698 C. R. KEYES^ MID-CO'NTINENTAL EOLATION 



California is a notable example. In the great depression to the north- 

 ward, now occupied by the Mojave Desert and Death Valley, the fine 

 Tertiary and Quaternary deposits, with thicknesses of over 4,000 to 8,000 

 feet, are believed to be marly dusts blown from the contiguous deserts 

 into an old arm of the ocean,^^ The process in all its details is well 

 shown today in the playas and "mud flats" which frequently occupy 

 temporarily the central parts of intermont plains. 



Another great but indeterminable volume of the desert dusts and sands 

 are borne into the few large rivers which on their way to the sea from 

 the mountains sometimes traverse the arid lands. The waters of these 

 streams are very muddy. During the sporadic downpours of rain which 

 occur at rare intervals large quantities of desert soil are also carried 

 directly into these rivers by the arroyos or tributaries that for much of 

 each year are perfectly dry. 



By far the largest volume of the finer rock waste is exported through 

 the atmosphere to regions beyond the boundaries of the desert, mainly 

 in a direction opposite to that of the prevailing winds. The total bulk 

 thus transported on the wings of the wind doubtless exceeds many times 

 that removed in an equal time through the general erosion and lowering 

 of a similar area in a normally moist or wet country. The distance to 

 which this great bulk of the desert dust is conveyed is determined by the 

 approach to moist climate conditions. 



In the instance of the American desert region we are able to measure 

 some of the important factors. We should expect, lying to the eastward 

 or northeastward of the great arid tract, an extensive belt where the ex- 

 ported desert dusts come to rest and accumulate. In the broad semi- 

 arid area bordering the desert we should also expect to find a belt of 

 considerable width where eolative processes are still in the ascendency, 

 where the surface is no longer occupied by a rock-floor worn out on the 

 beveled edges of the strata, and where eolic deposition is taking place 

 instead of eolic removal. Still farther out we should expect to encounter 

 a third broad belt where aeroposition is quite extensive, but where, on 

 account of the nearness to wet climate conditions, the settled dusts are 

 removed by the rains and the streams nearly as fast as they are deposited. 



With these ideal expectations the facts observed on the Great Plains 

 seem strictly to agree. 



Abundant vegetation seems to be a most essential factor concerned in 

 the deposition of eolic dusts on land surfaces. The thickly matted sods 

 which cover the surface of the central belt of the Great Plains exem- 



