592 W- M. DAVIS PENEPLAINS AND THE GEOGRAPHICAL CYCLE 



details and complications which they hold to be indispensable. Passarge, 

 for example, ascribes so great a value to past changes of climate that 

 he is disinclined, to say the least, to regard any existing mature or old 

 land form as the product of a one-climate erosion cycle; not merely that 

 the climate would change during the progress of a cycle from cold and 

 rainy on the initial highlands of an uplifted region to warmer and drier 

 on the old worn-down lowlands ; but that, by reason of the shifting of 

 climatic zones, the tilting of the earth's axis, and other possible causes, 

 the processes of "normal" subaerial erosion in middle latitudes, for 

 example, are not likely to endure through so long a period of time as an 

 erosion cycle. He is therefore unwilling to look upon most existing 

 land forms as the product of the prolonged action of their present-day 

 erosional processes. He questions the sufficiency of soil creep on the 

 forested slopes of subdued mountains, like those of North Carolina, to 

 continue the degradation that such mountains have already suffered, 

 and ascribes their present form to the action of some other-than-normal 

 processes in the past; similarly, he discredits the explanation of the 

 Inselberglandschaften of sub-arid east Africa — isolated residual moun- 

 tains rising sharply over degraded rock-plains, like those of southeastern 

 California — by the processes now in operation upon them, and calls 

 in the aid of one or more pluvial periods, even though the visible forms 

 give no evidence of pluvial action. 



That there have been climatic changes during the progress of certain 

 cycles of erosion is plain enough in glaciated regions, and also in certain 

 subarid regions where the records of extinct lakes are still preserved ; 

 but if the Great Basin of Utah and Xevada be taken as a sample of 

 such a subarid region, the remarkable thing about it is the relatively 

 trifling amount of work that was done there during the Quaternary 

 humid epochs; they were mere transitory episodes in the immensely 

 longer period or cycle of erosion that would be required for the general 

 degradation of the region to a low peneplain. Where the work of the 

 humid episodes is not recorded in lake-shore terraces or cliffs or in basin- 

 floor sediments, it is not recognizable. The forms of the Great Basin 

 are essentially those of a long-continued subarid cycle of erosion, on 

 which the details produced during the brief episodes of humidity are of 

 very subordinate value. Hence, while the effects of changes of climate 

 should, of course, be considered wherever accessary, it is practically help- 

 ful to discuss steady-going cycles of erosion during which a normal 

 (humid), or subarid, or other climate has prevailed, and to apply such 

 discussions to the description of regions like the North Carolina mount- 



