640 R. PUMPELLY EVOLUTION OF OASES AND CIVILIZATIONS 



The western system of land-locked basins covers a great part of west- 

 ern Asia. Extending west from the Tienshan ranges, it is limited on 

 the south by the Persian plateau and the Caucasus and on the north by 

 the low Siberian elevation that forms the water divide toward the Arctic 

 ocean. On the west, from a hydrographic, but to a lesser extent from a 

 climatic point of view, this system includes the Black sea, with the areas 

 drained by the Volga, Don and Dneiper (a large part of Eussia), and by 

 the lower Danube. 



The Persian plateau itself forms an independent high-lying system of 

 arid land-locked basins. 



Of this great western system a part near the Caspian sea lies below 

 the level of the ocean. A large part of the whole system is so situated 

 in reference to the barriers that separate it from the oceans that, given 

 a sufficient quantity of water and the closing of the Bosphorus channel, 

 there would* be a land-locked sea several hundred feet deep and larger 

 than the Mediterranean. It is potentially a sea, of which the Black sea, 

 Caspian, and Aral remain as three larger residuary bodies of water. 

 This is due to climatic conditions, under which the precipitation over 

 the region, together with the water brought by the streams from without, 

 is offset by the intense evaporation over the heated arid surface. 



With a sufficiently long-continued inflow of water in excess of evap- 

 oration and a restoration of the barrier at the Bosphorus, the Black sea 

 and the Caspian would coalesce and, after extending to include the Aral, 

 would rise till an overflow should be reached, either into the Mediter- 

 ranean or into the Arctic ocean, and our potential sea would become a 

 reality. 



If, on the other hand, there should exist a sufficiently long-continued 

 condition, in which evaporation should be in excess of inflow of water, 

 then a time would come when, instead of a sea, there would be only a 

 region of barren deserts. 



Our basin is, therefore, potentially both a sea and a desert. At present 

 the two controlling factors — water and evaporation — are about in a state 

 of equilibrium. 



The existing residuary seas are therefore, in the rising and lowering 

 of their surfaces, gauges recording the cyclical climatic changes as they 

 occur over the great catch-basins that supply them with water. 



Of these catch-basins the northern and western ones are the great 

 hydrographic systems of European Eussia and the smaller river systems, 

 chiefly of the Caucasus. The rest lie almost wholly in the lofty moun- 

 tain chains that stretch with increasing height and area as they go east- 

 ward to high Asia. The vast masses of snow and ice constantly accumu- 



