32 W. M. DAVIS 



The elementary presentation of the ideal cycle usually postulates a rapid 



uplift of a land mass, followed by a prolonged still stand The uplift 



may be of any kind and rate, but the simplest is one of uniform amount and 



rapid completion In my own treatment of the problem, the postulate 



of rapid uplift is largely a matter of convenience, in order to gain ready entrance 



to the consideration of sequential processes Instead of rapid uplift, 



gradual uplift may be postulated with equal fairness to the scheme, but with 

 less satisfaction to the student who is then first learning it ; for gradual uplift 

 requires the consideration of erosion during uplift. It is, therefore, preferable 

 to speak of rapid uplift in the first presentation of the problem, and afterward 

 to modify this elementary and temporary view by a nearer approach to the 

 probable truth; and this has been for some years past my habitual method 

 in teaching [p. 153]. 



A brief analysis of a case of slow uplift is then presented: 

 A special case necessitating explanation by slow uplift may easily be 

 imagined. If an even upland of resistant rocks be interrupted by broadly 

 open vaUeys, whose gently sloping, evenly graded sides descend to the stream 

 banks, leaving no room for flood plains, it would suggest slow uplift; the 

 absence of the flood plains would show that the streams have not yet ceased 

 deepening their valleys, and the graded valley sides would show that the 

 downward corrosion by the streams had not been so rapid that the relatively 

 slow process of slope grading could not keep pace with it. In such a case there 

 would have been no early stage of dissection in which the streams were inclosed 

 in narrow valleys with steep and rocky walls; the stage of youth would have 

 been elided and that of maturity wovdd have prevailed from the beginning, 

 but with constantly increasing relief as long as upUft continued [p. 154]. 



The closing statement as to "constantly increasing reHef as long as 

 uplift continued" evidently deserves correction and receives it in the 

 second phase of sharp-crested ridges in Penck's first ideal cycle. 

 A following statement: "Examples of this kind must be rare, "per- 

 haps needs correction also ; but except in the case of weak rocks that 

 remains to be proved. My writings are to my regret deficient in not 

 giving any special attention to the modifications that the scheme of 

 the erosion cycle should receive in the case of weak structures, such 

 as coastal plains with unconsolidated strata. 



Definition of the initial surface. — The fact that none of the above 

 brief allusions to cycles of slow initial uplift are accompanied by 

 detailed discussions of the pecuHar features that should distinguish 

 such cycles from others of rapid uplift, perhaps gives some color, 

 as far as the publications thus far cited are concerned, to Penck's 



