WEATHERING 21 



soon further oxidized so that the hydrated ferric oxide and sulfuric 

 acid are produced. The resulting oxide is much softer, more easily 

 removed by water and more bulky than the sulfide, hence becomes 

 quite absorl>ent of moisture and is then readily affected by freezing 

 and thawing. The expansion produced by the change tends to 

 loosen the crystals of the rock and make it very susceptible to other 

 agencies. The same is true in the case of iron existing in the ferrous 

 condition either as a carbonate or silicate. The resulting products 

 of decomposition tend to color the soil material, producing a yel- 

 lowish, brownish, or reddish color. 



(d) Deoxidation. Under certain conditions oxygen will be 

 removed from some compounds, but as a means for breaking down 

 rocks this is not very important. The chief agency in deoxidatiou 

 is organic acids. The great afl'mity of these acids for oxygen enables 

 them to take part or all of it from certain compounds, especially 

 those of iron, as oxides or sulfates producing a different mineral 

 with entirely different physical properties, the m,ost noticeable of 

 which are color and hardness. In swamps organic, acids frequently 

 reduce ferric oxides to ferrous oxides and sulfates to sul fides, result- 

 ing in a grayish or drab color in the subsoil. The gray subsurface 

 and subsoil of many of our poorly drained soils are undoubtedly 

 due to the process of deoxidation. The soil under a peat bed is 

 usually drab, indicating a reduction of iron. 



(e) Hydration. During the process of weathering certain of 

 the common minerals that compose igneous and metamorphic rocks 

 unite with water which not only changes the chemical composition, 

 but produces very important changes in the physical character of the 

 minerals that aid greatly in breaking them down into soil material. 

 This change is usually attended with more or less loss by solution. 

 One of the most important changes is ihc increase in volume, by 

 which there is a tendency to rupture the rock. If no loss took place 

 by solution, the change of granite into soil through various processes 

 of weathering would give an increase in bulk of as much as SS per 

 cent, a large part of which is due to hydration. At the same time 

 the hardness of the rock is lowered very materially, and this, of 

 course, gives other agencies a better chance. The absorption of 

 water will also be increased and free/ing and thawing will Ix 1 more 

 effective. The general result of hydration is to render the rock very 

 susceptible to other agencies. The process of hydration goes on to 

 great depth. Apparently solid but hydrated rock taken from many 



