. THE LA URENTIA N PENEPLA IN 6 5 I 



impossible to say. The character of the walls and the occurence 

 of Silurian sediments within the basin certainly suggest that it is 

 of the graben type. 



THE PENEPLAIN FACETS. 



The relations of the Paleozoic sediments around the southern 

 border of the plain to the peneplain surface suggest several 

 interesting problems. In a former paper (41). the writer drew 

 attention to the fact that there seem to be some grounds for 

 surmising the existence of at least two peneplains cut upon the 

 Archaean rocks and meeting at a very low angle. The data on 

 which this suggestion was based are in part repeated here, with 

 the addition of a few facts since obtained which bear on the 

 same subject. 



In the earlier part of this paper attention was drawn to the 

 remarkably even sky-line exhibited by the plain everywhere in 

 the interior, and it was mentioned that the average gradient in 

 the interior rarely exceeded four feet per mile. In the central 

 part of the province of Ontario it is found that radially from the 

 upland surface in the vicinity of Haliburton Lake (the highest 

 point in this part of central Ontario) to a number of points 

 along the base of the Ordovician escarpment between Georgian 

 Bay and Kingston, the average gradient over the surface of the 

 peneplain is nearly nine feet per mile. At Toronto a well boring 

 has shown that the crystallines are about eleven hundred feet 

 beneath the present surface. Two other borings — one at 

 Cobourg and the other in the township of South Fredericksburg — 

 indicate that the floor is over five hundred and over six hundred 

 feet respectively below the surface at these localities. The 

 average gradient beneath the sedimentary cover along a series of 

 lines from the foot of the Ordovician escarpment to the bottoms 

 of these borings indicates that the gradient beneath the cover 

 varies from twenty-two feet per mile in the western portion of 

 the district to over forty-one feet per mile at the eastern end. 

 The relative attitudes of these two surfaces are represented in 

 Fig. 14, where AB represents the edge of a cross-section of the 

 plain beneath, and BCt\\Q edge of a cross-section of that outside 

 the sedimentary area. Such a comparison is quite justifiable 



