86 C. R. KEYES — INTERMONT PLAINS OE THE ARID REGION 



this point the observations of Walther^^ in nortliern Africa are of great 

 interest. ' 



In the central Asian desert belts the planed surfaces of such flat-topped 

 mountain ranges as the Bural-bas-tau and other similar ridges in the 

 great Tian Shan sj'stem 'were formerl}^-^ considered as remnants of an old 

 peneplain uplifted to heights of 12,000 feet and over. Regarding them 

 the question has been more recently raised by Davis^* that they might 

 possibly be partly, if not wholly, due to leveling without baseleveUng, 

 imder conditions of arid climate ; but the existence of a peneplain, drain- 

 ing to the sea, in the northern part of Asia would, according to this au- 

 thor, seem to militate against this suggestion. Farther to the westward, 

 in the Caspian basin and also even in the southern provinces of European 

 Eussia, the beveled rock-floor of the steppes, even though lying so near 

 or even below sealevel, may be largely the result of desert-leveling rather 

 than of base-leveling. Bearing directly on this point, Penck^" has sug- 

 gested that so long as the waters of the sea do not have free access to the 

 area a desert surface may be readily excavated considerably below mean 

 tidelevel. 



The great desert plains of Australia are in need of careful scrutiny 

 with this idea constantly in mind of general leveling under conditions of 

 aridity instead of under conditions of normal peneplanation. 



PLANORASION 



In the normal cycle of evolution of land forms under conditions of a 

 humid climate the dominant process is stream corrasion. The ultimate 

 product of a general plain is the least important stage. The process is 

 known as peneplanation. In the geographic cycle under climatic condi- 

 tions of aridity stream action is of small consequence. A plains surface 

 is persistently maintained throughout the cycle, or at least from a time 

 very soon after its inauguration. The plains feature is, as fully attested 

 by the observations of Passarge, smoother than is possible with the pene- 

 plain. The ultimate product is a general plain but slightly diiferent 

 from that existing through the whole period during which the plains- 

 forming process has been in action. It is essentially planorasion. 



Geographic Cycle in an arid Climate 



The foregoing observations are intended as a contribution to our knowl- 

 edge of the substructure of the plains in the dry regions of the West, and 



I 



" Das Gesetz der Wiistenbildung, Berlin, 1900. ■ 



^ Appalachia, vol. x, 1904, pp. 277-284. ■ 

 =» .Tom-nal of Geology, vol. xiii, 1905, p. 405. 



=" American .Tournal of Science (4), vol. xix, 1905, pp. 1C5-174. i. 



