THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 405 



believes that the even uplands can have been truncated only by 

 peneplanation close to sea-level. As soon, however, as it is recog- 

 nized that leveling may be accomplished in an arid region without 

 baseleveling, it is no longer necessarily the case that truncated 

 uplands represent uplifted peneplains; the uplands may perhaps 

 be parts of ancient desert plains, originally denuded at their present 

 altitude; and until this possibility is excluded, their isolated position 

 may be explained by the depression of the surrounding lands, as 

 Suess has supposed, without corresponding change in the level of 

 the oceans and the other continents. 



In the case of the truncated uplands or horsts of central Germany, 

 there appears to be good geological reason for associating them with 

 the denuded areas of the Ardennes and of Brittany, as described by 

 Lapparent (b), and thus concluding that they were all low-lying 

 peneplains before they were uplifted. In the case of the plateaus 

 of northern Arizona, the evidence of normal peneplanation is less 

 complete; yet, as above stated, it still seems probable that these 

 plateaus were denuded with respect to normal baselevel, and that 

 the canyon was cut across their surface in consequence of a later 

 uplift with respect to sea-level. The Bural-bas-tau, a flat-topped 

 range, and the associated plateau-like highlands in the Tian Shan 

 system, also need reconsideration in view of the possibility of desert- 

 leveling. I have treated the Bural-bas-tau (e, /, g), and Huntington 

 has treated the associated highlands as uplifted peneplains. Fried- 

 erichsen, on the other hand, while recognizing the highland region 

 as a Denudationsflache, has hesitated to treat it as a once low-lying 

 peneplain, because of the possibility of its erosion above baselevel 

 in a region of inland drainage. If such were the case, it need not 

 have had that close relation to baselevel that is to be expected in a 

 normal peneplain. Nevertheless, the truncated highlands and 

 mountain tops in the Tian Shan seem to be closely related to the still 

 low-lying plains of erosion that are drained by the Hi river to Lake 

 Balkash, and also to the still lower-lying plain of erosion — apparently 

 a true peneplain — that is drained by the Irtysh and the Ob to the 

 Arctic ocean; hence the probability still seems great that the even 

 highlands of the Bural-bas-tau represent a greatly uplifted plain, 

 even though that plain may have been, at the time of its erosion, 



