30 



W. M. DAVIS 



setting forth the scheme of the cycle in this simple and elementary- 

 manner, I have quite as often extended the scheme by presenting 

 it in a more advanced manner, as opened by upheaval and erosion 

 acting together and completed by the continued action of erosion 

 after upheaval ceases. Nevertheless, the interaction of upheaval 

 and erosion has never been presented by myself or by anyone else 

 in the beautiful manner deduced by Penck in his first ideal cycle 

 of the "Gipfelflur'" essay; and for that reason his essay should be 

 regarded as marking an extension of the previous treatment of the 

 cycle scheme. In order to justify the opening statement of this para- 

 graph I desire to cite a number of passages from my earher writings. 

 My first contribution to the problem of the erosion cycle was 

 in 1884; it was then stated that valleys in their early stages "will 

 be narrow and steep walled in regions of relatively rapid elevation, 

 but broadly open in regions that have risen slowly, and I beheve 

 that rate of elevation is thus of greater importance than climatic 

 conditions in giving the canyon form to a valley."^ The last clause 

 of that statement was introduced to correct what seemed to be a 

 then prevaiHng misapprehension, namely, the explanation of the 

 narrowness of the Colorado canyon by the aridity of its region, 

 instead of chiefly by the recent elevation of the plateau in which it 

 is incised. In an essay on the "Rivers and Valleys of Pennsyl- 

 vania,"^ the successive deformations of the Appalachian belt in 

 that state are described as follows : " The great Permian deformation 

 .... may have begun at an earlier date, and may have continued 

 into Triassic time, its culmination seems to have been within 

 Permian limits" (p. 193). "During and for a long time after this 

 period of mountain growth, the destructive processes of erosion 

 wasted the land and lowered its surface" (p. 194). The post- 

 Triassic tilting " culminated in Jurassic" time (p. 196) ; the Tertiary 

 and Quaternary uplifts are merely dated in a general way without 

 specification of rate. In the following pages on the "general 

 conception of the history of a river," it is said: "For the sake of 

 simpHcity, let us suppose the land mass, on which an original river 



'"Geographic Classification, Illustrated by a Study of Plains, Plateaus and 

 Their Derivatives," Proc. Amer. Assoc. Adv. Sci., Vol. XXXIII (1884), pp. 428-32; 

 see p. 429. 



2 Nai. Geogr. Mag., Vol. I (1889), pp. 183-253. 



