CHEMICAL RELATION OF IRON AND MANGANESE. 357 



neous mass, resembling iron ore when iron is in the preponderance 

 and manganese ore when manganese predominates. In such 

 cases there appears to be no tendency to combine in one fixed pro- 

 portion, though, as iron is a much more abundant substance than 

 manganese, the mixture most commonly contains an excess of 

 iron, and exists in the form of a manganiferous iron ore. The 

 manganese, when not intimately mixed with the iron, may occur 

 in it in pockets or as scattered nodules and concretions. Such 

 occurrences as those described are frequent in the Lake Superior 

 iron region, the Appalachian Valley of the eastern states, in 

 Nova Scotia, Arkansas, Colorado, New Mexico and innumerable 

 other places. In Virginia very common occurrences are alternat- 

 ing layers of iron and manganese ore. The iron in such cases is 

 generally in the larger quantities and the more continuous 

 deposits ; while the manganese is often represented by thin len- 

 ticular layers or by bands of nodules. 



From such cases, where iron predominates, there are all 

 gradations in admixture, up to the rarer cases where manganese 

 predominates. Frequently a given geologic horizon is charac- 

 terized by both iron and manganese, though in one case it may 

 contain only iron, in another only manganese, and in still another 

 iron and manganese mixed in various proportions. A remarkable 

 case of this is seen in the iron and manganese horizons immedi- 

 ately above, or a short distance above, the Paleozoic quartzite, 

 on the east side of the Appalachian Valley, especially in the 

 Valley of Virginia.^ Here deposits of iron ore, of manganese 

 ore, and of both ores mixed, are found at various points along 

 the same geologic horizons. Similar alternations also occur in the 

 Lower Silurian novaculites of the Ouachita Mountains of Arkan- 

 sas,^ in Cebolla Valley, in Gunnison county, Colorado, 3 and in 



' The exact age of the iron and manganese deposits here referred to is, in some 

 cases, a little uncertain. Some may be Cambrian, others Silurian, but the exact 

 determination of the age of the horizon is not a part of the present discussion. 

 The matter has been discussed by the writer in Geological Survey of Arkansas, 1890, 

 Vol. I., pp. 376-379- 



'See Geological Survey of Arkansas, 1890, Vol. I., pp. j'2G-325. 



3 See Geological Survey of Arkansas, 1890, Vol. I., pp. 456-457. 



