THE WORK OF RUNNING WATER. 165 



it is hase-leveled} At all earlier stages its crest is uneven. After the 

 cycle represented by the remnants a, a', and a'' was completed, the 

 region suffered uplift. A new cycle represented by the plain h, V , 

 and h'^ was well advanced, though not completed, when the region 

 was again elevated, and the rejuvenated streams began to cut their 

 valleys d, d' , and d'^ in the plain of the previous incomplete cycle. 

 The elevations, c and c' (intermediate in elevation between a, a' , and 

 a", and h, V , and V) may represent either remnants of the first base- 

 level plain which were lowered, but not obliterated, while the plane 

 6, V , h" was developing; or they may represent a cycle intermediate 

 between that during which a, a', a'^ and h, h'', h" v/ere developed. If 

 the intermediate elevations (c, c) have a common height and level 

 crests, the presumption would be in favor of the latter interpretation. 

 If they be numerous and of varying heights, as is possible, they may in 

 the field obscure the planes (a, a', a" and h, h' , h^') developed in the 

 different cycles, which, in the figure, are distinct. 



If the strata involved be horizontal the determination of cycles 

 is sometimes less easy. Thus in Fig. 156, it is not possible to say whether 

 a and a' represent remnants of an old base-level, or whether they repre- 



FiG. 156. — Diagram to illustrate cycles of erosion where the beds are horizontal. 



sent the original surface from which degradation started. So, too, 

 the various benches below a, such as h, h' , and h" may readily be the 

 result of the superior hardness of the beds at this level. For the de- 

 termination of successive uplifts in the field it is necessary to consider 

 areas of considerable size, and to eliminate the topographic effects of 

 inequalities of hardness, and of certain other factors to be mentioned 

 presently. 



The inequalities in the depths of the young valleys in Figs. 155 

 and 156 may be explained on the supposition that the deeper ones 

 belong to main streams, and the shallower ones to tributaries. Such a 

 valley as that shown at e, Fig. 155, suggests rejuvenation at this point; 

 but farther up the stream which occupies this valley, rejuvenation 



* Another view has been advocated by Tarr, Am. Geo!. Vol. XXT, pp. 351-370. 



