SHALLOW-WATER DEPOSIT 195 



appears to he deposited in quantity only in the basins 

 of arid regions where concentration reaches an advanced state. I 

 Since normal sea-water is far from being saturated with common salt, \ 

 the latter is precipitated only in lagoons, in closed seas, or other 

 situations favorable to great concentration. This is, as a rule, 

 only in regions which are notably arid. It follows that deposits of 

 salt usually signify highly arid conditions, and where they occur 



I'iu. 201. Records of sun-crocks in sandstone. About three-eighths natural 



si/.c. 



over wide ranges in latitude and longitude, as in certain periods 

 of the past, general aridity of climate is inferred. Where confined 

 to limited areas, their climatic significance is less, for topographic 

 conditions may determine local aridity. The total area where salt 

 is now being precipitated is small, though on the whole the present 

 is probably a rather arid period of the earth's history. On the \ 

 other hand, ancient deposits of salt preserved in the sedimentary \ 

 strata show that the area of salt deposition has been much more 

 considerable than now' at one time and another in the earth's 

 history. The salt and gypsum deposits of the past seem, therefore, 

 to tell an interesting tale of the climates of bygone days. 



