396 THE VERMILION IRON-BEARING DISTRICT. 



From the foregoing statements we see that the Rove slates and 

 graywackes form the youngest member of the Animikie series in the 

 Vermihon district. The only rocks younger than it are the dolerite sills, 

 the Duluth gabbro, and the occasional basic and acid dikes which cut 

 through the gabbro. 



THICKNESS. 



Only a very small portion of the sediments which constitute the Rove 

 slates in the Lake Superior region are represented in the Vermilion district. 

 As has been shown by the distribution, only the apex of a V which rapidly 

 widens to the east is there present. The gabbro of Keweenawan age comes 

 in from the south and swings up northwestward, cutting across the east- 

 west striking slates, and producing the V above referred to. In the Ver- 

 milion district, then, the Rove slates vary from a minimum, at the point of 

 the V, up to a maximum for that district which attains a considerable thick- 

 ness. No attempt to measure the maximum thickness for this district has, 

 however, been made, as it would give merely the thickness of a portion of 

 the slate formation, and not that of the formation as a whole. The latest 

 estimate for that part of the slates present in the Vermilion district is 

 that made by the Greological and Natural History Survey of Minnesota. 

 According to this, the "G-raywacke Slate Member" has a thickness of 1,650 

 feet, the "Black Slate Member" a thickness of 950 feet, and the sills intruded 

 in these rocks a thickness of about 250 feet. This gives a total thickness for 

 the sediments of the Rove formation exposed in this district of 2,600 feet. 

 No statement is made as to the section on which the estimate of this thick- 

 ness was based, but it was presumably between Grunflint and North and 

 South lakes, just east of the limits of the area shown on the accompanying 

 map of the Vermilion district, PL II. The formation has been studied at 

 various places by a number of geologists, and varying estimates have been 

 made of its total thickness. According to estimates made by Irving," the 

 Animikie series of slates corresponding to the Rove slates of the report has 

 a thickness of 10,000 feet. Ingall has estimated this thickness at 12,000 feet* 



In 1892 Irving and Van Hise" gave an estimate of 11,000 feet as the 

 maximum thickness of the Animikie slates in the Penokee district. 



«Mon. U. S. Geol. Survey Vol. V, 1883, p. 380. 



''Geol. and Nat. Hist. Survey of Canada, Ann. Eept. for 1888, H, p. 26. 



'■Mon. U. S. Geol. Survey Vol. XIX, 1892, p. 299. 



