EROSION AND THE SUMMIT LEVEL OF THE ALPS 7 



ated. (It is noteworthy that, in spite of Passarge's dictum as to the 

 futihty of deducing the late stages of an erosion cycle under the 

 assumption of unchanging cHmate — because in his beHef climate 

 changes repeatedly during a period so long as a cycle of hard-rock 

 erosion — Penck nevertheless tacitly postulates a uniform chmate. 

 It would be better to say regarding the old age of a cycle that 

 downward erosion is more and more retarded than that it ceases, 

 for there must still be some downward erosion as long as the streams 

 have to lower their grades in order to adjust them to the decreasing 

 discharge of waste from the lowering swells. Gilbert has shown 

 that, for a far inland region, such as the plains of central Colorado, 

 not only the valley floors but the whole surface may be lowered 

 hundreds of feet as a result of continued but very slow downward 

 erosion by the streams even after the stage of peneplanation is 

 reached.) 



The most significant stage of this ideal cycle is held to be the 

 intermediate one in which the sharp ridge crests maintain a constant 

 absolute altitude as well as a constant relief, in consequence of a 

 balance having been struck between the rate of upheaval and the 

 rate of deepening the larger valleys ; but the duration of this stage 

 is shorter than that of the sharpened crests, which persist in the 

 preceding stages while crest altitude is increasing, as well as in 

 the following one while it is decreasing. For these three stages, 

 in which the sharpness of ridge crests is maintained, my term 

 "mature" as originally defined, seems appropriate, in view of the 

 maximum strength and variety of relief then acquired by the 

 mountain mass with its steep but graded slopes ; but Penck discards 

 the German equivalent, reif, of that term which he had previously 

 used, and speaks in paraphrase, "von einem ausgewachsenen 

 Gebirge mit dem Schneidenstadium der Entwickelung, welches 

 ein Gegenstiick zum Schluchtstadium der Taler darstellt, aber 

 von kiirzerer Dauer ist." (That the mature stage of sharpened 

 ridges with graded slopes down to the streams should be of shorter 

 duration than the young stage of flat interstream uplands and 

 rock-walled gorges does not seem to be proved.) 



Second and third ideal cycles. — In the second ideal cycle, strong 

 upheaval of short duration, and hence of moderate total amount, is 



