THE ORIGIN OF GUMBOTIL 113 



of gluing together vegetable earths into layers impervious to water. 

 Their action consists mainly in the removal of calcium, magnesium, 

 and iron, which are again precipitated at the lower limit of action, 

 either by soluble salts, by exchange of bases, or by loss of water. 

 The precipitated organic humus is finally oxidized and disappears, 

 depositing the base metals as hydroxides or carbonates. 



Summary of the mineral constituents of the soil solution. — In 

 summarizing, the mineral matter of the soil solution may be 

 divided into two classes. The more easily diffusible are those 

 comprising the soluble alkali salts, the soluble acid-carbonates 

 of calcium and magnesium, the sHghtly soluble ferrous compounds, 

 the ferric humates, and the semi-colloidal sodium and potassium 

 silicates. The less easily diffusible are the colloidal gelatinous 

 silicic acid, the gelatinous hydrated silicates, and the colloidal 

 hydrated ferric oxide. The solvent action of the alkaline-soil 

 solution and of the humic acids, aided by the abrasive effects of 

 the earth's displacements, slowly but surely transform the quartz 

 pebbles into colloidal silica. Under the influence of the decaying 

 organic matter the ferric compounds are reduced, temporarily at 

 least, to ferrous compounds. While the existence of ferrous com- 

 pounds in contact with the oxygenated soil atmosphere must 

 obviously be a short one, the alternate oxidation and reduction 

 permit the slow downward transportation of iron. The decompo- 

 sition of the original complex aluminum silicates leads ultimately 

 to the formation of the colloidal hydrated aluminum silicates. 

 These are the most complex, most resistant, and the least soluble of 

 all of the decomposition products produced by the disintegration of 

 silicate rocks. 



Hence in the leaching of the so-called weathering products of 

 the original glacial till one should expect to find a gradual relative 

 increase in the proportion of the soluble diffusible materials from 

 the surface downward. On the contrary, conditions permitting, 

 a gradual decrease in the proportion of alumina should be observed. 

 This is exactly what is found from a study of the results of the 

 chemical analyses of a complete series of strata in any single 

 complete cut. 



