Cairnes — Some Suggested New Physiographic Terms. 81 



interior drainage, during the early stages of the physiographic 

 cycle and under conditions just sufficiently arid to prevent the 

 streams from reaching the sea. As the climate becomes 

 increasingly arid, and in arid regions that are approaching a 

 late mature or old stage in the physiographic development, 

 more and more fine material tends to be exported by the wind. 

 However, in many regions possessing interior drainage, the 

 exportation of wind-borne dust is but a minor process, and 

 equiplanation dominates the situation. Passarge has described 

 various such districts, notable among which are certain regions 

 such as the Kordofan type of desert, of which the climate has 



Fig. 3. 



Fig. 3. A view of the Yukon plateau 3500 feet above sea level, along the 

 Yukon- Alaska boundary, at latitude 66° 51'. Equiplanation is active ; the 

 flat plain-like portion of the upland which here supports a typical arctic 

 vegetation and is underlain by partly frozen accumulations of debris, is dis- 

 tinctly seen to be encroaching on the limestone residual bluff in the right 

 hand portion of the picture. 



recently become modified and is now somewhat more humid 

 than formerly. In Kordofan and similar regions the rainfall 

 is now great enough to give rise to a steppe vegetation, but is 

 not sufficient to allow the rivers to reach the sea. "Washed 

 deposits spread over the lowlands, being coarsest about the 

 prominent residuals, and finest in the swampy areas between 

 the residuals, where a dark rich soil occurs. 



Am. Jour. Sci— Fourth Series, Vol. XXXIV, No. 199.— July, 1912. 

 6 



