260 i?. D. Irving — Is there a Ruronian Group ? 



The latest formation of the region is the Potsdam sandstone, 

 which shows in the extreme southwestern corner of the area 

 mapped, in the vicinity of the St. Louis River, between Fond 

 du Lac and Thomson, Minn. The sandstone, here of a gen- 

 erally reddish cast, and with a slight easterly dip, may be seen 

 directly overlying a series of folded and cleaved slates belong- 

 ing in the upper part of the Animike* series, as subsequently 

 explained. The contact of this sandstone with the Keweenawan 

 rocks, which are largely displayed in the vicinity of Duluth, is 

 concealed ; but in Douglass County, Wis., at a distance of ten 

 or fifteen miles southeast from Fond du Lac, the sandstone 

 may be seen in very distinct unconformable abutment against 

 the Keweenawan diabases of that district. Farther east the 

 entire coast of Lake Superior, to within two miles of the 

 mouth of the Montreal River, on the boundary line between 

 Michigan and Wisconsin, is formed of the same sandstone in a 

 horizontal position. 



TJnconformably beneath the Potsdam sandstone comes next 

 the great Keweenaw series, which in this region reaches an 

 aggregate thickness of upwards of twenty thousand feet, the 

 layers being mainly of an eruptive origin, but also in part of a 

 detrital nature. The upper detrital division* of the Keweenaw 

 series is, however, not represented in the region, being buried 

 here beneath the waters of Lake Superior. So far as the coast 

 line of the lake is concerned, the characters and divisions of 

 the Keweenawan rocks there displayed, I have set forth else- 

 where. f Much new material has since been gathered with 

 regard to these rocks in the interior of this region, but I will 

 take the time now only to note their general flat lakeward dip,;}: 

 the concentric and crescentic curves of the outcrops of the 

 beds or groups of beds — the highest layers appearing on the 

 Minnesota coast about midway its length — and the immense 

 development of coarse grained, stratiform olivine-gabbro at the 

 base of the series. These gabbros are the surface-rocks over 

 an area in the interior some 125 miles in length from northeast 

 to southwest, 25 miles in greatest width, and considerably 

 more than 1,000 square miles in extent. 



Next in downward order comes the Animike iron-bearing 

 slaty series, whose relations to the adjoining formations it is 

 now designed especially to set forth. These Animike rocks 

 are exposed in four distinct areas, one of which is separated 

 from the others by an overlap of the great gabbro which forms 

 the base of the Keweenaw series. The others are separated 



* See Monog. U. S. G. S., vol. v, pp. 152 and 266. 



f Copper-bearing Rocks of Lake Superior, Monographs of the United States 

 Geological Survey, vol. v, pp. 260-328 and Plates I-III, V-XY, XXVI, XXIX. 

 % Monog. U. S. G. S., vol. v. 



