THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 397 



of the earth's crust or the condition of climate that has been changed. 

 It is likewise important to scrutinize desert plains, now standing 

 above baselevel, to see if they may not have been formed normally 

 as lowland plains of erosion and afterward uplifted. It is therefore 

 necessary to inquire into these features by which baseleveled pene- 

 plains and rock- floored desert plains may be distinguished, even 

 though the former may be uplifted with a change to an arid climate, 

 or though the latter may be depressed with a change to a humid 

 climate. 



Passarge holds the opinion that the plains of the Inselbergland- 

 schajt are smoother than any peneplain can be; for he describes 

 the desert plains as true plains, not as gently undulating surfaces. 

 He states that water is not competent to produce such plains; 

 its power of erosion works chiefly downward, and only by exception 

 laterally; and he concludes that, although long-continued normal 

 erosion may produce a peneplain — that is, a low, undulating hilly 

 surface — it nevertheless cannot produce a surface like that of the 

 plains in the Inselberglandschaft. 1 But, however difficult it may 

 be to wait, in imagination, through the ages required to wear a low 

 hilly region down to less and less relief by the weakened processes 

 of weather and water erosion in the latest stages of the normal cycle, 

 there are certainly some truncated uplands, ordinarily taken to be 

 uplifted peneplains, whose interstream uplands are astonishingly 

 even, and whose surface must have been, before dissection, very 

 nearly plain over large areas; hence it does not seem to me altogether 

 certain that a greater and a less degree of flatness can be taken to 

 distinguish the two classes of plains. 



A plain of erosion lying close to sea-level in a region of normal 

 climate, and therefore traversed by rivers that reach the sea, but 

 that do not trench the plain, might conceivably be a depressed desert 

 plain standing long enough in a changed climate to have become 

 cloaked with local soils; but it is extremely unlikely that the depression 

 of a desert plain could place it so that it should slope gently to the 



1 Wasser ist nicht imstande solche Ebene zu erodieren. Seine Erosionskraft 



wirkt hauptsachlich in die Tiefe, nur ausnahmsweise in die Breite Bei sehr lang 



andauernder Abtragung kann wohl eine 'Peneplain' zustande kommen, d. h. ein 

 flaches welliges Hiigelland, aber keine Flache, wie die Ebenen der Inselberglandschaf- 

 ten" (b, p. 196). 



