DEPRESSIONS IN THE EARTH'S SURFACE. 151 



inflow is exhausted through evaporative agencies, and 

 not carried out by subterranean passages as was sup- 

 posed by the ancients. If the straits should com- 

 pletely close, as the tendency seems to be, the gulf 

 would become an isolated and independent sea, and 

 rapidly shrink to a restricted area perhaps to a dry 

 basin with the bottom incrusted with salt. 



The lower or southern third of the Caspian Sea is 

 very deep, its greatest depth being 3,000 feet; and the 

 water is much salter than in the upper third. It is 

 highly probable that the contents of the Black and 

 Caspian seas once commingled, but became divorced 

 through causes already mentioned, the Caspian lower- 

 ing gradually through excessive evaporation, till it is 

 now 85 feet below the level of its old associate. An 

 ancient waterway can still be traced between the two 

 seas, sand-ridges appearing here and there which were 

 once bars thrown up in shallow straits and by a pro- 

 cess now going on at the mouth of the Scythian gulf. 



If the Caspian Sea should, through diminished 

 evaporation, regain its ancient level, its contents would 

 obliterate the Volga as far up stream as Saratov, and 

 inundate plains of vast extent, so low and flat is the 

 country stretching northward. The water-shed be- 

 tween streams flowing into the Polar Sea and those de- 

 scending into the Caspian basin is so slight an elevation 

 that, at an earlier period, the waters of the White and 

 the Black seas may have connected with each other. 

 Lake Elton seems to be a remnant of the old com- 

 munion. Its waters are so intensely saline that it must 

 have been refilled many times with sea water after re- 

 peated lowerings through evaporation. Being very 

 shallow, it constitutes an excellent evaporating basin, 



