University of California. 



[Vol. 2. 



of the rock as regards its resistance to weathering. The angle 

 varies further with lack of homogeneity in the rock, with variation 

 in the structure and attitude of the rocks, and with differences in 

 climate and in amount and kind of vegetation. 



If the stream increases its rate of cutting, the part of the slope 

 next the stream has its angle increased, and this increase of angle 

 is gradually extended until the summit is reached, and the erosion 

 over the entire surface of the slope is once more uniform and 

 equivalent to that of the stream. If the rate of cutting of the 

 stream is decreased, and consequently that at the base of the slope, 

 the upper parts and summit, which are at first eroded more rapidly 

 than the stream is cutting its channel, will soon be so reduced in 

 their elevation above the stream, and the angle of slope, conse- 

 quently, so decreased, that equilibrium will be restored. 



A graded slope once established tends to maintain itself, and to 

 be lowered only as the stream bed is lowered. If the stream is 

 cutting rapidly, the summit is lowered rapidly; if the stream cuts 

 slowly, the summit is lowered slowly, and the angle of the graded 

 slope is less than in the former case. If the stream cuts at a 

 uniform rate, the summit is lowered at the same rate, and the slope 

 is maintained at a uniform angle. It would follow that in old age, 

 when the streams cut very slowly, the angle of the graded slopes 

 will be lower than in the earlier history, when the cutting was more 

 rapid, and the summits, therefore, will not be so far above the 

 stream beds. 



Differential Erosion. — Since soft rocks are eroded more readily 

 than hard ones, it would be necessary, in order that there should 

 be equality of erosion, on a given slope, in hard and soft rocks, that 

 the angle of slope be less in the soft rocks, and greater in the 

 hard ones. It is generally assumed that differential erosion always 

 takes place in areas containing both hard and soft rocks; but if 

 graded slopes are free to develop, then the difference in rate of 

 erosion between the hard and soft rocks will be compensated for, 

 to a greater or less degree, by difference in the angles of the slopes. 

 Such differences in slope, due to difference in resistance to erosion 

 of the various rocks, may be observed almost anywhere in moun- 

 tainous regions. An illustration is shown in Fig. i, where the hills 



