398 THE VERMILION IRON-BEARING DISTRICT. 



territory, heginniug with Logan," that the sedimentary rocks of this region, 

 slates, quartzites, and graywackes, have intercalated in them at various 

 horizons sheets of basic igneous rock ranging in thickness from 1 to 100 

 feet. These vary in character from distinctly gabbroic rocks in the 

 centers of the large masses through all gradations of tiuer-grained granular 

 and porphyritic ■ rocks to the very fine-grained basaltic phases which form 

 the thin sheets and occur as well-formed selvages of many of the thicker 

 sheets. These are the intrusive sheets which have been called the Logan 

 sills by Lawson,*" in recognition of the geological work done by that 

 pioneer of investigation in this field. 



DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. 



Distribution. — The Duluth gabbro forms the southern boundary of the 

 pre-Keweenawan rocks throughout the greater portion of the Vermilion 

 district. The westernmost point at which the Duluth gabbro touches the 

 district is in sees. 26 and 35, T. 63 N., R. 10 W., and section 3, T. 62 

 N., R. 10 W. From these sections on along the Kawishiwi River the Duluth 

 gabbro swings off to the northeast with a broad sweep, extending just 

 within the area mapped as far east as the vicinity of Paulson's mine, in 

 sec. 28, T. 66 N., R. 4 W. From this place its edge trends to the southeast, 

 passing beyond the limits- of the area mapped toward Lake Superior. 

 A couple of small isolated outliers have been found north of Grobbemichi- 

 gamma Lake. The southernmost one is only a quarter of a mile from the 

 northern edge of the main mass of the gabbro, northwest of Paulson 

 Lake, and the other is about three-fourths of a mile from the nearest point 

 on the edge of the gabbro and lies in the NW. 5 sec. 29, and NE. ^ sec. 

 30, T. 65 N., R. 5 W. 



The sills lie well within the district to the north of the edge of the gabbro 

 mass, varying in distance from this edge. The first exposure of such a sill 

 was noticed on the southwest side of Gobbemichigamma Lake, but this can 

 not be traced far. The next one was seen near Bingoshick Lake. This sill 

 has been followed to the east for several miles to a point east of Paulson's 

 mine, having throughout this distance an almost continuous outcrop. Par- 

 allel to this sill several small and relatively unimportant sills have been 



« Geological Survey of Canada, 1846^7, p. 13. 



6 The lacolitic sills of the northwest coast of Lake Superior, by A. C. Lawson: Geol. and Nat. 

 Hist. Survey of Minnesota, Bull. No. 8, pt. 2, 1893, pp. 48. 



