122 GEORGE F. KAY AND J. NEWTON PEARCE ' 



Kansan drift averages about five feet, attaining in a few places a 

 thickness of about seven feet. That of the lUinoian has an average 

 thickness not to exceed six feet. 



After the leaching of the calcium and magnesium carbonates 

 there follows a third step. When the concentrations of the pre- 

 cipitating calcium and magnesium ions have been reduced below 

 their critical coagulating values new processes occur within the 

 leaching zone. The coagulated iron passes into solution either as 

 colloidal ferric hydroxide by peptization or defiocculation by the 

 emulsoidal organic humous material or through the influence of 

 peptizing ions, or as ferrous compounds by reduction by organic 

 matter. Thus, either by colloidal flow, by alternate reduction 

 and oxidation, or through the medium of its soluble or slightly 

 soluble salts, the iron is leached and slowly passes downward. 

 The silica either in the form of the colloidal gelatinous silicic 

 acid or as the alkaline silicates also moves downward. Likewise, 

 through various peptizing influences the colloidal clays and the 

 simpler colloidal silicates begin to swell and deflocculate. Ulti- 

 mately some of these pass into suspensions of colloidal particles. 

 They are caught also in the downward current and carried by it to 

 lower levels, where they are again coagulated. Only a very slight 

 amount of this kind of material is leached away. There is left above 

 only the more resistant, less mobile, complex colloidal aluminum 

 silicates. 



The stratum now forming, deprived of practically all of its 

 sodium and potassium, of most of its calcium and magnesium, and 

 some of its iron and silica, is the present residuum of the whole 

 chemical leaching process. This is the gumbotil. 



Physical and chemical properties of the gumhotih. — The properties 

 of the gumbotil are largely those which one might predict from a 

 knowledge of the colloidal chemistry of clays. Like certain colloids 

 it becomes very hard and tenacious when dry; it swells when wetted 

 and then to some extent passes spontaneously into colloidal sus- 

 pension. It becomes sticky and sometimes so slippery that under 

 the pressure of the earth above it oozes or slides out of the sides 

 of the hills. It is gray when dry, dark when wet. The char- 

 acteristic color changes of the gumbotil are those imparted to it 



