36 W. M. DAVIS 



to 7,000 feet, or 3,000 feet higher than before, and with the valleys 

 more deeply incised and more widely opened at the top of their 

 slopes, so that they have reduced most of the inter-valley upland 

 surfaces to sharpened ridges. The fourth drawing, hmited to the 

 western or south-north part of the arch, shows the greatly uplifted 

 mass to be completely carved into mountain forms, some of which 

 exceed 10,000 feet in altitude; all the summits that rise along the 

 axis of the vanished arch, as well as all the ridges that radiate from 

 the summits, now have sharp crests, but none of the valleys, not 

 even that of the large antecedent river, have any flood plains. 

 The fifth drawing shows the south-north mountains subdued to 

 rounded forms, the smaller valleys opened to gentler slopes, and the 

 valley of the antecedent river with a flood plain. The sixth repre- 

 sents the east-west part of the curved range reduced to hills of 

 various altitudes, between which even the small-stream valleys 

 have flood plains; and for a seventh stage, reference is made back 

 to the first drawing. The cycle is thus carried from an initial 

 lowland through high mountains to an ultimate lowland, three of 

 the drawings representing the interaction of upheaval and erosion, 

 and three more — or four if the first is counted over as the seventh — ■ 

 representing the continuance of erosion after upheaval has ceased. 

 Inasmuch as these exercises were planned for use in secondary 

 schools, the lowland which is upheaved to be carved into mountains 

 and then worn down to a lowland again is assumed, for the sake 

 of simpKcity, to be composed of massive rocks, and as a result the 

 highly characteristic but somewhat compHcated process of the 

 development of subsequent streams at the cost of pre-existent 

 streams, and to the profit of the adjustment of drainage and relief 

 to weak and hard structures, is excluded. The same simpHfying 

 assumption is made by Penck in the first ideal cycle of his " Gipfel- 

 flur" essay; for although the actual Alps have a good number of 

 subsequent ridges and valleys, and a much larger number of subse- 

 quent valley-side ravines which exercise a considerable measure of 

 control upon the altitude of ridge crests by serrating them with 

 subsequent notches, and thus reducing the relief of the intervening 

 subsequent knobs, no mention is made of variations of structure 

 or of the development of subsequent valleys in Penck 's analysis of 



