The Origin of Roch-Salt. 31 



which flow through areas in which evaporation goes on at a 

 high rate, large quantities of the salt, in process of being 

 transported seaward by the river, are left upon the land. 

 Hence the salt lakes, salinas, and salt-encrusted lands found 

 in the lower parts of the river courses in all areas where 

 such geographical conditions obtaiu. 



But in the areas which are wholly those of inland drainage, 

 the quantity carried down by the rivers, and left by the 

 evaporation of their waters, is, from the nature of the case, 

 much greater still. Hence the great deposits of salt left in 

 such areas as those of the Aralo- Caspian area, in the Salt 

 Lake of Utah, in the desert regions of South America, 

 Australia, and in all other regions where such conditions 

 as those mentioned are found. Some of these we may pro- 

 ceed to consider in more detail. 



The Caspian area is especially worthy of study in this 

 respect. It was formerly part of a greater Mediterranean, 

 which extended far east into Central Asia. Terrestrial 

 changes affecting the surface have broken up the marine 

 area, and have left the Caspian, estimated at being now 

 about 180,000 square miles in extent, as an area into which 

 flows the drainage of the Volga, the Ural, the Kuma, the 

 Terek, the Kur, the Emba, the Araxes, and many smaller 

 rivers. Notwithstanding the enormous volume of river-water 

 annually poured into this inland lake, its level is not raised 

 in the least, because the whole of the inflow is carried off 

 by evaporation. As the Caspian lies between the summer 

 isotherms of 70° and 80°, and it is, further, traversed by very 

 dry winds for a large proportion of the year, this excessive 

 evaporation is, perhaps, hardly greater than might have been 

 expected. The southern portion of the lake has a depth of 

 2953 feet below the level of the Black Sea, while the northern 

 portion shallows by an almost imperceptible slope to the 

 surface. In the shallows the evaporation is greater than the 

 influx, so that a current from the south sets in to make up 

 the loss. The quantity of salts there amounts to 13 per 1000, 

 of which, on an average, 62'7 is chloride of sodium, and 23*8 

 sulphate of magnesia. On the eastern side, in the narrow 

 bay of Kai-Dak, evaporation increases the proportion of salts 



