392 Sources of the Constituents of Minnesota Soils. 



geological change affecting a whole region. This change sug- 

 gests another fact within observation: a given type of plant 

 growth will continue to live generation after generation upon the 

 same place. This is the case because the peculiar food constit- 

 uents which are favorable to the growth and maturity of this par- 

 ticular species are continually loosened and separated from the 

 rocks in which they lie, and in the course of time they are com- 

 minuted by temperature changes and rain water at so slow a rate 

 as to furnish plants food at about the same rate as they exhaust it 

 by assimilating it. The great diversity of composition and phys- 

 ical characters which soils present are really primary traits and 

 they depend to a great degree on the nature of the rocks beneath 

 the surface and the topographical features of the country in which 

 they are found. Other traits are secondary; briefly enumerated, 

 they are due : — to depositions of vegetable and animal matter ; to 

 the action of moles, worms, insects; to the growth of roots of 

 trees ; the wind which, in leveling countries, moves great masses 

 of the finer soil, and to those slow but unceasing chemical activi- 

 ties which transform entire rock formations and extend over the 

 continental areas of the globe. 



Darwin fifty years ago noted the peculiar creeping of soils 

 by which lower layers would seem to change places with upper 

 ones, and this apparently without the aid of worms or moles. 

 This phenomenon is probably explained by the capillary action 

 possible through the pressure of water. Underground waters 

 also equalize the temperature of soils to a most remarkable de- 

 gree, a fact of great moment in the steady growth of crops in a 

 changing climate like ours. 



The ivork of the sea in the forming of soils. — While it is not 

 necessary to call attention to those soils formed directly by the sea, 

 still we note that indirectly the work of the sea in the building of 

 all soils is an imposing one. The beginning of this work lies in 

 those silent and ceaseless ages in which the contents of the vast 

 oceanic basins were brought into one compound after another, and 

 at the close of many transformations — some of which transfor- 

 mations are made in the tissues of plants and animals and others 

 in the vast abysmal precipitations by which alumina, iron, silica, 

 and the alkalies,— are packed down on the bed of the sea. Thus 

 the waters of the sea play no insignificant part in soil building, 

 although it is a very indirect one. 



