I20 GEORGE F. KAY AND J. NEWTON PEARCE 



clay is a subsequent formation, it also will have been leached of 

 some of its CaO and MgO. This means a slight increase in the 

 proportions of these two in the stratum below, the leaching of which 

 has not been completed. 



The leaching process. — The siHca and iron are less diffusible and 

 hence less subject to leaching than are the carbonates of calcium 

 and magnesium, and a much longer time is required for complete 

 leaching. In every instance the proportion of iron in the gumbotil 

 is less than it is in the oxidized and leached stratum just below. 

 Except for the case of the Nebraskan (Table III), the proportion of 

 SiOz is greater in the oxidized and leached stratum than in the 

 gimibotil. It should be observed that the Nebraskan at locaHty 

 "C" underlies the Kansan, and the apparent discrepancy may 

 be accounted for in the transfusion of the alkaline silicates from 

 the Kansan into the upper strata of the Nebraskan below. On 

 the basis of parts per loo parts of AI2O3 not only CaO and MgO 

 but also SiOa show distinct evidence of leaching even in the oxidiz . 

 and leached zone, and this is true for practically every series. On 

 the same basis the evidence points to a leaching of the iron into 

 the oxidized and leached zone from the gumbotil, but the time 

 allowed was not sufficient for the subsequent leaching of the iron 

 from the oxidized and leached zone into the one below. This is 

 not surprising, since, as will be shown later, the iron is the last 

 constituent to be leached away. 



The oxygenated and carbonated water falls upon a uniformly 

 level, more or less uniformly constituted, blue to blue-black drift. 

 Percolating downward, it dissolves a portion of the rock material. 

 Hydrolysis follows, and there are liberated successively the hydrox- 

 ides of sodium or potassium, then of calcium or magnesium, and 

 finally the more or less difficultly soluble hydroxides of ferrous and 

 ferric iron, depending upon the natura of the iron in the original 

 silicate. The ferrous iron throughout the depth penetrated by the 

 dissolved oxygen is immediately oxidized and the deposit assumes 

 the typical iron color of the yellow clays. 



The calcium and magnesium hydroxides combine with the 

 carbon dioxide of the soil atmosphere to form the insoluble car- 

 bonates. These crystalHze out as calcareous concretions. The 



