igoo-i.] Physical Geology of Central Ontario. 145 



to the north. The surface is too irregular to justify any general 

 comparison. In the area studied, that portion lying east of a line 

 running a little to the east of north from the west side of Balsam lake 

 is inclined towards the southeast. West of this line the inclination is 

 towards the west. 



The meagre data available thus indicate that beneath the sediment- 

 ary cover towards the western end of the area the average gradient is 

 more than double that of the uncovered portions, while at the eastern 

 end it is about four times as great. Near the western end, the gradient 

 from the summit over the crystallines to the Black River escarpment, 

 and over the surface of the Palaeozoic strata, is nearly the same. This 

 gradient is less than half the average gradient of the northern side of 

 the basin of Lake Ontario (23.7 feet). 



The relative positions of the three plains suggest certain problems 

 which may be summarized thus : — 



1. Do these three plains represent three distinct periods of 

 planation ? 



2. Are AB and BC of the same age, but now discordant by warping ? 



3. Did the plain AB formerly extend upward in the direction BF ; 

 is BC of the same age as DB, or is it younger? 



4. Is the discordance between AB or DB and BC produced by 



warping ? 



The accordance of the plains DB and BC towards the western end 

 of the area is suggestive of warping elsewhere. Data of a detailed 

 character as to the gradient upon the uncovered crystalline areas and 

 upon the sedimentary outliers of the plain AB between the point 

 represented by B and the front of the escarpment a have not been 

 obtained. Without them the evidence available is inadequate to solve 

 the problems. 



It should be added that the relative arrangement of the three plains, 

 represented as meeting in a broad angle at B, is purely fortuitous. The 

 data in hand are not enough to determine whether AB and BC represent 

 two intersecting plains, or portions of a continuous arc, and whether all 

 three have a common point of intersection. 



Similar relations between two plains of denudation upon crystalline 

 rocks, meeting at low angles, have been found by Van Hise in Wiscon- 

 sin ('96), and by Smyth in the region south of Lake Superior ('99). 



