PREEXISTING TOPOGRAPHY AND DATE OF DENUDATION 219 



that, so far as accessible contacts are concerned, they are probably parts 

 of sheets. Of course nothing is positively known about their cores, but 

 one is justified in assuming uniformity on the Archean topography. In 

 many cases, between the detached remnants of sheets or between these 

 remnants and the main mass of diabase, are wide dikes now unconnected 

 with any sheets, but of similar rock. These dikes probably mark some 

 of the channels through which the molten diabase ascended. 



The diabases are so widespread that we must infer that they were in- 

 truded at many different points, either simultaneously or successively. 

 While only a few possible points of intrusion are known, we can infer 

 the existence of others. Still it must not be forgotten that in the Snake 

 River fields flows of between 50 and 60 miles are recorded for lavas 

 which solidified into basalt. Where the lava was fluid (as is indicated 

 by the coarse crystalline structure and absence of flow structure) and 

 remained quiescent for a long time subsequent to extrusion, and where 

 the outflow was so great that even the small remnants of the sheets now 

 left show a minimum thickness of over 600 feet for a belt more than 60 

 miles in length — under such conditions, very few openings would suffice. 



Character of the preexisting Topography and Date of 



Denudation 



Previous to the extravasation of the diabase the section of country 

 lying south of the present lake Nipigon was underlain largely by Ke- 

 weenawan sedimentary rocks, with here and there exposures of older 

 crystallines. In the basin of lake Mpigon and northward there were 

 numerous sedimentary remnants, outliers upon the Archean. The 

 Archean itself presented that undulating and mammillated topography 

 so characteristic everywhere along the margin between the sedimentaries 

 and the earlier rocks. 



In fact, the topography was that of a belted coastal plain with the 

 Archean oldland to the northeast with numerous sedimentary outliers in 

 the basin of lake Nipigon. The southwest part of the present basin 

 represents a portion of the inner lowland. The main area of sediments 

 lay south and southwest of the lake, and the first cuesta, that facing the 

 oldland, can be traced from near the Canadian Pacific Railway line, lying 

 to the west of the present Black Sturgeon valley, and continuing north- 

 west to a point about 20 miles southwest of Nipigon House, where it 

 turns southwestward again. The present Black Sturgeon river runs 

 along the lowland in front of the cuesta to within a few miles of the 

 present Canadian Pacific Railway bridge, where it turns and enters the 

 cuesta, crossing to lake Superior through a gorge similar to that of a 

 consequent stream. 



