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THE JOURNAL OF GEOLOGY.

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.

The iron and manganese contained in sedimentary strata may be considered as derived primarily from the decay of pre-existing rocks. Some of the later sedimentary rocks may have derived a part or all of their iron from older sedimentary rocks, which, in turn, had derived their iron and manganese from still older rocks. In this way the iron and manganese in a given geologic horizon may have formed a part of various older horizons before they reached their present resting place, but, in every case, their primary source can be traced back to the original materials from which sedimentary rocks were first formed. In certain cases the sea water has supplied a certain amount of iron and manganese to sedimentary rocks, but in such cases the sea water acts only as a carrier of these materials from the land areas or from submarine sources to the strata then forming.

THE TRANSPORTATION OF IRON AND MANGANESE IN NATURE.

The process that goes on in this interchange of iron and manganese from older to younger rocks is as follows:

  • (1) The conversion, by surface agencies, of the minerals containing iron and manganese into forms that can be taken into solution by surface waters.
  • (2) The solution of the iron and manganese in surface waters, acidulated with organic and sometimes inorganic acids, and their transportation in this form from the areas of older rocks to areas over which younger rocks are being deposited.
  • (3) Finally, the precipitation in one or more of several ways of the iron and manganese contained in solution.
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