. Bulletin of the Geological Society of America. Geology. 212 A. W. 0. WILSON TRAP SHEETS OF LAKE NIPIGON BASIN the most striking of Lawson's illustrations is reproduced 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 conside
. Bulletin of the Geological Society of America. Geology. 212 A. W. 0. WILSON TRAP SHEETS OF LAKE NIPIGON BASIN the most striking of Lawson's illustrations is reproduced 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
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