201 



222. The structure of the soil. If 



pure coarse sand is examined under 

 magnification, it is readily seen to con- 

 sist of many small rock particles. Some 

 are flattish, others are cuboid or ob- 

 long, some have sharp corners, and 

 some are well rounded. The surfaces 

 of these small particles of rock are 

 glistening and glass-like. In fact, glass 

 is made by melting pure sand so that 

 the silica of which sand is composed 

 flows together in a solid mass (for 

 articles on glass-making see the ency- 

 clopedias). An examination of sand 

 of different degrees of coarseness 

 readily shows that the difference is 

 due to variation in the size of the 

 rock particles of which the sand con- 

 sists. In sand that is fine enough to 

 be blown by the winds the particles 

 are usually so small as to look like 

 dust, but upon magnification their 

 rock-like nature appears. 



Clay consists chiefly of extremely 

 small particles which fit together so 

 as to make a compact soil so com- 

 pact that, when quite dry, pieces of 

 this soil are sometimes said to be " as 

 hard as rock." A high power of 

 magnification is required in order to 

 distinguish the smallest rock particles 

 in clay soils. Mixed with the finest 

 clay particles we sometimes find sand, 

 peat, or gravel, the result being sandy, 

 peaty, or gravelly clay. 



FIG. 99. Soil particles 



Particles assorted by settling 

 long in water. Gravel at bot- 

 tom ; fine sand and silt at top 



