CHEMICAL RELATION OF IRON AND MANGANESE. 359 



The iron and manganese thus chemically precipitated may be 

 deposited either with mechanical sediments, such as sand, clay 

 etc., or without them. If the deposition of mechanical sedi- 

 ments is largely in excess of the precipitation of iron and man- 

 ganese, the final products will be beds of ferruginous shale, 

 sandstone, etc., common in many geologic horizons. If the pre- 

 cipitation of iron and manganese is in excess of the deposition 

 of mechanical sediments, the resulting products are deposits of 

 more or less pure iron and manganese ore. Between these two 

 extremes there are all gradations in the admixture of the iron 

 and manganese with mechanical sediments. 



Frequently the iron and manganese which were originally 

 finely disseminated through shale, sandstone, etc., are subse- 

 quently concentrated into bodies of comparatively pure ore, and 

 very commonly this concentration takes place by a process of 

 re-solution of the iron and manganese and re-deposition by 

 replacement with limestone, or, more rarel}^ with some other 

 material. The limestone or other material which thus acts as a 

 precipitant is often in the same series of strata from which the 

 iron and manganese were removed, and thus these two sub- 

 stances, which were once in a finely disseminated condition, may 

 be converted into deposits of comparatively pure ore and yoX, 

 remain in the same general series of strata in which they were 

 originally deposited. A remarkable case of this is seen in the iron 

 deposits of the Penokee series in Michigan and Wisconsin,^ to be 

 mentioned again on page 370. It has also been suggested by 

 H. D. Rogers^ that certain siderite deposits in the Coal Meas- 

 ures were formed by the conversion of finely disseminated ses- 

 quioxide of iron into carbonate of iron by organic matter, and 

 the subsequent segregation of the carbonate as now found in 

 layers and nodules. 



The surface waters that carr}- the iron and manganese to the 

 strata being deposited at a given time are sometimes derived 



•R. D. Irving and C. R. Van Hise, U. S. Geol. Survey', Tenth Ann. Report, 1888- 

 1889, Vol. I, pp. 409-422. 



^Geol. Survey of Penn., Vol. II, 185S, p. 739. 



