200 C. K. LEITH 



crystalline, and the iron is magnetite instead of hematite. The rocks 

 consist chiefly of jaspers, amphibole (grtinerite) schists, greenish sili- 

 ceous slates, cherts, cherty carbonates, and magnetite slates. It is 

 believed that these rocks were originally glauconitic green-sands ; that 

 the ore has been derived from the iron in the glauconite, and that the 

 ore bodies result from concentration and replacement. In this part 

 of the Mesabi range no ore bodies have yet been found which are at 

 the same time both rich enough and large enough for profitable mining, 

 although vast quantities of magnetite ore occur at or near the surface. 

 The dip of this formation varies from an average of 45 ° to 50 ° on 

 the west to less than 15 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 the 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 metamor- 

 phism, and usually complete recrystallization. The gabbro varies from 

 a nearly pure plagioclase rock to titaniferous magnetite. 



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

 clature used by the United States Geological 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 Algon- 

 kian, and. the gabbro as the lower part of the Keweenawan series of 

 the Algonkian. 



Winchell, Alexander, 1 gives a detailed petrographical description 

 of the Koochiching granite occurring on the north boundary of Min- 

 nesota, about two miles west of Rainy Lake. The rock is a biotite- 

 hornblende granite of eruptive origin, and is assigned to the Lauren- 

 tian. 



'The Koochiching Granite, by Alexander Winchell : Am. Geol., Vol. XX, 

 1897, pp. 293-299. 



