SOIL-FORMING PROCESSES 27 



in another way — as water of combination.^ The process is 

 called hydration. While hydration usually proceeds or ac- 

 companies oxidation and carbonation, thus making them pos- 

 sible, it often, unlike these transformations, occurs at great 

 depths and may be practically the only change that the rock 

 minerals have undergone. Many minerals, especially the oliv- 

 ine, feldspar and mica groups, are so affected. They become 

 soft and lose their luster and elasticity on the assumption of 

 this chemically combined water. Considerable increase of 

 bulk occurs during the transition of the rock to soil. The 

 latter change has no small physical significance. This hydra- 

 tion is particularly effective in encouraging other kinds of 

 chemical decay. In addition to the examples already cited, 

 the change of hematite to limonite, which occurs to a greater 

 or less degree in every soil where the sesquioxide is present, 

 is worthy of note : 



2Pe203 + 3HoO =z 2Fe203 . SH^O 

 Hematite Water Limonite (yelfow) 



When the products of weathering dry out due to varying 

 weather conditions, dehydration may occur. Thus limonite 

 may readily reduce to a lower hydrate or to hematite. 



19. Solution. — It is quite evident that while weathering 

 and erosion produce many compounds of a very complex char- 

 acter, there is a tendency toward simplification and, as water 

 is universally present, some solution occurs. Such bases as 

 calcium, magnesium, sodium and potassium are found in the 

 water that circulates in rocks, soil materials and soils. These 

 bases, when in solution, are generally combined as chlorides, 

 phosphates, nitrates, carbonates, and the like. Carbon dioxide 

 intensifies to a marked degree the solvent action of water and 

 consequently increases its power as a weathering agent. The 



^JSTote carefully the difference between hydration and the production 

 of an hydroxide. The former is the more important as a soil phenome- 

 non. 



