PHYSICAL STRUCTURE OF THE SOIL 289 



material so compact that when moist it may be pressed into 

 balls which will hold together when thrown with great force. 

 This is known as clay. " Silt " is the name applied to the 

 fine, rich soil that is deposited by currents of water. It is 

 commonly deposited on broad bottom lands at times of high 

 water. In low, swampy ground plants may grow in great 

 quantity and fall down year after year. Their bodies par- 

 tially or completely decay and become pressed into a porous 

 soil known as peat. It must be obvious that these different 

 kinds of soils may be found mixed together in various ways ; 

 indeed, any one of them is usually found with one or more of 

 the others. 



289. Structure of the soil. If pure, coarse sand is ex- 

 amined under magnification, it is readily seen to consist 

 of many small rock particles. Some are flattish, some are 

 cubical or oblong, some have sharp corners, and some are well 

 rounded. The surfaces of these small particles of rock are 

 glistening and glasslike. In fact, glass 'is made by melting 

 pure sand together with other substances. 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 consists. 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 rocklike 

 nature appears. 



Clay consists chiefly of extremely small particles which fit 

 together so as to make a compact soil so compact 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. 



We may therefore have many gradations in soil: from 

 finest clay to coarse clay ; from finest sand to coarse sand, 

 and on to coarse gravel and even to rocky soil ; from sandy 



