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



The ancient belted coastal plain of North America, so well and charac- 

 teristically developed nearly continuously around the whole convex south- 

 ern, southwestern, and western border of the Archean oldland, is usually 

 considered to be of post-Cretaceous age, it being inferred that the dis- 

 section by which the belted features of the topography were developed 

 took place subsequent to that long period of post-Paleozoic planation 

 which continued to the close of the Cretaceous. 



As the Nrpigon belted coastal plain — only a mere remnant clinging to 

 the edge of the Laurentian peneplain, it is true — lies midway between 

 the coastal plain of the Great lakes of the Saint Lawrence system and 

 the similar coastal plain of the Mackenzie and Saskatchewan river sys- 

 tems, it seems not unreasonable to suppose that the conditions which were 

 so uniform and similar on all sides of this area (for a similar belted 

 coastal plain seems also to have developed north of the Archean oldland 

 in the Hudson Bay basin) should have also prevailed here, and that the 

 Mpigon coastal plain remnant may also be of post-Cretaceous age. 



The incursion of diabase, judging by the nature of the contacts and 

 from the character of the pre-diabase dissection and relief, took place at 

 a time when the relief was at its maximum development. Xumerous 

 outliers lay in front of the main cuesta on the newly uncovered lowland, 

 and remnants of many of these outliers are still preserved as lava-capped 

 mesas standing well out on the lowland or even on the oldland. Had it 

 not been for the protecting lava cover, it is very probable, in view of the 

 enormous amount of erosion that has certainly taken place since the ad- 

 vent of the diabase, that the number and area of these outliers would be 

 much less. In fact, it is quite possible that little, if any, of the Ke- 

 weenawan sediments would have been preserved north of lake Superior. 



Distribution of the Diabase 



The main sheet of diabase occupies the ancient lowland depression of 

 the Xipigon coastal plain, and roughly forms a crescentic mass bounding 

 lake Mpigon on the south, southwest, and west. "With small gaps where 

 subsequent dissection has curved deep valleys/ in the bottoms of which 

 the earlier rocks are exposed, it extends in a crescent form from Gull 

 lake on the east to beyond the Wabinosh river on the west. Southward 

 and southwestward remnants of the sheet form mesa caps on manv of 

 the prominent ridges found scattered widely over the area on which the 

 main mass of the sediments is still preserved, between lake Mpigon and 

 lake Superior. Northward, practically the whole of the basin of the 



8 Pijitawabikong bay, Nipigon River gorge, Black Sturgeon passage, and several minor 



