PHYSICAL DEVELOPMENT OF CENTRAL ASIA 643 



than the Mediterranean and fed by the larger rivers that flow from the 

 snow and ice capped regions. 



You see the rivers building great deltas where they enter the sea, 

 while above these they spread their silts far and wide over the aggrading 

 plains. 



Eemember that while yon look, in your time-perspective millenniums 

 are as seconds. Even now the Glacial period has passed and the reac- 

 tion has begun, and you see the beginning of a general trend toward 

 desolation. The ice-cap is gone from Russia and the great glaciers on 

 the southern mountains are diminishing in extent. Evaporation is now 

 more rapid than inflow of water, and the sea is shrinking and breaking 

 up into smaller basins. With each lapse of thousands of years you see 

 the larger rivers grow smaller, while many of those coming from the 

 southern mountains fail to reach the receding sea. Those great gyrating 

 columns that are coursing the surface of the earth show that the dried 

 silts have become the prey of the winds. 



And now, if you will look closer, you will see at their work all the 

 controlling agencies that are the life of the great geographic organism 

 that we call an arid inner-continental region. You observe that the 

 floodplains and deltas and the drying beds of seas are covered with 

 dried silts of clay, sands, and gravels. 



The winds are working these over and classifying them according to 

 size of grain. The finest material is easily lifted and carried afar, and 

 it is this that forms those massive yellow clouds that are darkening those 

 plains in their progress, and those gyrating columns — vortices in the 

 heart of the sweeping whirlwind. 



Of the coarser silts the winds move only the sands, and these only 

 slowly, along the surface of the plain where you see them, forming great 

 seas of sand waves or dunes, in places more than 100 feet high. These 

 waves progress as each high wind, lifting sand from the windward side, 

 deposits it on the lee side. As the winds vary in direction during the 

 seasons, so does the progress of the dust and of dune waves. But it is 

 an important fact for us that both dust and dunes make an absolute 

 progress during the year in the direction of the predominant winds. 



Watch those columns and clouds of dust ; as the wind falls, they disap- 

 pear, settling on the surface to wait to be borne on the wings of the next 

 wind-storm. 



Look now toward the grass-covered plains bordering the deserts; no 

 clouds rise from these; on the contrary, the volumes of dust that fall 

 here, fall to remain under the protecting vegetation; the grass is nour- 

 ished perennially by the dust, and under this reciprocal process the sur- 



