AGE OF GYPSUM OF CENTRAL IOWA 



735 



the densities of 1.1315 and 1.2 1, whereas the deposition of salt 

 does not begin until the latter point is reached. The remaining- 

 one-fourth is precipitated with the salt, but constitutes so small 

 a part of the whole that the commercial value of the salt is not 

 appreciably lowered. The normal order of deposition on evap- 

 oration from sea water, aensitif.-'^-^' 



CaCO. Fc.O. 

 /.osoq- 



1/cl. 



/.Z4(, 



/■JJ7 



CoStj 



MgCl^ 



MgSO^ KCl Mgtlr-^ 



beginning with the first 

 which occurs, of course, 

 at the bottom of the 

 deposit, is : 



1 . Limestone with limo- 

 nite, CaCOs and aFezOg 

 3H2O. 



2. Gypsum CaS04 

 2H2O. 



3. Sodium chloride 

 (common salt) NaCl. 



4. The bitter salts (in 



carnallite) KCl MgClj 



6H2O. Fig. I. — Sketch showing the order of precipitation 



Practicallvthis order °^ ^^^'■^ from sea water, with increase in density due to 

 evaporation. 

 IS observed m the great 



salt deposits of Stassfurt, Germany. 



While gypsum has been formed and is still forming in all of 

 the ways described, most of these explanations are manifestly 

 not adapted to the Webster county deposit. The definite lami- 

 nation and layermg of the gypsum indicate an aqueous origin. 

 Pointing to the same conclusion is the fact that no limestones 

 are associated with the gypsum which by alteration could yield 

 gypsum. Salt may be regarded as absent from the Iowa gypsum 

 area. If it existed, it would probably have been detected in some 

 of the many wells and prospect holes. Its absence is not sur- 

 prising, for the degree of concentration necessary for a salt 

 deposit may never have been reached, or the salt, after having 

 been deposited, may have been removed by subsequent erosion 

 and solution. 



The great thickness of some occurrences of gypsum and salt 

 must be considered in seeking to determine their origin. The 



