172 



University of California. 



[Vol. 2. 



lowering the summits of the adjacent slopes, and as the latter is 

 replaced by the former, the adjacent graded slopes are able to 

 maintain a similar angle. 



If, on the whole, the rate of vertical corrasion decreases pro- 

 gressively as one goes down stream (see page 165), till it reaches 

 zero at sea-level, then, if the cutting were wholly vertical, with 

 decrease in its rate, the adjacent slopes must present decreased 

 angles, other things being equal. With decrease in the angle 'of 

 slope there must be also a decrease in the vertical distance between 

 the stream channel and the adjacent ridges ; thus the altitude of 

 the ridges will decrease more rapidly than the altitude of the stream 

 beds. When the vertical is replaced, in greater or less degree, by 

 lateral cutting, the altitude of the divides tends to decrease still 

 more rapidly. For since, as shown above, the lateral cutting is as 

 effective as the vertical in lowering the summits, the result is the 

 same as if the stream channel continued to be deepened by vertical 

 cutting, and the altitude of the divides followed the descent of this 

 hypothetical stream bed. 



Professor Davis makes the criticism that " tilting has no an- 

 nounced place in Professor Tarr's theory;"* but there is no neces- 

 sity for introducing tilting if the features in question can be as well 

 explained without it. ' An application of the principles already 

 given will, I think, sufficiently account for the relation of upland 

 and stream profile, which is commonly ascribed to tilting of the 

 region. Assuming for purposes of discussion that "a rough 

 equality of mountain heights" has been established, then, "the 

 master streams having gained a graded slope," with the erosion on 

 the slopes following the cutting of the streams, we might expect, 

 other tilings being equal, to find the elevation above the stream bed 

 of all points along the divide roughly the same. But it has been 

 already shown that as between the lower and upper stretches 

 (" coast " and " interior " regions) of a stream, other things are not 

 equal. " The streams are larger and the valleys are necessarily 

 lower and broader near the coast than in the interior," f and "the 

 interstream uplands are," therefore, not " under essentially similar 



* Loc. cit., p. 237. 

 t Ibid. 



