206 A. W. G. WILSON TRAP SHEETS OF LAKE NIPIGON BASIX 



north side of the river valley and descending on the other side, at a dis- 

 tance varying from 1 to 2 miles from the river and at a height of between 

 145 and 160 feet above it the granite gneiss is again encountered. The 

 north edge of the diabase sheet thus is shown to abut against the side of 

 an ancient valley in the Archean, and the depth of this valley was at least 

 150 feet. The preserved portion of the diabase sheet between the valley 

 of the Wabinosh and the Archean highlands to the northeast of it now 

 lies on the side of this old Archean valley. 



OMBABIKA NARROWS 



At the northeast angle of lake Xipigon is found one of the most inter- 

 esting contacts of the district. The remnants of the diabase sheet have, 'n 

 the vicinity of Ombabika narrows, a thickness of about 400 feet. On the 

 north side of the narrows, just opposite the small island which divides 

 the channel into two parts, the diabase flowed over rocks which had pre- 

 viously been eroded in such a way as to produce an undulating surface (or 

 a warped surface). Subsequent erosion and glaciation have removed 

 almost all of the diabase. Because of the position of the high ridges of 

 diabase on either side of the narrows and because of the relations which 

 the fronts of these cliffs bore to the direction from which the movement 

 of the ice-sheet took place, erosion was especially active at this point, and 

 the present surface, also a warped surface, is remarkably smooth. It has 

 happened that over an area of several hundred square feet glaciation 

 stopped at such a point that the present surface intersects the old pre- 

 diabase warped surface in such a way that small areas of trap now occupy 

 small hollows in the old surface, and, in a low cliff, sections of contacts 

 in both vertical and horizontal planes are exposed. 



At the west end of the island, in the narrows, there are a .number of 

 large and small granite boulders, derived from the immediately under- 

 lying Archean rocks, included in the diabase, with their upper surfaces 

 planed off by the glaciation. 



"Within 2 miles of the narrows, toward the south, there are at two 

 points very small exposures of a white quartzite with the diabase over- 

 lying, actual contacts being observed. Along the shore for 8 miles south 

 of the narrows there is a narrow but nearly continuous strip of granite- 

 gneiss exposed along the shore, the diabase rising in a ridge behind it. 



On the northwest side of Humboldt bay, 8 miles southeast of the nar- 

 rows and across the ridge of diabase which forms the main axis of the 

 South peninsula of Ombabika, there are several areas both of granite- 

 gneisses and Kewatin schists overlain by traps, actual contacts being 

 seen. 



