GEOLOGICAL STUDY OF SOILS 59 



ing action during transportation. The area of glacial 

 soils in the northern portion of the United States is 

 quite large. These soils are, as a rule, fertile because of 

 the pulverization and mixing of a great variety of rock. 



58. Chemical Action of Water. The chemical action 

 of water is an important factor in soil formation. While 

 nearly all rocks are practically insoluble in water there 

 is always some material dissolved, evidenced by the fact 

 that all spring water contains dissolved mineral matter. 

 When charged with carbon dioxide and other gases, 

 water acts as a solvent upon rocks ; it converts many 

 oxides, as ferrous oxid'e, into hydroxides, and produces 

 new compounds more soluble or readily disintegrated, 

 as deposits of clay, which have been formed from feld- 

 spar rock by the chemical and physical action of water. 

 Rock decay is often dependent upon chemical change ; 

 the addition of water, or hydration of the molecule, par- 

 ticularly of the silicates, is one of the most important 

 chemical changes. When rocks, as feldspar, disinte- 

 grate, there is an addition of 12 to 14 per cent of water 

 of hydration to the disintegrated products. This chem- 

 ical union of water with the rock materials entirely 

 changes their properties and often prepares the way 

 for other chemical changes. Water takes as prominent 

 a part in the decay of rocks as in the decay of vegeta- 

 ble matter. Dissolved minerals produce many chemical 

 changes in both rocks and soils. The chemical action 

 of fertilizers, known as fixation, can take place only 



