734 FRANK A. WILDER 



+ FcgOg + 2 COg, the original form of the iron being FeSg 

 Cerussite and smithsonite with gypsum in a mineral vein are 

 similarly accounted for. Anhydrite when exposed to air con- 

 taining moisture gradually takes on water and forms gypsum. 



2. That great quantities of gypsum in all parts of the world 

 and at different times in geological history have been derived 

 from sea water by evaporation is generally recognized. Sea water 

 contains 3^ per cent, of mineral matter, distributed as follows: 



Chloride of sodium ------ 77.758^' 



Chloride of magnesium ----- 10.878 



Sulphate of magnesium - - - - - 4.737 



Sulphate of calcium (gypsum) . - - 3.600 



Sulphate of potassium ----- 2,465 



Carbonate of lime ------ 0.345 



Bromide of magnesium ----- 0.217 



Total ------- 100.000^ 



Gypsum is deposited from typical sea water when 80 per cent, 

 of the water has evaporated, whereas common salt is not deposited 

 until the bulk of the water is reduced more than 90 per cent. 

 Gypsum deposits are more widespread than salt, but salt usually 

 occurs in thicker beds. These facts, taken with the relative 

 amount of each salt in sea water and the amount of evaporation 

 necessary for precipitation in each case, accord with the theory 

 which regards the evaporation of sea water as the cause of most 

 salt and gypsum deposits. It is evident that conditions allowing 

 the 80 per cent, of evaporation necessary for a gypsum deposit 

 would occur more often than those giving rise to 90 per cent, of 

 evaporation and a deposit of rock salt. When the amount of 

 evaporation necessary for a salt deposit took place, however, the 

 high percentage of salt in the water would yield a stratum of 

 notable thickness as compared with gypsum. 



The accompanying diagram indicates the relation of deposi- 

 tion to density in the case of the salts common to sea water. 

 The magnesium chloride alone is not actually precipitated, but 

 remains in solution always under ordinary atmospheric condi- 

 tions. It may be precipitated, however, with potassium chloride 

 as carnallite. Three-fourths of the gypsum is deposited between 



