BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 20, pp. 197-222 June 5, 1909 



TRAP SHEETS OF THE LAKE NIPIGON BASIN 1 



BY ALFRED AY. G. WILSON 



(Read before the Society December 31, 1908) 



CONTENTS 



Tage 



Introduction and general review 198 



Distribution of the traps 108 



Petrography of the traps 198 



Views as to their origin 200 



Scope of the present discussion 201 



Descriptions of critical areas 202 



In general 202 



Gorge of the Nipigon river 202 



Tchiatang bluff 203 



Spruce River gorge 204 



Nipigon House outliers 205 



Wabinosh valley 205 



Ombabika narrows 206 



Possible remnants of old soils in situ 207 



Discussion of the evidence 209 



Surface flows versus intrusions 209 



Unconformities of three types 211 



General characteristics 211 



Examples of each type 211 



Discussion .213 



Columnar structure 215 



Summary of the evidence available 215 



In favor of intrusive sills 215 



In favor of surface flows 210 



Balancing the evidence 21 <; 



Source and nature of the flows 2^ ~ 



Character of the preexisting topography and date of denudation 219 



Distribution of the diabase 220 



Summary of the post-Cretaceous geologic history 222 



Published by permission of the Director of the Geological Survey of Ganada. 

 Manuscript received by the Secretary of the Society February 2, 1909. 



XVII— Bull. Geol. Soc. Am., Yol. 20, 1908 (197) 



