10 BULLETIN OF THE NEW YORK STATE MUSEUM 



solutions of salt, and, on the other, the rivers replace the 

 evaporated water, but in inland seas, especially those into 

 which but little river water flows, the conditions are the 

 same as in a glass vessel, since in them a saturated solution 

 of salt sinks to the bottom, from which rock-salt crystallizes 

 out and by continual accumulation a layer of salt is formed. 



In periods of abundant rainfall when the tributary streams 

 are muddy, beds of saliferous clay are deposited upon the 

 layers of rock-salt. If the mud is calcareous a deposit of 

 carbonate of lime is formed. 



On the other hand, in periods of drought when more water 

 evaporates than is contributed by streams, deposits of pure 

 rock-salt are formed ; consequently in the course of the year 

 alternate layers of rock-salt and clay are deposited. If the 

 water of a bay, in which such a process of evaporation and 

 deposition is going on, contains sulphate of lime, inasmuch 

 as the saturation of water with sulphate of lime occurs 

 earlier than with chloride of sodium, the former must sepa- 

 rate before the separation of the salt begins. This com- 

 mences when the sea water is concentrated to a saturated 

 solution of salt. Consequently beds of rock-salt very com- 

 monly rest upon beds of gypsum or anhydrite, so that layers 

 of gypsum and anhydrite appear between the beds of salt, 

 corresponding to the beginnings of the process of evapora- 

 tion, interrupted from time to time. Therefore, deposits of 

 gypsum over rock-salt always occur in consequence of a new 

 influx of sea water. 



If layers of gypsum and anhydrite occur alone and are not 

 accompanied by layers of rock-salt, either the evaporation 

 has not advanced to the saturation point of the brine or the 

 separated rock-salt has been redissolved by an influx of 

 water. That sulphate of lime separates sometimes free 

 from water as anhydrite and sometimes with water of crys- 

 tallization as gypsum, may depend upon the amount of the 

 pressure and consequently upon the depth of the enclosing 

 basin for it is estimated that a pressure of ten atmospheres 



