GENERAL FEATURES OF A PENEPLAIN 589 



A peneplain formed in a region of varied structure will be first devel- 

 oped in areas of weak rocks, wherever they are situated; it will be later 

 developed in areas of moderately resistant rocks; and by that time 

 the weak-rock areas may be degraded to true plains. The most resistant 

 rocks will long survive as knobs or ridges, now commonly known as 

 monadnocks. 



Subsequent Streams and Valleys 



The development of subsequent streams along belts of weak structure 

 and the corresponding diminution of consequent streams is a character- 

 istic feature of a well advanced cycle of erosion; and while subsequent 

 streams are developing along belts of weak structure consequent divides 

 will be largely replaced by subsequent divides on belts of resistant struc- 

 ture. But be it noted that the physiographic value of these two stream 

 terms does not lie so much in the indications that they give of stream 

 and valley origins as in the suggestions that they offer regarding the 

 relation of the streams and valleys to their surroundings. A consequent 

 stream or valley following the original slope of a body of inclined strati- 

 fied rocks will usually have similar rocks and similar forms on both 

 sides; but subsequent streams and valleys are usually characterized by 

 different rocks and by different forms on the two sides; and, further- 

 more, in a region of slanting structure, where a master consequent is 

 joined by one subsequent stream, it will usually be joined also by another 

 subsequent stream coming from the opposite direction, both subsequents 

 being developed on the same tranverse weak belt. 



It is curious to note that the term subsequent, which has been proposed 

 to name a class of streams and valleys in southern Ireland clearly 

 described by Jukes in 1862, has not gained general acceptance among 

 American geologists and physiographers, not even among those who 

 adopt Powell's trio of 1875 — consequent, antecedent, and superimposed 

 (shortened by McGee to superposed). Yet it has been pointed out that, 

 by reason of Powell's failure to extend the trio to a quartet by adding 

 subsequent as a fourth member, he was led into serious errors in the inter- 

 pretation of the streams of the Uinta Mountains ; and that Dutton made 

 similar errors in the region of the Colorado Canyon of Arizona for the 

 same reason. Even Gilbert referred to Powell's trio as if it contained 

 a complete genetic classification of streams. Whether the name "subse- 

 quent" should be used for the fourth member of the group is unim- 

 portant; but that no fourth member should be recognized is a serious 

 mistake. The spontaneous development of subsequent streams in regions 

 of tilted, strong and weak strata is an essential process in the advance 



