The Soil and Soil Water 27 



present. From this great watershed the ice sheet moved Where the 

 slowly but irresistibly down into what is now the United camT/rom 

 States. The Middle West and the North owe much* 

 of their fertility and depth of soil to the excellent work 

 done by this great ice mill. The grist it ground was 

 rock, but the soil that resulted has produced the grist 

 for many a flour mill which at present rumbles in that 

 country. 



Constituents of the soil. The mineral particles of 

 the soil are classified in accordance with their fineness, 

 as gravel, sand, silt, and clay (Fig. 18). All four come 

 from the same source, rock, and they differ from one 

 another only in the size of the particles composing them. 

 If twenty-five grains of the coarsest sand are laid side 

 by side, they reach an inch. Anything coarser than 

 that is called gravel. It would require five hundred The size oj 

 grains of the finest sand to extend one inch. Silt grains 

 are so small that anywhere from five hundred to five 

 thousand would be needed to reach an inch. Any soil 

 in which the grains are smaller than this is clay. (Exp. 4 

 and Exp. 5.) In ordinary garden soil there are about 

 a hundred billion grains to each ounce. Besides the Humus 

 mineral matter, gravel, sand, silt, and clay, there is 

 present in all soils a small quantity of partially decayed 

 organic matter, chiefly of vegetable origin. This is 



o 



4 56 



FIG. 18. Comparative sizes of soil grains: i, medium sand; 2, fine sand; 

 3, very fine sand ; 4, silt; 5, fine silt; 6, clay. (All enlarged about 100 times.) 



