i6 W. M. DAVIS 



the name of a particular residual mountain that has come to be 

 used for such mountains in general. 



It may be added that young, mature, and old seem to be best 

 used in connection with features that persist, but with appropriate 

 changes, all through a cycle, such as forms of relief, rivers, and shore 

 lines; and not with features of shorter endurance, like lakes and 

 waterfalls, which are characteristic only of a part of a cycle. 



As to terms for rivers and valleys, "antecedent," "consequent," 

 and "superimposed" are due to Powell; the last was shortened to 

 "superposed" by McGee. "Subsequent," which Jukes used in a 

 descriptive sense, I have used similarly in a restricted and technical 

 sense for streams that have grown headward by retrogressive erosion 

 along belts of weak structure, and also for streams which, having 

 been thus developed in one cycle, persist in the same courses in a 

 following cycle. It may be noted that, while consequent rivers and 

 valleys commonly exhibit a bilateral symmetry, subsequent rivers 

 and valleys as commonly do not; also that subsequent streams are 

 very commonly found in pairs on opposite sides of a consequent 

 stream, thus constituting characteristic elements of its bilateral 

 symmetry. McGee's "autogenetic" I have replaced by "insequent," 

 in order to bring it into the sequential group of terms, to which I 

 later added "obsequent" and "resequent." 



It is worth noting that the terms of the sequential series may 

 be used for other features than streams and valleys. Thus an 

 anticlinal arch may be called a consequent ridge as long as it 

 retains its axial eminence. When it is breached along the axis by 

 the excavation of weak strata underlying a hard cover, so that 

 the initially single crest is divided by a subsequent anticlinal valley 

 into two lateral crests, they. may be called subsequent ridges; 

 their outer slopes are still consequent slopes; their inner slopes 

 are obsequent slopes. If the continuation of erosion reveals along 

 the axis of the breached anticHne another anticlinal arch of hard 

 strata, it will gain rehef as the first-developed anticHnal subsequent 

 valley is divided into two lateral or monoclinal subsequent valleys; 

 the anticlinal ridge between them is then a resequent ridge. The 

 Appalachian mountains of Pennsylvania and Virginia give abundant 



