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



low, filled with loose soil, obscures it. The contact seemed to dip awav 

 from the cliff face and stands approximately at an angle of 45 degrees, 

 dipping toward the northeast. Since the gneiss here is part of the 

 Archean ridge to the south, that has unquestionably been buried in a 

 flow of diabase, it is possible that the preexisting topography was such 

 as to bring about the relations here described without any intrusion from 

 below in this locality. 



A third locality of interest is at Camp Alexander. About one mile 

 below the steamboat landing, at the mouth of Bass creek, on the west side 

 of the river, a nearly vertical contact between diabase and granite occurs. 

 Since the granite outcrops on the east side of the river, the width of the 

 diabase mass is about 400 yards, or less than a quarter of a mile. A num- 

 ber of outcrops show that it extends northward for about 2 miles. At 

 the north end, part of it is spread out over granite as a capping sheet. 

 Xo conclusive evidence was found to show that the diabase at Camp 

 Alexander and vicinity occurs as a dike. There is independent evidence 

 to show that it occurs at the bottom of an old Archean valley and there 

 is reason to suspect that the mass of diabase here preserved occupies a 

 depression that was once a gorge in the valley floor. 



Adjacent to the shores of lake Xipigon there are several localities 

 where wide dikes of diabase similar to that in the sheets occur. There 

 are also numerous outlying large and small masses of diabase on the sum- 

 mits of Archean ridges, on their sides, and in the valley bottoms. In 

 many instances it was impossible to find any actual contacts. In quite 

 a few cases, particularly where the diabase lies at the summit or on the 

 side of a ridge, sufficient actual contacts were found to show that the 

 remnants were parts of sheets. With regard to some large masses of 

 diabase, very coarse in texture (in one instance we found olivines more 

 than one inch across), no contacts were found. While the coarse texture 

 suggests slow cooling, the mass may have cooled slowly, because it lay at 

 the bottom of a hollow and was thus deeper below the surface than the 

 portions of the sheet lying above the adjacent hills : or it may have been 

 more closely associated with a feeding dike and been the last to cool and 

 solidify. In the majority of cases the textures of the more or less iso- 

 lated and outlying masses of diabase are neither coarser nor finer than 

 the textures of the different parts of the sheets in the main area. 



While it seems probable that the diabase flowed out over the area from 

 many fissures, very few of these fissures are known. The numerous out- 

 lying masses and ridges of diabase, scattered everywhere over the Archean, 

 to the north and northeast of the main area close to the bed of the pres- 

 ent lake Xipigon, are often supposed to be remnants of an extensive 

 svstem of dikes. Detailed examinations of many of them have shown 



