A few words concerning the relation of manganese to iron in nature will perhaps make the following discussion clearer. One of the most common modes of occurrence of manganese is with iron, though extensive deposits containing manganese more or less free from iron often occur. When associated with iron, manganese occurs with it in various ways. Sometimes the two are intimately mixed, so that they have the appearance of a homogeneous 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 proportion, 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 alternating 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 lenticular 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 characterized 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 immediately above, or a short distance above, the Paleozoic quartzite, on the east side of the Appalachian Valley, especially in the Valley of Virginia.[13] 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 Arkansas,[14] in Cebolla Valley, in Gunnison county, Colorado,[15] and in many other places. In many cases certain horizons are characterized over large areas by iron alone, and but little manganese, as is well seen in the Clinton formation and in the Tertiary iron-ore horizons of Arkansas and Texas; while, on the other hand, some areas of certain horizons contain considerable quantities of manganese and very little iron, as is seen in parts of the Marine limestone in New Brunswick and Nova Scotia, and also in parts of the metamorphosed Cretaceous shales of California. THE SOURCE OF IRON AND MANGANESE IN SEDIMENTARY ROCKS.