854 PRINCIPLES OF STRATIGRAPHY 



monadnocks still survive." (i2:jpj.) This condition of wide- 

 spread desert-leveling has actually been reached in the Kalahari 

 region of South Africa, as described by Passarge^and these ex- 

 amples of the final stage, Davis holds, justify the assumption that 

 the various stages, through which they must have passed to reach 

 this last stage, and the characters of which can easily be deduced 

 theoretically, may actually find representation in the arid regions 

 of the world. Furthermore, fossil examples of such desert-leveled 

 plains, as well as examples of stages which precede the final stage, 

 ought to be looked for in the sections of the earth's crust, and we 

 can no longer assume that any level plain, recent or fossil, is a 

 normal peneplain ; the possibility that it may be a high-level desert 

 plain must not be overlooked. 



Some criteria for distinguishing modern peneplains from desert 

 plains are given by Davis. 



"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 



Fig. 224. Erosion-buttes (Zeugenberge) near Guelb-el-Zerzour. The erosion 

 is mainly eolian. (After Walther.) 



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 de- 

 pression of a desert plain could place it so that it should slope 

 gently to the seashore, and that its new-made rivers should not 

 dissect it, and that there should be no drifted sands and loess 

 sheets on adjoining areas, and no signs of submergence on neigh- 

 boring coasts. An untrenched plain of erosion in such an attitude 

 would be properly interpreted as the result of normal processes, long 

 and successfully acting with, respect to normal baselevel." (12: 



397-39S.) • 



"In the same way a high-standing plain of erosion in a desert 

 region might be possibly explained as an evenly uplifted peneplain 

 whose climate had in some way been changed from humid to arid, 

 whose deep weathered soils had been removed and replaced by 

 thin sheets of stony, sandy, or saline waste, and whose residual 

 reliefs had been modified to the point of producing shallow basins. 

 But in this case there should be some indications of recent uplift 

 around the margin of the area, either in the form of uplifted marine 



