656 ALFRED W. G. WILSON 



with the second problem which suggests itself: What is the 

 origin of the peneplain surface as to the time and process? At 

 present it is quite certain that the evidence available is inade- 

 quate to solve the problem. 



2 . The siibaerial origin of the pe?ieplai?i and the date of origin of 

 the principal plain. — As already noted, the stratigraphic studies 

 of Walcott, Schuchert, Ulrich, and others have shown that at 

 the beginning of Ordovician time the greater portion of the 

 Canadian shield was above sea-level, and that since early Cam- 

 brian time it has never been wholly submerged. The work of 

 Laflamme, Adams, Lawson, and others has shown that along the 

 southern margin of the peneplain from Lake St. John to Lake 

 Winnipeg, where the plain passes beneath the Ordovician sedi- 

 ments, it possesses the same hummocky character and fresh rock 

 surface as it does in districts remote from the margin. Between 

 the farthest outliers (exclusive of the outlying areas which may 

 owe their preservation to the downthrow of fault blocks) there is a 

 belt of the uncovered presedimentary surface, which shows all 

 the features everywhere characteristic of the modified peneplain. 

 Hence not only must the portion of the plain adjacent to the 

 areas now overlaid by Paleozoic sediments be of pre-Ordovician 

 date, but the dissection which it has undergone must also be 

 pre-Ordovician. The dissection is regarded as of different date 

 from the planation, because the gradient curves of the valley 

 sides are not accordant with the flatter curves of the surface of 

 the plain, often meeting the latter at an angle. The valleys are 

 regarded as younger than the plain, because in no single instance 

 do they contain sediments that have been derived from the 

 adjacent plain. Along the margin of the plain their filling is 

 always of the same materials as that with which the plain itself 

 is covered — usually Ordovician limestone. The forms of the 

 valleys and their relations to the plain are such that they would 

 not be attributed to marine erosion. 



During the transgression of the Ordovician sea it is conceivable 

 that the earlier surface of subaerial origin may have been 

 replaced by one of marine planation, assuming a time interval 

 long enough. It is now generally admitted that during a time 



