igoo-i.] Physical Geology of Central Ontario. 155 



to describe the sky-line of the upland plain, where for long distances, so 

 far as the eye can judge, it appears with no marked irregularity. In 

 many parts of the area the surfaces of large lakes offer horizontal lines 

 for comparison. Occasional irregularities occur, and these frequently 

 make abrupt changes in the otherwise even line. Upland plain has 

 been used to indicate that imaginary surface whose elevations accord 

 with the elevations of the even sk\'-lines, as seen in many parts of the 

 area. Upland indicates portions of the present land surface whose 

 elevation accords closely with the upland plain, and whose surface 

 presents only minor irregularities, as compared with the greater irregu- 

 larities of the surface of the region as a whole (figure 7). The present 



Figure 7. — Diagram to illustrate the definitions of terms. 



topography is such that although the slopes are frequently graded, there 

 are few areas to which the corresponding term lowland should be 

 applied. The change in gradient from the valley side to the upland is 

 frequently so marked as to justify the use of the term shoulder to 

 describe the place where the change occurs. 



Third Problem, Conditions of Erosion. — Two hypotheses 

 have been offered to account for the origin of topography of this nature. 

 The one would consider it as the product of a single cycle, the other as 

 the product of two or more (n-f-i) cycles. The first, the "bevelling" 

 hypothesis, would consider the present features as those of an ancient 

 mountain system reduced to maturity and possibly re-elevated and 

 made more rugged. The second would consider that the even uplands 

 (produced during a long interval of time, at a period when the land 

 stood relatively near base level) are remnants of the upland plain, and 

 that the present valleys and lowlands were due to an increased activity 

 of the agencies of degradation and denudation because of subsequent 

 elevation. 



If the area is part of an old mountain system reduced to its present 

 form by bevelling, the present elevations must have once been higher 

 and more rugged than they now are (figure 8, ABCDEF). In the 

 process of degradation the ruggedness would be reduced and the slopes 

 become graded by the removal of waste from the mountain sides and 

 its transportation to the valleys, where it would either remain or be 



