198 a. w. g. wilson trap sheets of lake nipigon basin 



Introduction and general Eeview 



DISTRIBUTION OF THE TRAPS 



Along the north shore of lake Superior from the Slate islands to 

 Pigeon river, and extending over an area reaching to more than 130 

 miles north of the Canadian Pacific railwa}% the most prominent geologic 

 feature is the occurrence of large areas in which trap sheets predominate. 

 Within this area practically every salient feature of the topography is 

 found to be associated with these trap sheets. Along the southern edge 

 of the district, and extending for about 40 miles north of the Canadian 

 Pacific Railway line, the traps are constantly found in association with 

 Keweenawan and Animikie sediments. In the northern part of the area, 

 in the basin of lake Mpigon, residual patches of Keweenawan sediments 

 are frequently found associated with the traps, but there are numerous 

 localities where the igneous rock rests directly upon the older Archean 

 rocks. 



The variegated, bold, and picturesque topography seen along the line 

 of the Canadian Pacific railway from Rossport to West Fort William, 

 Eed Rock at the mouth of the Mpigon river, McKay mountain at Fort 

 William, Pie island, and Thunder cape are a few of the many salients 

 familiar to any one who has journeyed by boat or rail to Port Arthur or 

 Fort William. The gorge of the Mpigon river from north of lake Jessie 

 is cut through one of these immense sheets, and the less well known but 

 more picturesque canyon which forms Pijitawabikong bay, a few miles 

 east of the Nipigon river, is also cut through the same sheet. 



Usually these trap sheets are nearly horizontal and of great extent. 

 The largest single continuous area, so far explored, lies in the basin of 

 lake Mpigon, south and southwest of the lake itself. The sheets occur 

 either as sills from 4 to more than 50 feet in thickness, intercalated within 

 the sandstones, shales, or dolomites, or in the form of capping sheets 

 from 12 to more than 500 feet in thickness. These caps stand at the 

 summit of the local stratigraphic column and their upper surface usually 

 is a tableland or mesa. 



PETROGRAPHY OF THE TRAPS 



As to the petrographic characters of the trap sheets it may be noted 

 that diabase is by far the most abundant rock, olivine being present fre- 

 quently in large amount. Structurally it may pass from a typical 

 diabase, the prevailing rock, to a coarse gabbro or olivine gabbro, or 

 locally to a fine grained porphyrite. The ophitic structure is the most 

 prominent and widespread. The texture, except at the contacts with the 



