202 A. W. G. WILSON TKAP SHEETS OF LAKE NIPIGON BASIN 



a matter of inference only. Possibly a few of the "caps" belong to the 

 first great group of sheets in which the character of the upper surface 

 can be directly inferred from evidence available. The greater number of 

 caps exhibit no direct evidence as to the character of their upper surface, 

 and it is with these that the present discussion deals. 



Negatively they exhibit none of those characteristics which are usually 

 associated with volcanic flows — there are no pyroclastics, they are never 

 either glassy or amygdaloidal, never exhibit ropy or wrinkled surfaces, 

 have no associated lava breccias, and the nature of the contact with the 

 underlying rocks shows that the latter were hard and brittle before traps 

 came into the area. So far as negative evidence is concerned, they ex- 

 hibit none of those features which are usually considered as characteristic 

 of volcanic flows. 



Descriptions of critical Areas 

 in general 



The nature of the basal contacts of these sheets and the general char- 

 acter of the sheets can be best understood from the study of a selected 

 few from a very much larger number of concrete examples. The exam- 

 ples which are cited in the succeeding paragraphs are only a few typical 

 instances in which the relations of the traps to the adjacent rocks are 

 clearly depicted. They have been selected because they are in localities 

 readily accessible and because they are all, with the exception of Red 

 Eock, within the Mpigon basin and are directly associated with the prin- 

 cipal area of diabase. 



(JORGE OF THE NIPIGON RIVER 



At Island portage, on the Mpigon river, about 5 miles above lake 

 Jessie and in the heart of the gorge, Archean gneisses are exposed in the 

 channel of the river. On the east there is a high cliff of gneiss rising 

 about 350 feet above the river, which has, I believe, usually been mistaken 

 for diabase, as it is nearly continuous with the diabase cliffs which swing 

 in from the east at lake Jessie and continue up the east side of the Mpi- 

 gon gorge almost to lake Mpigon. On the west side of the river, at 

 Island portage, is a coarse pegmatitic granite rising about 250 feet above 

 the water. 



The area of Archean here exposed in the gorge of the Mpigon, com- 

 pletely surrounded and partly capped by diabase, is about 4 square miles. 

 At lake Jessie the diabase is found nearly 300 feet lower, with possibly 

 not more than 100 feet of sediments between it and the underlying 

 Archean. The south edge of this sheet has been traced from a point 



