194 GENERAL GEOLOGY. 



and it is here that the epsomite (native epsom salts) mentioned 

 on another page has been fonnd. The explanation of its 

 prodnction there seems to be this : The magnesian portion of 

 the Kinderhook strata contain carbonate of magnesia and 

 also snlphnret of iron, in the form of common pyrites, in a 

 finely divided condition. The decomposition of the latter 

 upon exposure to the weather, produces snlphnric acid. 

 This uniting with the magnesia by displacing the carbonic 

 acid, with which it originally combined, forms the epsomite. 

 This substance is doubtless often formed by similar decompo- 

 sition of magnesian limestones elsewhere; but wherever their 

 decomposing surfaces are exposed to rains or moving-water, 

 the epsomite, being very soluble, is removed as fast as it is 

 formed. Here, however, it is protected by the overhanging 

 rocks from the access of any water, except such as the 

 atmosphere contains, but which is quite sufficient to aid in 

 the natural production of the salt without removing it by 

 solution. 



All the beds enumerated in the foregoing section, may be 

 distinctly recognized at, and in the immediate vicinity of 

 Burlington, wherever the formation is exposed; but beds No. 

 2, 3, 4 and 5, may be regarded as local only. Nos. 1, 6 and 

 7 are, however, more or less characteristic of the formation 

 throughout its extent in Iowa. The oolitic member is espe- 

 cially constant and characteristic, and the last to disappear 

 at its northern limit. The most southerly exposure of the 

 Kinderhook beds in Iowa is near the mouth of Skunk river, 

 in Des Moines county, where they pass beneath the Burling- 

 ton limestone and the Mississippi river also, by their southerly 

 dip. The most northerly exposure of the formation now 

 known is in the eastern part of Pocahontas county, more than 

 two hundred miles from the locality first named. 



The principal exposures of this formation, are along the 

 bluffs which border the Mississippi and Skunk rivers, where 

 they form the eastern and northern boundary of Des Moines 

 county; along English river, in Washington county; along 

 the Iowa river, in Tama, Marshall, Hardin, and Franklin 



