212 A. W. 0. WILSON TRAP SHEETS OF LAKE NIPIGON BASIN 



the most striking of Lawson's illustrations is reproduced here. 6 The un- 

 conformity in the gorge of the Spruce river also belongs to this group. 



The explanations offered by Lawson and Ingall on the one hand and 

 by the writer on the other, so far as they apply to these major uncon- 

 formities, are diametrically opposite. In so far as these unconformities 

 alone are concerned, the writer's verdict would be "not proven;" but 

 when the various other unconformities are also considered, the balance 

 of evidence seems to be strongly in favor of the latter explanation. 



Contacts of the second class are numerous all around lake Mpigon. 

 One of the most striking, that at Ombabika narrows, has been described 

 in some detail, and attention has been called to the inclusion of boulders 

 of the underlying rock in the diabase. Another example from near 



Figure 4. — Diagrammatic Section near the Extremity of the Point between Big Trout 



and Pigeon Bays 



Showing the relation of the trap sills to the Animikie strata. (After Lawson) 



Mpigon House has also been cited and the occurrence of included boul- 

 ders noted. There are also a number of other localities where masses of 

 diabase, covering areas varying from about 1 acre to over 10 square 

 miles, show actual contacts with an eroded Archean surface, and in some 

 instances they contain detached blocks of Archean rocks similar to the 

 bedrock on which they rest. The diabase masses occur on the tops of 

 ridges and on the sides of valleys cut in the Archean (as at Wabinosh 

 river, Kipigon House, and Tchiatang bluff) as well as in the bottoms of 

 valleys. Archean valleys with a relief of a minimum of 150 feet are 

 found to have masses of diabase located on their sides. 



The contacts of some of the smaller remnants of diabase sheets and 

 the underlying rock show in detail the nature of the surface over which 

 the diabase flowed and show that it was very intricate. There are many 

 instances in which it can be shown that what appear to a traveler in the 



6 Lawson : Op. cit., figures 1, 2, 3, 4, and 5. 



