Sources of the Consfitueufs of Minnesota Soils. 38c) 



gaseous portion — the air and the water. It is, therefore, the zone 

 of chemical activity and represents that era of profound change, 

 under which rocks break in pieces and re-form under new physi- 

 cal conditions and chemical composition ; under which water is 

 absorbed by the rocks or, unabsorbed, becomes loaded with every 

 conceivable chemical element and compound as it passes on in its 

 devious wanderings. 



The Formation of Soils. Touching the formation of soils 

 two things may be noted: (i) The soil is made of debris of 

 weathered and comminuted rocks mingled with the remains of 

 plants; (2) Since the composition of vegetation is always nearly 

 the same, the varying conditions of soils must depend upon two 

 things, the proportion of vegetable remains in the vegeto-mineral 

 mixture and the variation of the rock constituents out of whose de- 

 composition the rnineral portion is obtained. Vegetation in its de- 

 composition plays rather a chemical than a geological part. In the 

 course of years, large quantities of vegetable matter are broken 

 down and are subjected to the various processes of decay. In 

 some instances peat is formed ; in others, the vegetable matter is 

 mingled with mineral to such an extent that it becomes a carbon- 

 aceous rock. 



Mineral matters in the mixture usually take the leading place 

 and are of prime importance in considering the character of soil. 

 The two extremes of infertility in soils are clear, comminuted 

 quartz, kaolin and calcite, — that is, sandstone, clay and limestone, 

 — on the one hand, and clean vegetable mold on the other ; and all 

 the stages of fertility lie between these extremes, where the sand- 

 stone, shale or the limestone, and mold are judiciously ming- 

 led and associated with proper proportions of alumina and the 

 alkalies. 



The process of rock alteration. — The process by which the 

 chemical condition of rocks is changed so that they may become 

 constituents of soils is rock alteration, and this is one of the most 

 constant processes in nature. It is going on wherever water and 

 air can come in contact with the rocks ; so it is not only along the 

 comminuted surface that is exposed to the sunshine, where we 

 call it weathering, but along the deep fissures which extend for 

 many feet — even miles — through the rocks in vertical and hori- 

 zontal directions. The processes by which alteration is effected 

 vary under different conditions. This change is effected in warm 



