THE LA URENTIA N PEAEPLA IN 653 



enough to determine whether AB and BC represent two inter- 

 secting plains, or are portions of a continuous curve. It may also 

 be noted that the data given above, taken in conjunction with 

 the progressive overlap of higher and higher geologic horizons 

 upon the peneplained surface, as one proceeds toward the north- 

 west, suggest that there are even more than two peneplains. 

 Each plain may be regarded as a facet cut upon the Canadian 

 shield. During the long interval of Paleozoic time a succession 

 of these facets intersecting at very low angles would be formed 

 as the land slowly sank and the successive formations were laid 

 down. The great central part of the area upon whose surface 

 the remarkably low gradients have been observed would repre- 

 sent the youngest of the facets, the region upon which the 

 planation processes had acted longest. The partial removal of 

 the Paleozoic covering has already exposed several of the older 

 facets. 



Similar relations to those noted above as occurring in Ontario 

 between two plains of degradation upon the crystalline rocks 

 have been found by Van Hise in Wisconsin (38), and by Smyth 

 in the region south of Lake Superior (31). Darton has described 

 a somewhat similar case in Virginia (10). In the Grand Canyon 

 of the Colorado we have an actual transverse section of two such 

 intersecting plains, both older than the Cambrian, but meeting at 

 a much higher angle than that noted for the two plains near the 

 margin of the Canadian shield. 



SUMMARY. 



From a physiographic standpoint the Canadian shield can be 

 described as an ancient peneplain which has undergone differ- 

 ential elevation; has been denuded, and subsequently slightly 

 incised around the uplifted margin. At several places on the 

 margin, as exposed today, the dissection may be regarded as sub- 

 mature. Around the southern margin between Montreal and 

 Winnipeg there are traces of a peneplain (or probably more than 

 one) of still earlier date, upon which Paleozoic sediments were 

 laid down, and which has been uncovered by processes of degra- 

 dation and denudation since the differential uplift of the latest 



