BARKHANS OF THE KARA-KUM. 
57 
The right-handed shifting and bhiff-cutting of the Amu, as a result of which 
the admirably irrigated and fertile oasis of Charjui is on its left side, has been taken 
as an illustration of the deflective force of the earth's rotation (cf Walther, 6r, 114)- 
but it is still questionable whether this small force is sufficient to produce the results 
that are ascribed to it. The doubt on this point has greatly increased in my mind by 
reason of the measured deflection of the Mississippi, as detennined recently by one 
of my students from the maps published by the Mississippi River Commission, he- 
iii& greater on the east (left) than on the west (right) in a period of twelve years. 
In any case, the bluff"-cutting by the Amu must be determined by some other cause 
Fig. 33. — Sand Dunes south of Charjui, looking northwest. 
besides the earth's rotation, inasmuch as it involves not mereh- a right-handed 
shifting, but a degrading action at the same time; and degradation by such a 
ri\er implies some alteration in individual regime, such as climatic change or crustal 
mo\ement would produce. 
The railroad crosses a tract of typical crescentic dunes (barkhans) before reach- 
ing the oasis of Chaijui on the Amu. Some areas had scattered bushes among the 
sands; others were essentially without vegetation, and there the dunes were at their 
best (fig. 7,2,). They were under the influence of northerly winds, for their longer 
slopes were to the north, and their steep scarps and lateral horns were to the south. 
According to Russian obsen,ers, the fonn of the barkhans has a seasonal variation, 
following the change in the prevailing winds. Besides the seasonal variation of 
dune fonn, there appears to be a secular extension of the dune-covered area, in some 
places averaging 20 feet a year (Walther, 1900, 119). 
