570 MARIUS R. CAMPBELL 



quently the history of this portion of the cycle is recorded, not 

 alone in the sculptured forms of the land, but also in the 

 changed alignment of the streams. 



As the stream approaches extreme old age, its gradient grows 

 less and less, and the divides between adjacent streams become 

 so low that many adjustments are required before a perfect bal- 

 ance prevails between the contending streams. This is due to 

 the fact that in this portion of the cycle the streams are but 

 slightly prepared to defend themselves, and any stream handi- 

 capped by a circuitous route to the sea will eventually suffer loss 

 of drainage area at the hands of a neighboring stream. 



It would be almost impossible to read the history of drainage 

 developments, unless we could go back for our beginning to 

 some period in which drainage conditions are known. During 

 its period of youthful development, a stream leaves but few 

 traces by which, in after years, we may judge of the extent of 

 its drainage basin, or of the conditions under which it labored ; 

 during its maturity its records are equally unintelligible, for they 

 tell us nothing of the local conditions which surrounded it ; but 

 in its old age we can say with confidence that, so far as it is 

 untrammeled by local obstacles (and local obstacles are rare in 

 this stage of erosion), it is evenly balanced against the surround- 

 ing streams. If then we find traces of a stream having reached 

 old age, we can calculate with reasonable certainty the extent of 

 its basin, or if its basin does not extend to the limit which 

 should mark the contending streams, we may be assured that 

 local obstacles interfered with its normal development. Thus 

 the last stage of the life history of a river implies certain phys- 

 ical conditions, consequently it is the period to which we must 

 refer in undertaking to read the history of drainage develop- 

 ments. 



Class 2. Adjustments due to rock character and geologic structure. 

 — Drainage modifications which fall within this class have 

 received the attention of our ablest physiographers, hence their 

 mode of origin is well understood. Gilbert has shown how 

 streams, flowing on inclined beds of alternating hard and soft 



