B42 R. PUMPELLY EVOLUTION OF OASES AND CIVILIZATIONS 



throughout the mountains bordering the great basin on the south and 

 east. 



Each of these epochs of glacial expansion must have had its echo in a 

 corresponding expansion of the water area and in a reaction on the 

 climate of the basin region itself, in the direction of local precipitation 

 and amelioration of the desert conditions. 



During the glacial and interglacial phases of the Glacial period there 

 must have existed a continuity of broad and perhaps alternately tundra 

 and grass-covered steppes along the whole length of central Asia into 

 Europe. 



The great "Central" basin system resembles the ocean in that it is the 

 sink into which all the solid and dissolved products of the destruction 

 of the surrounding country are brought. In the ocean all such detritus 

 is classified by gravity, wave action, and currents, which distribute the 

 graded material over wide areas. On the dry surface of the desert 

 plains this classification and distribution is begun by the rivers and 

 finished by the winds. 



While in the ocean the sand is deposited to become stratified beds of 

 sandstone, and the clays to form ultimately beds of slate, in the arid 

 basin the sand accumulates in moving hills and the finest silts are borne 

 off by the winds to form the remarkable and economically important 

 deposits of loess. 



We have seen that the lofty mountains intercept most of the moisture 

 brought by air currents from the ocean, and that the fiery column of air 

 rising from the heated barren plains prevents precipitation except in 

 winter; but there is a zone between the deserts and the mountains on 

 which sufficient moisture falls in spring to nourish the grasses of a semi- 

 arid region. In Mongolia, where the intercepting mountains are low, 

 the zone is broad. In Turkestan it is narrow or in places now almost 

 wanting. During the cold Glacial period it was wide. 



I will ask you now to consider this central region as an organic whole. 



Imagine yourselves, if you please, looking down over this great ex- 

 panse and, foreshortening space and the vista back through untold cen- 

 turies, able to view the successive phases of its life during a short period 

 of geological time. 



First, you are in the Glacial period. On the south you see the giant 

 mountains, from the Caucasus to China, covered with snow and, on the 

 higher masses, great domes of ice and far-reaching glaciers. 



Far away in the northwest you see the cap of continental ice spread 

 thousands of feet thick over nearly all of European Eussia. Between 

 these limits your sight wanders over the blue waters of a sea greater 



