320 H. C. COOKE 



between the tuffs and the Timiskaming series. About 5 miles to 

 the west, however, in Otto Township, where the tuffs approach the 

 Timiskaming most closely, there is a width of 8,250 feet between 

 the base of the Timiskaming and the base of the band of tuffs. 

 The sum of these two numbers, 8,500 and 8,250, therefore repre- 

 sents the minimum width of the Keewatin between the center of 

 the anticlinal fold and the base of the Timiskaming. Assuming an 

 average dip for the Keewatin of 60 degrees, which is smaller than 

 the average measurement, we arrive at a minimum thickness for 

 the Keewatin of approximately 15,000 feet. 



Overlying the Keewatin there was at least 3,600 feet of 

 Timiskaming series so that there were at least 18,600 feet of strata 

 in all removed by erosion before the deposition of the Cobalt 

 series. The actual amount may have been much more than this, 

 however. 



AGE 



The descriptions of preceding sections show definitely that the 

 Timiskaming series is post-Keewatin and pre-granitic in age, and 

 antedates the Cobalt series by a period sufficient for the penepla- 

 nation of a mountainous area, with removal of at least 18,600 feet 

 of rock. Its age may, however, be more closely delimited by the 

 following consideration. 



The Cobalt series outcrops in an almost continuous sheet from 

 Larder Lake southwest to the north shore of Lake Huron. 

 Throughout this whole section it lies in gentle open folds, with 

 dips rarely exceeding 20 degrees, and rests on a correspondingly 

 gently warped peneplain that bevels closely folded rocks cut by 

 granitic intrusives. Near Lake Huron, however, this peneplain is 

 directly overlain by the Bruce series, and the Cobalt series over- 

 lies the Bruce. There is an erosional unconformity between the 

 Cobalt and Bruce series, representing a time interval sufficient 

 for the removal of 1,700 feet of sediments,^ but little or no structural 

 unconformity; so that both series, where not disturbed by the 

 Keewanawan folding and intrusion, lie in the same open gentle 

 folds as the Cobalt to the north. It seems evident therefore that 



^ W. H. Collins, Geol. Surv. Can., Museum Bulletin No. 8, 1914. 



