PLAYA DEPOSITS 85 



buried. With few exceptions, sucla reported observations must be sub- 

 mitted to further scrutiny before they can be fully relied upon. The 

 recent assertions of Tight^* and of Lee^^ are categorical in the extreme, 

 and have to be taken, in the light of recently observed facts, with ex- 

 treme caution. In the case of the Albuquerque deep well being 700 feet 

 in soft mesa deposits, it is significant that the main water supply of this 

 well comes from levels less than one-half of this distance from the sur- 

 face, and that the strata beneath this level appear strangely like the 

 uppermost Cretaeic sands of the local geologic section. It would not be 

 surprising to find also an extensive section of the Eocene Puereo deposits 

 at a level only a short distance beneath the river bed. 



On the whole, the ascribed thicknesses of aggraded materials in the 

 central portions of the basins under consideration is, as yet, largely a 

 result of inference to fit an hypothesis rather than a matter of careful 

 observation on actual conditions. There are probably, in different parts 

 of the arid country, all gradations, from the basins with deep aggraded 

 substructure to those in which there is merely a veneer or even bare rock. 

 The significant fact remains that in the majority of cases in which there 

 have been careful observations made there is a rock-floor at very shallow 

 depths. Such facts are in perfect accord with the evolution of the gen- 

 eral relief features under conditions of an arid climate, where wind, and 

 not water, is the dominant factor in giving expression to the earth's sur- 

 face. Facts of this class are also in harmony with our ideas regarding 

 complete leveling of high-lying surfaces without baseleveling. 



ARID PLAINS OF OTHER REGIONS 



The idea of the planing off of a high-lying land-mass without being 

 first reduced to baselevel was developed mainly by Passarge^^ for the vast 

 dry plains of South Africa. To this region reference has been already 

 made in some detail. 



Bornhardt's description^'' of the elevated plains of German East Africa 

 indicates that the main epoch of planation was one of aridity. In north- 

 ern Africa the Libian and N"ubian deserts present in many places a firm 

 rock-fioor at slight depths, the full significance of which has never been 

 appreciated until Passarge emphasized the fact elsewhere. From the 

 accounts of travelers the interior of the Sahara desert have characteristics 

 not unlike those of some of the plains districts of western America. On 



^ American Geologist, vol. xxxvi, 1905, p. 279. 



-^ U. S. Geological Survey, Water Supply and Irrigation Paper, no. 188, p. 190. 



-^ Zur Oberflachengestallung u. Geologie Deutsch-Ostaf riUas, Berlin, 1900. 



