358 THE JOURNAL OF GEOLOGY. 



many other places. In many cases certain horizons are charac- 

 terized 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 man- 

 ganese 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-exist- 

 ing 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 origi- 

 nal 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 : 



(i) The conversion, by surface agencies, of the minerals con- 

 taining 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 mansfanese contained in solution. 



