io6 GEORGE F. KAY AND J. NEWTON PEARCE 



gumbotils and underlying oxidized and leached tills have been 

 formed by chemical weathering and leaching of till which was 

 originally unoxidized and unleached. 



The abundant field evidence supporting this theory has been 

 presented. Emphasis has been put upon the gradation of gum- 

 botil into underlying till, the variations in the sizes of pebbles in 

 the related zones, and the presence of remnants of thoroughly 

 disintegrated and decomposed bowlders in the transition zone 

 between gumbotil and oxidized and unleached till. 



There have been profound changes involving chemical processes 

 which operated during immense lengths of time, and which occurred 

 long ages ago. These chemical processes are subject to a few 

 definite, general physical laws of nature which are independent 

 of time or place. The laws of stress and strain, of the degradation 

 of energy, of hydrolysis, of mass-action, or of solution in general 

 are as lasting as the universe itself. 



The role of water in geochemical changes. — The dominant 

 factor in all of these geo-physico-chemical changes is water, more 

 especially the aerated water. When the rain falls upon the ground 

 one part, the "run-off,"^ flows over the surface and escapes by 

 way of the natural drainage channels. It is this form which pro- 

 duces erosion. A second part, the "fly-off," immediately evapo- 

 rates into the air, while the third part, the "cut-off," penetrating 

 the soil by way of the soil interstices, flows downward under the 

 influence of gravity. Of these the cut-off water is the only form 

 which is directly effective in geochemical transformations. It 

 moves through the soil and its substrata with comparative rapidity, 

 reappearing elsewhere as seepage water or as springs. 



The rain and surface waters contain dissolved oxygen, nitrogen, 

 and carbon dioxide, each in proportion to its partial pressure in the 

 atmosphere. " The chemical and physical processes which are con- 

 tinually taking place below the surface involve the absorption and 

 formation of carbon dioxide and the disappearance of oxygen and 

 nitrogen. These gases impart to the soil an atmosphere, and their 

 concentrations in the soil solution follow more or less slowly the 

 barometric changes above the surface. The soil bacteria and 



' This terminology was proposed by McGee. 



