726 FRANK A. WILDER 



ness the deposit varies from ten to thirty feet. Instead of thin- 

 ning out gradually through a considerable area, it seems to 

 diminish but slightly before it abruptly gives place to shale. At 

 Kohle's Brewery, for instance, ten feet of gypsum appear, while 

 half a mile further north, in the clay pit of the Fort Dodge Brick 

 & Tile Co., only drift and Coal-Measure shales are found. 

 Everywhere in the Webster county gypsum the laminae alternate 

 regularly in color from green to white. The gypsum is remark- 

 ably pure calcium sulphate (CaS042H20). The lower layers, 

 generally the lower three feet, are not as pure as the upper and 

 are not used in the manufacture of plaster. Even in these lower 

 layers, however, the amount of the impurities is so small that 

 they would hardly injure the plaster. An analysis of the upper 

 layers shows :' 



Calcium sulphate (CaS04, - - - - 78.44 



Water crystallization (calculated) - - - 20.76 



Insoluble matter (impurities) - - - - 0.65 



An analysis made by Professor J. B. Weems of gypsum taken 

 from the lower, middle, and upper part of those layers that are 

 rejected in making plaster shows : 



Silicia, SiO .--.-.- i.g2 



Alumina, A 1 2O3 i.oo 



Calcium sulphate, CaSO^ - - - - 76.28 



Water - - - - - - - -20. 72 



Total 99-92 



With traces of magnesia and carbon dioxide. 



When made into plaster this lower layer, while soft, will not 

 adhere to the laths satisfactorily. After hardening it is as firm 

 and durable as the plaster made from the upper layers. The 

 gypsum is crystalline throughout, the slender needlelike crystals 

 being arranged at right angles to the plane of sedimentation. 

 Though the gypsum is now well preserved by the thick mantle 

 of drift that overlies it, at one time it formed the surface rock, 

 and in consequence suffered considerably from erosion and solu- 

 tion. When the overlying drift is removed, the surface of the 

 gypsum everywhere seems deeply trenched, some of the trenches 



^Analysis by G. E. Patrick, Iowa Geological Survey, Vol. Ill, p. 291. 



