous rocks of later date than the Animikie. Near the contact -with 

 the gabbro the Animikie rocks show marked metamorphism and usually 

 complete recrystallization. The gabbro varies from a nearly pure plagio- 

 clase rock to titaniferous magnetite. 



The pre- Animikie rocks here described, according to the nomenclature 

 used by the United States Greological Survey, belong to the Lower Huronian 

 series of the Algonkian system, and probably also in part to the older 

 Archean or Basement complex; the Animikie is regarded as the equivalent 

 of the Upper Huronian series of the Algonkian, and the gabbro as a part 

 of the Keweenawan series of the Algonkian. 



Sardeson, F. W. Report of secretary of the Geological Club of the 

 University of Minnesota: Science, Vol. IX, pp. 412-4:13. 



Prof. C. W. Hall discusses "The extent and distribution of the Archean 

 in Minnesota." The following quotations are from the secretary's report: 



Accepting the Archean as that original "crust" or solidified portion of the 

 earth, . ... he defined it as an era of igneous origins, whose rocks represent the 

 original crystallization of earth matter added to from below by successive solidifica- 

 tion and many subsequent intrusions. By this definition all overlying elastics or 

 irruptions into or through the elastics are excluded from the Archean. . . . 



Between Rainy Lake and Lake Superior there are several belts of schists, with 

 alternating granites and other rocks having a general northeast-and-southwest trend. 

 Concerning one of these, Irving noted in 1886 "that we have among the rocks . . . 

 two types, in one of which the crystalline structure is complete and in which there is 



