126 THE GEOLOGY OF MINNESOTA. 



[Soils and subsoils. 



The character and composition of the soils of the state are dependent 

 on two general causes; (1) the nature of the subsoil, (2) the local modify- 

 ing circumstances, the chief of which is in the nature of the local drainage. 



(1) The nature of the subsoil. As an element in the production and 

 modification of soil, the subsoil is most potential. Into it vegetation sends 

 its principal roots, and from it rise, by osmose the salts that renew the sur- 

 face soil when impoverished by cropping or by unfavorable drainage, or by 

 drouth. The soil may be said to be the comminuted and modified upper 

 surface of the subsoil. The subsoils may be grouped under four divisions: 



Subsoils of blue till. 

 Subsoils of red till. 

 Subsoils of gravel or of sand. 

 Subsoils of clay or of clay-loam. 



Whether the subsoil consist of blue till or of red, its physical characters 

 are nearly the same, except that the blue till is generally closer and more 

 impervious than the red, and is less stony; but its chemical characteristics 

 will differ considerably according as it is blue or red. There is nothing 

 of importance in the difference of color. The color simply indicates the 

 origin of the till, and its accompanying qualities. The blue till, in gen- 

 eral, is derived from the disintegration of the (.'rt'fiin'oiiH, and the red from 

 Cambrian, though there are exceptions, and a blue till is also produced 

 by the other formations. It so happens, however, that in Minnesota a large 

 proportion of the clayey parts of the blue till can be referred to the Creta- 

 ceous, with as much certainty as the red to the Cambrian. The Creta- 

 ceous being a marine deposit, of an age when the ocean's waters in the 

 interior of North America were charged with the salts of the alkalies and 

 of the alkaline earths, the till resulting from its disintegration and distri- 

 bution necessarily exhibits the same qualities; and as the soil is dependent 

 largely on the subsoil for its characteristic chemical qualities, it follows 

 that soils based primarily on the blue till in Minnesota will exhibit the 

 same alkaline characters. Such is the case. Soils based directly on the 

 Cretaceous rocks, without the intervention of any sheet of drift, as in west- 

 ern Dakota and in Montana, exhibit these chemical qualities still more 



