AGE OF GYPSUM OF CENTRAL IOWA 72y 



cutting half-way through the entire deposit. At times the gyp- 

 sum is wholly cut out, and records of drillings at points wholly 

 surrounded by gypsum show only gravel. Trenches are fre- 

 quently encountered in mining the gypsum when they cause con- 

 siderable trouble. Where exposed along ravines, the gypsum is 

 decayed on the surface to a depth of three or four inches, and 

 picturesquely grooved and fluted. 



AGE OF THE GYPSUM AND ASSOCIATED DEPOSITS. 



In considering the age of the gypsum, the red shales which 

 accompany it must be kept in mind, for they are very closely 

 associated, as shown by exposures along Soldier Creek, where 

 thin layers of gypsum are found in the shales. Wherever they 

 were not removed by preglacial erosion, these shales overlie the 

 gypsum conformably. Their extent is greater than that of the 

 gypsum, and in the pit of the Fort Dodge clay works, as already 

 described, they may be seen resting unconformably on the Coal- 

 Measures. In Douglas township, sec. 8, there is a good expos- 

 ure of these red shales which iasix miles northwest of any known 

 gypsum. Their color is striking, often brilliant, and for this 

 reason they have been used to some extent as a natural pigment. 



The fact that the gypsum and the red shales lie unconform- 

 ably on the Coal Measures is good ground for believing that, if 

 they belong to the Paleozoic era, they were formed near its close, 

 during the Permian. The Permian beds of Kansas, Indian Terri- 

 tory and Texas, which contain quantities of gypsum, are so highly 

 and so characteristically colored that they are known as the "red 

 beds." These red beds, like the red shales and gypsum of 

 Iowa, are nearly destitute of fossils, due probably to the fact that 

 the climatic conditions favoring deposition of gypsum were hos- 

 tile to organic life. Aridity is the climatic characteristic most 

 essential for great deposits of gypsum, and the redness of the 

 sandstones and shales usually accompanying gypsum deposits of 

 all ages and localities may fairly be assumed to be an effect of cli- 

 mate, direct or indirect, on the iron content of th*" soil. All of 

 these considerations — namely, the arid climate that prevailed 

 during the Permian, shown by great gypsum deposits associated 



