736 



FRANK A. WILDER 



combined series of Strassfurt amounts to more than i,000 feet, 

 and at Sperenberg to more than 3,000 feet. To yield even fifteen 



feet of gypsum, the average thickness in the Iowa field, an 

 immense amount of water must have been evaporated. A cubic 

 foot of gypsum weighs 140 pounds, and the amount of gypsum 



