THE SOIL PARTICLE 99 



The sands and the gravel, because of their sizes, func- 

 tion as separate particles. They aFe irregular and rounded, 

 the continual rubbing that they have received being 

 sufficient to have effaced their angular character. They 

 exhibit very low plasticity and cohesion, and as a con- 

 sequence are little influenced by changes in water content. 

 Their water-holding capacity is low, and because of the 

 large size of the spaces between each separate particle 

 the passage of water is rapid. They therefore facilitate 

 drainage and encourage good air movement. In all 

 the grades of sand the separate particles are visible to 

 the naked eye, a condition impossible with the silt and 

 clay groups. Soil containing much sand or gravel, 

 therefore, is of an open character, possessing good 

 drainage and aeration, and is usually in a loose friable 

 condition. 



74. Mineralogical characteristics of the separates. — 

 From the mineralogical standpoint there are usually 

 considerable differences in the soil separates. These 

 differences would naturally be expected to occur par- 

 ticularly in residual soils, because of the differentiating 

 tendencies of weathering. Quartz would naturally per- 

 sist, and because of its slow solubility would very soon 

 make up most of the larger soil grains. Other minerals, 

 such as the feldspars, hornblende, augite, and the like, 

 being less persistent as the law of mineral resistance 

 has already taught us, would be worn to fine shreds 

 and be found as the main constituents of the silts 

 and the clays. The following data sustain this as- 

 sumption regarding the mineralogical characteristics of 

 some of the soil groups as designated by the Bureau of 

 Soils as well as furnish some interesting comparisons of 

 some important soil provinces : — 



