164 
EOLIAN GEOLOGY 
tion by which the lowering of a continent may be reduced to years, 
is thus found to be only the difference between upbuilding and 
the amount of rain-wash. In the instance of the Humphrey and 
Abbot areal unit, their figures are much too small for the west 
half of the region, and much too large for the east half. All this 
is quite independent of diastrophic movement which provides other 
disturbing factors which are never evaluated. 
Wind-borne dusts from western deserts are alone today prob¬ 
ably depositing materials over the entire Mississippi Valley faster 
that the great stream and its tributaries are carrying this rock 
waste to the sea. In recent geological times also the western half 
of the basin actually receives upon its surface deposits to a thick¬ 
ness of not less than 1,000 feet. The Mississippi River has not 
only not been equal to the task of doing its normal amount of 
work but it has been so incapacitated as to permit this prodigious 
volume of debris to accumulate until its original Tertic surface is 
already carried far beneath sea-level. Nowhere on earth is there 
finer exemplification of vast continental sedimentation. 
K^yKS. 
Desert Soil Sorting by Winds. In the arid regions of the globe 
separation of the finer from the coarser materials of rock waste 
by air in motion is comparable in every way to the elutriation of 
sediment by streams or water currents. In humid lands where 
at rare intervals conditions are such as to permit soil-flowing to 
take place, it is convenient to recognize, according to size of com¬ 
ponents, several grades of concentrates. Lag-gravels include all 
rock-waste, the fragments of which are too large to be moved by 
the winds. Coarse sands and fine sands ricochet over the surface 
of the ground until completely separated from the heavy gravels. 
The dust content of the soil is at once deflated and exported. 
With the lag-gravels and rock debris left behind are also the 
heavy rock-forming minerals however small may be the crystals. 
These include magnetite, pyrite, augite, and the like. In addition 
there is gold which seems to be so universally distributed as vir¬ 
tually to form a golden carpet of the desert. 
When the future of the precious metals is taken into consider¬ 
ation the potentialities of desert countries for great supplies of 
gold are never seriously reckoned. As a distinctly novel field of 
