CONTRASTED FEATURES OF TOPOGRAPHIC JUVENILITY 553 



national Geologic Congress, with what utter astonishment I noted in so 

 many localities on the steppes of southern Russia and on the Kirghiz 

 steppes the rock-floor exposed in situations where water could not pos- 

 sibly have operated. Persian deserts left me with similar new impres- 

 sions. When a little later I had occasion to visit the Saharan region, the 

 bedrock peeping out from under the soils and sands of the Nubian and 

 Lybian deserts convinced me then and there that the true explanation 

 lay not in any phase of water action, for here the annual rainfall was 

 less than one inch. The leveling and general lowering of arid tracts it 

 seemed must be attributable mainly to eolic action, if not to the winds 

 alone. Peneplanation without the aid of water became as real to me then 

 as was peneplanation by means of water. 



The alleged enormous depths of basinal wash as reported from time to 

 time in deep well drillings have been already discussed. In all of these 

 cases which I have personally investigated there manifestly have been 

 mistaken for wash materials a great thickness of the little indurated 

 Tertiary beds. In several such instances the Tertiary bedrocks were 

 standing on end and the surface wash was but a few feet in thickness; 

 in other cases the drill began in soft Cretaceous strata and there were 

 reported nearly 2,000 feet of "wash." I do not doubt but there are in 

 many localities wash deposits of considerable thicknesses; but it is also 

 evident that before the usual data, and especially well-logs furnished by 

 the average driller, are to be implicitly depended on they shall have to be 

 critically examined anew in the light of recent determinations. 



At a distance and on the hypothesis of normal water action we de- 

 ductively should expect transferrence of the rock-waste from a basin-rim 

 to central interment depression. In the absence of direct stream con- 

 nection with other and lower basins, we likewise should expect great ac- 

 cumulations of finer waste in the middle portions of the higher basins. 

 On a basis of deflation thick basinal deposits are inexplicable ; thin soil 

 coverings are demanded. By wind the relief of an arid basin does not 

 appear to be slowly diminished in the beginning by the removal of waste 

 from the highlands and its deposition on the lower gentle slopes or on 

 the basin-bed. In the case of the latter the presence of a rock-floor but 

 thinly veneered by soils seems to be the strongest evidence that the basin 

 itself is being rapidly lowered, not raised. The median line of a basin 

 can hardly be regarded, therefore, a local baselevel of stream action. 



In another place^^ I have shown that the geologic work of the ephem- 

 eral streams, sporadic sheetfloods, and transitory playas of arid plains 



w Bull. Geol. Soc. America, vol. 19, 1908, p. 78. 



