162 
EOLIAN GEOLOGY 
The relief features are even more puzzling. Plains form the 
dominant characteristics; they are as vast and apparently as 
smooth as glassy ocean. The mountains which often rise so 
abruptly are rugged and lofty, and devoid of fringing foothills; 
they are enisled by level earth as perfectly as they could be by 
the sea. Vast lake-basins abound with never a sheen of water. 
There are deep trenches which never saw sign of river; and long 
pitching valleys branching as often at their lower ends as at their 
upper extremities. Despite their high gradients the intermontane 
plains remain devoid of traceable drainage-channels. 
Evidences of prodigious rock-wasting are apparent on every 
hand yet there are phenomenally small amounts of debris, and no 
residual mantle whatever. Decomposition of the rocks proceeds 
from below instead of from above. Often large areas of soil are 
covered by a layer of small stones set closely together as in a 
mosaic, which is polished or ‘Vanished” like a drawing-room floor. 
There are vast forests turned to stone. A coloquial saying that 
in the arid land one digs for wood and climbs for water, is liter¬ 
ally true. 
Throughout the moister regions of the globe, or those parts 
with which the majority of us are most familiar, moving water is 
so universally regarded as the chief agent of denudation that other 
erosional means seldom receive more than bare mention. In arid 
districts there appears to be a reversal of the relative activities of 
the erosive processes. Water-action is of quite secondary conse¬ 
quence ; wind-scour is not only the most vigorous, but often almost 
the sole erosive power. 
^ Keye:s. 
Measure of Bolic Depletion of Great Plains. Relations of the 
dust clouds along the Missouri River to the adjoining loess mantle 
are now well understood. On a much larger scale is the dust 
storm which ofitimes enters the humid region from the neighbor¬ 
ing arid country. Occasionally such storms are of almost desert 
fierceness; and not only traverse the Great Plains but the grass- 
clad prairies beyond. So remarkable are these dust storms that 
they have been minutely described. 
One of unusual duration and severity took place in the Missis¬ 
sippi Valley in March, 1902. During a period of three days the 
sun was completely obscured, a dense dry fog hung as a dark pall 
