THE ORIGIN OF GUMBOTIL I2i 



soluble alkalies and the alkaline silicates are carried downward 

 by the moving free-water. The negative colloidal siHcates and 

 silicic acid are coagulated and thus rendered motionless by the 

 positively charged calcium and magnesium ions. As hydrolysis 

 proceeds the mass of the insoluble material thus formed increases 

 and probably does continue to increase until all of the easily 

 available, hydrolyzable materials are used up or removed. 



The second stage of the leaching process now follows. Obvi- 

 ously those insoluble substances which are most easily attacked 

 will be the first to be leached away. These are the carbonates of 

 calcium and magnesium. Although only very sHghtly soluble, 

 the dissolved portions of these combine with the carbonic acid 

 of the soil solution to form the soluble acid carbonates. These 

 are carried downward to lower levels, where in fissures and crevices 

 they again crystalHze as irregular concretions. In this way were 

 formed all of those calcareous concretions which are found in the 

 ^.^^zed zone. 



According to the law of mass action, the activity, or the solvent 

 effect, of the carbon dioxide will be greatest at points where its 

 concentration is a maximum. This obviously will be at the upper 

 level of the initially unleached calcareous zone. Owing to its 

 diffusion power some of the carbon dioxide may escape combina- 

 tion at the upper level only to combine at a slightly lower level. 

 Ultimately there will be a lower limit beyond which the carbon 

 dioxide entering from the atmosphere will not penetrate, or its 

 concentration in the soil solution will be too slight to produce any 

 appreciable chemical effect. These two limits of maximum and 

 minimum activity represent the boundaries of the dynamic zone 

 of carbonic acid activity — the oxidized and leached zone. As time 

 goes on the concretions at the upper level disappear, and the levels 

 of maximum and minimum activity move downward simultaneously. 



This dynamic zone has played an important role in all drift 

 transformations. It spreads horizontally like a continuous sheet 

 of more or less uniform thickness. It is found always directly 

 upon the oxidized and unleached drift and directly below the gimi- 

 botil. In the Nebraskan drift it is thin, less than two feet to some- 

 what more than four feet. The oxidized leached zone of the 



