TYPES OF SOILS 31 



and a cubic foot of it weighs from seventy-five to eighty 

 pounds. Clay is powdery when dry, sticky when wet, and is 

 easily molded. Silt consists of particles somewhat coarser than 

 clay. They range in diameter from five thousandths to five 

 hundredths of a millimeter. When moist, silt becomes a soft 

 mud, and in drying inclines to crumble. Sand consists of loose 

 hard grains from five hundredths of a millimeter to one milli- 

 meter in diameter, resulting from the weathering of sandstones 

 and quartzes. The grains may be angular or rounded, but are 

 always harsh and granular to the touch. A cubic foot of sand 

 weighs from one hundred to one hundred ten pounds. Wet 

 sand is held together by the moisture ; when dry, the grains 

 at once fall apart. Crravel is a mixture of many kinds of rock 

 fragments and differs from sand chiefly in the size of the parti- 

 cles composing it. The smaller fragments are called pebbles ; 

 the larger, bowlders. Glacial pebbles are angular in shape. 

 When pebbles are rounded it is an indication that they have 

 been worked over by water. Gravel is usually accompanied 

 by varying amounts of sand and clay, and often forms rich 

 soils. Peat, muck, marl, and humus are all of organic origin ; 

 clay, silt, sand, and gravel are inorganic. The nature of the 

 soil greatly influences the plants that grow on it. This is shown 

 from the fact that plants on the same kind of soil in different 

 parts of the world resemble one another. 



Sand and clay contrasted. The best soil for ordinary crops 

 is a mixture of clay, sand, silt, and humus. Owing to the 

 contrasting characters of clay and sand, the soil is heavy or 

 light, cold or warm, moist or dry, worked with difficulty or 

 easily worked, according to whether one or the other predom- 

 inates. Neither forms a good soil by itself, but intermingled 

 in various proportions they give a wide range of soils from 

 which the farmer and gardener can select one suited to the 

 crop he proposes to grow. Clay consists of the finest of soil 

 particles. It would require 400,000 of the smallest, side by 



