214 National Geographic Magazine. 



20. Opportunity for new adjustments vnth revival. — One of 

 the most common effects of the revival of a river by general ele- 

 vation is a new adjustment of its course to a greater or less 

 extent, as a result of the new relation of baselevel to the hard 

 and soft beds on which the streams had adjusted themselves in 

 the previous cycle. Synclinal mountains are most easily ex- 

 |ilained as results of drainage changes of this kind [Science, Dec. 

 2 1st, 1888]. Streams thus rearranged may be said to be adjusted 

 through elevation or revival. It is to be hoped that, as our study 

 advances, single names of brief and appropriate form may 

 replace these paraphrases ; but at present it seems advisable to 

 keep the desired idea before the mind by a descriptive phrase, 

 even at the sacrifice of brevity. A significant example may be 

 described. 



Let it be supposed that an originally consequent river system 

 has lived into advanced maturity on a surface whose structure is, 

 like that of Pennsylvania, composed of closely adjacent anticlinal 

 and synclinal folds with rising and falling axes, and that a series 

 of particularly resistant beds composes the upper members of the 

 folded mass. The master stream. A, fig. 19, at maturity still 

 resides whei*e the original folds were lowest, but the side streams 

 have departed more less from the axes of the synclinals that they 

 first followed, in accordance with the principles of adjustment 

 presented above. The relief of the surface is moderate, except 

 around the synclinal troughs, where the rising margins of the 

 hard beds still appear as ridges of more or less prominence. The 

 minute hachures in figure 19 are drawn on the outcrop side of 

 these ridges. Now suppose a general elevation of the region, 

 lifting the synclinal troughs of the hard beds up to baselevel or 

 even somewhat above it. The deepening of the revived master- 

 stream will be greatly retarded by reason of its having to cross 

 so many outcrops of the hard beds, and thus excellent opportunity 

 will be given for readjustment by the growth of some diverting 

 stream, B, whose beginning on adjacent softer rocks Avas already 

 made in the previous cycle. This will capture the main river at 

 some up-stream point, and draw it nearly all away from its hard 

 path across the synclinal troughs to an easier path across the low- 

 lands that had been opened on the underlying softer beds, leaving 

 only a small beheaded remnant in the lower course. The final 

 re-arrangement may be indicated in fig. 20. It should be noted 

 that every capture of branches of the initial main stream made 



