204 A. W. G. WILSON TRAP SHEETS OF LAKE NIPIGON BASIN 



whether there are any sheets of diabase intrusive in these beds. East of 

 the south end of Black Sturgeon lake the same sheet of diabase rests 

 directly upon Kewatin rocks. Hence in the vicinity of Black Sturgeon 

 bay and Black Sturgeon lake we find the upper diabase sheet resting 

 directly upon Archean gneisses and granites, Kewatin schists and Ke- 

 weenawan sandstones and dolomites, five different types of rocks of three 

 formations, and in addition there are some inclusions of sandstones in 

 the diabase standing in various attitudes from nearly horizontal to 

 vertical. 



SPRUCE RIVER GORGE 



The main stream running southeast from Black Sturgeon lake is a 

 river of the same name. Through most of its course from the lake to 

 Black bay, on lake Superior, the Black Sturgeon river flows along a low- 

 land bounded on the southwest by a cuesta formed of Keweenawan sand- 

 stones and dolomites, the strata having a slight southwesterly dip. On 

 the northeast side of the lowland is an extensive area of Archean oldland. 



Figure 2. — Section on Spruce River, 10 Miles southwest of Black Sturgeon Lnhc 

 The profile of the stream bed is shown at "a" 



The cuesta on the wesl rises about 300 or more feet above the river, and in 

 places it is capped by remnants of diabase sheets. At one point in the 

 bed of the river there is also an outcrop of diabase, but the relations of 

 this isolated mass to the surrounding rocks are unknown. The escarpment 

 which forms the edge of the cuesta can be traced from just above- the 

 Canadian Pacific Railway bridge to a point some 15 miles west of the 

 south end of lake Xipigon. where it terminates. Outlying remnants, of 

 the sediments are known in several localities still farther northwest. 



About 10 miles southwest of the Black Sturgeon lake the Spruce river, 

 an important tributary of the Black Sturgeon river, flows down the face 

 of the escarpment through a gorge cut in diabase, and eventually it 

 reaches Black Sturgeon lake. Above the gorge the river flows in a chan- 

 nel cut in red Keweenawan dolomites. On approaching the edge of the 



